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Air ions are the key components for a series of atmospheric physicochemical interactions, such as ion-catalyzed reactions, ion-molecule reactions, and ion-induced new particle formation (NPF). They also control atmospheric electrical properties with effects on global climate. We performed molecular-level measurements of cluster ions at the high-altitude research station Chacaltaya (CHC; 5240 m a.s.l.), located in the Bolivian Andes, from January to May 2018 using an atmospheric-pressure-interface time-of-flight mass spectrometer. The negative ions mainly consisted of (H2SO4)0–3⚫HSO4-, (HNO3)0–2⚫NO3-, SO5-, (NH3)1–6⚫(H2SO4)3–7⚫HSO4-, malonic-acid-derived, and CHO / CHON⚫(HSO4- / NO3-) cluster ions. Their temporal variability exhibited distinct diurnal and seasonal patterns due to the changes in the corresponding neutral species' molecular properties (such as electron affinity and proton affinity) and concentrations resulting from the air masses arriving at CHC from different source regions. The positive ions were mainly composed of protonated amines and organic cluster ions but exhibited no clear diurnal variation. H2SO4–NH3 cluster ions likely contributed to the NPF process, particularly during the wet-to-dry transition period and the dry season, when CHC was more impacted by air masses originating from source regions with elevated SO2 emissions. Our study provides new insights into the chemical composition of atmospheric cluster ions and their role in new particle formation in the high-altitude mountain environment of the Bolivian Andes.

Measurements of the molecular composition of organic aerosol (OA) constituents improve our understanding of sources, formation processes, and physicochemical properties of OA. One instrument providing such data at a time resolution of minutes to hours is the chemical ionization time-of-flight mass spectrometer with filter inlet for gases and aerosols (FIGAERO-CIMS). The technique collects particles on a filter, which are subsequently desorbed, and the evaporated molecules are ionized and analyzed in the mass spectrometer. However, long-term measurements using this technique and/or field deployments at several sites simultaneously require substantial human and financial resources. The analysis of filter samples collected outside the instrument (offline) may provide a more cost-efficient alternative and makes this technology available for the large number of particle filter samples collected routinely at many different sites globally. Filter-based offline use of the FIGAERO-CIMS limits this method, albeit to particle-phase analyses, which is likely at a reduced time resolution compared to online deployments. Here we present the application and assessment of offline FIGAERO-CIMS, using Teflon and quartz fiber filter samples that were collected in autumn 2018 in urban Beijing. We demonstrate the feasibility of the offline application with a “sandwich” sample preparation for the over 900 identified organic compounds with (1) high signal-to-noise ratios, (2) high repeatability, and (3) linear signal response to the filter loadings. Comparable overall signals were observed between the quartz fiber and Teflon filters for 12 and 24 h samples but with larger signals for semi-volatile compounds for the quartz fiber filters, likely due to adsorption artifacts. We also compare desorption profile (thermogram) shapes for the two filter materials. Thermograms are used to derive volatility qualitatively based on the desorption temperature at which the maximum signal intensity of a compound is observed (Tmax⁡). While we find that Tmax⁡ can be determined with high repeatability (±5.7 ∘C) from the duplicate tests for one filter type, we observe considerable differences in Tmax⁡ between the quartz and Teflon filters, warranting further investigation into the thermal desorption characteristics of different filter types. Overall, this study provides a basis for expanding OA molecular characterization by FIGAERO-CIMS to situations where and when deployment of the instrument itself is not possible.

Turbulence is the key process transporting material and energy in the atmosphere. Furthermore, turbulence causes concentration fluctuations, influencing different atmospheric processes such as deposition, chemical reactions, formation of low-volatile vapours, formation of new aerosol particles and their growth in the atmosphere, and the effect of aerosol particles on boundary-layer meteorology. In order to analyse the connections, interactions and feedbacks relating those different processes require a deep understanding of atmospheric turbulence mechanisms, atmospheric chemistry and aerosol dynamics. All these processes will further influence air pollution and climate. The better we understand these processes and their interactions and associated feedback, the more effectively we can mitigate air pollution as well as mitigate climate forcers and adapt to climate change. We present several aspects on the importance of turbulence including how turbulence is crucial for atmospheric phenomena and feedbacks in different environments. Furthermore, we discuss how boundary-layer dynamics links to aerosols and air pollution. Here, we present also a roadmap from deep understanding to practical solutions.

The size distributions of urban atmospheric aerosols convey important information on their origins and impacts. Their long-term characteristics, especially for sub-3 nm particles, are still limited. In this study, we examined the characteristics of atmospheric aerosol size distributions down to ∼1 nm based on 4-year measurements in urban Beijing. Using cluster analysis, three typical types of number size distributions were identified, i.e., daytime new particle formation (NPF) type, daytime non-NPF type, and nighttime type. Combining a power law distribution and multiple lognormal distributions can well represent the sharp concentration decrease of sub-3 nm particles with increasing size and the modal characteristics for those above 3 nm in the submicron size range. The daytime NPF type exhibits high concentrations of sub-3 nm aerosols together with other three modes. However, both the daytime non-NPF type and the nighttime type have a low abundance of sub-3 nm aerosol particles together with only two distinct modes. In urban Beijing, the concentration of H2SO4 monomer during the daytime with NPF is similar to that during the daytime without NPF, while significantly higher than that during the nighttime. The concentration of atmospheric sub-3 nm particles on NPF days has a strong seasonality while their seasonality on non-NPF days is less pronounced. In addition to NPF as the most important source, we show that vehicles can emit sub-3 nm particles as well, although their influence on the measured aerosol population strongly depends on the distance from the road.

The understanding of new particle formation and growth processes is critical for evaluating the role of aerosols in climate change. One of the knowledge gaps is the ion-particle interaction during the early growth process, especially in the sub-3 nm range, where direct observations are sparse. While molecular interactions would imply faster growth rates of ions compared to neutral particles, this phenomenon is not widely observed in the atmosphere. Here, we show field measurements in the boreal forest indicating a smaller apparent growth rate of the ion population compared to the total particles. We use aerosol dynamics simulations to demonstrate that this effect can be caused by a changing importance of ion-induced nucleation mechanisms during the day. We further compare these results with chamber experiments under similar conditions, where we demonstrate that this effect critically depends on the abundance of condensable vapors and the related strength of ion-induced nucleation. Our results imply that atmospheric ion growth rate measurements below 3 nm need to be evaluated very carefully as they do not represent condensational growth alone but are influenced by ion-particle population interactions.

Atmospheric cluster ions are important constituents in the atmosphere, and their concentrations and compositions govern their role in atmospheric chemistry. However, there is currently limited quantitative research on atmospheric ion compositions, sources, and sinks, especially in the urban atmosphere where pollution levels and human populations are intense. In this study, we measured the compositions of negative cluster ions and neutral molecules using an atmospheric pressure interface high-resolution time-of-flight mass spectrometer (APi-TOF) and a chemical ionization mass spectrometer in urban Beijing. Quantitative analysis of cluster ions was performed by their comparison with condensation sink (CS), reagent ions, and neutral molecules. We demonstrate the feasibility of quantifying cluster ions with different compositions using in situ-measured ion mobility distributions from a neutral cluster and air ion spectrometer (NAIS). The median concentration of negative cluster ions was 85 (61–112 for 25 %–75 %) cm−3 during the measurement period, which was negatively correlated with CS. The negative cluster ions mainly consisted of inorganic nitrogen-containing ions, inorganic sulfur-containing ions, and organic ions in the form of adducts with NO3- or HSO4-. The CHON-related organic ions accounted for over 70 % of the total organic ions. Although the molecules clustered with NO3- and HSO4- had similar compositions, we found that HSO4- clustered more efficiently with CHO and CHONnonNPs species (CHON excluding nitrated phenols), while NO3- clustered more efficiently with nitrated phenols (CHONNPs). Additionally, most organic ions were positively correlated with neutral molecules, resulting in similar diurnal cycles of organic ions and neutral molecules. However, an exception was found for CHONNPs, the concentration of which is also significantly influenced by the reagent ions NO3-. The charge fractions are generally higher for molecules with higher molecular weight and a higher oxidation state, and the opposite diurnal variations in charging fractions between H2SO4 and organic species indicate a charging competition between them. Finally, we choose HSO4- and C3H3O4- as representatives to calculate the contribution of different formation and loss pathways. We found their losses are condensational loss onto aerosol particles (73 %–75 %), ion–molecule reaction losses (19 %), and ion–ion recombination losses (6 %–8 %).

Several studies have investigated new particle formation (NPF) events from various sites ranging from pristine locations, including forest sites, to urban areas. However, there is still a dearth of studies investigating NPF processes and subsequent aerosol growth in coastal yet semi-urban sites, where the tropospheric layer is a concoction of biogenic and anthropogenic gases and particles. The investigation of factors leading to NPF becomes extremely complex due to the highly dynamic meteorological conditions at the coastline especially when combined with both continental and oceanic weather conditions. Herein, we engage in a comprehensive study of particle number size distributions and aerosol-forming precursor vapors at the coastal semi-urban site in Helsinki, Finland. The measurement period, 25 June–18 August 2019, was timed with the recurring cyanobacterial summer bloom in the Baltic Sea region and coastal regions of Finland. Our study recorded several regional/local NPF and aerosol burst events during this period. Although the overall anthropogenic influence on sulfuric acid (SA) concentrations was low during the measurement period, we observed that the regional or local NPF events, characterized by SA concentrations on the order of 107 molec.cm-3, occurred mostly when the air mass traveled over the land areas. Interestingly, when the air mass traveled over the Baltic Sea, an area enriched with algae and cyanobacterial blooms, high iodic acid (IA) concentration coincided with an aerosol burst or a spike event at the measurement site. Further, SA-rich bursts were seen when the air mass traveled over the Gulf of Bothnia, enriched with cyanobacterial blooms. The two most important factors affecting aerosol precursor vapor concentrations, and thus the aerosol formation, were speculated to be (1) the type of phytoplankton species and intensity of bloom present in the coastal regions of Finland and the Baltic Sea and (2) the wind direction. During the events, most of the growth of sub-3 nm particles was probably due to SA, rather than IA or methane sulfonic acid (MSA); however much of the particle growth remained unexplained indicative of the strong role of organics in the growth of particles, especially in the 3–7 nm particle size range. Further studies are needed to explore the role of organics in NPF events and the potential influence of cyanobacterial blooms in coastal locations.

