Abstract. This paper summarizes the main results from the scientific project “Boundary layer Evolution Through Harmonization of Aerosol measurements at Ny-Ålesund research stations” (BETHA-NyÅ), in which aerosol measurements of two Arctic atmospheric observatories located near Ny-Ålesund (Svalbard) at different elevations were harmonized: at the Gruvebadet atmospheric laboratory (61 m a.s.l.) and Zeppelin observatory (472 m a.s.l.). This approach allows for a better understanding of how atmospheric layering may affect the variability of aerosol observations in the Ny-Ålesund area. From February 2022 to March 2023, a coordinated sampling campaign enabled a direct comparison of optical, chemical, and physical aerosol properties, integrated with meteorological data from the Amundsen-Nobile Climate Change Tower. Results reveal a strong seasonal coherence between the two sites for two topical markers such as sulfate and ammonium, with clear evidence of the winter–spring Arctic Haze phenomenon. Local differences emerged mainly for biogenic tracers (e.g., arabitol and mannitol) which were detected at higher concentrations at Gruvebadet compared to Zeppelin observatory, highlighting the role of near-surface sources and aerosol stratification. The analysis of trace elements, lead isotopic ratios, and organic markers helped us to distinguish natural from anthropogenic contributions, confirming the dominant role of long-range transport and the persistence of isotopic signatures consistent with Eurasian sources. The systematic comparison across the two observatories demonstrates the robustness of the harmonized protocol and emphasizes the importance of an integrated monitoring network for evaluating the evolution of atmospheric processes in the Arctic.

Organic aerosol (OA) is a major component of atmospheric particulate matter (PM), affecting both human health and climate. However, high-resolution estimates of OA exposure needed for exposure analysis remain scarce. Here, we integrate a chemical transport model (CAMx) with a random forest (RF) machine learning approach to bias-correct and downscale daily OA concentrations across Europe. CAMx OA simulations at ∼15 km resolution show moderate agreement with observations (r = 0.55). By combining these outputs with high-resolution land-use data and training the RF model on ∼48,000 daily OA measurements from 137 sites, prediction accuracy improved (r = 0.65), with ∼l5% reduction in root mean square error. The resulting maps provide European daily OA concentrations at ∼250 m resolution for alternate years from 2011 to 2019. The model captures key spatial features, including elevated OA in the Po Valley, Southeastern, and Central Europe, as well as intracity variations due to local hotspots. Seasonal analysis reveals higher concentrations in winter, while long-term trends indicate a general decline in OA levels. Exposure estimates show that half of the European population experiences OA levels above 3 µg/m3, and ∼50 million people are exposed to more than 5 µg/m3, which is the current guideline level recommended by the world health organization for total PM2.5. These high-resolution OA maps offer vital critical support for epidemiological research and air quality policy.

Volatile organic compounds (VOCs), precursors to secondary air pollutants such as ozone and secondary organic aerosols (SOA) were measured for the first time in Milan and in Bologna, two urban areas located in the highly polluted Po Valley in northern Italy. Sixteen VOCs were quantified seasonally in 2023 in Milan, and in September 2024 in Bologna, using a Vocus chemical ionization time-of-flight mass spectrometer. The two sites had comparable summed mixing ratios, hydroxyl radical (OH) reactivities, ozone and SOA formation potentials (OFP, SOAP) derived from the targeted VOCs. Acetone and acetaldehyde were the most abundant chemical species in the Po Valley urban air. Oxygenated VOCs in Milan and biogenic VOCs in Bologna were the largest contributors to the OH reactivity; while OFP was dominated by oxygenated and anthropogenic VOCs in Milan and by oxygenated and biogenic VOCs in Bologna. Anthropogenic VOCs dominated SOAP, with substantial contributions from biogenic VOCs, particularly sesquiterpenes. This study highlights the similarities across the Po Valley urban atmosphere and the atmospheric impact of biogenic VOCs within urban environments.

