Abstract. Laboratory experiments addressing complex phenomena such as atmospheric new-particle formation and growth typically involve numerous instruments measuring a range of key coupled variables. In addition to independent calibration, the combined dataset provides not just constraints on the parameters of interest but also on the critical instrument calibrations. Here we find good agreement between production and loss rates of sulfuric acid (H2SO4) in an experiment performed at the CERN CLOUD chamber involving oxidation of sulfur dioxide (SO2) in the presence of ammonia (NH3) at 58 % relative humidity, driving new-particle formation and growth of particles by H2SO4 + NH3 nucleation initiated by O3 photolysis via several light sources. This closure requires consistency across numerous parameters, including: the particle number and size distribution; their condensation sink for H2SO4; the particle growth rates; the concentration of H2SO4; and the nucleation coefficients for both neutral and ion-induced pathways. Our study shows that accurate agreement can be achieved between production and loss of condensable vapors in laboratory chambers under atmospheric conditions, with accuracy ultimately tied to particle number measurement (i.e. a condensation particle counter). This, in turn implies parameters such as the H2SO4 concentration and particle size distributions can be determined to a comparable precision.

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.

Uncertainty in estimations of the net contribution of anthropogenic aerosol particles, particularly of aerosol-cloud interactions (ACIs) to the Earth's radiation budget, limits our ability to understand past and project future climate change. Earth System Models (ESMs) are among the key tools for assessing the magnitude and impacts of changes in various forcing agents on the global climate system. Hence, improving aerosol and cloud descriptions in ESMs is an important way forward to increase the confidence in estimates of climate impacts of aerosol perturbations in the past, present and future. In the framework of the FORCeS project, experimental and theoretical approaches were combined to bridge the current key gaps in the fundamental understanding of essential aerosol and cloud processes and their descriptions in selected European ESMs. Regarding aerosol types and processes, we focused on organic aerosol, particulate nitrate, absorbing aerosols, and ultrafine aerosol sources including new particle formation and growth. In terms of cloud processes, we targeted cloud droplet activation, hydrometeor growth and evaporation, ice formation and multiplication as well as aerosol processing and scavenging by clouds. The selection was made based on the identified knowledge gaps in the scientific understanding of these processes and/or their current representation in ESMs, as well as a novel perturbed parameter ensemble approach to detecting potential structural deficiencies in an ESM. Here, we review the state-of-the-art, outline our approach for arriving at recommendations for improving the representation of key aerosol and cloud processes within ESMs, and then provide such recommendations applicable in models operating at the Earth system scale. The limitations of the recommendations, applicability, as well as alternative approaches and future research directions are discussed. Overall, the findings highlight the need for continuous efforts towards smart ways for representing the aerosol number size distribution as well as consistent representations of key parameters (e.g., liquid water content and cloud droplet number concentration). Furthermore, we provide guidance for future ESM evaluation emphasising, in particular, the need for exploring the consistency of key parameters, process-based (as opposed to parameter-based), and the complementarity of in-situ and remote-sensed measurements for model evaluation.