The formation of ice particles in Earth's atmosphere strongly influences the dynamics and optical properties of clouds and their impacts on the climate system. Ice formation in clouds is often triggered heterogeneously by ice-nucleating particles (INPs) that represent a very low number of particles in the atmosphere. To date, many sources of INPs, such as mineral and soil dust, have been investigated and identified in the low and mid latitudes. Although less is known about the sources of ice nucleation at high latitudes, efforts have been made to identify the sources of INPs in the Arctic and boreal environments. In this study, we investigate the INP emission potential from high-latitude boreal forests in the mixed-phase cloud regime. We introduce the HyICE-2018 measurement campaign conducted in the boreal forest of Hyytiala, Finland, between February and June 2018. The campaign utilized the infrastructure of the Station for Measuring Ecosystem-Atmosphere Relations (SMEAR) II, with additional INP instruments, including the Portable Ice Nucleation Chamber I and II (PINC and PINCii), the SPectrometer for Ice Nuclei (SPIN), the Portable Ice Nucleation Experiment (PINE), the Ice Nucleation SpEctrometer of the Karlsruhe Institute of Technology (INSEKT) and the Microlitre Nucleation by Immersed Particle Instrument (mu L-NIPI), used to quantify the INP concentrations and sources in the boreal environment. In this contribution, we describe the measurement infrastructure and operating procedures during HyICE-2018, and we report results from specific time periods where INP instruments were run in parallel for inter-comparison purposes. Our results show that the suite of instruments deployed during HyICE-2018 reports consistent results and therefore lays the foundation for forthcoming results to be considered holistically. In addition, we compare measured INP concentrations to INP parameterizations, and we observe good agreement with the Tobo et al. (2013) parameterization developed from measurements conducted in a ponderosa pine forest ecosystem in Colorado, USA., Atmospheric Chemistry and Physics, 22 (8), ISSN:1680-7375, ISSN:1680-7367

Co-occurrences of high concentrations of PM2.5 and ozone (O3) have been frequently observed in haze-aggravating processes in the North China Plain (NCP) over the past few years. Higher O3 concentrations on hazy days were hypothesized to be related to nitrous acid (HONO), but the key sources of HONO enhancing O3 during haze-aggravating processes remain unclear. We added six potential HONO sources, i.e., four ground-based (traffic, soil, and indoor emissions, and the NO2 heterogeneous reaction on ground surface (Hetground)) sources, and two aerosol-related (the NO2 heterogeneous reaction on aerosol surfaces (Hetaerosol) and nitrate photolysis (Photnitrate)) sources into the WRF-Chem model and designed 23 simulation scenarios to explore the unclear key sources. The results indicate that ground-based HONO sources producing HONO enhancements showed a rapid decrease with height, while the NO + OH reaction and aerosol-related HONO sources decreased slowly with height. Photnitrate contributions to HONO concentrations were enhanced with aggravated pollution levels. The enhancement of HONO due to Photnitrate on hazy days was about 10 times greater than on clean days and Photnitrate dominated daytime HONO sources (∼ 30 %–70 % when the ratio of the photolysis frequency of nitrate (Jnitrate) to gas nitric acid (JHNO3) equals 30) at higher layers (>800 m). Compared with that on clean days, the Photnitrate contribution to the enhanced daily maximum 8 h averaged (DMA8) O3 was increased by over 1 magnitude during the haze-aggravating process. Photnitrate contributed only ∼ 5 % of the surface HONO in the daytime with a Jnitrate/JHNO3 ratio of 30 but contributed ∼ 30 %–50 % of the enhanced O3 near the surface in NCP on hazy days. Surface O3 was dominated by volatile organic compound-sensitive chemistry, while O3 at higher altitudes (>800 m) was dominated by NOx-sensitive chemistry. Photnitrate had a limited impact on nitrate concentrations (

Better monitoring, reporting, and verification (MRV) of the amount, additionality, and persistence of the sequestered soil carbon is needed to understand the best carbon farming practices for different soils and climate conditions, as well as their actual climate benefits or cost efficiency in mitigating greenhouse gas emissions. This paper presents our Field Observatory Network (FiON) of researchers, farmers, companies, and other stakeholders developing carbon farming practices. FiON has established a unified methodology towards monitoring and forecasting agricultural carbon sequestration by combining offline and near-real-time field measurements, weather data, satellite imagery, modeling, and computing networks. FiON's first phase consists of two intensive research sites and 20 voluntary pilot farms testing carbon farming practices in Finland. To disseminate the data, FiON built a web-based dashboard called the Field Observatory (v1.0, https://www.fieldobservatory.org/, last access: 3 February 2022). The Field Observatory is designed as an online service for near-real-time model–data synthesis, forecasting, and decision support for the farmers who are able to monitor the effects of carbon farming practices. The most advanced features of the Field Observatory are visible on the Qvidja site, which acts as a prototype for the most recent implementations. Overall, FiON aims to create new knowledge on agricultural soil carbon sequestration and effects of carbon farming practices as well as provide an MRV tool for decision support.

Lung-deposited surface area (LDSA) has been considered to be a better metric to explain nanoparticle toxicity instead of the commonly used particulate mass concentration. LDSA concentrations can be obtained either by direct measurements or by calculation based on the empirical lung deposition model and measurements of particle size distribution. However, the LDSA or size distribution measurements are neither compulsory nor regulated by the government. As a result, LDSA data are often scarce spatially and temporally. In light of this, we developed a novel statistical model, named the input-adaptive mixed-effects (IAME) model, to estimate LDSA based on other already existing measurements of air pollutant variables and meteorological conditions. During the measurement period in 2017–2018, we retrieved LDSA data measured by Pegasor AQ Urban and other variables at a street canyon (SC, average LDSA = 19.7 ± 11.3 µm2 cm−3) site and an urban background (UB, average LDSA = 11.2 ± 7.1 µm2 cm−3) site in Helsinki, Finland. For the continuous estimation of LDSA, the IAME model was automatised to select the best combination of input variables, including a maximum of three fixed effect variables and three time indictors as random effect variables. Altogether, 696 submodels were generated and ranked by the coefficient of determination (R2), mean absolute error (MAE) and centred root-mean-square difference (cRMSD) in order. At the SC site, the LDSA concentrations were best estimated by mass concentration of particle of diameters smaller than 2.5 µm (PM2.5), total particle number concentration (PNC) and black carbon (BC), all of which are closely connected with the vehicular emissions. At the UB site, the LDSA concentrations were found to be correlated with PM2.5, BC and carbon monoxide (CO). The accuracy of the overall model was better at the SC site (R2=0.80, MAE = 3.7 µm2 cm−3) than at the UB site (R2=0.77, MAE = 2.3 µm2 cm−3), plausibly because the LDSA source was more tightly controlled by the close-by vehicular emission source. The results also demonstrated that the additional adjustment by taking random effects into account improved the sensitivity and the accuracy of the fixed effect model. Due to its adaptive input selection and inclusion of random effects, IAME could fill up missing data or even serve as a network of virtual sensors to complement the measurements at reference stations.

Wetlands cover only 3g% of the global land surface area, but boreal wetlands are experiencing an unprecedented warming of four times the global average. These wetlands emit isoprene and terpenes (including monoterpenes (MT), sesquiterpenes (SQT), and diterpenes (DT)), which are climate-relevant highly reactive biogenic volatile organic compounds (BVOCs) with an exponential dependence on temperature. In this study, we present ecosystem-scale eddy covariance (EC) fluxes of isoprene, MT, SQT, and DT (hereafter referred to together as terpenes) at Siikaneva, a boreal fen in southern Finland, from the start to the peak of the growing season of 2021 (19 May 2021 to 28 June 2021). These are the first EC fluxes reported using the novel state-of-the-art Vocus proton transfer reaction mass spectrometer (Vocus-PTR) and the first-ever fluxes reported for DTs from a wetland. Isoprene was the dominant compound emitted by the wetland, followed by MTs, SQTs, and DTs, and they all exhibited a strong exponential temperature dependence. The Q10 values, the factor by which terpene emissions increases for every 10gg C rise in temperature, were up to five times higher than those used in most BVOC models. During the campaign, the air temperature peaked above 31gg C on 21-22 June 2021, which is abnormally high for boreal environments, and the maximum flux for all terpenes coincided with this period. We observed that terpene emissions were elevated after this abnormally "high-temperature stress period", indicating that past temperatures alter emissions significantly. The standardized emission factor (EF) of the fen for isoprene (EFiso) was 11.1g±g 0.3gnmolgm-2gs-1, which is at least two times higher than in previous studies and as high as the emission factors typical for broadleaf and other forests in the lower latitudes. We observed EFMT of 2.4g±g 0.1gnmolgm-2gs-1, EFSQT of 1.3g±g0.03gnmolgm-2gs-1, higher than typical for needle leaf and broadleaf tree functional types, and EFDT of 0.011g±g0.001gnmolgm-2gs-1. We also compared the landscape average emissions to the model of emissions of gases and aerosols from nature (MEGAN) v2.1 and found that the emissions were underestimated by over 9 times for isoprene, over 300 times for MTs, and 800 times for SQTs. Our results show that due to very high EFs and high sensitivity to increasing temperatures, these high-latitude ecosystems can be a large source of terpenes to the atmosphere, and anthropogenic global warming could induce much higher BVOC emissions from wetlands in the future., Lejish Vettikkat was financially supported by the UEF EPHB doctoral degree program. This work was financially supported by the Academy of Finland Flagship program (grant nos. 337550 and 337549) and Academy of Finland project no. 310682. Simon Schallhart was supported by Academy of Finland project no. 323255. Tuukka Petäjä was supported by Academy of Finland project nos. 1325681 and 1328616. Elisa Männistö and Eeva-Stiina Tuittila were supported by Academy of Finland project no. 330840. Roger Seco was supported by a Ramón y Cajal grant (RYC2020-029216-I) funded by MCIN/AEI/10.13039/501100011033 and by “ESF Investing in your future”, as well as project PID2021-122892NA-I00 funded by MCIN/AEI and by “ERDF A way of making Europe”. IDAEA-CSIC is a Severo Ochoa Centre of Research Excellence (MCIN/AEI, Project CEX2018-000794-S). Alex B. Guenther was supported by the US National Science Foundation award ANS-2041250.

Dimethyl sulfide (DMS) is the primary natural contributor to the atmospheric sulfur burden. Observations concerning the fate of DMS oxidation products after long-range transport in the remote free troposphere are, however, sparse. Here we present quantitative chemical ionization mass spectrometric measurements of DMS and its oxidation products sulfuric acid (H2SO4), methanesulfonic acid (MSA), dimethylsulfoxide (DMSO), dimethylsulfone (DMSO2), methanesulfinic acid (MSIA), methyl thioformate (MTF), methanesulfenic acid (MSEA, CH3SOH), and a compound of the likely structure CH3S(O)2OOH in the gas phase, as well as measurements of the sulfate and methanesulfonate aerosol mass fractions. The measurements were performed at the Global Atmosphere Watch (GAW) station Chacaltaya in the Bolivian Andes located at 5240 m above sea level (a.s.l.). DMS and DMS oxidation products are brought to the Andean high-altitude station by Pacific air masses during the dry season after convective lifting over the remote Pacific ocean to 6000–8000 m a.s.l. and subsequent long-range transport in the free troposphere (FT). Most of the DMS reaching the station is already converted to the rather unreactive sulfur reservoirs DMSO2 in the gas phase and methanesulfonate (MS−) in the particle phase, which carried nearly equal amounts of sulfur to the station. The particulate sulfate at Chacaltaya is however dominated by regional volcanic emissions during the time of the measurement and not significantly affected by the marine air masses. In one of the FT events, even some DMS was observed next to reactive intermediates such as methyl thioformate, dimethylsulfoxide, and methanesulfinic acid. Also for this event, back trajectory calculations show that the air masses came from above the ocean (distance >330 km) with no local surface contacts. This study demonstrates the potential impact of marine DMS emissions on the availability of sulfur-containing vapors in the remote free troposphere far away from the ocean.