Air pollution and fog are closely connected, influencing both visibility and human health. As relative humidity rises, aerosol particles absorb water and grow hygroscopically, potentially activating into fog droplets when supersaturation is reached. Distinguishing between hydrated (non-activated) aerosols and activated particles is critical, as their differing thermodynamic states influence fog chemistry and dissipation. This study quantifies the impact of hydrated aerosol particles on fog microphysical properties and visibility in the Po Valley, one of Europe’s most polluted regions. We analyzed detailed aerosol–fog observations from the FAIRARI 2021/2022 campaign at San Pietro Capofiume, Italy, using κ–Köhler theory and the Large Eddy Simulation (LES) model MIMICA. The median hygroscopicity parameter (κ) of fog residuals (0.45) exceeded that of interstitial particles (0.40) and out-of-fog aerosols (0.34), reflecting enhanced inorganic content in fog droplets. Hygroscopic growth calculations show that hydrated particles can reach several micrometers in diameter, significantly influencing inferred fog microphysical properties. Excluding hydrated aerosols led to an 81 % increase in effective diameter (from 11.6 to 21.0 µm) and an 87 % decrease in cloud droplet number concentration (from 97.4 to 12.4 cm−3). Hydrated particles contributed, on average, 21 % to liquid water content and accounted for 36 % of sub-kilometer visibility events without droplet activation. LES results emphasize that fog prediction depends strongly on the largest dry aerosol particles. Our findings demonstrate the need to distinguish between hydrated and activated particles when interpreting fog observations and modeling fog development in polluted environments.

Organic aerosol particles (OA) can absorb solar radiation with varying efficiencies depending on their chemical composition and physical properties. This light-absorbing fraction of OA, commonly referred to as brown carbon (BrC), is difficult to accurately represent in climate models due to the inherent diversity of its optical properties. This variability arises from differences in emission sources and atmospheric processing, as well as from variations in experimental design and the analytical methods used to quantify BrC absorption. As a result, the climate effect of BrC remains uncertain. Here, we studied the light absorption properties of surface ambient OA using measurements from 17 sites across Europe. Combining multi-wavelength absorption measurements from filter-based photometers with OA mass concentrations and source apportionment derived from ACSM/AMS data, we derive empirical estimates of the OA mass absorption cross section (MACOA), its wavelength dependence (AAEOA), the OA density (⍴OA), and the MAC associated with different primary and secondary OA sources. We further develop parameterizations that relate MACOA, AAEOA and ⍴OA to the ambient black carbon-to-organic aerosol ratio (eBC/OA) and propose a corresponding parameterization for the imaginary refractive index (kOA). Given the widespread availability of eBC and OA measurements in global monitoring networks, the framework presented here provides a practical approach for estimating the absorptive properties of surface OA particles under real-world conditions.

The Antarctic coastal zones are among the most biologically productive areas on Earth. The effect of marine microbiota on the emissions of sea spray particles, a critical factor for global climate and clouds, remains an open and actively researched question. Here, by means of in situ ship-borne bubble-bursting SSA production experiments at multiple locations around the Antarctic Peninsula, we show a 2-fold variability in the 10-500 nm size-resolved SSA number concentrations. We observed that the organic chemical composition of seawater (SW) and surface microlayers (SML) clearly impacts SSA number concentrations. SW and SML samples with saccharides, proteins, and N-osmolytes were less efficient at emitting SSA compared with waters rich in biotic material originating from lipids, such as fatty acids and polyols. We found that the dissolved organic carbon (DOC) fraction containing lipid degradation products and polyols indicates higher SSA production. Our results indicate that low concentration organic components, rather than the most abundant classes of biomolecules, influence the ability to be aerosolized, with strong chemical selectivity affecting SSA production.

