Search

New particle formation (NPF) in the tropical free troposphere (FT) is a globally important source of cloud condensation nuclei, affecting cloud properties and climate. Oxidized organic molecules (OOMs) produced from biogenic volatile organic compounds are believed to contribute to aerosol formation in the tropical FT, but without direct chemical observations. We performed in-situ molecular-level OOMs measurements at the Bolivian station Chacaltaya at 5240 meters above sea level, on the western edge of Amazonia. For the first time, we demonstrate the presence of OOMs, mainly with 4-5 carbon atoms, in both gas-phase and particle-phase (in terms of mass contribution) measurements in tropical FT air from Amazonia. These observations, combined with air mass history analyses, indicate that the observed OOMs are linked to isoprene emitted from the rainforests hundreds of kilometers away. Based on particle-phase measurements, we find that these compounds can contribute to NPF, at least the growth of newly formed nanoparticles, in the tropical FT on a continental scale. Thus, our study is a fundamental and significant step in understanding the aerosol formation process in the tropical FT.

Carbonaceous soot particles are formed during incomplete combustion of fossil fuels, biofuels or biomass and are considered to be a significant proportion of aerosol emission, especially in polluted areas, and to contain light-absorbing carbon fractions. The light-absorbing carbon components make these particles exhibit positive radiative forcing and thus they contribute to atmospheric warming processes. The exact contribution to this process however still has significant uncertainties. One of the sources of uncertainty is related to the capacity to accurately describe soot spectral optical properties (MAC/MSC/MEC - mass absorption/scattering/extinction cross-sections in m2g-1; CRI - complex refractive index), due to significant uncertainties for key measurements like the absorption coefficient or absorbing mass fraction. A second source is considered to be the change of the optical properties with the variable physico-chemical state of soot (e.g. chemical composition, morphology, primary particle size, aggregate size distribution, coating and mixing state) which depends on (i) the combustion conditions/sources and is (ii) known to change during atmospheric lifetime due to ageing and mixing processes.The present work aims at providing new measurements of soot spectral optical properties and investigating their dependence on particles’ physico-chemical state and the role of measurement uncertainties. For this, a set of original experiments was performed in the 4.3m3 CESAM atmospheric simulation chamber (https://cesam.cnrs.fr/) on soot aerosols generated by a commercial propane diffusion flame soot generator (miniCAST model 6204 TYPE C, JING). In these experiments, the variability of soot properties due to (i) generation and (ii) atmospheric ageing was explored. Different combustion conditions going from fuel–leaner to fuel-richer were set to produce soot aerosol with different effective densities, EC/TC-ratios ranging from 0.8 ± 0.1 to 0.0 ± 0.1 and size distributions with median diameters between 30 and 120 nm. Selected aerosols were subjected to ageing in a N2/O2 atmosphere under simulated atmospheric conditions (humid, with/without illumination, up to 26 hours lifetime). In these conditions, physical and chemical ageing, under the presence of different gaseous phases (O3, SO2) and addition of a second aerosol phase produced by photo-oxidation of SO2 or the ozonolysis of α-pinene, led to changes in the physico-chemical properties of the soot.State-of-the-art techniques were used to generate an extensive dataset of physico-chemical parameters (mass concentration, morphology, effective density, composition, size distribution) and spectral optical properties (absorption, scattering, extinction coefficients) for different soot aerosols and different ageing states of these. The absorption coefficient in particular was measured by both filter-based (AE - Aethalometer, MAAP - Multi-Angle Absorption Photometer, MWAA – Multi-Wavelength Absorbance Analyzer, PP_UniMI - Polar Photometer by University of Milano) and extinction minus scattering techniques. The MAC, MSC and MEC datasets, retrieved by combining these measurements, will be presented for the ensemble of chamber experiments and the variability of these parameters in link with variations in the particles’ physico-chemical properties will be discussed together with key relevant uncertainties.

Early growth of atmospheric particles is essential for their survival and ability to participate in cloud formation. Many different atmospheric vapors contribute to the growth, but even the main contributors still remain poorly identified in many environments, such as high-altitude sites. Based on measured organic vapor and sulfuric acid concentrations under ambient conditions, particle growth during new particle formation events was simulated and compared with the measured particle size distribution at Chacaltaya Global Atmosphere Watch station in Bolivia (5240 m a.s.l.) during April and May 2018, as a part of the SALTENA (Southern Hemisphere high-ALTitude Experiment on particle Nucleation and growth) campaign . The simulations showed that the detected vapors were sufficient to explain the observed particle growth, although some discrepancies were found between modelled and measured particle growth rates. This study gives an insight on the key factors affecting the particle growth on the site. Low volatile organic compounds were found to be the main contributor to the particle growth, covering on average 65 % of simulated particle mass in particle with diameter of 40 nm In addition, sulfuric acid had a major contribution to the particle growth, covering at maximum 39 % of simulated particle mass in 40 nm particle during periods when volcanic activity was detected on the area, suggesting that volcanic emissions can greatly enhance the particle growth.

PM2.5 haze pollution driven by secondary inorganic NO3- has been a great concern in East Asia. It is, therefore, imperative to identify its sources and oxidation processes, for which nitrogen and oxygen stable isotopes are powerful tracers. Here, we determined the δ15N (NO3-) and Δ17O (NO3-) of PM2.5 in Seoul during the summer of 2018 and the winter of 2018–2019 and estimated quantitatively the relative contribution of oxidation pathways for particulate NO3- and investigated major NOx emission sources. In the range of PM2.5 mass concentration from 7.5 µg m−3 (summer) to 139.0 µg m−3 (winter), the mean δ15N was −0.7 ‰ ± 3.3 ‰ and 3.8 ‰ ± 3.7 ‰, and the mean Δ17O was 23.2 ‰ ± 2.2 ‰ and 27.7 ‰ ± 2.2 ‰ in the summer and winter, respectively. While OH oxidation was the dominant pathway for NO3- during the summer (87 %), nighttime formation via N2O5 and NO3 was relatively more important (38 %) during the winter, when aerosol liquid water content (ALWC) and nitrogen oxidation ratio (NOR) were higher. Interestingly, the highest Δ17O was coupled with the lowest δ15N and highest NOR during the record-breaking winter PM2.5 episodes, revealing the critical role of photochemical oxidation process in severe winter haze development. For NOx sources, atmospheric δ15N (NOx) estimated from measured δ15N (NO3-) considering isotope fractionation effects indicates vehicle emissions as the most important emission source of NOx in Seoul. The contribution from biogenic soil and coal combustion was slightly increased in summer and winter, respectively. Our results built on a multiple-isotope approach provide the first explicit evidence for NO3- formation processes and major NOx emission sources in the Seoul megacity and suggest an effective mitigation measure to improve PM2.5 pollution.