Aerosol properties, loading, trends, and variability in the upper troposphere are key to understanding the evolving state of the atmosphere and the role of aerosols in climate and cloud processes. However, long-term in-situ aerosol observations at high altitudes remain scarce worldwide, particularly in the Global South. This observational gap limits our ability to develop a global perspective on aerosol sources, processes, and impacts within the climate system.Here we present 13 years (2012–2024) of continuous aerosol-related measurements conducted at the world’s highest Global Atmosphere Watch (GAW) station, located on Mount Chacaltaya (CHC) in the central Andes of Bolivia at an elevation of 5.2 km a.s.l. This dataset is one of the longest in existence on the South American continent and therefore provides a unique opportunity to evaluate trends in aerosol concentrations and properties. These trends and properties are influenced by, for example, biomass burning in the Amazon, the transport of pollution from the conurbation of La Paz and El Alto, located 18 km to the south, and the subsidence of air masses from the upper troposphere.We focus on particle number size distributions (PNSD), equivalent black carbon (eBC), and related meteorological and chemical tracers, including water vapor mixing ratio (WVMR) and carbon monoxide (CO). We characterize aerosol properties and loading by combining traditional time-series analysis (e.g., separation by hour of day, season, and year) with an unsupervised k-means clustering approach that disentangles the dominant atmospheric regimes influencing aerosol properties at CHC. The clustering uses PNSD, eBC, and WVMR as input variables and identifies seven distinct categories of days, hereafter referred to as atmospheric regimes, which represent significantly different source regions and aerosol processing pathways (e.g., cloud processing, wet deposition, and new particle formation). The performance of the clustering is evaluated using independent tracers, namely CO concentrations and HYSPLIT back trajectories. For each regime, the individual days grouped within it exhibit internally consistent CO levels and air-mass provenance that are clearly distinct from those of other regimes. This result is particularly encouraging given that neither CO nor back trajectories were included as inputs to the clustering algorithm.One regime is particularly noteworthy, representing a persistent free-tropospheric state characterized by extremely low WVMR, CO, and eBC, along with signatures of early-morning new particle formation. We find that the concentration of particles in this regime has significantly decreased over the 13-year period which indicates a declining upper-tropospheric particle concentration. A second notable regime is associated with biomass burning. We find that its occurrence has increased over time, from ~10% of days during the biomass-burning season (August–November) in the first years to ~50% in the last years. This suggests an increment on the number of biomass burning episodes measured at the station. Additional categories capture aerosol–cloud processing during Amazonian boundary-layer uplift, local eBC influence from the La Paz–El Alto metropolitan area, and strong nucleation under dry, coastal/Altiplano air masses. Overall, these results emphasize a region in rapid change and the importance and utility of long-term measurements in under sampled areas.

Methanesulfonic acid (MSA; CH3SO3H), produced by oxidation of dimethylsulfide (DMS; CH3SCH3), is a key precursor for aerosols in the marine boundary layer and free troposphere. Laboratory experiments show that MSA contributes to both particle nucleation and subsequent growth, often together with sulfuric acid (H2SO4) and base vapours such as ammonia (NH3). However, the influence of relative humidity (RH) on MSA condensation to particles remains uncertain. Here, in experiments conducted under low NH3 conditions (<4 parts per trillion by volume, pptv) at the CERN Cosmics Leaving OUtdoor Droplets (CLOUD) chamber, we find that RH critically regulates the participation of MSA in the initial growth of newly formed particles. Between +10 °C and -10 °C, MSA drives rapid particle growth at high RH (>50%), whereas its contribution at low RH (<16%) is negligible. When comparing with aerosol process models such as the Model for Acid-Base Chemistry in Nanoparticle Growth (MABNAG), we find that the model fails to match our laboratory results. In particular, our measurements show that MSA drives rapid particle growth at substantially warmer temperatures and lower relative humidities than predicted by the Extended Aerosol Inorganic Model (E-AIM). This highlights the importance of further experiments to fully quantify the effect of RH on MSA particle growth and evaporation, and incorporating these measurements in aerosol models. This will be essential for accurately representing the important role of MSA in marine aerosols in global models, particularly in cold, low-NH3 environments like polar regions and the free troposphere.

Employing high-resolution Large Eddy Simulation (LES) coupled with interactive aerosol and cloud microphysics schemes, this study investigates the influence of aerosol and droplet microphysics on the life cycle and properties of wintertime radiation fog in the Po Valley, Italy. For the simulated case, the results show that the main drivers of radiation fog onset and dissipation are nocturnal longwave cooling and surface warming, respectively. Increasing aerosol loading increases droplet number concentration, liquid water content, and fog optical thickness, which reduces droplet sedimentation rates and prolongs fog duration by up to 54 min. Overall, the microphysical influence of aerosols and droplets weakens under heavily polluted conditions. We also show that non-activated hydrated aerosols have a limited influence on total liquid water content and fog-layer mixing. However, they critically affect visibility and fog duration prediction, underscoring the importance of explicitly incorporating hydrated particles in fog forecasting and accurately representing aerosol composition. Additional sensitivity experiments reveal that the prescribed droplet spectral shape parameters significantly influence fog characteristics. Parameter settings that represent a broad droplet size spectrum overestimate the number of large droplets compared to observations, which increases mean droplet sedimentation rates and decreases mean liquid water content by up to 104 % and 78 %, respectively, compared to the settings that best represent the observed spectrum.