27 pages, 12 figures, supplement https://doi.org/10.5194/acp-23-389-2023.-- Data availability: All data sets used in this work that were obtained during the MOSAiC campaign will be made publicly available by 1 January 2023 via PANGAEA (https://www.pangaea.de/, last access: June 2022) or are already publicly available in the Department of Energy Atmospheric Radiation Measurement program (ARM) user facility data discovery tool (https://adc.arm.gov/discovery/#/, last access: March 2022). Data from the PANGAEA archive include the following. Meteorological observations from Polarstern: Schmithüsen, H.: Continuous meteorological surface measurement during POLARSTERN cruise PS122/1. Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, Bremerhaven, PANGAEA [data set], https://doi.org/10.1594/PANGAEA.935221, 2021a. Schmithüsen, H.: Continuous meteorological surface measurement during POLARSTERN cruise PS122/2. Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, Bremerhaven, PANGAEA [data set], https://doi.org/10.1594/PANGAEA.935222, 2021b. Schmithüsen, H.: Continuous meteorological surface measurement during POLARSTERN cruise PS122/3. Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, Bremerhaven, PANGAEA [data set], https://doi.org/10.1594/PANGAEA.935223, 2021c. Schmithüsen, H.: Continuous meteorological surface measurement during POLARSTERN cruise PS122/4. Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, Bremerhaven, PANGAEA [data set], https://doi.org/10.1594/PANGAEA.935224, 2021d. Schmithüsen, H.: Continuous meteorological surface measurement during POLARSTERN cruise PS122/5. Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, Bremerhaven, PANGAEA [data set], https://doi.org/10.1594/PANGAEA.935225, 2021e. Black carbon (BC): Heutte, B., Beck, I., Quéléver, L., Jokinen, T., Laurila, T., Dada, L., Schmale, J.: Equivalent black carbon concentration in 10 minutes time resolution, measured in the Swiss container during MOSAiC 2019/2020, PANGAEA [data set], https://doi.org/10.1594/PANGAEA.952251, 2022. Particle number concentration (CPC3025): Beck, I., Quéléver, L., Laurila, T., Jokinen, T., and Schmale, J.: Continuous corrected particle number concentration data in 10 sec resolution, measured in the Swiss aerosol container during MOSAiC 2019/2020, PANGAEA [data set], https://doi.org/10.1594/PANGAEA.941886, 2022b. The ARM data include the following: Kuang, C., Singh, A., and Howie, J.: Scanning mobility particle sizer (AOSSMPS), ARM [data set], https://doi.org/10.5439/1476898, 2022. Kuang, C., Salwen, C., Boyer, M., and Singh, A.: Condensation Particle Counter (AOSCPCF), ARM [data set], https://doi.org/10.5439/1046184, 2022. The land-based PNSD data: Alert: Personal communication from Tak Chan and Sangeeta Sharma, 2022 (for details, see Croft et al., 2016). Villum: Personal communication from Jakob Boyd Pernov, 2022 (for details, see Nguyen et al., 2016). Zeppelin: Personal communication from Peter Tunved, 2022 (for details, see Tunved et al., 2013). Tiksi: Personal communication from Eija Asmi, 2022 (for details, see Asmi et al., 2016). Utqiaġvik: Freud, E., Krejci, R., Tunved, P., Leaitch, W. R., Nguyen, Q. T., Massling, A., Skov, H., and Barrie, L.: Hourly mean homogenised (dry diameter range 20 to 500 nm) observations of aerosol number size distributions from station Barrow, 2007-09-20 to 2015-07-09, PANGAEA [data set], https://doi.org/10.1594/PANGAEA.877329, 2017b. The land-based BC data: Villum: Personal communications from Daniel Thomas, Jakob Klenø Nøjgaard, and Andreas Massling, 2022 (for details, see Thomas et al., 2022). NOAA Barrow Atmospheric Baseline Observatory (Utqiaġvik) during 2020: personal communication from Elisabeth Andrews, 2022 (for processing details, see Schmale et al., 2022). Gruvebadet. The PSAP data are accessible at the Italian Arctic Data Center operated by the National Research Council of Italy: https://data.iadc.cnr.it/erddap/tabledap/ebc_2010_2020.html (last access: December 2021, refer to Schmale et al., 2022, for additional details). All other BC data sets used in this work will be made publicly available on EBAS (http://ebas-data.nilu.no/, last access: December 2021, refer to Schmale et al., 2022, for additional details). The Arctic Oscillation (AO) data are publicly available from the NOAA and the National Weather Service: https://www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_ao_index/ao.shtml (last access: June 2022). An archive of the FLEXPART model output and quick looks for the whole campaign can be found at https://img.univie.ac.at/webdata/mosaic (last access: March 2022), The Arctic environment is rapidly changing due to accelerated warming in the region. The warming trend is driving a decline in sea ice extent, which thereby enhances feedback loops in the surface energy budget in the Arctic. Arctic aerosols play an important role in the radiative balance and hence the climate response in the region, yet direct observations of aerosols over the Arctic Ocean are limited. In this study, we investigate the annual cycle in the aerosol particle number size distribution (PNSD), particle number concentration (PNC), and black carbon (BC) mass concentration in the central Arctic during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition. This is the first continuous, year-long data set of aerosol PNSD ever collected over the sea ice in the central Arctic Ocean. We use a k-means cluster analysis, FLEXPART simulations, and inverse modeling to evaluate seasonal patterns and the influence of different source regions on the Arctic aerosol population. Furthermore, we compare the aerosol observations to land-based sites across the Arctic, using both long-term measurements and observations during the year of the MOSAiC expedition (2019–2020), to investigate interannual variability and to give context to the aerosol characteristics from within the central Arctic. Our analysis identifies that, overall, the central Arctic exhibits typical seasonal patterns of aerosols, including anthropogenic influence from Arctic haze in winter and secondary aerosol processes in summer. The seasonal pattern corresponds to the global radiation, surface air temperature, and timing of sea ice melting/freezing, which drive changes in transport patterns and secondary aerosol processes. In winter, the Norilsk region in Russia/Siberia was the dominant source of Arctic haze signals in the PNSD and BC observations, which contributed to higher accumulation-mode PNC and BC mass concentrations in the central Arctic than at land-based observatories. We also show that the wintertime Arctic Oscillation (AO) phenomenon, which was reported to achieve a record-breaking positive phase during January–March 2020, explains the unusual timing and magnitude of Arctic haze across the Arctic region compared to longer-term observations. In summer, the aerosol PNCs of the nucleation and Aitken modes are enhanced; however, concentrations were notably lower in the central Arctic over the ice pack than at land-based sites further south. The analysis presented herein provides a current snapshot of Arctic aerosol processes in an environment that is characterized by rapid changes, which will be crucial for improving climate model predictions, understanding link, This research has been supported by the Academy of Finland (grant no. 337552), Horizon 2020 (EMME-CARE, grant no. 856612), the US Department of Energy (grant no. DE-SC0022046), and the Swiss Polar Institute (grant no. 188478), With the institutional support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S)

Most trees emit volatile organic compounds (VOCs) continuously throughout their life, but the rate of emission and spectrum of emitted VOCs become substantially altered when the trees experience stress. Despite this, models to predict the emissions of VOCs do not account for perturbations caused by biotic plant stress. Considering that such stresses have generally been forecast to increase in both frequency and severity in the future climate, the neglect of stress-induced plant emissions in models might be one of the key obstacles for realistic climate change predictions, since changes in VOC concentrations are known to greatly influence atmospheric aerosol processes. Thus, we constructed a model to study the impact of biotic plant stresses on new particle formation and growth throughout a full growing season. We simulated the influence on aerosol processes caused by herbivory by the European gypsy moth (Lymantria dispar) and autumnal moth (Epirrita autumnata) feeding on pedunculate oak (Quercus robur) and mountain birch (Betula pubescens var. pumila), respectively, and also fungal infections of pedunculate oak and balsam poplar (Populus balsamifera var. suaveolens) by oak powdery mildew (Erysiphe alphitoides) and poplar rust (Melampsora larici-populina), respectively. Our modelling results indicate that all the investigated plant stresses are capable of substantially perturbing both the number and size of aerosol particles in atmospherically relevant conditions, with increases in the amount of newly formed particles by up to about an order of magnitude and additional daily growth of up to almost 50 nm. We also showed that it can be more important to account for biotic plant stresses in models for local and regional predictions of new particle formation and growth during the time of infestation or infection than significant variations in, e.g. leaf area index and temperature and light conditions, which are currently the main parameters controlling predictions of VOC emissions. Our study thus demonstrates that biotic plant stress can be highly atmospherically relevant. To validate our findings, field measurements are urgently needed to quantify the role of stress emissions in atmospheric aerosol processes and for making integration of biotic plant stress emission responses into numerical models for prediction of atmospheric chemistry and physics, including climate change projection models, possible.