Measurements of methane concentrations were made at the Chacaltaya GAW station (16.3ºS, 68.1ºW, 5240m a.s.l.) in the Andean Cordillera from 2015 to date. During this period two high-precision Picarro-CRDS analyzers were used at the station, regularly calibrated with internationally certified gases (WMO X2004A) via the LSCE primary scale. The site has a privileged location not only due to its altitude but also because air masses arriving from the near Altiplano (3 800 m a.s.l.), the Amazon (so-called low-lands), the Pacific Ocean, and the nearby metropolitan area of La Paz/El Alto (~2 million of inhabitants) can be sampled there. The complex topography of the region represents a challenge for deconvoluting the origin of the air masses and therefore to understanding the sources and/or processes associated with the measurements made at Chacaltaya. Here we show some results based on re-analysis data as well as on high and medium - resolution back trajectories in order to identify the influence of different regions on the station. In addition, satellite products and satellite-derived databases, from TROPOMI and GFED4.1s and WAD2M are used to characterize and interpret daily, seasonal and interannual behavior of the methane concentrations observed in Chacaltaya. The influence of the local atmospheric planetary boundary layer is clearly seen in the measurements, especially in the late morning, but collocated measurements of other atmospheric components such as carbon monoxide or equivalent black carbon have proven that identifying free-tropospheric air masses is not an easy task. The contributions of the largest human conglomerate of the region are also discussed in this context.

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.

La Paz and El Alto are two fast-growing high-altitude Bolivian cities forming the second largest metropolitan area in the country, located between 3200 and 4050 m a.s.l. Together they host a growing population of around 1.8 million people. The air quality in this conurbation is strongly influenced by urbanization. However, there are no comprehensive studies that have assessed the sources of air pollution and their impacts on health. Despite being neighboring cities, the drastic change in altitude and topography between La Paz and El Alto together with different socio-economic activities lead to different sources, dynamics and transport of particulate matter (PM). In this investigation, PM10 samples were collected at two urban background stations located in La Paz and El Alto between April 2016 and June 2017. The samples were later analyzed for a wide range of chemical species including numerous source tracers (OC, EC, water-soluble ions, sugar anhydrides, sugar alcohols, trace metals, and molecular organic species). The US-EPA Positive Matrix Factorization (PMF v.5.0) receptor model was then applied for source apportionment of PM10. This is the first source apportionment study in South America that incorporates a large set of organic markers (such as levoglucosan, polycyclic aromatic hydrocarbons – PAH, hopanes and alkanes) together with inorganic species. The multisite PMF resolved 11 main sources of PM. The largest annual contribution to PM10 came from two major sources: the ensemble of the four vehicular emissions sources (exhaust and non-exhaust), together responsible for 35 % and 25 % of the measured PM in La Paz and El Alto, respectively, and dust contributing 20 % and 32 % to the total. Secondary aerosols contributed 22 % (24 %) in La Paz (El Alto). Agriculture-related smoke from biomass burning originated in the Bolivian lowlands and neighboring countries contributed to 8 % (7 %) of the total PM10 mass annually. This contribution increased to 17 % (13 %) between August–October. Primary biogenic emissions were responsible for 13 % (7 %) of the measured PM10 mass. Finally, it was possible to identify a profile related to open waste burning occurring between the months of May and August. Despite the fact that this source contributed only to 2 % (5 %) of the total PM10 mass, it constitutes the second largest source of PAHs, compounds potentially hazardous to health. Our analysis resulted in the identification of two specific traffic-related sources. In addition, we also identified a lubricant source (not frequently identified) and a non-exhaust emissions source. This study shows that PM10 concentrations in La Paz and El Alto region are mostly impacted by a limited number of local sources. In conclusion, dust, traffic emissions, open waste burning and biomass burning are the main sources to target in order to improve air quality in both cities.

International audience

Black carbon (BC) is a significant forcing agent in the Arctic, but substantial uncertainty remains to quantify its climate effects due to the complexity of the different mechanisms involved, in particular related to processes in the snowpack after deposition. In this study, we provide detailed and unique information on the evolution and variability in BC content in the upper surface snow layer during the spring period in Svalbard (Ny-Ålesund). A total of two different snow-sampling strategies were adopted during spring 2014 (from 1 April to 24 June) and during a specific period in 2015 (28 April to 1 May), providing the refractory BC (rBC) mass concentration variability on a seasonal variability with a daily resolution (hereafter seasonal/daily) and daily variability with an hourly sampling resolution (hereafter daily/hourly) timescales. The present work aims to identify which atmospheric variables could interact with and modify the mass concentration of BC in the upper snowpack, which is the snow layer where BC particles affects the snow albedo. Atmospheric, meteorological and snow-related physico-chemical parameters were considered in a multiple linear regression model to identify the factors that could explain the variations in BC mass concentrations during the observation period. Precipitation events were the main drivers of the BC variability during the seasonal experiment; however, in the high-resolution sampling, a negative association has been found. Snow metamorphism and the activation of local sources (Ny-Ålesund was a coal mine settlement) during the snowmelt periods appeared to play a non-negligible role. The statistical analysis suggests that the BC content in the snow is not directly associated to the atmospheric BC load.