While aerosol–cloud interactions have been extensively investigated, large knowledge gaps still exist. Atmospheric organic nitrogen (ON) species and their formation in the aqueous phase are potentially important due to (1) their influence on aerosol optical and hygroscopic properties and (2) their adverse effects on human health. This study aimed to characterize the wintertime aerosol and fog chemical composition, with a focus on the formation of ON, at a rural site in the Italian Po Valley. Online chemical characterization of interstitial aerosol (nonactivated particles) and fog residuals (dried fog droplets) were performed in parallel. Fog residuals were sampled using a ground-based counterflow virtual impactor (GCVI) inlet and analyzed by a soot particle aerosol mass spectrometer (SP-AMS), while the interstitial aerosol was characterized by a high-resolution time-of-flight AMS (HR-ToF-AMS). Our results revealed an enhancement of nitrate (NO3-; 43.3 % vs. 34.6 %), ammonium (NH4+; 15.2 % vs. 11.7 %), and sulfate (SO42-; 10.5 % vs. 6.6 %) in the fog residuals compared to the ambient non-fog aerosol, while organic aerosol (OA; 27.6 % vs. 39.4 %) and refractory black carbon (rBC; 2.3 % vs. 6.3 %) were less abundant. An enrichment of ON was observed in the fog, mainly consisting of CxHyN1+ ions, partly originating from amines in the fog. CxHyN2+ ions, fragments linked to imidazoles, were overproportionally present in the fog, which was verified by proton nuclear magnetic resonance (1H-NMR) spectroscopy, suggesting aqueous-phase formation. This study demonstrates that fogs and clouds are potentially important sinks for gaseous nitrogen species and media for the aqueous production of nitrogen-containing organic aerosol in the atmosphere.

Remote from most human influences, the Southern Ocean (SO) is one of the most pristine regions on Earth and a window to preindustrial atmospheric conditions (Hamilton, 2015). Currently, many unknowns remain about atmospheric and oceanographic processes in this region and their relations. This is largely due to the poor understanding of aerosol sources and processes in this region.Sub-micrometer aerosol samples were collected onboard the Italian RV Laura Bassi cruising the Southern Ocean and the Ross Sea, in the framework of the PNRA (Programma Nazionale di Ricerca in Antartide) project CAIAC (oCean Atmosphere Interactions in the Antarctic regions and Convergence latitude). The aim is to characterize the marine aerosol chemical composition in different ecoregions, with a particular interest for organic aerosols and their formation processes in relation with the patterns of oceanic biological activity.Samples were collected by a high volume sampler (TECORA, ECHO-HIVOL, 500 LMP) from mid-January to mid-February 2023, deploying a wind direction selection system to avoid ship contaminations. A total of 9 samples were collected. The samples have been analysed for their water-soluble Carbon and Nitrogen content by a C-N elemental analyzer (Shimadzu) and for the ionic composition (including low molecular weight acids and amines) by ion chromatography (Dionex). The characterization of the water-soluble organic fraction in terms of tracers and functional group abundance was performed by 1H NMR (Proton Nuclear Magnetic Resonance) spectroscopy (Decesari et al, 2020).The samples show variable contributions in terms of primary and secondary components, mostly depending on back trajectory origin and wind speed, with a general predominance of secondary species. Sulfate resulted generally the most abundant aerosol component, while water soluble organic matter (WSOM) showed a non-negligible contribution from 5 to 14% of the analysed mass. NMR spectra show the complexity of the WSOM composition, even though all the spectra were dominated by the MSA signal, which contribution in terms of carbon to WSOM spans from 8 to 64%.Analysis of organic aerosol sources is in progress by back-trajectory analysis and statistical analysis of the NMR spectra. Acknowledgements: CAIAC (oCean Atmosphere Interactions in the Antarctic regions and Convergence latitude) PNRA project. Decesari, S. et al. (2020), Atmos. Chem. Phys., 20, 4193–4207, https://doi.org/10.5194/acp-20-4193-2020Hamilton, D. S. Weather 2015, 70 (9), 264– 8, DOI: 10.1002/wea.254

