Semi-volatile compounds such as organics, nitrate, chloride, and ammonium are ubiquitous in atmospheric aerosols. Their gaseous precursors (organics, HNO₃, HCl, NH₃) co-condense with water vapour when ambient relative humidity (RH) increases, thus enhancing hygroscopic growth under sub-saturated conditions and facilitating activation as cloud condensation nuclei (CCN) to cloud droplets. In this study, we investigate the co-condensation effect on CCN activation for inorganics, organics, and their combination in a boreal forest site in autumn with our cloud parcel model that includes non-ideality of organic-inorganic mixtures. The volatility distribution of organics is highly uncertain but critically important to estimate the co-condensation effect. We compare two distinct volatility basis sets (VBS) established from experimental and modelling data at 25 °C, which we amended with a volatility bin of saturation concentration C^* = 10⁴ µg m⁻³, which proved to be highly relevant for CCN activation. The combined co-condensation of organics and inorganics increases CDNC by up to 44 % in simulations initialized with RH of 80 %, depending on VBS distribution and updraft velocity during the air parcel uplifts. Non-ideality of the system is relevant for considering the co-condensation effect realistically. For the ideal case, the maximum CDNC enhancement due to the combined co-condensation effect is 53 % while it is 44 % for the non-ideal case. The combined enhancement in CDNC of inorganic and organic species exceeds the sum of individual effects and should be further constrained in different environments in cloud parcel models as a basis for regional and global simulations.

In this study, we combine aerosol observations with high-resolution Eulerian (WRF-CHIMERE) and Lagrangian (FLEXPART) modelling to investigate the source regions, emission sources, transport pathways, and chemical transformation of sulphate aerosols at the high-altitude Monte Cimone station during July 2017. Our analysis shows that marine air masses are linked to higher levels of sulphate at Monte Cimone. In particular, the sea plays a dominant role in enhancing the oxidation of sulphur dioxide (SO2) into sulphate due to prolonged exposure to elevated hydroxyl radical (OH) concentrations over the sea. At the same time, sensitivity simulations reveal that industrial emissions contribute significantly to sulphate levels at Monte Cimone, even when air masses have spent a long time travelling over the sea. Furthermore, examination of vertical atmospheric dynamics indicates that free tropospheric air masses favour higher concentrations of sulphuric acid likely due to lower condensation sink (CS) conditions in the free troposphere (FT). In contrast, boundary layer conditions were found to enhance the transport of dimethyl sulphide (DMS) oxidation products, meaning that, over the Mediterranean Sea, DMS and its oxidation products do not reach the FT efficiently. Our results highlight the complex interaction between marine and terrestrial sources, atmospheric chemistry, and transport mechanisms in shaping sulphate aerosol levels at high-altitude sites. They also provide valuable insights into sulphate sources and transport processes over large geographical areas.

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.

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 (SO₄^2-; 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 C_xH_yN₁⁺ ions, partly originating from amines in the fog. C_xH_yN₂⁺ 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.

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 (PM_2.5), 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.

Some atmospheric compounds are emitted by both natural and anthropogenic sources, making them hard to distinguish. For instance, sawmills emit large amounts of volatile organic compounds, which are released from wood in different processing stages. SMEAR II (Station for Measuring Ecosystem–Atmosphere Relations) is located near sawmills in Korkeakoski (6 km southeast from SMEAR II) and Vilppula (21 km northeast from SMEAR II). Long-term measurements show that monoterpene concentrations were more than three times higher when the wind was from the Korkeakoski direction. Concentrations of NO_x, O₃, and Aitken mode-sized particles were also impacted by the sawmills. Perturbations were less clear from the Vilppula direction, likely because it was less frequent wind direction, and the transport distance was longer. We recommend considering the influence of the sawmills on the above-mentioned variables in future analyses of biosphere–atmosphere interactions, to avoid a distinct anthropogenic influence from the nearest sawmills.

Three independent particulate matter (PM) mass concentration measurements and their long-term (2005–2020) trends were compared at the Station for Measuring Ecosystem–Atmosphere Relations (SMEAR II, Hyytiälä, Finland). The different methods (a gravimetric method with a cascade impactor, an online method with a Synchronized Hybrid Ambient Real-time Particulate monitor (SHARP; only PM₁₀), and a calculated PM concentration from a combined particle number size distribution data of a differential mobility particle sizer (DMPS) and an aerodynamic particle sizer (APS)) showed good correlation (Pearson's correlation coefficient of approximately 0.8) in all size classes (PM₁, PM_2.5, and PM₁₀). The mass concentrations in all PM classes were the highest in summer and the lowest in autumn and winter. Statistically significant (Mann–Kendall test) declining annual trends were observed in DMPS+APS and impactor data in all size classes, ranging from −0.021 to −0.036 µg m⁻³ yr⁻¹. While the DMPS+APS method also indicated a statistically significant decline in all seasons, the decline in impactor data was statistically significant only in spring and winter. SHARP data could not be used for trend estimation due to the change in the inlet heating temperature, affecting the measured PM₁₀ concentrations. Seasonally, the decline was smallest in summer, which follows the trends also observed in SO₂ and NO_x concentrations. The results underline both the summertime dominance of biogenic sources for the aerosol mass concentration in the rural boreal forest environment and the reduction of anthropogenic pollution due to the EU-level restrictions for improved air quality.