Funding Information: This research was supported by the National Research Foundation of Korea (NRF), funded by the Ministry of Science and ICT (MSIT) (NRF2020M3G1A111499813). S. Lim was supported by the National Research Foundation of Korea (NRF) from the Ministry of Science and ICT (2018R1D1A1B07050849 and 2021R1C1C2011543). M. Lee thanks to the support by the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (NRF2020R1A2C301459213). S.-W. Kim was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education (2017R1D1A1B06032548). Funding to K.-S. Kang was provided by the National Institute of Environmental Research (NIER-RP2017-166). Funding Information: This research was supported by the National Research Foundation of Korea (NRF), funded by the Ministry of Science and ICT (MSIT) (NRF2020M3G1A111499813). S. Lim was supported by the National Research Foundation of Korea (NRF) from the Ministry of Science and ICT (2018R1D1A1B07050849 and 2021R1C1C2011543). M. Lee thanks to the support by the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (NRF2020R1A2C301459213). S.-W. Kim was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education (2017R1D1A1B06032548). Funding to K.-S. Kang was provided by the National Institute of Environmental Research (NIER-RP2017-166). Publisher Copyright: © 2022 The Author(s) Northeast Asia has suffered from severe PM2.5 pollution and the exact mechanisms have yet to be fully understood. Here, we investigated the transformation processes of submicron aerosols using a 4-year data set obtained at Jeju, a Northeast Asian background site. The diurnal-cycle constrained empirical orthogonal function analysis of nanoparticle size-number distribution distinguished 2 modes: burst of nucleation-Aitken particles and increase in accumulation mode particles, representing “new particle formation and growth” and “PM2.5 mass increase,” respectively. In these events, aerosol and meteorological characteristics changed progressively over several days, revealing that the PM2.5 mass increase is an episodic event occurring on a regional scale. The increase in PM2.5 mass was accompanied by an increase in aerosol liquid water content, which correlated well with SO4-2 and NO3, and a decrease in incoming solar radiation (-14.1 Wm-2 day-1) constituting a positive feedback. The “transport/haze” episode of KOREA-U.S. Air Quality campaign corresponds to “PM2.5 mass increase,” during which the vertical evolution of particles demonstrates that nanoparticles ≥3.5 nm were entrained into the shallow boundary layer upon vertical mixing and converted to accumulation-mode particles ≥0.3 mm at relative humidity (RH) exceeding the deliquescence RH of secondary inorganic aerosol (SIA). Coincidently, at ground, the coating thickness of refractory black carbon (rBC) (48 ± 39 nm) and SIA concentration increased. Furthermore, the diameter of rBC (180-220 nm)-containing particle in core-shell configuration linearly increased with PM2.5 mass, reaching 300-400 nm at PM2.5 ≥ 40 mg m-3.This observational evidence suggests that the thick coating of rBCs resulted from the active conversion of condensable gases into the particulate phase on the rBC surface, thereby increasing the mass of the accumulation-mode aerosol. Consequently, this result complies with the strategy to reduce primary emissions of gaseous precursors for SIA and particulates such as rBC as a way to effectively mitigate haze pollution as well as climate change in Northeast Asia.

The discovery of atmospheric micro(nano)plastic transport and ocean-atmosphere exchange points to a highly complex marine plastic cycle, with negative implications for human and ecosystem health. Yet, observations are currently limited. In this Perspective, we quantify the processes and fluxes of the marine-atmospheric micro(nano)plastic cycle, with the aim of highlighting the remaining unknowns in atmospheric micro(nano)plastic transport. Between 0.013 and 25 million metric tons per year of micro(nano)plastics are potentially being transported within the marine atmosphere and deposited in the oceans. However, the high uncertainty in these marine-atmospheric fluxes is related to data limitations and a lack of study intercomparability. To address the uncertainties and remaining knowledge gaps in the marine-atmospheric micro(nano)plastic cycle, we propose a future global marine-atmospheric micro(nano)plastic observation strategy, incorporating novel sampling methods and the creation of a comparable, harmonized and global data set. Together with long-term observations and intensive investigations, this strategy will help to define the trends in marine-atmospheric pollution and any responses to future policy and management actions. Atmospheric transport of microplastics could be a major source of plastic pollution to the ocean, yet observations currently remain limited. This Perspective quantifies the known budgets of the marine-atmospheric micro(nano)plastic cycle and proposes a future global observation strategy.

This study presents a multiparameter analysis of aerosol trends over the last 2 decades at regional and global scales. Regional time series have been computed for a set of nine optical, chemical-composition and mass aerosol properties by using the observations from several ground-based networks. From these regional time series the aerosol trends have been derived for the different regions of the world. Most of the properties related to aerosol loading exhibit negative trends, both at the surface and in the total atmospheric column. Significant decreases in aerosol optical depth (AOD) are found in Europe, North America, South America, North Africa and Asia, ranging from −1.2 % yr−1 to −3.1 % yr−1. An error and representativity analysis of the spatially and temporally limited observational data has been performed using model data subsets in order to investigate how much the observed trends represent the actual trends happening in the regions over the full study period from 2000 to 2014. This analysis reveals that significant uncertainty is associated with some of the regional trends due to time and space sampling deficiencies. The set of observed regional trends has then been used for the evaluation of 10 models (6 AeroCom phase III models and 4 CMIP6 models) and the CAMS reanalysis dataset and of their skills in reproducing the aerosol trends. Model performance is found to vary depending on the parameters and the regions of the world. The models tend to capture trends in AOD, the column Ångström exponent, sulfate and particulate matter well (except in North Africa), but they show larger discrepancies for coarse-mode AOD. The rather good agreement of the trends, across different aerosol parameters between models and observations, when co-locating them in time and space, implies that global model trends, including those in poorly monitored regions, are likely correct. The models can help to provide a global picture of the aerosol trends by filling the gaps in regions not covered by observations. The calculation of aerosol trends at a global scale reveals a different picture from that depicted by solely relying on ground-based observations. Using a model with complete diagnostics (NorESM2), we find a global increase in AOD of about 0.2 % yr−1 between 2000 and 2014, primarily caused by an increase in the loads of organic aerosols, sulfate and black carbon.