This research was supported by the European Commission's Horizon 2020 Framework Programme (ACTRIS2 (grant agreement no. 654109)), the University of Helsinki, the Finnish Meteorological Institute, the Department of Science and Innovation of South Africa, the Academy of Finland Centre of Excellence programme (project no. 272041), the Academy of Finland project Greenhouse gas, aerosol and albedo variations in the changing Arctic (project no. 269095), the Novel Assessment of Black Carbon in the Eurasian Arctic: From Historical Concentrations and Sources to Future Climate Impacts (NABCEA, project no. 296302), the Korea Meteorological Administration Research and Development Program "Development of Monitoring and Analysis Techniques for Atmospheric Composition in Korea" (grant no. KMA2018-00522), the National Research Foundation of Korea (grant no. 2017R1D1A1B06032548), the Korea Meteorological Administration Research and Development Program (grant no. KMI2018-01111), the Taiwan Environmental Protection Administration, the China Meteorological Administration, the National Scientific Foundation of China (41675129, 41875147), the National Key R&D Program of the Ministry of Science and Technology of the People's Republic of China (grant no. 2016YFC0203305 and 2018YFC0213204), the Chinese Academy of Meteorological Sci-ences (2020KJ001), the Innovation Team for Haze-fog Observation and Forecasts of MOST and CMA, CNRS-INSU, the French Ministry for Research under the ACTRIS-FR national research infrastructure, the French Ministry of the Environment, MeteoSwiss (GAW-CH aerosol monitoring programme), the Swiss State Secretariat for Education, Research and Innovation (SERI), the Ministry of Education, Youth and Sports of CR within National Sustainability Program I (NPU I, grant no. LO1415), ERDF "ACTRISCZ RI" (grant no. CZ.02.1.01/0.0/0.0/16_013/0001315), CRISOL (CGL2017-85344-R MINECO/AEI/FEDER, UE), TIGAS-CM (Madrid Regional Government Y2018/EMT-5177), AIRTECCM (Madrid Regional Government P2018/EMT4329), REDMAAS2020 (RED2018-102594-T CIENCIA), Red de Excelencia ACTRIS-ESPANA (CGL2017-90884-REDT), the Spanish Ministry of Economy, Industry and Competitiveness, FEDER funds (project HOUSE, grant no. CGL2016-78594-R), the Generalitat de Catalunya (AGAUR 2017 SGR41 and the DGQA), the National Institute for Aerospace Technology, the Ministerio Espanol de Economia, Industria y Competitividad (MINECO), the Spanish Ministry of Economy and Competitiveness (projects no. CGL2016-81092-R, CGL2017-90884-REDT, RTI2018-097864-BI00 and PGC2018-098770-B-I00), the Andalusia Regional Government (project no. P18-RT-3820), the PANhellenic infrastructure for Atmospheric Composition and climate change (MIS 5021516), Research and Innovation Infrastructure, Competitiveness, Entrepreneurship and Innovation (grant no. NSRF 20142020), the Italian Ministry of Research and Education, the Norwegian Environment Agency, Swedish FORMAS, the Swedish Research Council (VR), the Magnus Bergvall foundation, the Marta och Erik Holmberg foundation, and the Swedish EPA., Aerosol particles are a complex component of the atmospheric system which influence climate directly by interacting with solar radiation, and indirectly by contributing to cloud formation. The variety of their sources, as well as the multiple transformations they may undergo during their transport (including wet and dry deposition), result in significant spatial and temporal variability of their properties. Documenting this variability is essential to provide a proper representation of aerosols and cloud condensation nuclei (CCN) in climate models. Using measurements conducted in 2016 or 2017 at 62 ground-based stations around the world, this study provides the most up-to-date picture of the spatial distribution of particle number concentration (N-tot) and number size distribution (PNSD, from 39 sites). A sensitivity study was first performed to assess the impact of data availability on N-tot's annual and seasonal statistics, as well as on the analysis of its diel cycle. Thresholds of 50% and 60% were set at the seasonal and annual scale, respectively, for the study of the corresponding statistics, and a slightly higher coverage (75 %) was required to document the diel cycle. Although some observations are common to a majority of sites, the variety of environments characterizing these stations made it possible to highlight contrasting findings, which, among other factors, seem to be significantly related to the level of anthropogenic influence. The concentrations measured at polar sites are the lowest (similar to 10(2) cm(-3)) and show a clear seasonality, which is also visible in the shape of the PNSD, while diel cycles are in general less evident, due notably to the absence of a regular day-night cycle in some seasons. In contrast, the concentrations characteristic of urban environments are the highest (similar to 10(3)-10(4) cm(-3)) and do not show pronounced seasonal variations, whereas diel cycles tend to be very regular over the year at these stations. The remaining sites, including mountain and non-urban continental and coastal stations, do not exhibit as obvious common behaviour as polar and urban sites and display, on average, intermediate N-tot (similar to 10(2)-10(3) cm(-3)). Particle concentrations measured at mountain sites, however, are generally lower compared to nearby lowland sites, and tend to exhibit somewhat more pronounced seasonal variations as a likely result of the strong impact of the atmospheric boundary layer (ABL) influence in connection with the topography of the sites. ABL dynamics also likely contribute to the diel cycle of N-tot observed at these stations. Based on available PNSD measurements, CCN-sized particles (considered here as either >50 nm or >100 nm) can represent from a few percent to almost all of N-tot, corresponding to seasonal medians on the order of similar to 10 to 1000 cm(-3), with seasonal patterns and a hierarchy of the site types broadly similar to those observed for N-tot. Overall, this work illustrates the importance of in situ measurements, in particular for the study of aerosol physical properties, and thus strongly supports the development of a broad global network of near surface observatories to increase and homogenize the spatial coverage of the measurements, and guarantee as well data availability and quality. The results of this study also provide a valuable, freely available and easy to use support for model comparison and validation, with the ultimate goal of contributing to improvement of the representation of aerosol-cloud interactions in models, and, therefore, of the evaluation of the impact of aerosol particles on climate., European Commission's Horizon 2020 Framework Programme (ACTRIS2) 654109, University of Helsinki, Finnish Meteorological Institute, Department of Science and Innovation of South Africa, Academy of Finland 272041, Academy of Finland project Greenhouse gas, aerosol and albedo variations in the changing Arctic 269095, Novel Assessment of Black Carbon in the Eurasian Arctic: From Historical Concentrations and Sources to Future Climate Impacts (NABCEA) 296302, Korea Meteorological Administration Research and Development Program "Development of Monitoring and Analysis Techniques for Atmospheric Composition in Korea" KMA2018-00522, National Research Foundation of Korea 2017R1D1A1B06032548, Korea Meteorological Administration Research and Development Program KMI2018-01111, Taiwan Environmental Protection Administration, China Meteorological Administration, National Natural Science Foundation of China (NSFC) 41675129 41875147, National Key R&D Program of the Ministry of Science and Technology of the People's Republic of China 2016YFC0203305 2018YFC0213204, Chinese Academy of Meteorological Sciences 2020KJ001, Innovation Team for Haze-fog Observation and Forecast of MOST Innovation Team for Haze-fog Observation and Forecast of CMA Innovation Team for Haze-fog Observation and Forecast of CNRS-INSU, French Ministry for Research under the ACTRIS-FR national research infrastructure, French Ministry of the Environment, MeteoSwiss (GAW-CH aerosol monitoring programme), Swiss State Secretariat for Education, Research and Innovation (SERI), Ministry of Education, Youth and Sports of CR within National Sustainability Program I (NPU I) LO1415, ERDF "ACTRISCZ RI" CZ.02.1.01/0.0/0.0/16_013/0001315, CRISOL CGL2017-85344, TIGAS-CM (Madrid Regional Government) Y2018/EMT-5177, AIRTECCM (Madrid Regional Government) P2018/EMT4329, REDMAAS2020 RED2018-102594-T, Red de Excelencia ACTRIS-ESPANA CGL2017-90884-REDT, Spanish Ministry of Economy, Industry and Competitiveness, FEDER funds CGL2016-78594-R, Generalitat de Catalunya, General Electric AGAUR 2017 SGR41, National Institute for Aerospace Technology, Ministerio Espanol de Economia, Industria y Competitividad (MINECO) Spanish Government CGL2017-90884-REDT CGL2016-81092-R RTI2018-097864-BI00 PGC2018-098770-B-I00, Andalusia Regional Government P18-RT-3820, PANhellenic infrastructure for Atmospheric Composition and climate change MIS 5021516, Research and Innovation Infrastructure, Competitiveness, Entrepreneurship and Innovation NSRF 20142020, Ministry of Education, Universities and Research (MIUR), Norwegian Environment Agency, Swedish FORMAS, Swedish Research Council, Magnus Bergvall foundation, Marta och Erik Holmberg foundation, Swedish EPA

The concentration of atmospheric ions affects the total aerosol particle number concentrations in the atmosphere as well as atmospheric new particle formation via ion-induced nucleation, ion–ion recombination, and effects on condensational growth. In this study, we investigate the concentrations and long-term trends of atmospheric ions in a boreal forest environment using 16 years of cluster ion (0.8–2 nm) and intermediate ion (2–7 nm) measurements and characterize the most important factors that explain those trends. We found that the median concentration of cluster ions in a boreal forest was 710 cm−3, the median concentration of 2–4 intermediate ions was 14 cm−3, and the median concentration of 4–7 nm intermediate ions was 9 cm−3. The concentrations of both cluster and intermediate ions have been increasing over the 16-year measurement period, with cluster ion concentrations increasing by about 1 % yr−1 and intermediate ion concentrations increasing 1.7 %–3.9 % yr−1. The increase in cluster ion concentrations can be best explained by the decrease in the coagulation sink caused by larger aerosol particles. Meanwhile, the dependence of intermediate ion concentrations on meteorological conditions is evident, but ionization sources and the coagulation sink do not seem to explain the increasing trend. This is likely because the dynamics of intermediate ions are more complicated, so that ionization sources and the coagulation sink alone cannot directly explain the variation. Season-specific analysis of the ion concentrations suggests that while the coagulation sink is the limiting factor for the ion concentrations in spring and summer, the dynamics are different in autumn and winter. Based on our findings, we recommend that a more comprehensive analysis is needed to determine if the increase in ambient ion concentrations, increasing temperature, and changing abundance of condensable vapors makes ion-mediated and ion-induced nucleation pathways in the boreal forest more relevant in the future.

Atmospheric new particle formation (NPF) events are regularly observed in urban Beijing, despite high concentrations of background particles which, based on theory, should inhibit NPF due to high values of coagulation sink (CoagS). The survival probability, which depends on both CoagS and particle growth rate (GR), is a key parameter in determining the occurrence of NPF events as it describes the fraction of newly formed particles that survive from a smaller diameter to a larger diameter. In this study, we investigate and compare survival probabilities from 1.5 to 3 nm (J3/J1.5), from 3 to 6 nm (J6/J3), and from 6 to 10 nm (J10/J6) based on analytical formulae, cluster population simulations, and atmospheric observations from Beijing. We find that survival probabilities based on the cluster population simulations and one of the analytical formulae are in a good agreement. However, at low ratios between the background condensation sink (CS) and GR, and at high concentrations of sub-3 nm clusters, cluster–cluster collisions efficiently lower survival probabilities in the cluster population simulations. Due to the large concentrations of clusters and small particles required to considerably affect the survival probabilities, we consider it unlikely that cluster–cluster collisions significantly affect atmospheric survival probabilities. The values of J10/J6 observed in Beijing show high variability, most likely due to influences of primary particle emissions, but are on average in relatively good agreement with the values based on the simulations and the analytical formulae. The observed values of J6/J3 are mostly lower than those predicted based on the simulations and the analytical formulae, which could be explained by uncertainties in CS and GR. The observed values of J3/J1.5 at high CS / GR are much higher than predicted based on the simulations and the analytical formulae. We argue that uncertainties in GR or CS are unlikely to solely explain the observed values of J3/J1.5 under high CS conditions. Thus, further work is needed to better understand the factors influencing survival probabilities of sub-3 nm atmospheric particles in polluted environments.

The survival probability of freshly nucleated particles governs the influences of new particle formation (NPF) on atmospheric environments and the climate. It characterizes the probability of a particle avoiding being scavenged by the coagulation with pre-existing particles and other scavenging processes before the particle successfully grows up to a certain diameter. Despite its importance, measuring the survival probability has been challenging, which limits the knowledge of particle survival in the atmosphere and results in large uncertainties in predicting the influences of NPF. Here we report the proper methods to retrieve particle survival probability using the measured aerosol size distributions. Using diverse aerosol size distributions from urban Beijing, the Finnish boreal forest, a chamber experiment, and aerosol kinetic simulations, we demonstrate that each method is valid for a different type of aerosol size distribution, whereas misapplying the conventional methods to banana-type NPF events may underestimate the survival probability. Using these methods, we investigate the consistency between the measured survival probability of new particles and the theoretical survival probability against coagulation scavenging predicted using the measured growth rate and coagulation sink. With case-by-case and time- and size-resolved analysis of long-term measurement data from urban Beijing, we find that although both the measured and theoretical survival probabilities are sensitive to uncertainties and variations, they are, on average, consistent with each other for new particles growing from 1.4 (the cluster size) to 100 nm.

A multitude of biogeochemical feedback mechanisms govern the climate sensitivity of Earth in response to radiation balance perturbations. One feedback mechanism, which remained missing from most current Earth System Models applied to predict future climate change in IPCC AR6, is the impact of higher temperatures on the emissions of biogenic volatile organic compounds (BVOCs), and their subsequent effects on the hydroxyl radical (OH) concentrations. OH, in turn, is the main sink term for many gaseous compounds including methane, which is the second most important human-influenced greenhouse gas in terms of climate forcing. In this study, we investigate the impact of this feedback mechanism by applying two models, a one-dimensional chemistry-transport model, and a global chemistry-transport model. The results indicate that in a 6 K temperature increase scenario, the BVOC-OH-CH4 feedback increases the lifetime of methane by 11.4% locally over the boreal region when the temperature rise only affects chemical reaction rates, and not both, chemistry and BVOC emissions. This would lead to a local increase in radiative forcing through methane (ΔRFCH4) of approximately 0.013 Wm−2 per year, which is 2.1% of the current ΔRFCH4. In the whole Northern hemisphere, we predict an increase in the concentration of methane by 0.024% per year comparing simulations with temperature increase only in the chemistry or temperature increase in chemistry and BVOC emissions. This equals approximately 7% of the annual growth rate of methane during the years 2008–2017 (6.6 ± 0.3 ppb yr−1) and leads to an ΔRFCH4 of 1.9 mWm−2 per year.

Oxygenated organic molecules (OOMs) are crucial for atmospheric new particle formation and secondary organic aerosol (SOA) growth. Therefore, understanding their chemical composition, temporal behavior, and sources is of great importance. Previous studies on OOMs mainly focus on environments where biogenic sources are predominant, yet studies on sites with dominant anthropogenic emissions, such as megacities, have been lacking. Here, we conducted long-term measurements of OOMs, covering four seasons of the year 2019, in urban Beijing. The OOM concentration was found to be the highest in summer (1.6×108 cm−3), followed by autumn (7.9×107 cm−3), spring (5.7×107 cm−3) and winter (2.3×107 cm−3), suggesting that enhanced photo-oxidation together with the rise in temperature promote the formation of OOMs. Most OOMs contained 5 to 10 carbon atoms and 3 to 7 effective oxygen atoms (nOeff=nO-2×nN). The average nOeff increased with increasing atmospheric photo-oxidation capacity, which was the highest in summer and the lowest in winter and autumn. By performing a newly developed workflow, OOMs were classified into the following four types: aromatic OOMs, aliphatic OOMs, isoprene OOMs, and monoterpene OOMs. Among them, aromatic OOMs (29 %–41 %) and aliphatic OOMs (26 %–41 %) were the main contributors in all seasons, indicating that OOMs in Beijing were dominated by anthropogenic sources. The contribution of isoprene OOMs increased significantly in summer (33 %), which is much higher than those in the other three seasons (8 %–10 %). Concentrations of isoprene (0.2–5.3×107 cm−3) and monoterpene (1.1–8.4×106 cm−3) OOMs in Beijing were lower than those reported at other sites, and they possessed lower oxygen and higher nitrogen contents due to high NOx levels (9.5–38.3 ppbv – parts per billion by volume) in Beijing. With regard to the nitrogen content of the two anthropogenic OOMs, aromatic OOMs were mainly composed of CHO and CHON species, while aliphatic OOMs were dominated by CHON and CHON2 ones. Such prominent differences suggest varying formation pathways between these two OOMs. By combining the measurements and an aerosol dynamic model, we estimated that the SOA growth rate through OOM condensation could reach 0.64, 0.61, 0.41, and 0.30 µg m−3 h−1 in autumn, summer, spring, and winter, respectively. Despite the similar concentrations of aromatic and aliphatic OOMs, the former had lower volatilities and, therefore, showed higher contributions (46 %–62 %) to SOA than the latter (14 %–32 %). By contrast, monoterpene OOMs and isoprene OOMs, limited by low abundances or high volatilities, had low contributions of 8 %–12 % and 3 %–5 %, respectively. Overall, our results improve the understanding of the concentration, chemical composition, seasonal variation, and potential atmospheric impacts of OOMs, which can help formulate refined restriction policy specific to SOA control in urban areas.