Absorbing aerosol species, such as Black (BC) and Brown (BrC) Carbon, are able to warm the atmosphere. The role of aerosols is one of the least clear aspects in the so called “Arctic Amplification” (AA) and up to now this was mostly modelled [1,2]. For this reason, we took part in four scientific cruises (AREX, Arctic-Expedition, summer 2018, 2019, 2021 and EUREC4A, 2020) in the North Atlantic, eastward and south-eastward of Barbados, aiming at the determination of the aerosol chemical composition and properties from the Tropics to the North Pole.The Heating Rate (HR) was experimentally determined at 1 minute time-resolution along different latitudes by means of an innovative methodology [3], obtained by cumulatively taking into account the aerosol optical properties, i.e. the absorption coefficients (measured by AE33 Aethalometer) and incident radiation (direct, diffuse and reflected) across the entire solar spectrum. The HR computed along AREX and in Milan (in the same period) were used to determine the energy gradient, due to the LAA induced heat storage at mid-latitudes, which contributes to AA through the atmospheric heat transport northward.Moreover, aerosol chemical composition was achieved by means of sampling via high volume sampler (ECHO-PUF Tecora) and analysis via ion chromatography, TCA08 for Total Carbon content, Aethalometer AE33 (for BC), ICP-OES for elements.A clear latitudinal behaviour in Black Carbon concentrations, with the highest values at low latitudes (e.g. average BC concentration in Gdansk up to 1507±75 ng/m3) and a progressive decrease moving northwards and away from the big Arctic settlements (Black Carbon concentrations within the 81st parallel: 5±1 ng/m3).According to the latitudinal behaviour of BC concentrations and solar radiation (decreases towards the north while the diffuse component increases), HR decreases noticeably towards the Arctic: e.g. higher in the harbor of Gdansk (0.290±0.010 K/day) followed by the Baltic Sea (0.04±0.01 K/day), the Norvegian Sea (0.010±0.010 K/day) and finally with the lowest values in the pure Arctic Ocean (0.003±0.001 K/day). Accordingly, the energy density added to the system by the aerosol, a positive forcing that differs by 2 orders of magnitude between mid-latitudes and North Pole was found: 347.3 ± 11.8 J/m3 (Milan), 244.8 ± 12.2 J/m3 (Gdansk) and 2.6 ± 0.2 J/m3 (80°N). These results highlight the presence of a great energy gradient between mid-latitudes and Arctic that can trigger a heat transport towards the Arctic. Moreover this was strengthen by the HR value for EUREC4A in Barbados that was 0.175±0.003 K/day. Finally, preliminary results from Antarctica collected onboard the Italian RV Laura Bassi cruising the Southern Ocean and the Ross Sea will be shown. Acknoledgements: GEMMA Center, Project TECLA MIUR – Dipartimenti di Eccellenza 2023–2027. JPI EUREC4A-OA project. CAIAC (oCean Atmosphere Interactions in the Antarctic regions and Convergence latitude) PNRA project References[1] Navarro, J. C. A. et al. (2016) Nat. Geosci. 9, 277–281.[2] Shindell, D. and Faluvegi, G. (2009) Nat. Geosci. 2, 294–300.[3] Ferrero, L. et al. (2018) Environ. Sci. Technol. 52, 3546 3555.

Addressing the planetary crisis associated with climate change, biodiversity loss, global pollution, and public health requires novel and holistic approaches. Here, we present the methodology and initial results of an experiment conducted in Rome within the framework of the National Biodiversity Future Center (NBFC) project, Spoke 6. The major objective of this study was to outline the planetary health approach as a lens to assess urban health. This transdisciplinary case study explored the relationship between urban traffic-related external exposome and pro-oxidative responses in humans and plants. This methodology is based on the integration of atmospheric dynamics modeling, state-of-the-art aerosol measurements, biomonitoring in human cohorts, in vitro cellular assays, and the assessment of functional trait markers in urban trees. The results indicate that short-term exposure to urban aerosols, even at low concentrations, triggers rapid oxidative and inflammatory responses in bronchial epithelial cells, modulates gene and miRNA expression, alters gut microbiota diversity, and induces functional trait changes in urban trees. This study also highlights the feedback mechanisms between vegetation and atmospheric conditions, emphasizing the role of urban greenery in modulating microclimate and exposure. The methodology and initial results presented here will be further analyzed in future studies to explore proof of a cause–effect relationship between short-term exposure to traffic-related environmental stressors in urban areas and oxidative stress in humans and plants, with implications for chronic responses. In a highly urbanized world, this evidence could be pivotal in motivating the widespread implementation of planetary health approaches for assessing urban health.