Aerosol-cloud interactions (ACI) are a major source of uncertainty in climate science, critically affecting our ability to project near-term climate evolution and assess societal risks. These interactions influence effective radiative forcing, cloud dynamics, and precipitation patterns, yet remain insufficiently constrained due to limitations in observations, modeling, and process understanding. This uncertainty hampers robust policy advice across multiple domains-from estimating remaining carbon budgets and climate sensitivity, to anticipating regional extreme events and evaluating climate interventions such as solar radiation modification. In many cases, the influence of ACI is either underappreciated or excluded from decision-making frameworks due to its complexity and lack of quantification. This perspective outlines a path forward to overcome these barriers by leveraging emerging opportunities in satellite remote sensing, ground-based and airborne observations, high-resolution climate modeling, and machine learning. We identify key areas where rapid progress is feasible, including improved retrievals of cloud microphysical properties, better representation of natural aerosols in a warming world, and enhanced integration of observational and modeling communities. Even as anthropogenic aerosol and its impacts on clouds is reducing owing to emissions controls, addressing ACI uncertainties remains essential for refining climate projections, supporting effective mitigation and adaptation strategies, and delivering actionable science to policymakers in a rapidly changing climate system.

Transpiration drives up to 40 % of terrestrial precipitation, with forests playing a critical role. This study combines long-term sap flow measurements and surface-based cloud observations to examine how different clouds affect tree transpiration across boreal and temperate European forests. Under specific cloudy conditions, sap flow can exceed clear-sky levels, reflecting distinct radiation effects of various cloud types. However, overall cloudiness reduces maximum sap flow by up to 40 %. A key finding is the contrasting influence of low- and highaltitude clouds: low-altitude clouds suppress transpiration by limiting incoming radiation, while high-altitude clouds have negligible impact even when overcast. Structural equation modelling further indicates a pathway linking transpiration to cloudiness when other meteorological and site factors are accounted for. Satellite data show a decline in low-altitude cloud fraction over boreal forests, and a highly simplified model-based order-of-magnitude estimate suggests a potential associated increase in transpiration equivalent to -0.6-1.2 mm of precipitation annually. These results emphasize how climate change driven changes in cloud cover and type may alter the moisture flux to the atmosphere, impacting regional and global water cycles.

The years 2023 and 2024 were characterized by unprecedented warming across the globe, underscoring the urgency of climate action. Robust science advice for decision makers on subjects as complex as climate change requires deep cross- and interdisciplinary understanding. However, navigating the ever-expanding and diverse peer-reviewed literature on climate change is enormously challenging for individual researchers. We elicited expert input through an online questionnaire (188 respondents from 45 countries) and prioritized 10 key advances in climate-change research with high policy relevance. The insights span a wide range of areas, from changes in methane and aerosol emissions to the factors shaping citizens' acceptance of climate policies. This synthesis and communications effort forms the basis for a science-policy report distributed to party delegations ahead of the 29th session of the Conference of the Parties (COP29) to inform their positions and arguments on critical issues, including heat-adaptation planning, comprehensive mitigation strategies, and strengthened governance in energy-transition minerals value chains.

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.

Abstract. New particle formation (NPF) is the phenomenon wherein gaseous precursors form critical clusters of barely a few nanometres in diameter, after which, under favourable conditions these particles can grow to climate-relevant sizes. Here we present measurements from 2022 to 2024 of particle and ion number size distributions from the Zeppelin Observatory (ZEP), an Arctic research station situated on the western edge of Svalbard. NPF events begin in April and continue occurring into November. The events at the start of the NPF season (i.e. April/May) are considerably stronger (i.e. a larger production of nucleation mode particles). The peaks in NPF strength coincide with peaks in the solar insolation experienced by arriving air masses. During the summer period NPF events occur on 20–40 % of days each month, however, there is a consistent decline in June. We show that the combined influence of solar radiation and the surface area of pre-existing aerosols (i.e. condensation sink, CS) are strong predictors for the likelihood of NPF. We develop a simplified predictive model which matches the frequency of NPF events identified via the classification schemes used in this study. We show that NPF events occur during the polar night (i.e. when the Sun does not pass above horizon), and speculate that these events are linked to high altitude air masses. Furthermore, we detail the likely geographic origins of nucleation within the Arctic, as measured at ZEP. We show that NPF events are considerably more likely to originate from the marine regions towards the west of Svalbard, particularly the Greenland Sea which presented the greatest likelihood that arriving air masses from this marine region would be linked to an NPF day. We also remark on the proportion of the Aitken mode particles within the Arctic that could originate from NPF; we show that NPF events lead to an increase in the number of Aitken mode particles. We measure over 50 NPF events where the nucleation mode particles grew beyond 25 nm, a diameter representing the minimum activation diameter for particles to act as cloud condensation nuclei. Overall, we present a concise picture of the lifecycle of nucleation mode particles in the Arctic, including the effect wet scavenging has in reducing the condensation sink, which in turn encourages NPF events to occur.

