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The outbreak of SARS-CoV-2 and subsequent spread of the disease COVID-19 became classified as a pandemic in March of 2020, leading to global safety measures introduced to limit the impact of the virus. This combination of safety measures has become commonly referred to as "lockdown". The associated industry and lifestyle changes led to reductions in the anthropogenic emission of atmospheric pollutants such as black carbon (BC), which is transported from the mid-latitudes into the Arctic during the winter and spring. Measurements of BC and other anthropogenic pollutants are of increasing importance in the Arctic due to the rapid warming observed there in the past few decades. It is believed that BC has a significant role in this warming, and so understanding the Arctic's response to reduced BC emissions at lower latitudes will provide insight into how future changes might mitigate further warming. Reductions in BC have been reported worldwide, and so in this study, the impact of these reductions on BC concentrations at the High Arctic site Villum Research Station was investigated. The effect was examined from March 2020, around when global lockdowns began, to June 2020, when the Arctic haze period ended and BC levels were once again low. Firstly, the Danish Eulerian Hemispheric Model (DEHM) was used to assess this impact on BC concentrations by adjusting global anthropogenic pollution emission inventories to simulate those observed during the lockdown period and comparing the results to a similar model run with standard emission inventories. Secondly, equivalent BC data from an aethalometer at Villum Research Station were analysed, comparing the concentrations during the lockdown period to both aethalometer data from previous years and DEHM results from the lockdown period. It was found that when adjusted DEHM emission inventories were introduced from the 1st of March, the model predicted a reduction in BC concentrations beginning on the 10th of March and reached a 10% reduction by the 1st of April. This reduction fluctuated around 10% until the end of the Arctic haze period. Aethalometer data did not show any significant change from previous years, and no concentration reduction could be concluded from its comparison with DEHM results. This is likely because the predicted reduction of 10% is smaller than both the inter-annual and intra-annual variability of measured BC concentrations at Villum. • DEHM model and aethalometer used to assess effect of lockdown on Villum BC levels. • DEHM model simulated a 10% reduction in BC at Villum during April–May 2020. • Similar reductions are expected throughout the Arctic region (between 7% and 11%). • The aethalometer showed no response to the lockdown due to inherent BC variability. [ABSTRACT FROM AUTHOR]

Sea spray aerosol (SSA) contributes significantly to natural aerosol particle concentrations globally, in marine areas even dominantly. The potential changes of the omnipresent inorganic fraction of SSA due to atmospheric ageing is largely unexplored. In the atmosphere, SSA may exist as aqueous phase solution droplets or as dried solid or amorphous particles. We demonstrate that ageing of liquid NaCl and artificial sea salt aerosol by exposure to ozone and UV light leads to a substantial decrease in hygroscopicity and cloud activation potential of the dried particles of the same size. The results point towards surface reactions on the liquid aerosols that are more crucial for small particles and the formation of salt structures with water bound within the dried aerosols, termed hydrates. Our findings suggest an increased formation of hydrate forming salts during ageing and the presence of hydrates in dried SSA. Field observations indicate a reduced hygroscopic growth factor of sub-micrometre SSA in the marine atmosphere compared to fresh laboratory generated NaCl or sea salt of the same dry size, which is typically attributed to organic matter or sulphates. Aged inorganic sea salt offers an additional explanation for such a measured reduced hygroscopic growth factor and cloud activation potential. [ABSTRACT FROM AUTHOR]