Adverse health effects associated with fine particulate matter (PM2.5) in urban areas can occur even at PM2.5 concentrations below current regulatory limits - a situation increasingly observed in high-income countries. However, the underlying biological mechanisms remain poorly understood. In this study, we investigated the molecular and cellular responses in human bronchial epithelial cells exposed to low PM2.5 using a novel methodology. We first identified specific meteorological conditions that favor low PM2.5 mass concentrations (<10 μg m-3) combined with high traffic-related aerosol emissions, which we found to correspond to a highly oxidant atmosphere. Under these conditions, PM2.5 samples were collected in the urban background of Rome. The cells were exposed in vitro using a novel methodological approach based on a direct filter-contact model. Our focus was on associating measurable aerosol properties with gene expression pathways related to oxidative stress, inflammation, and their epigenetic modulation through microRNAs. Our findings indicate that exposure to fresh traffic-related aerosols under low PM2.5 concentrations elicited a biphasic gene expression response. The initial response involved the activation of genes such as NRF2, NF-κB, CAT1, SOD1, HIF-1α, and HMOX1; while a secondary response involved TNF-α and GPX4. A strong association was observed between these biological effects and black carbon metrics related to fossil fuel, implicating fresh traffic emissions as key contributors. Additionally, a significant modulation of air pollution-associated microRNAs was observed, even at early times of exposure, suggesting an epigenetic dimension to the cellular stress response. These findings have important implications for future air quality regulations. We provide mechanistic insights into oxidative and epigenetic responses underlying PM2.5 induced biological effects at low PM2.5 levels, emphasizing that neither PM2.5 mass concentration nor its oxidative potential - two metrics currently considered by legislation - are sufficient on their own to explain the observed effects.

Exposures to fine particulate matter (PM1) have been associated with health impacts, but our understanding of PM1 concentration-response (PM1-CR) relationships remains incomplete. This study investigates the association between particulate-bound reactive oxygen species (PB-ROS) and cellular oxidative stress responses induced by urban nanoparticles. PB-ROS were measured using the DCFH assay, both on PTFE filters (24-hour resolution, PB-ROSfilter) and PB-ROS measured using a Particle-Into-Liquid Sampler (2-hour resolution, PB-ROSPILS). We compared these with oxidative stress markers in BEAS-2B human cell lines exposed directly to ambient air at the air-liquid interface. A comparative analysis of PB-ROSfilter and PB-ROSPILS revealed significant differences in the types of PB-ROS detected, mainly due to the temporal resolution of sampling and the measurement techniques. In most cases, PB-ROSfilter levels were reduced by more than 90% compared to PB-ROSPILS. PB-ROSfilter predominantly identified long-lived species, which are more stable and indicative of aged aerosols. In contrast, PB-ROSPILS provided insights into transient PB-ROS, which correlated with urban nanoparticles. A low condensation sink played a decisive role, suggesting atmospheric conditions in which condensable compounds (including ROS) did not rapidly deposit onto pre-existing accumulation-mode particles but instead bound to nanoparticles. Finally, we demonstrate that gene expression patterns for oxidative stress in BEAS-2B human cell lines correlate with PB-ROSPILS, but not with PB-ROSfilter.

It is important to study aerosols and their origins, as they pose various negative health and environmental impacts. In this study, we combined year-long datasets from 15 different countries with Trajectory Statistical Methods (TSMs) for the first time at this comprehensive scale. We found possible source regions and seasonal variations of various particulate matter (PM) components in Europe, including total organic aerosol (OA), biomass burning OA (BBOA), oxygenated OA (OOA), ammonium (NH4), nitrate (NO3), and sulphate (SO4). We found that for all of the studied components, Eastern Europe was among the highest contributors. For NO3, other important source regions were Northern France and the Benelux, while for SO4 there were significant contributions from the Mediterranean region. We also compared our measurement-based model with simulated concentrations of an atmospheric chemistry transport model (CAMx). We observed a satisfactory agreement in regions where we had sufficient coverage with air pollution monitoring stations. The main deviations for OA were found around the Po Valley, where CAMx consistently estimated higher concentrations, while the TSM analysis did not highlight it as a hotspot because long-term monitoring datasets in this region are lacking. CAMx also underestimated the concentrations around Poland, mainly from residential burning. Our results provide opportunities to refine European emission inventories and deliver valuable information on long-range transported air pollutants. This work suggests that policies mitigating air pollution in Eastern Europe and the Benelux could help improve overall air quality in entire Europe more efficiently.