Aerosol-cloud interactions play a crucial role in the Arctic’s radiative budget. During the campaign ‘Atmospheric rivers and the onset of sea ice melt’ (ARTofMELT 2023) we aimed to improve our understanding of aerosol-cloud interactions by conducting in-situ measurements of microphysical and chemical properties of aerosols, cloud droplets, and precipitation in the Arctic during the onset of sea ice melt. A ground-based fog and aerosol spectrometer (GFAS) and a fog monitor (FM-120) from Droplet Measurement Technologies (DMT) were used to measure among other things droplet size, number concentration, and liquid water content. Precipitation was measured with a meteorological particle spectrometer (MPS, DMT). Throughout the campaign, we observed several fog and blowing snow events, along with occasional precipitation. These events provided an opportunity to investigate and compare the distinctive microphysical properties associated with each event. Our findings reveal significant variations in the size distribution and particle phase of blowing snow, precipitation, and fog.

Black Carbon (BC) aerosols are known to be important for the Earth’s climate, yet their exact role to the changing of the Earth’s climate and Arctic amplification remains unclear. An accurate description of the BC life cycle in general circulation models (GCMs) can help reduce the uncertainties due to BC aerosols and specify BC's role in the Arctic.In this study, several GCMs (ECHAM6.3-HAM2.3, ECHAM6.3-HAM2.3-P3, ECHAM6.3-HAM2.3-SALSA2 and UKESM1.0) are compared in terms of their representation of BC mass in the Arctic within the AeroCom project GCM Trajectory. A novel Lagrangian framework is employed to examine the history of air masses reaching the observational station Zeppelin, Svalbard. Therfore the removal processes were analysed along the trajectory and the GCMs compared with each other. The analysis emphasises the impact of remote emissions on local BC concentrations in the Arctic, indicating a longer BC lifetime compared to the global average. This underlines the importance of dry and wet scavenging parametrisations in the GCMs.

The Po Valley in northern Italy is a densely populated region, with a variety of anthropogenic emissions of primary aerosol particles and precursor gases from industrial, agricultural, and urban activities. Especially in the winter, the orographic and meteorological conditions in this region are favorable to result in pollution levels among the highest in Europe (Daellenbach et al., 2020). The combination of these high aerosol loadings with prevalent cold temperatures in winter cause the formation of fog. Efforts over the last decades led to an increased characterization and understanding of this fog (Fuzzi et al., 1992); however, from an aerosol perspective, this applies mainly to the physical characteristics of the fog while the role of the chemical composition of aerosols remains much less determined.To address this lack in understanding the chemical characteristics, state-of-the-art chemical mass spectrometers measuring aerosol particles as well as trace gases were deployed during the Fog and Aerosol InteRAction Research Italy (FAIRARI) field campaign. The FAIRARI campaign covered the winter and spring 2022 in San Pietro Capofiume in the Po Valley in northern Italy with the overall aim to comprehensively investigate aerosol-fog interactions in this polluted environment by covering the entire size range, from gas molecules up to hydrometeors.First results from the FAIRARI campaign on the bulk chemical composition measurements indicate no clear pattern in the difference in composition when comparing periods of fog with periods where no fog was formed. The bulk aerosol composition was dominated by nitrate and organics during both conditions. In contrast to the ambient aerosol particles, the aerosol particles left when drying the fog droplets show presence of organonitrates. In this presentation, we will dive into the molecular-level chemical composition of organic molecules present in the particulate and gas phase in fog episodes, as well as these organonitrates, potentially formed in the fog droplets.Understanding the different phases and their composition will help to better determine potential sources driving fog formation in this region, which is relevant for both climate and health mitigations. ReferencesDaellenbach, K. R., Uzu, G., Jiang, J., Cassagnes, L.-E., Leni, Z., Vlachou, A., Stefenelli, G., Canonaco, F., Weber, S., Segers, A., Kuenen, J. J. P., Schaap, M., Favez, O., Albinet, A., Aksoyoglu, S., Dommen, J., Baltensperger, U., Geiser, M., El Haddad, I., Jaffrezo, J.-L., and Prévôt, A. S. H.: Sources of particulate-matter air pollution and its oxidative potential in Europe, Nature, 587, 414–419, https://doi.org/10.1038/s41586-020-2902-8, 2020.Fuzzi, S., Facchini, M. C., Orsi, G., Lind, J. A., Wobrock, W., Kessel, M., Maser, R., Jaeschke, W., Enderle, K. H., Arends, B. G., Berner, A., Solly, I., Kruisz, C., Reischl, G., Pahl, S., Kaminski, U., Winkler, P., Ogren, J. A., Noone, K. J., Hallberg, A., Fierlinger-Oberlinninger, H., Puxbaum, H., Marzorati, A., Hansson, H.-C., Wiedensohler, A., Svenningsson, I. B., Martinsson, B. G., Schell, D., and Georgii, H. W.: The Po Valley Fog Experiment 1989., Tellus B, 44, 448–468, https://doi.org/10.1034/j.1600-0889.1992.t01-4-00002.x, 1992.