A k‐means cluster analysis of a 5‐year aerosol particle size distribution data set from northeast Greenland is combined with measurements of coincident shorter field studies of aerosol equivalent black carbon (eBC) content, hygroscopic growth factor (HGF), and cloud condensation nuclei (CCN) concentrations. This led to five clusters strongly controlled by natural emissions (eBC 8–15 ng/m3) and three anthropogenic clusters with larger particle concentrations in the accumulation mode (eBC 29–77 ng/m3). The HGF and CCN properties of the eight aerosol clusters differ drastically. Anthropogenic clusters feature high growth factors (1.62–1.81) and low CCN κ values (0.10–0.46), while natural clusters show lower HGF (1.38–1.70) but remarkably higher κ values (0.35–0.51). Extrapolating the CCN properties on the basis of the cluster analysis to annual timescales suggests that biogenic organic aerosol may drive Arctic aerosol production during summer. Plain Language Summary: The variability of the warming Arctic—particularly the decreasing sea ice extent—is likely to boost the control of the ocean on the atmospheric composition. Key factors controlling Arctic aerosol‐climate interactions are sources, chemical transformations, and removal mechanisms of aerosols and their precursor gases. However, knowledge on the composition and sources of summer aerosols is insufficient. To tackle this problem, we use an approach where aerosol particle size distributions, aerosol equivalent black carbon, hygroscopic growth factor, and cloud condensation nuclei measurements at Villum Research Station within the period 2012–2016 are synergistically combined. Our results suggest that marine sources of new particle formation and growth dominate summer Arctic background conditions. These aerosols are organic rich in nature. Elevated cloud condensation nuclei concentrations occurred when secondary Arctic biogenic organic aerosols were detected. Key Points: An extended analysis of marine biogenic new particle formation and growth dominating summer Arctic background conditions is presentedDirect measurements of low aerosol hygroscopic growth factors (HGFs) dominate the aerosol ultrafine population—aerosols is organic richElevated cloud condensation nuclei concentrations occurred when Arctic background biogenic aerosols—secondary in origin—were detected [ABSTRACT FROM AUTHOR]

The Arctic region is warming about four times faster than the rest of the globe, and thus it is important to understand the processes driving climate change in this region. Aerosols are a significant component of the Arctic climate system as they form ice crystals and liquid droplets that control the dynamics of clouds and also directly interact with solar radiation, depending on the compositions and mixing states of individual particles. Here, we report on the characteristics of submicron-sized aerosol particles using transmission electron microscopy obtained at two high Arctic sites, northeast Greenland (Villum Research Station) and Svalbard (Zeppelin Observatory), during spring 2018. The results showed that a dominant compound in the submicron-sized spring aerosols was sulfate, followed by sea salt particles. Both model simulations and observations at the Zeppelin Observatory showed that sea salt particles became more prevalent when low-pressure systems passed by the station. Model simulations indicate that both sampling sites were affected by diffused and diluted long-range transport of anthropogenic aerosols from lower latitudes with negligible influences of biomass burning emissions during the observation period. Overall, the composition of measured aerosol particles from the two Arctic sites was generally similar and showed no apparent variation except for the sea salt fractions. This study shows a general picture of high Arctic aerosol particles influenced by marine sources and diffused long-range transport of anthropogenic sources during the Arctic spring period. These results will contribute to a better knowledge of the aerosol composition and mixing state during the Arctic spring, which helps to understand the contributions of aerosols to the Arctic climate. [Display omitted] • Electron microscopy reveals composition and mixing state of submicron aerosol particles in the Arctic spring. • Sea salt particles demonstrate a size-dependent increase and exhibit correlations with specific elements. • Regional Arctic aerosol particles mainly from marine sources and diffused long-range transport are shown. [ABSTRACT FROM AUTHOR]

Air pollution is associated with increased risk of myocardial infarction (MI), but it is unresolved to what extent the association is modified by factors such as socioeconomic status, comorbidities, financial stress, residential green space, or road traffic noise. We formed a cohort of all (n = 1,964,702) Danes, aged 50–85 years, with 65,311 cases of MI during the followed-up period 2005–2017. For all participants we established residential five-year running average exposure to particulate matter <2.5 μm (PM 2.5), ultrafine particles (UFP, <0.1 μm), elemental carbon (EC) and nitrogen dioxide (NO 2). We evaluated risk in population strata, using Aalen additive hazards models to estimate absolute risk and Cox proportional hazards models to estimate relative risk of MI with 95% confidence intervals (CI). PM 2.5 and the other pollutant were associated with MI. Lower education and lower income were associated with higher absolute risks of MI from air pollution, whereas no clear effect modification was apparent for relative risk estimates. For example, 5 μg/m3 higher PM 2.5 was associated with HR for MI of 1.16 (95% CI: 1.10–1.22) among those with only mandatory education and 1.13 (95% CI: 1.03–1.24) among those with long education. The corresponding rate differences per 100,000 person years were 243 (95% CI: 216–271) and 358 (95% CI: 338–379), respectively. Higher level of comorbidity was consistently across all four pollutants associated with both higher absolute and relative risk of MI. In conclusion, people with comorbid conditions or of lower SES appeared more vulnerable to long-term exposure to air pollution and more cases of MI may be prevented by focused interventions in these groups. • Air pollution is positively associated with risk of myocardial infarction. • People of low socioeconomic status or with comorbidities are more susceptible. • Effect modification appears to be most apparent with absolute risk estimates. [ABSTRACT FROM AUTHOR]