In this study, we modeled the aerosol particle formation along air mass trajectories arriving at the remote Arctic research stations Gruvebadet (67 m a.s.l.) and Zeppelin (474 m a.s.l.), Ny-Ålesund, during May 2018. The aim of this study was to improve our understanding of processes governing secondary aerosol formation in remote Arctic marine environments. We run the Lagrangian chemistry transport model ADCHEM, along air mass trajectories generated with FLEXPART v10.4. The air masses arriving at Ny-Ålesund spent most of their time over the open ice-free ocean. In order to capture the secondary aerosol formation from the DMS emitted by phytoplankton from the ocean surface, we implemented a recently developed comprehensive DMS and halogen multi-phase oxidation chemistry scheme, coupled with the widely used Master Chemical Mechanism (MCM). The modeled median particle number size distributions are in close agreement with the observations in the marine-influenced boundary layer near-sea-surface Gruvebadet site. However, while the model reproduces the accumulation mode particle number concentrations at Zeppelin, it overestimates the Aitken mode particle number concentrations by a factor of ∼5.5. We attribute this to the deficiency of the model to capture the complex orographic effects on the boundary layer dynamics at Ny-Ålesund. However, the model reproduces the average vertical particle number concentration profiles within the boundary layer (0–600 m a.s.l.) above Gruvebadet, as measured with condensation particle counters (CPCs) on board an unmanned aircraft system (UAS). The model successfully reproduces the observed Hoppel minima, often seen in particle number size distributions at Ny-Ålesund. The model also supports the previous experimental findings that ion-mediated H2SO4–NH3 nucleation can explain the observed new particle formation in the marine Arctic boundary layer in the vicinity of Ny-Ålesund. Precursors resulting from gas- and aqueous-phase DMS chemistry contribute to the subsequent growth of the secondary aerosols. The growth of particles is primarily driven via H2SO4 condensation and formation of methane sulfonic acid (MSA) through the aqueous-phase ozonolysis of methane sulfinic acid (MSIA) in cloud and deliquescent droplets.

The occurrence frequency of regional atmospheric new aerosol particle formation and consecutive growth events (fNPF) were studied with respect to vegetation activity, aerosol properties, air pollutants and meteorological data in Budapest over the time interval from 2008 to 2018. The data set evaluated contained results of in situ measurements on the land surface that were mostly performed at the Budapest platform for Aerosol Research and Training Laboratory, of satellite-based products recorded by MODIS on Terra and of modelled vegetation emission-related properties from an advanced regional biogeochemical model. The annual mean relative occurrence frequencies were considerable (with an overall mean of 21 %), remained at a constant level (with an overall SD of 5 %) and did not exhibit tendentious change over the years. The shape of the distributions of monthly mean fNPF exhibited large variability from year to year, while the overall average distribution already possessed a characteristic pattern. The structure of the new particle formation (NPF) occurrence distributions was compared to those of environmental variables including concentrations of gas-phase H2SO4, SO2, O3, NO, NO2, CO, PM10 mass and NH3; particle numbers in the size fractions of 6–1000, 6–100 and 100–1000 nm; condensation sink; air temperature (T); relative humidity (RH); wind speed (WS); atmospheric pressure (P); global solar radiation (GRad); gross primary production (GPP) of vegetation; leaf area index (LAI); and stomatal conductance (SCT). There were no evident systematic similarities between fNPF on the one hand and all of the variables studied on the other hand, except for H2SO4 and perhaps NH3. The spring maximum in the NPF occurrence frequency distribution often overlapped with the time intervals of positive T anomaly in vegetated territories. The link between the potential heat stress exerted on plants in sultry summer intervals and the summer fNPF minimum could not be proven. The relevance of environmental variables was assessed by their ratios on NPF event days and on non-event days. The gas-phase H2SO4 concentration showed the largest monthly ratios, followed by O3. The WS, biogenic precursor gases and SO2 can generally favour NPF events, although their influence seemed to be constrained. An association between the fNPF and vegetation growth dynamics was clearly identified.

The occurrence of new particle formation (NPF) events detected in a coastal agricultural site, at Qvidja, in Southwestern Finland, was investigated using the data measured with a nitrate ion-based chemical-ionization atmospheric-pressure-interface time-of-flight (CI-APi-TOF) mass spectrometer. The binned positive matrix factorization method (binPMF) was applied to the measured spectra. It resulted in eight factors describing the time series of ambient gas and cluster composition at Qvidja during spring 2019. The most interesting factors related to the observed NPF events were the two factors with the highest mass-to-charge ratios, numbered 7 and 8, both having profiles with patterns of highly oxygenated organic molecules with one nitrogen atom. It was observed that factor 7 had elevated intensities during the NPF events. A variable with an even better connection to the observed NPF events is fF7, which denotes the fraction of the total spectra within the studied mass-to-charge ratio range between 169 and 450 Th being in a form of factor 7. Values of fF7 higher than 0.50±0.05 were observed during the NPF events, of which durations also correlated with the duration of fF7 exceeding this critical value. It was also observed that factor 8 acts like a precursor for factor 7 with solar radiation and that the formation of factor 8 is associated with ozone levels.

Short summary We summarize results during the last 5 years in the northern Eurasian region, especially from Russia, and introduce recent observations of the air quality in the urban environments in China. Although the scientific knowledge in these regions has increased, there are still gaps in our understanding of large-scale climate–Earth surface interactions and feedbacks. This arises from limitations in research infrastructures and integrative data analyses, hindering a comprehensive system analysis. The Pan-Eurasian Experiment (PEEX) Science Plan, released in 2015, addressed a need for a holistic system understanding and outlined the most urgent research needs for the rapidly changing Arctic-boreal region. Air quality in China, together with the long-range transport of atmospheric pollutants, was also indicated as one of the most crucial topics of the research agenda. These two geographical regions, the northern Eurasian Arctic-boreal region and China, especially the megacities in China, were identified as a “PEEX region”. It is also important to recognize that the PEEX geographical region is an area where science-based policy actions would have significant impacts on the global climate. This paper summarizes results obtained during the last 5 years in the northern Eurasian region, together with recent observations of the air quality in the urban environments in China, in the context of the PEEX programme. The main regions of interest are the Russian Arctic, northern Eurasian boreal forests (Siberia) and peatlands, and the megacities in China. We frame our analysis against research themes introduced in the PEEX Science Plan in 2015. We summarize recent progress towards an enhanced holistic understanding of the land–atmosphere–ocean systems feedbacks. We conclude that although the scientific knowledge in these regions has increased, the new results are in many cases insufficient, and there are still gaps in our understanding of large-scale climate–Earth surface interactions and feedbacks. This arises from limitations in research infrastructures, especially the lack of coordinated, continuous and comprehensive in situ observations of the study region as well as integrative data analyses, hindering a comprehensive system analysis. The fast-changing environment and ecosystem changes driven by climate change, socio-economic activities like the China Silk Road Initiative, and the global trends like urbanization further complicate such analyses. We recognize new topics with an increasing importance in the near future, especially “the enhancing biological sequestration capacity of greenhouse gases into forests and soils to mitigate climate change” and the “socio-economic development to tackle air quality issues”.

Secondary aerosols are a major component of PM2.5, yet their formation mechanisms in the ambient atmosphere are still unclear. Based on field measurements in downtown Beijing, we show that the photolysis of nitrous acid (HONO) may promote the formation of organic and nitrate aerosols in winter in Beijing, which is supported by the fact that the mass concentrations of organic and nitrate aerosols linearly increase as a function of HONO consumed from early morning to noon. The increased nitrate content also leads to the formation of ammonium particulate matter through enhancing the neutralization of nitrate and sulfate by ammonia. We further illustrate that during pollution events in winter in Beijing, over 50 % of the ambient HONO may be related to traffic-related emissions, including direct emissions and formation via the reaction between OH and vehicle-emitted NO. Overall, our results indicate that traffic-related HONO may play an important role in the oxidative capacity and in turn contribute to haze formation in winter in Beijing. The mitigation of HONO and NOx emissions from vehicles may be an effective way to reduce the formation of secondary aerosols and severe haze events in winter in Beijing.

Although secondary particulate matter is reported to be the main contributor of PM2.5 during haze in Chinese megacities, primary particle emissions also affect particle concentrations. In order to improve estimates of the contribution of primary sources to the particle number and mass concentrations, we performed source apportionment analyses using both chemical fingerprints and particle size distributions measured at the same site in urban Beijing from April to July 2018. Both methods resolved factors related to primary emissions, including vehicular emissions and cooking emissions, which together make up 76 % and 24 % of total particle number and organic aerosol (OA) mass, respectively. Similar source types, including particles related to vehicular emissions (1.6±1.1 µg m−3; 2.4±1.8×103 cm−3 and 5.5±2.8×103 cm−3 for two traffic-related components), cooking emissions (2.6±1.9 µg m−3 and 5.5±3.3×103 cm−3) and secondary aerosols (51±41 µg m−3 and 4.2±3.0×103 cm−3), were resolved by both methods. Converted mass concentrations from particle size distributions components were comparable with those from chemical fingerprints. Size distribution source apportionment separated vehicular emissions into a component with a mode diameter of 20 nm (“traffic-ultrafine”) and a component with a mode diameter of 100 nm (“traffic-fine”). Consistent with similar day- and nighttime diesel vehicle PM2.5 emissions estimated for the Beijing area, traffic-fine particles, hydrocarbon-like OA (HOA, traffic-related factor resulting from source apportionment using chemical fingerprints) and black carbon (BC) showed similar diurnal patterns, with higher concentrations during the night and morning than during the afternoon when the boundary layer is higher. Traffic-ultrafine particles showed the highest concentrations during the rush-hour period, suggesting a prominent role of local gasoline vehicle emissions. In the absence of new particle formation, our results show that vehicular-related emissions (14 % and 30 % for ultrafine and fine particles, respectively) and cooking-activity-related emissions (32 %) dominate the particle number concentration, while secondary particulate matter (over 80 %) governs PM2.5 mass during the non-heating season in Beijing.