Ten years of almost continuous observations at the highest Global Atmosphere Watch Regional station in the world are presented here. The Chacaltaya observatory (5240 m asl, 16.3ºS, 68.1ºW) was set up in December 2011. It is currently the only operational station characterizing optical and chemical properties of climate-relevant aerosol and gases in Bolivia and in a radius of about 1500 kilometers from the station. The observations show a clear influence of the well-marked dry and wet meteorological seasons. In addition, the impact on the Andean mountains of long and mid-range transport of biomass burning products from the lowlands is clearly recorded in different parameters measured at the station. Furthermore, the nearby presence of the largest metropolitan area in the region (~1.8 million inhabitants) is observed almost on a daily basis, and therefore different campaigns were carried out to characterize the area and its influence on our measurements. Specific results from these campaigns are discussed elsewhere. Finally, the topographic complexity represents an important challenge for modeling efforts in order to understand sources and sinks (and associated processes) of the observed parameters, requiring not only high spatial resolution and the correct choice of model options, but a novel way of interpreting these results. The decade of collaboration of an international consortium made it possible to keep the station running successfully. The challenge is now to preserve its functioning for the coming decades in a region with historically few high-quality observations while disrupting environmental and socio-economic changes take place.

Observations of aerosol and trace gases in the remote troposphere are vital to quantify background concentrations and identify long-term trends in atmospheric composition on large spatial scales. Measurements made at high altitude are often used to study free-tropospheric air; however such high-altitude sites can be influenced by boundary layer air masses. Thus, accurate information on air mass origin and transport pathways to high-altitude sites is required. Here we present a new method, based on the source-receptor relationship (SRR) obtained from backwards WRF-FLEXPART simulations and a k-means clustering approach, to identify source regions of air masses arriving at measurement sites. Our method is tailored to areas of complex terrain and to stations influenced by both local and long-range sources. We have applied this method to the Chacaltaya (CHC) GAW station (5240 m a.s.l.; 16.35 degrees S, 68.13 degrees W) for the 6-month duration of the "Southern Hemisphere high-altitude experiment on particle nucleation and growth" (SALILNA) to identify where sampled air masses originate and to quantify the influence of the surface and the free troposphere. A key aspect of our method is that it is probabilistic, and for each observation time, more than one air mass (cluster) can influence the station, and the percentage influence of each air mass can be quantified. This is in contrast to binary methods, which label each observation time as influenced by either boundary layer or free-troposphere air masses. Air sampled at CHC is a mix of different provenance. We find that on average 9 % of the air, at any given observation time, has been in contact with the surface within 4 d prior to arriving at CHC. Furthermore, 24 % of the air has been located within the first 1.5 km above ground level (surface included). Consequently, 76 % of the air sampled at CHC originates from the free troposphere. However, pure free-tropospheric influences are rare, and often samples are concurrently influenced by both boundary layer and free-tropospheric air masses. A clear diurnal cycle is present, with very few air masses that have been in contact with the surface being detected at night. The 6-month analysis also shows that the most dominant air mass (cluster) originates in the Amazon and is responsible for 29 % of the sampled air. Furthermore, short-range clusters (origins within 100 km of CHC) have high temporal frequency modulated by local meteorology driven by the diurnal cycle, whereas the mid- and long-range clusters' (> 200 km) variability occurs on timescales governed by synoptic-scale dynamics. To verify the reliability of our method, in situ sulfate observations from CHC are combined with the SRR clusters to correctly identify the (pre-known) source of the sulfate: the Sabancaya volcano located 400 km north-west from the station.

PM2.5 haze pollution driven by secondary inorganic NO3− has been a great concern in East Asia. It is, therefore, imperative to identify its sources and oxidation processes, for which nitrogen and oxygen stable isotopes are powerful tracers. Here, we determined the δ15N (NO3−) and Δ17O (NO3−) of PM2.5 in Seoul from 2018 to 2019, and estimated quantitatively the relative contribution of oxidation pathways for particulate NO3− and major NOx emission sources. In the range of PM2.5 mass concentration from 7.5 g m−3 (summer) to 139.0 g m−3 (winter), the mean δ15N was −0.7 ± 3.3 ‰ and 3.8 ± 3.7 ‰, and the mean Δ17O was 23.2 ± 2.2 ‰ and 27.7 ± 2.2 ‰ in the summer and winter, respectively. While OH oxidation was the dominant pathway for NO3− during the summer (87 %), nighttime formation via N2O5 and NO3 was more important (38 %) during the winter, when aerosol liquid water content (AWLC) and nitrogen oxidation ratio (NOR) were higher. Interestingly, the highest Δ17O was coupled with the lowest δ 15N and highest NOR in record-breaking winter PM2.5 episodes, revealing the critical role of photochemical oxidation process in severe winter haze development. For NOx sources, vehicle emissions were confirmed as a main contributor, followed by biomass combustion from various activities. The contribution from biogenic soil and coal combustion was slightly increased in summer and winter, respectively. Our results built on multiple-isotope approach provide the first explicit evidence for NO3− formation processes and major NOx emission sources in Seoul megacity and suggest an effective mitigation measure to improve PM2.5 pollution.

Over the past few decades, northeast Asia has suffered from the extreme levels of PM2.5 (particulate matter with an aerodynamic diameter smaller than 2.5 μm). Despite extensive efforts and the scientific advances in understanding PM2.5 pollution, the fundamental mechanisms responsible for the occurrence of high PM2.5 concentrations have not been comprehensively understood. In this study, we investigated the physical and chemical drivers for the formation and transformation of atmospheric particles using a four-year dataset of nanoparticle number size distributions, PM2.5 chemical composition, gaseous precursors, and meteorological variables in northeast Asia outflows. The empirical orthogonal function (EOF) analyses of size-separated particle numbers extracted two modes representing a burst of nanoparticles (EOF1) and an increase in PM2.5 mass (EOF2) associated with persistent anticyclone and synoptic-scale stagnation, respectively. The vertical structure of the particles demonstrated that the synoptic conditions also affected the daily evolution of boundary layer, promoting either the formation of nanoparticles through deep mixing or conversion into accumulation-mode particles in shallow mixed layers. In the haze-development episode equivalent to EOF2 during the KORUS-AQ (KORea-US Air Quality) campaign, the PM2.5 mass reached 63 μg m−3 with the highest contribution from inorganic constituents, which was accompanied by a thick coating of refractory black carbon (rBC) that linearly increased with condensation-mode particles. This observational evidence suggests that the thick coating of rBC resulted from an active conversion of condensable gases into particle-phase on the BC surface, thereby increasing the mass of the accumulation-mode aerosol. Consequently, this result complies with the strategy to reduce black carbon as a way to effectively mitigate haze pollution as well as climate change in northeast Asia.