This paper presents a detailed emission inventory for NO x , particle mass (PM) and particle numbers (PN) for aircraft main engines, APU's and handling equipment at Copenhagen Airport (CPH) based on time specific activity data and representative emission factors for the airport. The inventory has a high spatial resolution of 5 m × 5 m in order to be suited for further air quality dispersion calculations. Results are shown for the entire airport and for a section of the airport apron area (“inner apron”) in focus. The methodology presented in this paper can be used to quantify the emissions from aircraft main engines, APU and handling equipment in other airports. For the entire airport, aircraft main engines is the largest source of fuel consumption (93%), NO x , (87%), PM (61%) and PN (95%). The calculated fuel consumption [ NO x , PM, PN ] shares for APU's and handling equipment are 5% [ 4%, 8%, 5% ] and 2% [ 9%, 31%, 0% ], respectively. At the inner apron area for handling equipment the share of fuel consumption [NO x , PM, PN] are 24% [ 63%, 75%, 2% ], whereas APU and main engines shares are 43% [ 25%, 19%, 54% ], and 33% [ 11%, 6%, 43% ], respectively. The inner apron NO x and PM emission levels are high for handling equipment due to high emission factors for the diesel fuelled handling equipment and small for aircraft main engines due to small idle-power emission factors. Handling equipment is however a small PN source due to the low number based emission factors. Jet fuel sulphur-PM sensitivity calculations made in this study with the ICAO FOA3.0 method suggest that more than half of the PM emissions from aircraft main engines at CPH originate from the sulphur content of the fuel used at the airport. Aircraft main engine PN emissions are very sensitive to the underlying assumptions. Replacing this study's literature based average emission factors with “high” and “low” emission factors from the literature, the aircraft main engine PN emissions were estimated to change with a factor of 14. [ABSTRACT FROM AUTHOR]

This cross-sectional study investigated the relationship between exposure to airborne indoor and outdoor particulate matter (PM) and cardiovascular and respiratory health in a population-based sample of 58 residences in Copenhagen, Denmark. Over a 2-day period indoor particle number concentrations (PNC, 10–300 nm) and PM 2.5 (aerodynamic diameter < 2.5 μm) were monitored for each of the residences in the living room, and outdoor PNC (10–280 nm), PM 2.5 and PM 10 (aerodynamic diameter < 10 μm) were monitored at an urban background station in Copenhagen. In the morning, after the 2-day monitoring period, we measured microvascular function (MVF) and lung function and collected blood samples for biomarkers related to inflammation, in 78 middle-aged residents. Bacteria, endotoxin and fungi were analyzed in material from electrostatic dust fall collectors placed in the residences for 4 weeks. Data were analyzed using linear regression with the generalized estimating equation approach. Statistically significant associations were found between indoor PNC, dominated by indoor use of candles, and lower lung function, the prediabetic marker HbA1c and systemic inflammatory markers observed as changes in leukocyte differential count and expression of adhesion markers on monocytes, whereas C-reactive protein was significantly associated with indoor PM 2.5 . The presence of indoor endotoxin was associated with lower lung function and expression of adhesion markers on monocytes. An inverse association between outdoor PNC and MVF was also statistically significant. The study suggests that PNC in the outdoor environment may be associated with decreased MVF, while PNC, mainly driven by candle burning, and bioaerosols in the indoor environment may have a negative effect on lung function and markers of systemic inflammation and diabetes. [ABSTRACT FROM AUTHOR]

Abstract: In this study we evaluated a forecast model for urban aerosol number concentrations against measurements made at five European cities: Helsinki, Stockholm, Copenhagen, Leipzig, and Athens. This model requires a learning data set with particle number concentrations, traffic densities and local meteorology. Additionally, in the forecasting process it requires the same parameters from the past week and forecasted values of weather and traffic. The model performance was tested based on the R 2, index of agreement (IA), mean square error (MSE), and bias. We tested three modelling approaches: one with fixed parameterisation and two with optimisations based on either the Deviance or the Akaike Information Criterion. Based on the hourly one-day forecasts at the urban background sites the IA ranged from 0.65 to 0.80 for accumulation mode particles and from 0.68 to 0.87 for ultrafine particles. The model performance was the best for Helsinki and Stockholm and the worst for Leipzig and Copenhagen. The main reason is the more pronounced diurnal variation in Helsinki and Stockholm. Another reason is that the traffic data for Leipzig and Copenhagen were not as complete as for the other cities. The three approaches yielded similar model performances, hence we conclude that the simplest one based on a fixed parametrisation is to be preferred. [Copyright &y& Elsevier]