A list of authors and their affiliations appears at the end of the paper New-particle formation is a major contributor to urban smog1,2, but how it occurs in cities is often puzzling3. If the growth rates of urban particles are similar to those found in cleaner environments (1–10 nanometres per hour), then existing understanding suggests that new urban particles should be rapidly scavenged by the high concentration of pre-existing particles. Here we show, through experiments performed under atmospheric conditions in the CLOUD chamber at CERN, that below about +5 degrees Celsius, nitric acid and ammonia vapours can condense onto freshly nucleated particles as small as a few nanometres in diameter. Moreover, when it is cold enough (below −15 degrees Celsius), nitric acid and ammonia can nucleate directly through an acid–base stabilization mechanism to form ammonium nitrate particles. Given that these vapours are often one thousand times more abundant than sulfuric acid, the resulting particle growth rates can be extremely high, reaching well above 100 nanometres per hour. However, these high growth rates require the gas-particle ammonium nitrate system to be out of equilibrium in order to sustain gas-phase supersaturations. In view of the strong temperature dependence that we measure for the gas-phase supersaturations, we expect such transient conditions to occur in inhomogeneous urban settings, especially in wintertime, driven by vertical mixing and by strong local sources such as traffic. Even though rapid growth from nitric acid and ammonia condensation may last for only a few minutes, it is nonetheless fast enough to shepherd freshly nucleated particles through the smallest size range where they are most vulnerable to scavenging loss, thus greatly increasing their survival probability. We also expect nitric acid and ammonia nucleation and rapid growth to be important in the relatively clean and cold upper free troposphere, where ammonia can be convected from the continental boundary layer and nitric acid is abundant from electrical storms4,5., Measurements in the CLOUD chamber at CERN show that the rapid condensation of ammonia and nitric acid vapours could be important for the formation and survival of new particles in wintertime urban conditions, contributing to urban smog.

The capabilities of the recently developed Vocus proton-transfer-reaction time-of-flight mass spectrometer (PTR-TOF) are reported for the first time based on ambient measurements. With the deployment of the Vocus PTR-TOF, we present an overview of the observed gas-phase (oxygenated) molecules in the French Landes forest during summertime 2018 and gain insights into the atmospheric oxidation of terpenes, which are emitted in large quantities in the atmosphere and play important roles in secondary organic aerosol production. Due to the greatly improved detection efficiency compared to conventional PTR instruments, the Vocus PTR-TOF identifies a large number of gas-phase signals with elemental composition categories including CH, CHO, CHN, CHS, CHON, CHOS, and others. Multiple hydrocarbons are detected, with carbon numbers up to 20. Particularly, we report the first direct observations of low-volatility diterpenes in the ambient air. The diurnal cycle of diterpenes is similar to that of monoterpenes and sesquiterpenes but contrary to that of isoprene. Various types of terpene reaction products and intermediates are also characterized. Generally, the more oxidized products from terpene oxidations show a broad peak in the day due to the strong photochemical effects, while the less oxygenated products peak in the early morning and/or in the evening. To evaluate the importance of different formation pathways in terpene chemistry, the reaction rates of terpenes with main oxidants (i.e., hydroxyl radical, OH; ozone, O3; and nitrate radical, NO3) are calculated. For the less oxidized non-nitrate monoterpene oxidation products, their morning and evening peaks have contributions from both O3- and OH-initiated monoterpene oxidation. For the monoterpene-derived organic nitrates, oxidations by O3, OH, and NO3 radicals all contribute to their formation, with their relative roles varying considerably over the course of the day. Through a detailed analysis of terpene chemistry, this study demonstrates the capability of the Vocus PTR-TOF in the detection of a wide range of oxidized reaction products in ambient and remote conditions, which highlights its importance in investigating atmospheric oxidation processes. The capabilities of the recently developed Vocus proton-transfer-reaction time-of-flight mass spectrometer (PTR-TOF) are reported for the first time based on ambient measurements. With the deployment of the Vocus PTR-TOF, we present an overview of the observed gas-phase (oxygenated) molecules in the French Landes forest during summertime 2018 and gain insights into the atmospheric oxidation of terpenes, which are emitted in large quantities in the atmosphere and play important roles in secondary organic aerosol production. Due to the greatly improved detection efficiency compared to conventional PTR instruments, the Vocus PTR-TOF identifies a large number of gas-phase signals with elemental composition categories including CH, CHO, CHN, CHS, CHON, CHOS, and others. Multiple hydrocarbons are detected, with carbon numbers up to 20. Particularly, we report the first direct observations of low-volatility diterpenes in the ambient air. The diurnal cycle of diterpenes is similar to that of monoterpenes and sesquiterpenes but contrary to that of isoprene. Various types of terpene reaction products and intermediates are also characterized. Generally, the more oxidized products from terpene oxidations show a broad peak in the day due to the strong photochemical effects, while the less oxygenated products peak in the early morning and/or in the evening. To evaluate the importance of different formation pathways in terpene chemistry, the reaction rates of terpenes with main oxidants (i.e., hydroxyl radical, OH; ozone, O-3; and nitrate radical, NO3) are calculated. For the less oxidized non-nitrate monoterpene oxidation products, their morning and evening peaks have contributions from both O-3(-) and OH- initiated monoterpene oxidation. For the monoterpene-derived organic nitrates, oxidations by O-3, OH, and NO3 radicals all contribute to their formation, with their relative roles varying considerably over the course of the day. Through a detailed analysis of terpene chemistry, this study demonstrates the capability of the Vocus PTR-TOF in the detection of a wide range of oxidized reaction products in ambient and remote conditions, which highlights its importance in investigating atmospheric oxidation processes.

The seasonality and interannual variability of terrestrial carbonyl sulfide (COS) fluxes are poorly constrained. We present the first easy-to-use parameterization for the net COS forest sink based on the longest existing eddy covariance record from a boreal pine forest, covering 32 months over 5 years. Fluxes from hourly to yearly scales are reported, with the aim of revealing controlling factors and the level of interannual variability. The parameterization is based on the photosynthetically active radiation, vapor pressure deficit, air temperature, and leaf area index. Wavelet analysis of the ecosystem fluxes confirmed earlier findings from branch-level fluxes at the same site and revealed a 3 h lag between COS uptake and air temperature maxima at the daily scale, whereas no lag between radiation and COS flux was found. The spring recovery of the flux after the winter dormancy period was mostly governed by air temperature, and the onset of the uptake varied by 2 weeks. For the first time, we report a significant reduction in ecosystem-scale COS uptake under a large water vapor pressure deficit in summer. The maximum monthly and weekly median COS uptake varied by 26 % and 20 % between years, respectively. The timing of the latter varied by 6 weeks. The fraction of the nocturnal uptake remained below 21 % of the total COS uptake. We observed the growing season (April–August) average net flux of COS totaling −58.0 g S ha−1 with 37 % interannual variability. The long-term flux observations were scaled up to evergreen needleleaf forests (ENFs) in the whole boreal region using the Simple Biosphere Model Version 4 (SiB4). The observations were closely simulated using SiB4 meteorological drivers and phenology. The total COS uptake by boreal ENFs was in line with a missing COS sink at high latitudes pointed out in earlier studies.

During the COVID-19 lockdown, the dramatic reduction of anthropogenic emissions provided a unique opportunity to investigate the effects of reduced anthropogenic activity and primary emissions on atmospheric chemical processes and the consequent formation of secondary pollutants. Here, we utilize comprehensive observations to examine the response of atmospheric new particle formation (NPF) to the changes in the atmospheric chemical cocktail. We find that the main clustering process was unaffected by the drastically reduced traffic emissions, and the formation rate of 1.5 nm particles remained unaltered. However, particle survival probability was enhanced due to an increased particle growth rate (GR) during the lockdown period, explaining the enhanced NPF activity in earlier studies. For GR at 1.5–3 nm, sulfuric acid (SA) was the main contributor at high temperatures, whilst there were unaccounted contributing vapors at low temperatures. For GR at 3–7 and 7–15 nm, oxygenated organic molecules (OOMs) played a major role. Surprisingly, OOM composition and volatility were insensitive to the large change of atmospheric NOx concentration; instead the associated high particle growth rates and high OOM concentration during the lockdown period were mostly caused by the enhanced atmospheric oxidative capacity. Overall, our findings suggest a limited role of traffic emissions in NPF.

Atmospheric new particle formation (NPF) is an important source of climate-relevant aerosol particles which has been observed at many locations globally. To study this phenomenon, the first step is to identify whether an NPF event occurs or not on a given day. In practice, NPF event identification is performed visually by classifying the NPF event or non-event days from the particle number size distribution surface plots. Unfortunately, this day-by-day visual classification is time-consuming and labor-intensive, and the identification process renders subjective results. To detect NPF events automatically, we regard the visual signature (banana shape) which has been observed all over the world in NPF surface plots as a special kind of object, and a deep learning model called Mask R-CNN is applied to localize the spatial layouts of NPF events in their surface plots. Utilizing only 358 human-annotated masks on data from the Station for Measuring Ecosystem–Atmosphere Relations (SMEAR) II station (Hyytiälä, Finland), the Mask R-CNN model was successfully generalized for three SMEAR stations in Finland and the San Pietro Capofiume (SPC) station in Italy. In addition to the detection of NPF events (especially the strongest events), the presented method can determine the growth rates, start times, and end times for NPF events automatically. The automatically determined growth rates agree with the manually determined growth rates. The statistical results validate the potential of applying the proposed method to different sites, which will improve the automatic level for NPF event detection and analysis. Furthermore, the proposed automatic NPF event analysis method can minimize subjectivity compared with human-made analysis, especially when long-term data series are analyzed and statistical comparisons between different sites are needed for event characteristics such as the start and end times, thereby saving time and effort for scientists studying NPF events.

Publisher Copyright: © 2022 Northern forest soils are a major carbon (C) reservoir of global importance. To estimate how the C balance in these soils will change, the roles of tree roots and soil microbes in C balance should first be decoupled. This study determined how the activity of heterotrophs and tree roots together with root-associated microbes in the rhizosphere varies in coniferous forest soils in boreal, hemiboreal, and temperate climates along a latitudinal gradient using a trenching approach. We created experimental plots without living tree roots, measured soil respiration (CO2 efflux) from these and from unmanipulated plots using the chamber technique, and partitioned the efflux into root-rhizosphere (RR) and heterotrophic (RH) respiration. The share of RR in ecosystem gross primary production (GPP) decreased from north to south in the Scots pine (Pinus sylvestris L.) and the Norway spruce (Picea abies (L.) Karst.) forests, with the exception of a mixed site, where the share of RR in GPP varied strongly between the years. RR per ground area and per root biomass were mainly independent of climate within the gradient. RH per ground area increased from north to south with temperature, while RH per soil C did not change with temperature. Soil moisture did not significantly affect the respiration components in the northernmost site, whereas soil moisture was positively connected with RH and negatively with RR in other Scots pine sites and positively connected with RR in pure Norway spruce stands. The dynamic ecosystem model LPJ-GUESS was able to capture the seasonal dynamics of RH and RR at the sites, but overall accuracy varied markedly between the sites, as the model underestimated RH in the southern site and RR elsewhere. Our study provides knowledge about the nature of soil respiration components. The valuable insights can be used in more accurate land-ecosystem modelling of forest ecosystems.