The identification of the severe COVID-19 virus in December 2019 led the World Health Organization to declare a global pandemic by March 2020. Up till recently with the first available vaccines, the only prevention measures include strict social, travel, and working restrictions in a so-called lockdown period that lasted for several weeks (mid-March to the end of April 2020 for most of Europe). This abrupt change in social behavior is expected to impact local but also regional atmospheric composition, and the environmental impact is potentially of high interest to policy and decision-makers.The Aerosol, Clouds, and Trace Gases Research Infrastructure (ACTRIS) is a pan-European research infrastructure producing high-quality data and information on short-lived atmospheric constituents and on the processes leading to the variability of these constituents in natural and controlled atmospheres. Realizing the crucial scientific value of ACTRIS observations of atmospheric composition changes across Europe, the ACTRIS community promptly actioned internal communication [AD1] thread to organize and set-up COVID-19 related activities. Such reactive internal involvement of ACTRIS partners generated timely outcomes. In fact, during the lockdown period in spring 2020, most of the ACTRIS observational and exploratory platforms were operational providing continuous access to data on air quality and atmospheric composition and, as a tailored service arrangement, to reinvent ACTRIS simulation chambers for testing mask filtering efficiencies. [AD2] ACTRIS response to the COVID-19 pandemic showcases multiple benefits to policy- and decision-makers focused on environmental and societal impacts of COVID-19 and the closing down of several sectors of society (e.g. transport, industry, services). To boost the visibility of ACTRIS COVID-19 response at the European level, ACTRIS actively engaged and collaborated with the wider community of Research Infrastructures (ESFRI, ENVRI, and ERF-AISBL) in Europe to support joint activities for SARS-CoV-2. The Open Science Session on COVID-19 during the ACTRIS week event brought together a broad audience and key-note speaker from major European agencies and organizations (ESA, ECMWF, FMI, ICOS). The online format of the event created the opportunity to open ACTRIS science to a broader public. At the national level, atmospheric scientists were interviewed on COVID impacts raising awareness on the work undertaken in the research infrastructure to the general public.ACTRIS aims at establishing further engagement and direct communication with decision and policy-makers and, for that, envisage the implementation of ad-hoc efforts. This presentation will showcase the various efforts and success stories to capture society as well as policy- and decision-makers.

The identification of the severe COVID-19 virus in December 2019 led the World Health Organization to declare a global pandemic by March 2020. Up till recently with the first available vaccines, the only prevention measures include strict social, travel and working restrictions in a so-called lockdown period that lasted for several weeks (mid-March to the end of April 2020 for most of Europe). This abrupt change in social behaviour is expected to impact local but also regional atmospheric composition, and the environmental impact is highly interesting to study.The Aerosol, Clouds and Trace Gases Research Infrastructure (ACTRIS) is a pan-European research infrastructure producing high-quality data and information on short-lived atmospheric constituents and on the processes leading to the variability of these constituents in natural and controlled atmospheres. ACTRIS integrates, harmonizes, and distributes datasets, activities, and services provided by the Central Facilities and National Facilities, located in 22 European countries. During the lockdown period in spring 2020 most of the ACTRIS observational were operational. The National Facilities performing the ambient measurements are generally regional background sites, with the aim to detect changes on regional level. Within the context of the current COVID-19 outbreak, ACTRIS has been continuously providing access to data on air quality and atmospheric composition. This is of particular interest and importance as it provides unique information measured from the ground to assess the European air quality and atmospheric composition during the lockdown complementing, in a fundamental way, satellite observations and modelling analysis. ACTRIS released a comprehensive and quality assured set of atmospheric measurement data during the COVID-19 pandemic spring 2020 – January– May 2020. This includes:- 30 sites with aerosol in situ measurements providing mainly absorption and scattering coefficient, size and/or number distribution. A few sites with high time solution aerosol chemical composition;- 12 sites with trace gases in situ data providing VOCs and NOX measurements;24 sites with aerosol remote sensing data providing profiles with backscattering and extinction coefficient;- 11 cloud remote sensing sites providing profile information of 9 various cloud properties.To facilitate studies, ACTRIS has compiled the data and coined a DOI for the data sets measured during the COVID-19 spring lockdown period, including an intensive aerosol remote sensing campaign in May. This presentation will present the data set and the potential applications and benefits using ACTRIS COVID-19 dataset for studying atmospheric composition changes during COVID-19 lockdown periods.