Abstract: Three years of aerosol measurements from four European cities were analysed to identify and quantify the parameters influencing the particle number size distribution (NSD) at different measurement sites (Copenhagen, Helsinki, Leipzig, London). A ‘triple-site’ approach was established with data from a roadside, an urban background and a rural measurement station, which was available for each city. The NSD measurements were conducted over a diameter size range of 8–700 nm. The average annual total number concentrations varied between 1700 cm−3 (Finnish rural site) and 23,000 particles cm−³ (London roadside). Highest variability on the diurnal course was found for the roadside sites and on the seasonal course for the rural sites. A first identification of the influencing parameters was found using correlation analysis. The most dominating factors were then quantified using a multiple linear regression approach documenting the highest influence for daytime (45%) followed by wind direction (18%), season (12%), wind speed (11%), temperature (8%) and solar radiation (6%), on average. The variation of influence between the different cities is analysed and discussed. Subsequently NSDs were modelled using the regression approach in order to verify how well the parameters are capable of ‘explaining’ the actual NSD, showing best performance for roadside NSD with correlation coefficients between 0.65 and 0.8 and limited performance for the rural sites (0.5–0.6). [Copyright &y& Elsevier]

The integrated model system DEHM/UBM/AirGIS, developed at Aarhus University, Department of Environmental Science, has been extended with the dynamic aerosol module M7 to account for particle number concentrations of particles with diameters below 1 μm in the atmosphere. The aim of this development is to quantify the spatial and temporal distribution of particle number concentration across Denmark and evaluate the results with available measurements. This article presents model results for particle number concentrations from the regional scale model DEHM and the urban scale model UBM, for comparison with measurements of particle number concentrations from European and Danish measurement stations. The deterministic modelling of particle number concentration has been vitiated by the lack of consistency between emission inventories, and the evaluation of the models is challenged by the lack of consistent long-term measurements data. The performance evaluation of the DEHM and UBM models shows that both models overestimate the level of the particle number concentrations at all stations, however, the results for the correlation coefficients are 0.86 for DEHM and in the range from 0.86 to 0.87, for UBM, for annual mean particle number concentrations at Danish measurement stations. We conclude that the inclusion of particle number concentration in DEHM and UBM shows some capability of reproducing observed patterns, when comparing the results of the models with available measurements, but that there is also room for improvement, especially with respect to the emission inventories and preprocessing of emissions and to the treatment of volatile organic compounds based on natural emissions during summer time. [ABSTRACT FROM AUTHOR]

Modelling of ambient particle number concentrations (PNC) has been implemented in the Danish air quality modelling system DEHM/UBM/AirGIS and evaluated with long-term measurements. We implemented particle dynamical processes in the regional scale model DEHM using the M7 aerosol dynamics module (presented in the accompanying article by Frohn et al., 2021), and we developed models for PNC at the local scale (UBM) and street scale (OSPM), in a first approximation without including the particle dynamics as presented in this article. Outdoor concentration estimates are provided at the front door of all residential address locations in Denmark for the past 40 years (1979–2018) with a spatial resolution of 1 km × 1 km taking all emission sectors in Denmark into account and additionally at the street location, with significant traffic (>500 vehicles/day). We evaluated our model with up to 18-year long measurement time series of particle number size distributions (PNSD) at Danish street, urban and rural background stations. Two particle size ranges were used for evaluation: PNC >10 (count of particles with diameter larger than 10 nm) and PNC 30_250 (diameter range 30–250 nm), in order to exclude nucleation events from the measurements and to obtain a consistent long-term measured time series. When comparing our model estimates with PNC 30_250 measurements, we obtain Pearson correlation coefficients (R p) in the range 0.39–0.95 depending on station location (street, urban background, rural) and averaging time (hour, day, month, year). The highest correlations were found for yearly averages at a monitoring station located at a street with dense traffic (R p = 0.95) whereas shorter time averages and comparisons with monitoring stations at urban and rural background locations provided lower correlations. The model performance for PNC in terms of correlation coefficients with respect to measurements is comparable to the performance for other pollutants such as NO X , PM 2.5 and better than the performance for PM 10. The model generally overestimated the observed concentrations, Normalized Mean Bias (NMB) was in the range 6%–190% compared to PNC >10 and 90%–290% compared to PNC 30_250. These relatively high NMBs are probably caused by uncertainties in the modelling process, especially the estimation of particle number emissions, which largely determine the ambient concentrations of PNC. Furthermore, uncertainties might as well originate from the complexity of modelling particle dynamical processes accurately and the great challenges in performing long-term PNC measurements. The presented model can estimate PNC at all Danish addresses over the last 40 years with a 1-h time resolution. The data seem to provide a good indication of the relative differences in PNC at Danish addresses. • Particle number concentrations (PNC) implemented in the Danish air quality modelling system DEHM/UBM/AirGIS. • Outdoor concentrations are provided at the front door of all residential addresses in Denmark for 40 years (1979–2018) • Spatial resolution of 1 km × 1 km taking all emission sectors in Denmark into account. • Additionally at the street locations consider the local traffic contribution, all done in 1 hour time resolution. • Comparing model with PNC 30_250 measurementsgives Pearson correlation coefficients in the range 0.39–0.95. • Model overestimated the observed concentrations, Normalized Mean Bias in the range 6%–190% compared to PNC >10. [ABSTRACT FROM AUTHOR]