Measurements of pre-industrial conditions are of paramount importance for understanding historical climate change. The Southern Ocean and Antarctic continent are some of the least polluted environments on planet Earth. Alkylamines can rapidly partition into aerosols, increasing their mass, as well as form new particles altogether. We demonstrate the importance of pelagic “open ocean” (OO) and sympagic “sea ice” (SI) regions in supplying distinct organic nitrogen aerosol components. In the aerosol phase, dimethylamine (DMA) and trimethylamine (TMA) are both secondary, though DMA likely originates mainly from pelagic regions, while TMA is associated mainly with sympagic regions. Parallel measurements in ice and surface waters reveal that melting sea ice contains a factor of four more TMA than coastal Antarctic Peninsula waters; and seventeen times more TMA than OO regions - suggesting additional coastal Antarctic sources. To better interpret future climate change, we recommend employing regional atmospheric chemistry models to understand these diverse aerosol sources.

The Italian Po Valley is one of the most polluted regions in Europe. During winter, meteorological conditions favor long and dense fogs, which strongly affect visibility and human health. In spring, the frequency of nighttime fogs reduces while daytime new particle formation events become more common. This transition is likely caused by a reduction in particulate matter (PM2. 5 ), leading to a decrease in the relevant condensation sink. The physics and chemistry of fog and aerosol have been studied at the San Pietro Capofiume site since the 1980s, but the detailed processes driving the observed trends are not fully understood. Hence, during winter and spring 2021/22, the Fog and Aerosol Interaction Research Italy (FAIRARI) campaign was carried out, using a wide spectrum of approaches, including in situ measurements, outdoor chamber experiments, and remote sensing. Atmospheric constituents and their properties were measured ranging from gas molecules and molecular clusters to fog droplets. One unique aspect of this study is the direct measurement of the aerosol composition inside and outside of fog, showing a slightly greater dominance of organic compounds in the interstitial compared to the droplet phase. Satellite observations of fog provided a spatial context and agreed well with in situ measurements of droplet size. They were complemented with in situ chamber experiments, providing insights into oxidative processes and revealing a large secondary organic aerosol-forming potential of ambient air upon chemical aging. The oxidative potential of aerosol and fog water inferred the impact of aerosol-fog interactions on particle toxicity. SIGNIFICANCE STATEMENT: Aerosol and fog influence our climate and can have adverse effects on human health and visibility. Through anthropogenic emissions, such as from industry or trans- port, humans can influence the physical and chemical properties of fog and aerosol. The FAIRARI campaign investigated the formation of aerosols and aerosol-fog interactions in the Italian Po Valley, one of the most polluted regions in Europe. State-of-the-art experimental methods revealed the details of aerosols and fogs from the molecular level to the droplet scale. We observed a clear transition from winter to spring, with fog-dominated periods followed by an intensified secondary aerosol formation period. Organic and nitrate compounds dominated the composition of aerosol and fog residuals. The results will contribute to various aerosol-fog interaction modeling activities.

Fine particulate matter (PM) poses a major threat to public health, with organic aerosol (OA) being a key component. Major OA sources, hydrocarbon-like OA (HOA), biomass burning OA (BBOA), and oxygenated OA (OOA), have distinct health and environmental impacts. However, OA source apportionment via positive matrix factorization (PMF) applied to aerosol mass spectrometry (AMS) or aerosol chemical speciation monitoring (ACSM) data is costly and limited to a few supersites, leaving over 80% of OA data uncategorized in global monitoring networks. To address this gap, we trained machine learning models to predict HOA, BBOA, and OOA using limited OA source apportionment data and widely available organic carbon (OC) measurements across Europe (2010-2019). Our best performing model expanded the OA source data set 4-fold, yielding 85 000 daily apportionment values across 180 sites. Results show that HOA and BBOA peak in winter, particularly in urban areas, while OOA, consistently the dominant fraction, is more regionally distributed with less seasonal variability. This study provides a significantly expanded OA source data set, enabling better identification of pollution hotspots and supporting high-resolution exposure assessments.