Within the framework of the AeroCom (Aerosol Comparisons between Observations and Models) initiative, the state-of-the-art modelling of aerosol optical properties is assessed from 14 global models participating in the phase III control experiment (AP3). The models are similar to CMIP6/AerChemMIP Earth System Models (ESMs) and provide a robust multi-model ensemble. Inter-model spread of aerosol species lifetimes and emissions appears to be similar to that of mass extinction coefficients (MECs), suggesting that aerosol optical depth (AOD) uncertainties are associated with a broad spectrum of parameterised aerosol processes. Total AOD is approximately the same as in AeroCom phase I (AP1) simulations. However, we find a 50 % decrease in the optical depth (OD) of black carbon (BC), attributable to a combination of decreased emissions and lifetimes. Relative contributions from sea salt (SS) and dust (DU) have shifted from being approximately equal in AP1 to SS contributing about 2∕3 of the natural AOD in AP3. This shift is linked with a decrease in DU mass burden, a lower DU MEC, and a slight decrease in DU lifetime, suggesting coarser DU particle sizes in AP3 compared to AP1. Relative to observations, the AP3 ensemble median and most of the participating models underestimate all aerosol optical properties investigated, that is, total AOD as well as fine and coarse AOD (AODf, AODc), Ångström exponent (AE), dry surface scattering (SCdry), and absorption (ACdry) coefficients. Compared to AERONET, the models underestimate total AOD by ca. 21 % ± 20 % (as inferred from the ensemble median and interquartile range). Against satellite data, the ensemble AOD biases range from −37 % (MODIS-Terra) to −16 % (MERGED-FMI, a multi-satellite AOD product), which we explain by differences between individual satellites and AERONET measurements themselves. Correlation coefficients (R) between model and observation AOD records are generally high (R>0.75), suggesting that the models are capable of capturing spatio-temporal variations in AOD. We find a much larger underestimate in coarse AODc (∼ −45 % ± 25 %) than in fine AODf (∼ −15 % ± 25 %) with slightly increased inter-model spread compared to total AOD. These results indicate problems in the modelling of DU and SS. The AODc bias is likely due to missing DU over continental land masses (particularly over the United States, SE Asia, and S. America), while marine AERONET sites and the AATSR SU satellite data suggest more moderate oceanic biases in AODc. Column AEs are underestimated by about 10 % ± 16 %. For situations in which measurements show AE > 2, models underestimate AERONET AE by ca. 35 %. In contrast, all models (but one) exhibit large overestimates in AE when coarse aerosol dominates (bias ca. +140 % if observed AE < 0.5). Simulated AE does not span the observed AE variability. These results indicate that models overestimate particle size (or underestimate the fine-mode fraction) for fine-dominated aerosol and underestimate size (or overestimate the fine-mode fraction) for coarse-dominated aerosol. This must have implications for lifetime, water uptake, scattering enhancement, and the aerosol radiative effect, which we can not quantify at this moment. Comparison against Global Atmosphere Watch (GAW) in situ data results in mean bias and inter-model variations of −35 % ± 25 % and −20 % ± 18 % for SCdry and ACdry, respectively. The larger underestimate of SCdry than ACdry suggests the models will simulate an aerosol single scattering albedo that is too low. The larger underestimate of SCdry than ambient air AOD is consistent with recent findings that models overestimate scattering enhancement due to hygroscopic growth. The broadly consistent negative bias in AOD and surface scattering suggests an underestimate of aerosol radiative effects in current global aerosol models. Considerable inter-model diversity in the simulated optical properties is often found in regions that are, unfortunately, not or only sparsely covered by ground-based observations. This includes, for instance, the Sahara, Amazonia, central Australia, and the South Pacific. This highlights the need for a better site coverage in the observations, which would enable us to better assess the models, but also the performance of satellite products in these regions. Using fine-mode AOD as a proxy for present-day aerosol forcing estimates, our results suggest that models underestimate aerosol forcing by ca. −15 %, however, with a considerably large interquartile range, suggesting a spread between −35 % and +10 %.

High-quality data of atmospheric mercury (Hg) is rare for South America, especially for its tropical part. In consequence, mercury dynamics are still highly uncertain in this region, a significant deficiency, as South America appears to play a major role in the global budget of this toxic pollutant. To address this issue, we performed nearly two years (July 2014–February 2016) of continuous high resolution total gaseous mercury (TGM) measurements at the Chacaltaya (CHC) mountain site in the Bolivian Andes, which is subject to a diverse mix of air masses coming predominantly from the Altiplano and the Amazon rainforest. For the first eleven months of measurements, we obtained a mean TGM concentration of 0.89±0.01 ng m−3, in good agreement with the sparse amount of data available from the continent. For the remaining nine months, we obtained a significantly higher TGM concentration of 1.34±0.01 ng m−3, a difference which we tentatively attribute to the strong El Niño event of 2015–2016. Based on HYSPLIT back-trajectories and clustering techniques, we show that lower mean TGM concentrations were linked to either westerly Altiplanic air masses or those originating from the lowlands to the south-east of CHC. Elevated TGM concentrations were related to northerly air masses of Amazonian or southerly air masses of Altiplanic origin, the former possibly linked to artisanal and small scale gold mining (ASGM), while the latter might be explained by volcanic activity. We observed a marked seasonal pattern, with low TGM concentrations in the dry season (austral winter), rising concentrations during biomass burning (BB) season, and highest concentrations at the beginning of the wet season (austral summer). With the help of simultaneously sampled equivalent black carbon (eBC) and carbon monoxide (CO) data, we use the clearly BB influenced signal during BB season (August to October) to derive a mean TGM/CO emission ratio of (2.3±0.6)·10−7 ppbvTGM ppbvCO−1, which could be used to constrain South American BB emissions. Through the link with in-situ measured CO2 and remotely sensed solar-induced fluorescence (SIF) as proxies for vegetation activity, we detect signs of a vegetation sink effect in Amazonian air masses and derive a best guess TGM/CO2 uptake ratio of 0.058±0.017 (ng m−3)TGM ppmCO2−1. Finally, significantly higher Hg concentrations in western Altiplanic air masses during the wet season as compared to the dry season point towards the modulation of atmospheric Hg by the Eastern Pacific Ocean.