Airborne microorganisms impact cloud formation and are involved in disease spreading. The ability of airborne cells to survive and express genes may be limited by reduced water availability in the atmosphere and depend on the ability of the cells to attract water vapor at subsaturated conditions, i.e., their hygroscopicity. We assessed hygroscopic properties of the plant pathogen Pseudomonas syringae , known to participate in cloud formation. We used a hygroscopicity tandem differential mobility analyzer to examine both hydration and dehydration behavior in the relative humidity (RH) range 5-90%. The cells were aerosolized either from Milli-Q water or from a 35 g L -1 NaCl solution, resulting in pure cells or cells associated with NaCl. Pure cells exhibited no deliquescence/efflorescence and a small gradual water uptake reaching a maximum growth factor (GF) of 1.09 ± 0.01 at 90% RH. For cells associated with NaCl, we observed deliquescence and a much larger maximum GF of 1.74 ± 0.03 at 90% RH. Deliquescence RH was comparable to that of pure NaCl, highlighting the major role of the salt associated with the cells. It remains to be investigated how the observed hygroscopic properties relate to survival, metabolic, and ice-nucleation activities of airborne P. syringae .

Ultrafine particles (UFP), commonly expressed as particle number concentrations (PNC), have been associated with harm to human health yet are currently not regulated or routinely monitored in many places. This has limited the potential for studies of health effects of long-term exposure to UFP. The present study aims to understand the spatial and temporal variation in façade-level UFP exposures in Copenhagen, Denmark. We measured PNC at the façades of 27 residences across the city for approximately 72 h each in two campaigns and continuously at an urban background reference site for twelve consecutive months, using portable monitors (miniature diffusion size classifiers [DiSCminis]). We estimated annual means at the residential sites based on temporal adjustment using reference site data. Furthermore, we co-located the DiSCminis at a regulatory monitoring station on three occasions and compared daily means from our reference site to those from seven fixed-site monitoring stations throughout the city. Annual mean PNC at the reference site was 4715 (SD of hourly mean: 3001) pt/cm 3 , while annual means at 27 residences were slightly higher with a mean of 5201 pt/cm 3 (SD: 807), ranging between 3735 and 6588 pt/cm 3 . The two individual adjusted campaign-specific means at 27 residential sites were weakly correlated (Spearman's correlation 0.11) and had an intra-class correlation coefficient of 0.06 (95%-confidence interval: -0.18, 0.28). Daily PNC at the reference site was highly correlated (R = 0.64-0.84) with PNC monitored at seven fixed-site stations throughout the city. We observed a seasonal trend at the reference site with the highest PNC in spring. Our measurement campaign revealed that façade-level PNC at residences in Copenhagen in 2021-2022 was relatively low with small spatial variability. The large variability in time suggests possibly longer and more frequent measurement campaigns to obtain more stable annual averages. Our study illustrates the challenges of UFP long-term exposure assessment., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Authors. Published by Elsevier Ltd.. All rights reserved.)