Dust particles from high latitudes have a potentially large local, regional, and global significance to climate and the environment as short-lived climate forcers, air pollutants, and nutrient sources. Identifying the locations of local dust sources and their emission, transport, and deposition processes is important for understanding the multiple impacts of high-latitude dust (HLD) on the Earth's systems. Here, we identify, describe, and quantify the source intensity (SI) values, which show the potential of soil surfaces for dust emission scaled to values 0 to 1 concerning globally best productive sources, using the Global Sand and Dust Storms Source Base Map (G-SDS-SBM). This includes 64 HLD sources in our collection for the northern (Alaska, Canada, Denmark, Greenland, Iceland, Svalbard, Sweden, and Russia) and southern (Antarctica and Patagonia) high latitudes. Activity from most of these HLD sources shows seasonal character. It is estimated that high-latitude land areas with higher (SI ≥0.5), very high (SI ≥0.7), and the highest potential (SI ≥0.9) for dust emission cover >1 670 000 km2, >560 000 km2, and >240 000 km2, respectively. In the Arctic HLD region (≥60∘ N), land area with SI ≥0.5 is 5.5 % (1 035 059 km2), area with SI ≥0.7 is 2.3 % (440 804 km2), and area with SI ≥0.9 is 1.1 % (208 701 km2). Minimum SI values in the northern HLD region are about 3 orders of magnitude smaller, indicating that the dust sources of this region greatly depend on weather conditions. Our spatial dust source distribution analysis modeling results showed evidence supporting a northern HLD belt, defined as the area north of 50∘ N, with a “transitional HLD-source area” extending at latitudes 50–58∘ N in Eurasia and 50–55∘ N in Canada and a “cold HLD-source area” including areas north of 60∘ N in Eurasia and north of 58∘ N in Canada, with currently “no dust source” area between the HLD and low-latitude dust (LLD) dust belt, except for British Columbia. Using the global atmospheric transport model SILAM, we estimated that 1.0 % of the global dust emission originated from the high-latitude regions. About 57 % of the dust deposition in snow- and ice-covered Arctic regions was from HLD sources. In the southern HLD region, soil surface conditions are favorable for dust emission during the whole year. Climate change can cause a decrease in the duration of snow cover, retreat of glaciers, and an increase in drought, heatwave intensity, and frequency, leading to the increasing frequency of topsoil conditions favorable for dust emission, which increases the probability of dust storms. Our study provides a step forward to improve the representation of HLD in models and to monitor, quantify, and assess the environmental and climate significance of HLD.

This study investigates the influence of the Chinese New Year (CNY) celebrations on local air quality in Beijing from 2013 through 2019. CNY celebrations include burning of fireworks and firecrackers, which consequently has a significant short-term impact on local air quality. In this study, we bring together comprehensive observations at the newly constructed Aerosol and Haze Laboratory at Beijing University of Chemical Technology – West Campus (BUCT-AHL) and hourly measurements from 12 Chinese government air quality measurement stations throughout the Beijing metropolitan area. These datasets are used together to provide a detailed analysis of air quality during the CNY over multiple years, during which the city of Beijing prohibited the use of fireworks and firecrackers in an effort to reduce air pollution before CNY 2018. Datasets used in this study include particulate matter mass concentrations (PM2.5 and PM10), trace gases (NOx, SO2, O3, and CO), and meteorological variables for 2013–2019; aerosol particle size distributions; and concentrations of sulfuric acid and black carbon for 2018 and 2019. Studying the CNY over several years, which has rarely been done in previous studies, can show trends and effects of societal and policy changes over time, and the results can be applied to study problems and potential solutions of air pollution resulting from holiday celebrations. Our results show that during the 2018 CNY, air pollutant concentrations peaked during the CNY night (for example, PM2.5 reached a peak around midnight of over 250 µg cm−3, compared to values of less than 50 µg cm−3 earlier in the day). The pollutants with the most notable spikes were sulfur dioxide, particulate matter, and black carbon, which are emitted in burning of fireworks and firecrackers. Sulfuric acid concentration followed the sulfur dioxide concentration and showed elevated overnight concentration. Analysis of aerosol particle number size distribution showed direct emissions of particles with diameters around 100 nm in relation to firework burning. During the 2019 CNY, the pollution levels were somewhat lower (PM2.5 peaking at around 150 µg cm−3 on CNY compared to values around 100 µg cm−3 earlier in the day), and only minor peaks related to firework burning were observed. During both CNYs 2018 and 2019 secondary aerosol formation in terms of particle growth was observed. Meteorological conditions were comparable between these 2 years, suggesting that CNY-related emissions were less in 2019 compared to 2018. During the 7-year study period, it appears that there has been a general decrease in CNY-related emissions since 2016. For example, the peak in PM2.5 in 2016 was over 600 µg cm−3, and in the years following, the peak was less each year, with a peak around 150 µg cm−3 in 2019. This is indicative of the restrictions and public awareness of the air quality issues having a positive effect on improving air quality during the CNY. Going into the future, long-term observations will offer confirmation for these trends.

Aerosol particles form in the atmosphere via the clustering of certain atmospheric vapors. After growing into larger particles by the condensation of low-volatility gases, they can affect the Earth's climate by scattering light and acting as cloud condensation nuclei (CCN). Observations of low-volatility aerosol precursor gases have been reported around the world, but longer-term measurement series and any Arctic data sets showing seasonal variation are close to nonexistent. Here, we present ∼7 months of aerosol precursor gas measurements performed with a nitrate-based chemical ionization atmospheric pressure interface time-of-flight (CI-APi-TOF) mass spectrometer. We deployed our measurements ∼150 km north of the Arctic Circle at the SMEAR I (Station for Measuring Ecosystem–Atmosphere Relations) continental Finnish subarctic field station, located in the Värriö strict nature reserve. We report concentration measurements of the most common compounds related to new particle formation (NPF): sulfuric acid (SA), methane sulfonic acid (MSA), iodic acid (IA) and the total concentration of highly oxygenated organic molecules (HOMs). At this remote measurement site, SA originates from both anthropogenic and biological sources and has a clear diurnal cycle but no significant seasonal variation. MSA shows a more distinct seasonal cycle, with concentrations peaking in the summer. Of the measured compounds, IA concentrations are the most stable throughout the measurement period, except in April during which time the concentration of IA is significantly higher than during the rest of the year. Otherwise, IA has almost identical daily maximum concentrations in spring, summer and autumn, and on NPF event or non-event days. HOMs are abundant during the summer months and low in the autumn months. Due to their low autumn concentrations and high correlation with ambient air temperature, we suggest that most HOMs are products of biogenic emissions, most probably monoterpene oxidation products. NPF events at SMEAR I happen under relatively low-temperature (1–8 ∘C) conditions, with a fast temperature rise in the early morning hours as well as lower and decreasing relative humidity (RH, 55 % vs. 80 %) during NPF days compared with non-event days. NPF days have clearly higher global irradiance values (∼450 m−2 vs. ∼200 m−2) and about 10 ppbv higher ozone concentrations than non-event days. During NPF days, we have, on average, higher SA concentrations, peaking at noon; higher MSA concentrations in the afternoon; and slightly higher IA concentration than during non-event days. In summary, these are the first long-term measurements of aerosol-forming vapors from SMEAR I in the subarctic region, and the results of this work will help develop an understanding of atmospheric chemical processes and aerosol formation in the rapidly changing Arctic.

This review presents how the boreal and the tropical forests affect the atmosphere, its chemical composition, its function, and further how that affects the climate and, in return, the ecosystems through feedback processes. Observations from key tower sites standing out due to their long-term comprehensive observations: The Amazon Tall Tower Observatory in Central Amazonia, the Zotino Tall Tower Observatory in Siberia, and the Station to Measure Ecosystem-Atmosphere Relations at Hyytiäla in Finland. The review is complemented by short-term observations from networks and large experiments.The review discusses atmospheric chemistry observations, aerosol formation and processing, physiochemical aerosol, and cloud condensation nuclei properties and finds surprising similarities and important differences in the two ecosystems. The aerosol concentrations and chemistry are similar, particularly concerning the main chemical components, both dominated by an organic fraction, while the boreal ecosystem has generally higher concentrations of inorganics, due to higher influence of long-range transported air pollution. The emissions of biogenic volatile organic compounds are dominated by isoprene and monoterpene in the tropical and boreal regions, respectively, being the main precursors of the organic aerosol fraction.Observations and modeling studies show that climate change and deforestation affect the ecosystems such that the carbon and hydrological cycles in Amazonia are changing to carbon neutrality and affect precipitation downwind. In Africa, the tropical forests are so far maintaining their carbon sink.It is urgent to better understand the interaction between these major ecosystems, the atmosphere, and climate, which calls for more observation sites, providing long-term data on water, carbon, and other biogeochemical cycles. This is essential in finding a sustainable balance between forest preservation and reforestation versus a potential increase in food production and biofuels, which are critical in maintaining ecosystem services and global climate stability. Reducing global warming and deforestation is vital for tropical forests.

Measurements of aerosol particles and clusters smaller than 3 nm in diameter are performed by many groups in order to detect recently formed or emitted nanoparticles and for studying the formation and early growth processes of aerosol particles. The Airmodus nano-Condensation Nucleus Counter (nCNC), consisting of a Particle Size Magnifier (PSM) and a Condensation Particle Counter (CPC) is a versatile tool to detect aerosol particles and clusters as small as ca. 1 nm in mobility diameter. It offers several different operation modes: fixed mode to measure the total particle number concentration with a fixed, but adjustable lower cut-off size and stepping and scanning modes for retrieving size-resolved information of ca. 1–4 nm particles. The size analysis is based on changing the supersaturation of the working fluid (diethylene glycol) inside the instrument, which changes the lowest detectable size. Here we present a standard operating procedure (SOP) for setting up, calibrating and operating the instrument for atmospheric field measurements. We will also present recommendations for data monitoring and analysis, and discuss some of the uncertainties related to the measurements. This procedure is the first step in harmonizing the use of the PSM/nCNC for atmospheric field measurements of sub-3 nm clusters and particles. Measurements of aerosol particles and clusters smaller than 3 nm in diameter are performed by many groups in order to detect recently formed or emitted nanoparticles and for studying the formation and early growth processes of aerosol particles. The Airmodus nano-Condensation Nucleus Counter (nCNC), consisting of a Particle Size Magnifier (PSM) and a Condensation Particle Counter (CPC) is a versatile tool to detect aerosol particles and clusters as small as ca. 1 nm in mobility diameter. It offers several different operation modes: fixed mode to measure the total particle number concentration with a fixed, but adjustable lower cut-off size and stepping and scanning modes for retrieving size-resolved information of ca. 1–4 nm particles. The size analysis is based on changing the supersaturation of the working fluid (diethylene glycol) inside the instrument, which changes the lowest detectable size. Here we present a standard operating procedure (SOP) for setting up, calibrating and operating the instrument for atmospheric field measurements. We will also present recommendations for data monitoring and analysis, and discuss some of the uncertainties related to the measurements. This procedure is the first step in harmonizing the use of the PSM/nCNC for atmospheric field measurements of sub-3 nm clusters and particles.

Atmospheric new particle formation (NPF) events have attracted increasing attention for their contribution to the global aerosol number budget and therefore their effects on climate, air quality and human health. NPF events are regarded as a regional phenomenon, occurring over a large area. Most observations of NPF events in Beijing and its vicinity were conducted in populated areas, whereas observations of NPF events on mountaintops with low anthropogenic emissions are still rare in China. The spatial variation of NPF event intensity has not been investigated in detail by incorporating both urban areas and mountain measurements in Beijing. Here, we provide NPF event characteristics in summer 2018 and 2019 at urban Beijing and a comparison of NPF event characteristics – NPF event frequency, formation rate and growth rate – by comparing an urban Beijing site and a background mountain site separated by ∼80 km from 14 June to 14 July 2019, as well as giving insights into the connection between both locations. During parallel measurements at urban Beijing and mountain background areas, although the median condensation sink during the first 2 h of the common NPF events was around 0.01 s−1 at both sites, there were notable differences in formation rates between the two locations (median of 5.42 cm−3 s−1 at the urban site and 1.13 cm−3 s−1 at the mountain site during the first 2 h of common NPF events). In addition, the growth rates in the 7–15 nm range for common NPF events at the urban site (median of 7.6 nm h−1) were slightly higher than those at the mountain site (median of 6.5 nm h−1). To understand whether the observed events were connected, we compared air mass trajectories as well as meteorological conditions at both stations. Favorable conditions for the occurrence of regional NPF events were largely affected by air mass transport. Overall, our results demonstrate a clear inhomogeneity of regional NPF within a distance of ∼100 km, possibly due to the discretely distributed emission sources.