In order to assess the global evolution of aerosol parameters affecting climate change, a long-term trend analyses of aerosol optical properties were performed on time series from 52 stations situated across five continents. The time series of measured scattering, backscattering and absorption coefficients as well as the derived single scattering albedo, backscattering fraction, scattering and absorption Ångström exponents covered at least 10 years and up to 40 years for some stations. The non-parametric seasonal Mann–Kendall (MK) statistical test associated with several prewhitening methods and with the Sen's slope were used as main trend analysis methods. Comparisons with General Least Mean Square associated with Autoregressive Bootstrap (GLS/ARB) and with standard Least Mean Square analysis (LMS) enabled confirmation of the detected MK statistically significant trends and the assessment of advantages and limitations of each method. Currently, scattering and backscattering coefficients trends are mostly decreasing in Europe and North America and are not statistically significant in Asia, while polar stations exhibit a mix of increasing and decreasing trends. A few increasing trends are also found at some stations in North America and Australia. Absorption coefficients time series also exhibit primarily decreasing trends. For single scattering albedo, 52 % of the sites exhibit statistically significant positive trends, mostly in Asia, Eastern/Northern Europe and Arctic, 18 % of sites exhibit statistically significant negative trends, mostly in central Europe and central North America, while the remaining 30 % of sites have trends, which are not statistically significant. In addition to evaluating trends for the overall time series, the evolution of the trends in sequential 10 year segments was also analyzed. For scattering and backscattering, statistically significant increasing 10 year trends are primarily found for earlier periods (10 year trends ending in 2010–2015) for polar stations and Mauna Loa. For most of the stations, the present-day statistically significant decreasing 10 year trends of the single scattering albedo were preceded by not statistically significant and statistically significant increasing 10 year trends. The effect of air pollution abatement policies in continental North America is very obvious in the 10 year trends of the scattering coefficient – there is a shift to statistically significant negative trends in 2010–2011 for all stations in the eastern and central US. This long-term trend analysis of aerosol radiative properties with a broad spatial coverage enables a better global view of potential aerosol effects on climate changes.

Black Carbon (BC) is a major forcing agent in the Arctic but substantial uncertainty remains to quantify its climate effects due to the complexity of mechanisms involved. In this study, we provide unique information on processes driving the variability of BC mass concentration in surface snow in the Arctic. Two different snow-sampling strategies were adopted during spring 2014 and 2015, focusing on the refractory BC (rBC) mass Ny-Ålesund concentration daily/hourly variability on a seasonal/daily time scale (referred to as 80-days and 3-days experiments). Despite the low rBC mass concentrations (never exceeding 22 ng g−1), a daily variability of up to 4.5 ng g−1 was observed. Atmospheric, meteorological and snow-related physico-chemical parameters were considered in multiple statistical models to understand the factors behind the observed variation of rBC mass concentrations. Results indicate that the main drivers of the variation of rBC are the precipitations events, snow metamorphism (melting-refreezing cycles, surface hoar formation and sublimation) and the activation of local sources (wind resuspension) during the snow melting periods. The rBC in the snow seems de-coupled with the atmospheric BC load. Our results highlighted a common association of snow rBC with coarse mode particles number concentration and with snow precipitation events.

Atmospheric aerosols are known to play a key role in Earth’s radiative budget, although the quantification of their climate forcing is still highly uncertain. In order to improve the scientific understanding of their climatic effect, in-situ ground-based aerosol properties observations are needed by the research community. Such data would also allow the global assessment of the effect of environmental policies over both the short and the long term.To develop a robust and consistent view over time of the worldwide variability of aerosol properties, data resulting from a fully-characterized value chain, including uncertainty estimation, is needed.The present work is part of a wider project, having among its goals the investigation of the variability of climate-relevant aerosol properties observed at all sites connected to the Global Atmospheric Watch network, whose data are publicly available from the World Data Centre for Aerosols and follow the aforementioned specifications.This work focuses on aerosol optical proprieties, i.e. the aerosol light scattering coefficient (σsp), the aerosol light absorption coefficient (σap), single scattering albedo (ωo) and both scattering and absorption Ångström exponents (åsp and åap).The analysis includes 108 yearly datasets collected either during 2016 or 2017 at different sites: 53 for absorption and 55 for scattering coefficient datasets, respectively. For 29 of these sites it was also possible to compute single scattering albedo.The spatial variability in extensive and intensive optical properties was analysed in terms of each site’s geographical location (either polar, continental, coastal or mountain) and its footprint (from pristine to urban, representing increasing levels of anthropogenic influence).The results highlight the impact of anthropogenic emissions and biomass burning on absolute levels and annual variability. The effect of sea spray or long range transport of dust is also evident for several sites, along with the influence of regional emissions. The largest seasonality in aerosol loading was observed at mountain sites under mixed footprint conditions, while the lowest seasonality occurred at urban sites. Urban sites also exhibited the highest σsp and σap values. The lowest levels in σsp and σap were observed at some polar sites, along with few coastal and mountain sites, despite their typically mixed footprint.AcknowledgementsThe authors acknowledge WMO-GAW World Data Centre on Aerosol for providing data available at http://ebas.nilu.no

In order to assess the global evolution of aerosol parameters affecting climate change, a long-term trend analyses of aerosol optical properties were performed on time series from 52 stations situated across five continents. The time series of measured scattering, backscattering and absorption coefficients as well as the derived single scattering albedo, backscattering fraction, scattering and absorption Ångström exponents covered at least 10 years and up to 40 years for some stations. The non-parametric seasonal Mann-Kendall (MK) statistical test associated with several prewhitening methods and with the Sen’s slope were used as main trend analysis methods. Comparisons with General Least Mean Square associated with Autoregressive Bootstrap (GLS/ARB) and with standard Least Mean Square analysis (LMS) enabled confirmation of the detected MK statistically significant trends and the assessment of advantages and limitations of each method. Currently, scattering and backscattering coefficients trends are mostly decreasing in Europe and North America and are not statistically significant in Asia, while polar stations exhibit a mix of increasing and decreasing trends. A few increasing trends are also found at some stations in North America and Australia. Absorption coefficients time series also exhibit primarily decreasing trends. For single scattering albedo, 52% of the sites exhibit statistically significant positive trends, mostly in Asia, Eastern/Northern Europe and Arctic, 18% of sites exhibit statistically significant negative trends, mostly in central Europe and central North America, while the remaining 30% of sites have trends, which are not statistically significant. In addition to evaluating trends for the overall time series, the evolution of the trends in sequential 10 year segments was also analyzed. For scattering and backscattering, statistically significant increasing 10 year trends are primarily found for earlier periods (10 year trends ending in 2010-2015) for polar stations and Mauna Loa. For most of the stations, the present-day statistically significant decreasing 10 year trends of the single scattering albedo were preceded by not statistically significant and statistically significant increasing 10 year trends. The effect of air pollution abatement policies in continental North America is very obvious in the 10 year trends of the scattering coefficient – there is a shift to statistically significant negative trends in 2010-2011 for all stations in the eastern and central US. This long-term trend analysis of aerosol radiative properties with a broad spatial coverage enables a better global view of potential aerosol effects on climate changes.