Aims: The three correlated environmental exposures (air pollution, road traffic noise, and green space) have all been associated with the risk of myocardial infarction (MI). The present study aimed to analyse their independent and cumulative association with MI., Methods and Results: In a cohort of all Danes aged 50 or older in the period 2005-17, 5-year time-weighted average exposure to fine particles (PM2.5), ultrafine particles, elemental carbon, nitrogen dioxide (NO2), and road traffic noise at the most and least exposed façades of residence was estimated. Green space around residences was estimated from land use maps. Cox proportional hazard models were used to estimate hazard ratios (HRs) and 95% confidence interval (CI), and cumulative risk indices (CRIs) were calculated. All expressed per interquartile range. Models were adjusted for both individual and neighbourhood-level socio-demographic covariates. The cohort included 1 964 702 persons. During follow-up, 71 285 developed MI. In single-exposure models, all exposures were associated with an increased risk of MI. In multi-pollutant analyses, an independent association with risk of MI was observed for PM2.5 (HR: 1.026; 95% CI: 1.002-1.050), noise at most exposed façade (HR: 1.024; 95% CI: 1.012-1.035), and lack of green space within 150 m of residence (HR: 1.018; 95% CI: 1.010-1.027). All three factors contributed significantly to the CRI (1.089; 95% CI: 1.076-1.101)., Conclusion: In a nationwide cohort study, air pollution, noise, and lack of green space were all independently associated with an increased risk of MI. The air pollutant PM2.5 was closest associated with MI risk., Competing Interests: Conflict of interest: none declared., (© The Author(s) 2023. Published by Oxford University Press on behalf of the European Society of Cardiology. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

The role of airborne nanoparticles in atmospheric chemistry and public health is largely controlled by particle size, morphology, surface composition, and coating. Aerosol mass spectrometry provides real-time chemical characterization of submicron atmospheric particles, but analysis of nanoplastics in complex aerosol mixtures such as sea spray is severely limited by challenges associated with separation and ionization of the aerosol matrix. Here we characterize the internal and external mixing state of synthetic sea spray aerosols spiked with 150 nm nanoplastics. Aerosols generated from pneumatic atomization and from a sea spray tank are compared. A humidified tandem differential mobility analyzer is used as a size and hygroscopicity filter, resulting in separation of nanoplastics from sea spray, and an inline high-resolution time-of-flight aerosol mass spectrometer is used to characterize particle composition and ionization efficiency. The separation technique amplified the detection limit of the airborne nanoplastics. A salt coating was found on the nanoplastics with coating thickness increasing exponentially with increasing bulk solution salinity, which was varied from 0 to 40 g kg -1 . Relative ionization efficiencies of polystyrene and sea salt chloride were 0.19 and 0.36, respectively. The growth-factor derived hygroscopicity of sea salt was 1.4 at 75% relative humidity. These results underscore the importance of separating airborne nanoplastics from sea salt aerosol for detailed online characterization by aerosol mass spectrometry and characterization of salt coatings as a function of water composition. The surface coating of nanoplastic aerosols by salts can profoundly impact their surface chemistry, water uptake, and humidified particle size distributions in the atmosphere.

Ultrafine particles (UFP; particulate matter <0.1 μm in diameter) may be more harmful to human health than larger particles, but epidemiological evidence on their health effects is still limited. In this study, we examined the association between short-term exposure to UFP and mortality and hospital admissions in Copenhagen, Denmark. Daily concentrations of UFP (measured as particle number concentration in a size range 11-700 nm) and meteorological variables were monitored at an urban background station in central Copenhagen during 2002-2018. Daily counts of deaths from all non-accidental causes, as well as deaths and hospital admissions from cardiovascular and respiratory diseases were obtained from Danish registers. Mortality and hospital admissions associated with an interquartile range (IQR) increase in UFP exposure on a concurrent day and up to six preceding days prior to the death or admission were examined in a case-crossover study design. Odds ratios (OR) with 95% confidence intervals (CI) per one IQR increase in UFP were estimated after adjusting for temperature and relative humidity. We observed 140,079 deaths in total, 236,003 respiratory and 342,074 cardiovascular hospital admissions between 2002 and 2018. Hospital admissions due to respiratory and cardiovascular diseases were significantly positively associated with one IQR increase in UFP (OR: 1.04 [95% CI: 1.01, 1.07], lag 0-4, and 1.02 [1.00, 1.04], lag 0-1, respectively). Among the specific causes, the strongest associations were found for chronic obstructive pulmonary disease (COPD) mortality and asthma hospital admissions and two-day means (lag 0-1) of UFP (OR: 1.13 [1.01, 1.26] and 1.08 [1.00, 1.16], respectively, per one IQR increase in UFP). Based on 17 years of UFP monitoring data, we present novel findings showing that short-term exposure to UFP can trigger respiratory and cardiovascular diseases mortality and morbidity in Copenhagen, Denmark. The strongest associations with UFP were observed with COPD mortality and asthma hospital admissions., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 The Authors. Published by Elsevier Ltd.. All rights reserved.)