The process of new particle formation from gas-phase precursors holds significant importance in Earth's atmosphere and introduces a notable source of uncertainty in climate change predictions. The growth of freshly formed molecular clusters should in theory be crucial for the climate impact of new particle formation, influencing the survival probability of these particles exponentially and determining their ability to act as cloud condensation nuclei. However, defining the fundamental aspects of nanoparticle growth is intricate. It involves a complex interplay of condensational and reactive vapor uptake, aerosol coagulation, sink processes, and a diverse array of potential gaseous precursors. Observational nanoparticle growth rates, derived from the evolution of the particle-size distribution, portray growth as a collective phenomenon. However, models often interpret these rates at a single-particle level, integrating them into simplified size-distribution representations (Stolzenburg et al., 2023). dditionally, many models only consider a limited subset of condensable vapors, while recent experimental observations identify an increasing number of potential contributors to new particle growth. Our objective here is to bridge the gap between experimental and modeling studies on nanoparticle growth. We compare three large-scale models (NorESM, ECHAM, and TM5) regarding their sensitivity to organic nanoparticle growth processes. Surprisingly, we find a much lower sensitivity than anticipated from box models. Through the inclusion of a sectional scheme into NorESM, we demonstrate that representing the complexity of size distribution dynamics leads to significantly different cloud condensation nuclei (CCN) levels. Furthermore, our results suggest that, on regional scales, sensitivity to organic growth is much higher. Inclusion of additional growth processes and/or a scaling of condensable vapor concentrations could yield a significantly altered climate response. In turn, comprehensive experimental observations from e.g. the open oceans are still lacking and we show that continental data exhibit surprisingly little variation in measured particle growth rates. The latter indicates limited sensitivity in current experimental approaches and potential unaccounted multi-phase chemistry in the growth process. Consequently, we propose specific guidance for future research to address questions regarding the buffered climate response in large-scale models and the unexpectedly low variation observed in global growth measurements. We advocate for more sensitivity studies and improved model-measurement comparisons. References: Stolzenburg, D., Cai, R., Blichner, S. M., Kontkanen, J., Zhou, P., Makkonen, R., Kerminen, V.-M., Kulmala, M., and Kangasluoma, J.: Atmospheric nanoparticle growth, Rev. Mod. Phys., 95, 045002, https://doi.org/10.1103/RevModPhys.95.045002, 2023.

We present direct observations of 2-methyltetrol (C5H12O4) in the gas- and particle phase from the deployment of a Filter Inlet for Gases and Aerosols coupled to a Time-of-Flight Chemical Ionization Mass Spectrometer (FIGAERO-CIMS) during the Southern Hemisphere High Altitude Experiment on Particle Nucleation and Growth (SALTENA), which took place between December 2017 and June 2018 at the high-altitude Global Atmosphere Watch station Chacaltaya (CHC) located at 5240 m a s l in the Bolivian Andes. 2-Methyltetrol signals were dominant in a factor resulting from Positive Matrix Factorization (PMF) identified as influenced by Amazon emissions. We combine these observations with investigations of isoprene oxidation chemistry and uptake in an isolated deep convective cloud in the Amazon using a photochemical box model with coupled cloud microphysics and show that, likely, 2-methyltetrol is taken up by hydrometeors or formed in situ in the convective cloud, and then transported in the particle phase in the cold environment of the Amazon outflow and to the station, where it partially evaporates.