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 time 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 observations 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 more than 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 measurements 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.

Within the framework of the AeroCom (Aerosol Comparisons between Observations and Models) initiative, the present day modelling of aerosol optical properties has been assessed using simulated data representative for the year 2010, from 14 global aerosol models participating in the Phase III Control experiment. The model versions are close or equal to those used for CMIP6 and AerChemMIP and inform also on bias in state of the art ESMs. Modelled column optical depths (total, fine and coarse mode AOD) and Angstrom Exponents (AE) were compared both with ground based observations from the Aerosol Robotic Network (AERONET, version 3) as well as space based observations from AATSR-SU instruments. In addition, the modelled AODs were compared with MODIS (Aqua and Terra) data and a satellite AOD data-set (MERGED-FMI) merged from 12 different individual AOD products. Furthermore, for the first time, the modelled near surface scattering (under dry conditions) and absorption coefficients were evaluated against measurements made at low relative humidity at surface in-situ GAW sites. Statistics are based mainly on normalised mean biases and Pearson correlation coefficients from colocated model and observation data in monthly resolution. Hence, the results are mostly representative for the regions covered by each of the observation networks. Model biases established against satellite data yield insights into remote continental areas and oceans, where ground-based networks lack site coverage. The satellite data themselves are evaluated against AERONET observations, to test our aggregation and re-gridding routines, suggesting relative AOD biases of −5 %, −6 %, +9 % and +18 % for AATSR-SU, MERGED-FMI, MODIS-aqua and MODIS-terra, respectively, with high correlations exceeding 0.8. Biases of fine and coarse AOD and AE in AATSR are found to be +2 %, −16 % and +14.7 % respectively, at AERONET sites, with correlations of the order of 0.8. The AeroCom MEDIAN and most of the participating models underestimate the optical properties investigated, relative to remote sensing observations. AERONET AOD is underestimated by 21 % ± 17 %. Against satellite data, the model AOD biases range from −38 % (MODIS-terra) to −17 % (MERGED-FMI). Correlation coefficients of model AODs with AERONET, MERGED-FMI and AATSR-SU are high (0.8–0.9) and slightly lower against the two MODIS data-sets (0.6–0.8). Investigation of fine and coarse AODs from the MEDIAN model reveals biases of −10% ± 20 % and −41 % ± 29 % against AERONET and −13 % and −24 % against AATSR-SU, respectively. The differences in bias against AERONET and AATSR-SU are in agreement with the established satellite bias against AERONET. These results indicate that most of the AOD bias is due to missing coarse AOD in the regions covered by these observations. Underestimates are also found when comparing the models against the surface GAW observations, showing AeroCom MEDIAN mean bias and inter-model variation of −44 % ± 22 % and −32 % ± 34 % for scattering and absorption coefficients, respectively. Dry scattering shows higher underestimation than AOD at ambient relative humidity and is in agreement with recent findings that suggest that models tend to overestimate scattering enhancement due to hygroscopic growth. Broadly consistent negative bias in AOD and scattering suggest a general underestimate in aerosol effects in current global aerosol models. The large diversity in the surface absorption results suggests differences in the model treatment of light absorption by black carbon (BC), dust (DU) and to a minor degree, organic aerosol (OA). Considerable diversity is found among the models in the simulated near surface absorption coefficients, particularly in regions associated with dust (e.g. Sahara, Tibet), biomass burning (e.g. Amazonia, Central Australia) and biogenic emissions (e.g. Amazonia). Regions associated with high anthropogenic BC emissions such as China and India exhibit comparatively good agreement for all models. Evaluation of modelled column AEs shows an underestimation of 9 % ± 24 % against AERONET and −21 % against AATSR-SU. This suggests that overall, models tend to overestimate particle size, with implications for lifetime and radiative transfer calculations. An investigation of modelled emissions, burdens and lifetimes, mass-specific-extinction coefficients (MECs) and optical depths (ODs) for each species and model reveals considerable diversity in most of these parameters. These are discussed in detail for each model individually. Inter-model spread of aerosol species lifetime appears to be similar to that of mass extinction coefficients, suggesting that AOD uncertainties are still associated to a broad spectrum of parameterised aerosol processes.

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 time 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 observations 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 more than 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 measurements 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.

This study presents a multi-parameter analysis of aerosol trends over the last two decades at regional and global scales. Regional time series have been computed for a set of nine optical, chemical composition and mass aerosol properties by using the observations of several ground-based networks. From these regional time series the aerosol trends have been derived for different regions of the world. Most of the properties related to aerosol loading exhibit negative trends, both at the surface and in the total atmospheric column. Significant decreases of aerosol optical depth (AOD) are found in Europe, North America, South America and North Africa, ranging from −1.3 %/yr to −3.1 %/yr. An error and representativity analysis of the incomplete observational data has been performed using model data subsets in order to investigate how likely the observed trends represent the actual trends happening in the regions over the full study period from 2000 to 2014. This analysis reveals that significant uncertainty is associated with some of the regional trends due to time and space sampling deficiencies. The set of observed regional trends has then been used for the evaluation of the climate models and their skills in reproducing the aerosol trends. Model performance is found to vary depending on the parameters and the regions of the world. The models tend to capture trends in AOD, column Angstrom exponent, sulfate and particulate matter well (except in North Africa), but show larger discrepancies for coarse mode AOD. The rather good agreement of the trends, across different aerosol parameters between models and observations, when co-locating them in time and space, implies that global model trends, including those in poorly monitored regions, are likely correct. The models can help to provide a global picture of the aerosol trends by filling the gaps in regions not covered by observations. The calculation of aerosol trends at a global scale reveals a different picture from the one depicted by solely relying on ground based observations. Using a model with complete diagnostics (NorESM2) we find a global increase of AOD of about 0.2 %/yr between 2000 and 2014, primarily caused by an increase of the loads of organic aerosol, sulfate and black carbon.