Chlorine radicals are strong atmospheric oxidants known to play an important role in the depletion of surface ozone and the degradation of methane in the Arctic troposphere. Initial oxidation processes of chlorine produce chlorine oxides, and it has been speculated that the final oxidation steps lead to the formation of chloric (HClO 3 ) and perchloric (HClO 4 ) acids, although these two species have not been detected in the atmosphere. Here, we present atmospheric observations of gas-phase HClO 3 and HClO 4 . Significant levels of HClO 3 were observed during springtime at Greenland (Villum Research Station), Ny-Ålesund research station and over the central Arctic Ocean, on-board research vessel Polarstern during the Multidisciplinary drifting Observatory for the Study of the Arctic Climate (MOSAiC) campaign, with estimated concentrations up to 7 × 10 6 molecule cm -3 . The increase in HClO 3 , concomitantly with that in HClO 4 , was linked to the increase in bromine levels. These observations indicated that bromine chemistry enhances the formation of OClO, which is subsequently oxidized into HClO 3 and HClO 4 by hydroxyl radicals. HClO 3 and HClO 4 are not photoactive and therefore their loss through heterogeneous uptake on aerosol and snow surfaces can function as a previously missing atmospheric sink for reactive chlorine, thereby reducing the chlorine-driven oxidation capacity in the Arctic boundary layer. Our study reveals additional chlorine species in the atmosphere, providing further insights into atmospheric chlorine cycling in the polar environment., (© 2023. The Author(s).)

The Arctic is a hot spot for climate change with potentially large consequences on a global scale. Aerosols, including bioaerosols, are important players in regulating the heat balance through direct interaction with sunlight and indirectly, through inducing cloud formation. Airborne bacteria are the major bioaerosols with some species producing the most potent ice nucleating compounds known, which are implicated in the formation of ice in clouds. Little is known about the numbers and dynamics of airborne bacteria in the Arctic and even less about their seasonal variability. We collected aerosol samples and wet deposition samples in spring 2015 and summer 2016, at the Villum Research Station in Northeast Greenland. We used amplicon sequencing and qPCR targeting the 16S rRNA genes to assess the quantities and composition of the DNA and cDNA-level bacterial community. We found a clear seasonal variation in the atmospheric bacterial community, which is likely due to variable sources and meteorology. In early spring, the atmospheric bacterial community was dominated by taxa originating from temperate and Subarctic regions and arriving at the sampling site through long-range transport. We observed an efficient washout of the aerosolized bacterial cells during a snowstorm, which was followed by very low concentrations of bacteria in the atmosphere during the consecutive 4 weeks. We suggest that this is because in late spring, the long-range transport ceased, and the local sources which comprised only of ice and snow surfaces were weak resulting in low bacterial concentrations. This was supported by observed changes in the chemical composition of aerosols. In summer, the air bacterial community was confined to local sources such as soil, plant material and melting sea-ice. Aerosolized and deposited Cyanobacteria in spring had a high activity potential, implying their activity in the atmosphere or in surface snow. Overall, we show how the composition of bacterial aerosols in the high Arctic varies on a seasonal scale, identify their potential sources, demonstrate how their community sizes varies in time, investigate their diversity and determine their activity potential during and post Arctic haze., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2022 Jensen, Glasius, Gryning, Massling, Finster and Šantl-Temkiv.)

Aerosols play an important yet uncertain role in modulating the radiation balance of the sensitive Arctic atmosphere. Organic aerosol is one of the most abundant, yet least understood, fractions of the Arctic aerosol mass. Here we use data from eight observatories that represent the entire Arctic to reveal the annual cycles in anthropogenic and biogenic sources of organic aerosol. We show that during winter, the organic aerosol in the Arctic is dominated by anthropogenic emissions, mainly from Eurasia, which consist of both direct combustion emissions and long-range transported, aged pollution. In summer, the decreasing anthropogenic pollution is replaced by natural emissions. These include marine secondary, biogenic secondary and primary biological emissions, which have the potential to be important to Arctic climate by modifying the cloud condensation nuclei properties and acting as ice-nucleating particles. Their source strength or atmospheric processing is sensitive to nutrient availability, solar radiation, temperature and snow cover. Our results provide a comprehensive understanding of the current pan-Arctic organic aerosol, which can be used to support modelling efforts that aim to quantify the climate impacts of emissions in this sensitive region., Competing Interests: Competing interestsThe authors declare no competing interests., (© The Author(s) 2022.)