This study investigates how chemical composition, atmospheric aging, and environmental conditions affect the oxidative potential (OP) and cellular toxicity of soot particles using an atmospheric simulation chamber (ASC). In the ASC ChAMBRe were simulated real-world summer and winter scenarios, exposing pure soot particles (generated by using the mini-inverted soot generator) and various secondary aerosol precursors (i.e., toluene, 2,5-dimethylfuran and α-pinene) alternatively to light or dark conditions and different oxidants. OP was assessed using multiple assays (namely, 2',7'-dichlorofluorescein - DCFH, Dithiothreitol - DTT and Ascorbic Acid - AA), revealing that soot particles exposed to light, especially in presence of toluene, exhibited higher OP. The presence of toluene also increased cellular reactive oxygen species (ROS) production, leading to elevated cytotoxicity, DNA damage, and release of the proinflammatory cytokine interleukin-8 (IL-8) in BEAS-2B cells. Ammonium sulfate addition reduced OP and do not enhance toxicological responses, suggesting that non-toxic components in aged particulate matter (PM) may mitigate some harmful effects. Toxicological assessment showed increased cytotoxicity, genotoxicity, oxidative stress, and inflammatory responses in soot generated under high irradiance conditions typical of summer and traffic environments, compared to low irradiance winter scenarios. Strong correlations were observed between OP and toxicological endpoints, such as ROS formation, LDH release, micronuclei formation, and IL-8 secretion underscoring the role of chemical composition and environmental aging in determining PM toxicity. The study highlights OP assays as a reliable tool for predicting PM-induced oxidative stress and potential health effects, emphasizing the importance of considering soot chemical composition and aging processes in urban air pollution assessments.

The Po Valley in northern Italy is an ideal laboratory to study fog-pollution interactions. The peculiar orography of the region (enclosed between the Alps and the Apennines) promotes stable meteorological conditions and radiation fog formation in wintertime. At the same time, high population density and the several agricultural and industrial activities are responsible for high levels of pollutants, among the highest in Europe. The interaction between those factors has been studied since the 1980s, however, the detailed microphysical processes behind the aerosol-fog interactions are still to be elucidated. Therefore, in winter 2021/22, the Fog and Aerosol InteRAction Research Italy (FAIRARI) campaign took place at the research station San Pietro Capofiume, in a rural area close to Bologna. Microphysical as well as chemical aerosol and fog processes from the molecular to the droplet scale were captured.Stockholm University’s mobile atmospheric laboratory simultaneously measured the total dried aerosol and the dried fog droplets (fog residuals), which then both were analyzed with respect to their size and (re-)activation behavior. Moreover, the chemical composition of the dried aerosol particles was determined. Meteorological parameters such as horizontal wind, updraft, and visibility were measured as well as the size distribution of the fog droplets.We will present and discuss the influence of aerosol particles on fog microphysics during FAIRARI. For example, hydrated but not activated aerosol particles contributed to more than 50% of the visibility reduction, having implications for the definition of the beginning of fog. This in turn impacted the fog describing parameters such as the effective diameter or liquid water content (LWC), which are crucial when comparing in-situ measurements to data retrieved from satellite observations and modelling predictions. During FAIRARI, if fog is defined as LWC > 0.01 g m-3, the in-fog median LWC increases by 28% (from 0.18 g m-3 to 0.23 g m-3), compared to if fog is defined by visibility < 1km. The hygroscopicity parameter κ was calculated to be around 0.36 in the ambient aerosol out of fog and about 0.47 in the fog residuals. Moreover, sensitivity tests with the large-eddy simulation model MIMICA showed that with the same amount of aerosol particles in the air, changes in the size distributions lead to significant modifications of fog microphysical properties. This work will contribute to constrain the role of aerosol parameters on fog properties and facilitate model improvements.Financial support from the European Union’s Horizon 2020 research and innovation program (project FORCeS No 821205 and H2020-INFRAIA-2020-1 under grant agreement No 101008004) and the European Research Council (Consolidator grant INTEGRATE No 865799) is gratefully acknowledged.