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.

In this study, we investigate atmospheric new particle formation (NPF) across 65 d in the Bolivian central Andes at two locations: the mountaintop Chacaltaya station (CHC, 5.2 km above sea level) and an urban site in El Alto–La Paz (EAC), 19 km apart and at 1.1 km lower altitude. We classified the days into four categories based on the intensity of NPF, determined by the daily maximum concentration of 4–7 nm particles: (1) high at both sites, (2) medium at both, (3) high at EAC but low at CHC, and (4) low at both. These categories were then named after their emergent and most prominent characteristics: (1) Intense-NPF, (2) Polluted, (3) Volcanic, and (4) Cloudy. This classification was premised on the assumption that similar NPF intensities imply similar atmospheric processes. Our findings show significant differences across the categories in terms of particle size and volume, sulfuric acid concentration, aerosol compositions, pollution levels, meteorological conditions, and air mass origins. Specifically, intense NPF events (1) increased Aitken mode particle concentrations (14–100 nm) significantly on 28 % of the days when air masses passed over the Altiplano. At CHC, larger Aitken mode particle concentrations (40–100 nm) increased from 1.1 × 103 cm−3 (background) to 6.2 × 103 cm−3, and this is very likely linked to the ongoing NPF process. High pollution levels from urban emissions on 24 % of the days (2) were found to interrupt particle growth at CHC and diminish nucleation at EAC. Meanwhile, on 14 % of the days, high concentrations of sulfate and large particle volumes (3) were observed, correlating with significant influences from air masses originating from the actively degassing Sabancaya volcano and a depletion of positive 2–4 nm ions at CHC but not at EAC. During these days, reduced NPF intensity was observed at CHC but not at EAC. Lastly, on 34 % of the days, overcast conditions (4) were associated with low formation rates and air masses originating from the lowlands east of the stations. In all cases, event initiation (∼ 09:00 LT) generally occurred about half an hour earlier at CHC than at EAC and was likely modulated by the daily solar cycle. CHC at dawn is in an air mass representative of the regional residual layer with minimal local surface influence due to the barren landscape. As the day progresses, upslope winds bring in air masses affected by surface emissions from lower altitudes, which may include anthropogenic or biogenic sources. This influence likely develops gradually, eventually creating the right conditions for an NPF event to start. At EAC, the start of NPF was linked to the rapid growth of the boundary layer, which favored the entrainment of air masses from above. The study highlights the role of NPF in modifying atmospheric particles and underscores the varying impacts of urban versus mountain top environments on particle formation processes in the Andean region.

The contribution of natural aerosol particles from boreal forests to total aerosol loadings may increase with reduction in anthropogenic emissions. Aitken and accumulation mode particles in boreal regions differ significantly in hygroscopicity, and ignoring this size dependence can cause large uncertainty in Cloud Condensation Nuclei (CCN) prediction. We applied κ-Köhler theory to a multi-year dataset (2016–2020) from Hyytiälä, Finland, to evaluate different representations of aerosol chemical composition for CCN prediction. Overpredictions by forward closures using either bulk chemical composition from an Aerosol Chemical Speciation Monitor (ACSM) or a constant κ= 0.18 were mitigated to a great extent by optimizing size-resolved composition using two inverse modeling approaches: (1) Nelder–Mead method with the size distribution fixed to its median during each 2 h CCN measurement cycle, and (2) MCMC (Markov Chain Monte Carlo) accounting also for the variability in the size distribution during each cycle. Both methods improved closure at SS =  0.2 %–1.0 % (with Geometric Mean Bias GMB values 1.12–1.20 and 0.95–1.05, respectively), with moderate improvement at 0.1 % (GMBs of 1.53 and 1.32, respectively). The Aitken mode was enriched in organics in 77 % of cases using method (1) and 46 % using method (2) – with typical κ values of ∼ 0.1 for Aitken and ∼ 0.3 for accumulation modes. The results generally align with known size-dependent chemical composition in Hyytiälä and indicate that variability in CCN hygroscopicity is largely driven by Aitken mode composition. Our results demonstrate the potential of inverse CCN closure methods for obtaining valuable information of the size-dependent chemical composition.