Magnesium and calcium chloride salts contribute to the global atmospheric aerosol burden via emission of sea spray and mineral dust. Their influence on aerosol hygroscopicity and cloud forming potential is important but uncertain with ambiguities between results reported in the literature. To address this, we have conducted measurements of the hygroscopic growth and critical supersaturation of dried, size selected nano-particles made from aqueous solution droplets of MgCl 2 and CaCl 2 , respectively, and compare experimentally derived values with results from state-of-the-art thermodynamic modelling. It is characteristic of both MgCl 2 and CaCl 2 salts that they bind water in the form of hydrates under a range of ambient conditions. We discuss how hydrate formation affects the particles' water uptake and provide an expression for hydrate correction factors needed in calculations of hygroscopic growth factors, critical super-saturations, and derived κ values of particles containing hydrate forming salts. We demonstrate the importance of accounting for hydrate forming salts when predicting hygroscopic properties of sea spray aerosol.

The radiative balance in the Arctic region is sensitive to in-cloud processes, which principally depend on atmospheric aerosols, including ice nucleating particles (INPs). High temperature INPs (active at ≥-15 °C) are common in the Arctic. While laboratory and limited in situ studies show that the high-temperature active INPs are associated with bioaerosols and biogenic compounds, there is still little quantitative insight into the Arctic biogenic INPs and bioaerosols. We measured concentrations of bioaerosols, bacteria, and biogenic INPs at the Villum Research Station (VRS, Station Nord) in a large number of snow (15) and air (51) samples. We found that INPs active at high subzero temperatures were present both in spring and summer. Air INP concentrations were higher in summer (18 INP m -3 at ≥-10 °C) than in spring (<4 INP m -3 at ≥-10 °C), when abundant INPs were found in snowfall (1.4 INP mL -1 at ≥-10 °C). Also, in summer, a significantly higher number of microbial and bacterial cells were present compared to the spring. A large proportion (60%-100%) of INPs that were active between -6 °C and -20 °C could be deactivated by heating to 100 °C, which was indicative of their predominantly proteinaceous origin. In addition, there was a significant linear regression between the summer air concentrations of INPs active at ≥-10 °C and air concentrations of bacterial-marker-genes ( p < 0.0001, R 2 = 0.999, n = 6), pointing at bacterial cells as the source of high-temperature active INPs. In conclusion, the majority of INPs was of proteinaceous, and possibly of bacterial, origin and was found in air during summer and in snowfall during springtime.

Background: Exposure to wood smoke is associated with respiratory symptoms, whereas knowledge on systemic effects is limited. We investigated effects on systemic inflammation, oxidative stress and microvascular function (MVF) after controlled wood smoke exposure., Methods: In a randomised, double-blinded, cross-over study 20 non-smoking atopic subjects were exposed at rest to 14, 220, or 354 μg/m3 of particles from a well-burning modern wood stove for 3 h in a climate controlled chamber with 2 week intervals. We investigated the level of oxidatively damaged DNA, inflammatory markers and adhesion molecules before and 0, 6 and 20 h after exposure. Six h after exposure we measured MVF non-invasively by digital peripheral artery tonometry following arm ischemia., Results: The MVF score was unaltered after inhalation of clean air (1.58 ± 0.07; mean ± SEM), low (1.51 ± 0.07) or high (1.61 ± 0.09) concentrations of wood smoke particles in atopic subjects, whereas unexposed non-atopic subjects had higher score (1.91 ± 0.09). The level of oxidatively damaged DNA, mRNA of ITGAL, CCL2, TNF, IL6, IL8, HMOX1, and OGG1 and surface marker molecules ICAM1, ITGAL and L-selectin in peripheral blood mononuclear cells were not affected by inhalation of wood smoke particles., Conclusions: Exposure to wood smoke had no effect on markers of oxidative stress, DNA damage, cell adhesion, cytokines or MVF in atopic subjects.