At SMEAR II research station in Hyytiälä, located in the Finnish boreal forest, the process of new particle formation and the role of ions has been investigated for almost 20 years near the ground and at canopy level. However, above SMEAR II, the vertical distribution and diurnal variation of these different atmospheric ions are poorly characterized. In this study, we assess the atmospheric ion composition in the stable boundary layer, residual layer, mixing layer, and free troposphere, and the evolution of these atmospheric ions due to photochemistry and turbulent mixing through the day. To measure the vertical profile of atmospheric ions, we developed a tailored set-up for online mass spectrometric measurements, capable of being deployed in a Cessna 172 with minimal modifications. Simultaneously, instruments dedicated to aerosol properties made measurements in a second Cessna. We conducted a total of 16 measurement flights in May 2017, during the spring, which is the most active new particle formation season. A flight day typically consisted of three distinct flights through the day (dawn, morning, and afternoon) to observe the diurnal variation and at different altitudes (from 100 to 3200 m above ground), to capture the boundary layer development from the stable boundary layer, residual layer to mixing layer, and the free troposphere. Our observations showed that the ion composition is distinctly different in each layer and depends on the air mass origin and time of the day. Before sunrise, the layers are separated from each other and have their own ion chemistry. We observed that the ions present within the stable layer are of the same composition as the ions measured at the canopy level. During daytime when the mixing layer evolved and the compounds are vertically mixed, we observed that highly oxidized organic molecules are distributed to the top of the boundary layer. The ion composition in the residual layer varies with each day, showing similarities with either the stable boundary layer or the free troposphere. Finally, within the free troposphere, we detected a variety of carboxylic acids and ions that are likely containing halogens, originating from the Arctic Sea.

As evidence of adverse health effects due to air pollution continues to increase, the World Health Organization (WHO) recently published its latest edition of the global air quality guidelines (World Health Organization, 2021). Although not legally binding, the guidelines aim to provide a framework in which policymakers can combat air pollution by formulating evidence-based air quality management strategies. In the light of this, the European Union has stated its intent to revise the current ambient air quality directive (2008/50/EC) to more closely resemble the newly published WHO guidelines (European Commission, 2020). This article provides an informed opinion on selected features of the air quality directive that we believe would benefit from a reassessment. The selected features include discussion about (1) air quality sensors as a part of a hierarchical observation network, (2) the number of minimum sampling points and their siting criteria, and (3) new target air pollution parameters for future consideration.

Tropospheric ozone (O-3) concentrations are observed to increase with temperature in urban and rural locations. We investigated the apparent temperature dependency of daytime ozone concentration in the Finnish boreal forest in summertime based on long-term measurements. We used statistical mixed effects models to separate the direct effects of temperature from other factors influencing this dependency, such as weather conditions, long-range transport of precursors, and concentration of various hydrocarbons. The apparent temperature dependency of 1.16 ppb ?(-1) based on a simple linear regression was reduced to 0.87 ppb ?(-1) within the canopy for summer daytime data after considering these factors. In addition, our results indicated that small oxygenated volatile organic compounds may play an important role in the temperature dependence of O-3 concentrations in this dataset from a low-NOx environment. Summertime observations and daytime data were selected for this analysis to focus on an environment that is significantly affected by biogenic emissions. Despite limitations due to selection of the data, these results highlight the importance of considering factors contributing to the apparent temperature dependence of the O-3 concentration. In addition, our results show that a mixed effects model achieves relatively good predictive accuracy for this dataset without explicitly calculating all processes involved in O-3 formation and removal.

Aerosol particles are essential constituents of the Earth’s atmosphere, impacting the earth radiation balance directly by scattering and absorbing solar radiation, and indirectly by acting as cloud condensation nuclei. In contrast to most greenhouse gases, aerosol particles have short atmospheric residence times, resulting in a highly heterogeneous distribution in space and time. There is a clear need to document this variability at regional scale through observations involving, in particular, the in situ near-surface segment of the atmospheric observation system. This paper will provide the widest effort so far to document variability of climate-relevant in situ aerosol properties (namely wavelength dependent particle light scattering and absorption coefficients, particle number concentration and particle number size distribution) from all sites connected to the Global Atmosphere Watch network. High-quality data from almost 90 stations worldwide have been collected and controlled for quality and are reported for a reference year in 2017, providing a very extended and robust view of the variability of these variables worldwide. The range of variability observed worldwide for light scattering and absorption coefficients, single-scattering albedo, and particle number concentration are presented together with preliminary information on their long-term trends and comparison with model simulation for the different stations. The scope of the present paper is also to provide the necessary suite of information, including data provision procedures, quality control and analysis, data policy, and usage of the ground-based aerosol measurement network. It delivers to users of the World Data Centre on Aerosol, the required confidence in data products in the form of a fully characterized value chain, including uncertainty estimation and requirements for contributing to the global climate monitoring system., European Commission Joint Research Centre 654109, European ERDF funds through different Spanish R&D projects of the Spanish Ministerio de Economia, Industria y Competitividad, NorthWest University, University of Helsinki, Academy of Finland 272041, Academy of Finland project Greenhouse gas 269095 296302, Korea Meteorological Administration Research and Development Program "Development of Monitoring and Analysis Techniques for Atmospheric Composition in Korea KMA2018-00522, National Research Foundation of Korea 2017R1D1A1B06032548, Korea Meteorological Administration Research and Development Program KMI2018-01111, Taiwan Environmental Protection Administration, Ministry of Research, France, French Ministry of the Environment, United States Environmental Protection Agency, MeteoSwiss (GAW-CH aerosol monitoring programme), Swiss State Secretariat for Education, Research and Innovation (SERI), Ministry of Education, Youth and Sports of CR within National Sustainability Program I (NPU I) LO1415, ERDF "ACTRISCZ RI" CZ.02.1.01/0.0/0.0/16_013/0001315 CGL2017-85344-R MINECO/AEI/FEDER, TIGAS-CM (Madrid Regional Government) Y2018/EMT-5177, AIRTECCM (Madrid Regional Government) P2018/EMT4329 REDMAAS2020 RED2018-102594-T CIENCIA, Spanish Ministry of Economy, Industry and Competitiveness, European Union (EU) CGL2016-78594-R, Generalitat de Catalunya AGAUR 2017 SGR41, National Institute for Aerospace Technology, Ministerio Espanol de Economia, Industria y Competitividad (MINECO) MIS 5021516, Competitiveness, Entrepreneurship and Innovation, NSRF, Ministry of Education, Universities and Research (MIUR), Norwegian Environment Agency, Swedish FORMAS; Swedish Research Council (VR), Magnus Bergvall foundation, Marta och Erik Holmberg foundation, Swedish EPA

Publisher Copyright: © 2021 Mao Xiao et al. New particle formation (NPF) is a significant source of atmospheric particles, affecting climate and air quality. Understanding the mechanisms involved in urban aerosols is important to develop effective mitigation strategies. However, NPF rates reported in the polluted boundary layer span more than 4 orders of magnitude, and the reasons behind this variability are the subject of intense scientific debate. Multiple atmospheric vapours have been postulated to participate in NPF, including sulfuric acid, ammonia, amines and organics, but their relative roles remain unclear. We investigated NPF in the CLOUD chamber using mixtures of anthropogenic vapours that simulate polluted boundary layer conditions. We demonstrate that NPF in polluted environments is largely driven by the formation of sulfuric acid-base clusters, stabilized by the presence of amines, high ammonia concentrations and lower temperatures. Aromatic oxidation products, despite their extremely low volatility, play a minor role in NPF in the chosen urban environment but can be important for particle growth and hence for the survival of newly formed particles. Our measurements quantitatively account for NPF in highly diverse urban environments and explain its large observed variability. Such quantitative information obtained under controlled laboratory conditions will help the interpretation of future ambient observations of NPF rates in polluted atmospheres.

Although the anthropogenic emissions of SO2 have decreased significantly in China, the decrease in SO42- in PM2.5 is much smaller than that of SO2. This implies an enhanced formation rate of SO42- in the ambient air, and the mechanism is still under debate. This work investigated the formation mechanism of particulate sulfate based on statistical analysis of long-term observations in Shijiazhuang and Beijing supported with flow tube experiments. Our main finding was that the sulfur oxidation ratio (SOR) was exponentially correlated with ambient RH in Shijiazhuang (SOR = 0.15+0.0032×exp⁡(RH/16.2)) and Beijing (SOR = -0.045+0.12×exp⁡(RH/37.8)). In Shijiazhuang, the SOR is linearly correlated with the ratio of aerosol water content (AWC) in PM2.5 (SOR = 0.15+0.40×AWC/PM2.5). Our results suggest that uptake of SO2 instead of oxidation of S(IV) in the particle phase is the rate-determining step for sulfate formation. NH4NO3 plays an important role in the AWC and the change of particle state, which is a crucial factor determining the uptake kinetics of SO2 and the enhanced SOR during haze days. Our results show that NH3 significantly promoted the uptake of SO2 and subsequently the SOR, while NO2 had little influence on SO2 uptake and SOR in the presence of NH3.

Aerosol particles cool the climate by scattering solar radiation and by acting as cloud condensation nuclei. Higher temperatures resulting from increased greenhouse gas levels have been suggested to lead to increased biogenic secondary organic aerosol and cloud condensation nuclei concentrations creating a negative climate feedback mechanism. Here, we present direct observations on this feedback mechanism utilizing collocated long term aerosol chemical composition measurements and remote sensing observations on aerosol and cloud properties. Summer time organic aerosol loadings showed a clear increase with temperature, with simultaneous increase in cloud condensation nuclei concentration in a boreal forest environment. Remote sensing observations revealed a change in cloud properties with an increase in cloud reflectivity in concert with increasing organic aerosol loadings in the area. The results provide direct observational evidence on the significance of this negative climate feedback mechanism., Vegetation emits organic vapors which can form aerosols in the atmosphere and influence cloud properties. Here, the authors show observational evidence that warmer temperatures lead to increased emissions of these aerosols in boreal forests which cause surface cooling, demonstrating a negative climate feedback mechanism.

In the aqueous phase, fine particulate matter can form reactive species (RS) that influence the aging, properties, and health effects of atmospheric aerosols. In this study, we explore the RS yields of aerosol samples from a remote forest (Hyytiälä, Finland) and polluted urban locations (Mainz, Germany; Beijing, China), and we relate the RS yields to different chemical constituents and reaction mechanisms. Ultra-high-resolution mass spectrometry was used to characterize organic aerosol composition, electron paramagnetic resonance (EPR) spectroscopy with a spin-trapping technique was applied to determine the concentrations of ⚫OH, O2⚫-, and carbon- or oxygen-centered organic radicals, and a fluorometric assay was used to quantify H2O2. The aqueous H2O2-forming potential per mass unit of ambient PM2.5 (particle diameter < 2.5 µm) was roughly the same for all investigated samples, whereas the mass-specific yields of radicals were lower for sampling sites with higher concentrations of PM2.5. The abundances of water-soluble transition metals and aromatics in ambient PM2.5 were positively correlated with the relative fraction of ⚫OH and negatively correlated with the relative fraction of carbon-centered radicals. In contrast, highly oxygenated organic molecules (HOM) were positively correlated with the relative fraction of carbon-centered radicals and negatively correlated with the relative fraction of ⚫OH. Moreover, we found that the relative fractions of different types of radicals formed by ambient PM2.5 were comparable to surrogate mixtures comprising transition metal ions, organic hydroperoxide, H2O2, and humic or fulvic acids. The interplay of transition metal ions (e.g., iron and copper ions), highly oxidized organic molecules (e.g., hydroperoxides), and complexing or scavenging agents (e.g., humic or fulvic acids) leads to nonlinear concentration dependencies in aqueous-phase RS production. A strong dependence on chemical composition was also observed for the aqueous-phase radical yields of laboratory-generated secondary organic aerosols (SOA) from precursor mixtures of naphthalene and β-pinene. Our findings show how the composition of PM2.5 can influence the amount and nature of aqueous-phase RS, which may explain differences in the chemical reactivity and health effects of particulate matter in clean and polluted air.