While climate change is a global issue, the change in itself is not homogeneously distributed over the globe. It is well established that near-surface Arctic warming is on average 3-4 times larger than the global-average warming. This so-called Arctic Amplification is due to a number of positive feedbacks in the Arctic, some of which are poorly understood. On a basic level, Arctic climate is determined by a balance between inflows of energy from the south and the net loss of energy by radiation at the top of the Arctic atmosphere. Both are large while their difference is small. The inflow of heat from the south occurs in both the atmosphere and ocean. From a numerical modeling perspective it occurs on sufficiently large spatial and temporal scales that it is considered resolved, however, a disproportionately large fraction of the heat transport into the Arctic happens in discrete localized events, sometimes referred to as atmospheric rivers. The net energy flux at the top of the atmosphere also has a very large annual cycle: positive but small in summer, when the solar radiation is at its maximum, but large and negative in winter when the sun is absent. The net radiative flux at TOA depends on a number of processes, including sea-ice cover, surface temperature and albedo, atmospheric chemical composition, clouds and aerosols etc. All of these have in common that they are not resolved in numerical models and hence have to be parameterized, described parametrically as functions of larger-scale resolved variables. Different in different models, models typically have substantial systematic but sometimes compensating errors in these descriptions and to a large extent this explains the spread in climate model projections of future climate and systematic errors in weather forecasts. Arctic Ocean near-surface air temperature, as a proxy for climate, goes through a substantial annual cycle with two main states; these can be characterized as either freezing or melting. Physically, in some sense, the Arctic Ocean surface only has two seasons – the melt season and the freeze season. In winter with surface temperature below the melting point, the surface temperature reacts to changes in the surface energy budget, hence, it features large and fast changes in response to changes mainly in incoming radiation. In summer, or the melt season, the surface temperature is prevented from increasing above the melting point by the phase change of melting, as long as there is substantial ice and snow remaining, and all excess energy goes into melting rather than into warming. Consequently, the summer near-surface air temperature varies only a little. How much ice melts over the melt season is directly related to the length of the melt period but also indirectly to what happens in winter. If the melt season becomes longer it follows that sea ice extent at its minimum in September will decrease. The length of the melt season is therefore one important component of the Arctic climate system, and studying and understanding the so-called shoulder seasons – the transition between melt and freeze both in spring and autumn – is of great interest in order to understand the Arctic climate system. Historically, icebreaker-based expeditions, capable of performing scientific-grade process-level observations have occurred in summer or early autumn because the ice is easier to navigate in the Arctic Ocean, when melting. Hence, a number of Oden expeditions have been able to observe the transition from surface melt to surface freeze in late August or early September. However, only a few have collected such observations at the melt onset. Hence, on a process level, there are no relevant observations of the melt onset. The ARTofMELT expedition was conceived to rectify this, studying the relationship between this onset and atmospheric rivers.

Dimethyl sulfide (DMS) is the major sulfur species emitted from the ocean. The gas-phase oxidation of DMS by hydroxyl radicals proceeds through the stable, soluble intermediate hydroperoxymethyl thioformate (HPMTF), eventually forming carbonyl sulfide (OCS) and sulfur dioxide (SO₂). Recent work has shown that HPMTF is efficiently lost to marine boundary layer (MBL) clouds, thus arresting OCS and SO₂ production and their contributions to new-particle formation and growth events. To date, no long-term field studies exist to assess the extent to which frequent cloud processing impacts the fate of HPMTF. Here, we present 6 weeks of measurements of the cloud fraction and the marine sulfur species methanethiol, DMS, and HPMTF made at the Atmospheric Radiation Measurement (ARM) research facility on Graciosa Island, Azores, Portugal. Using an observationally constrained chemical box model, we determine that cloud loss is the dominant sink of HPMTF in this region of the MBL during the study, accounting for 79 %–91 % of HPMTF loss on average. When accounting for HPMTF uptake to clouds, we calculate campaign average reductions in DMS-derived MBL SO₂ and OCS of 52 %–60 % and 80 %–92 % for the study period. Using yearly measurements of the site- and satellite-measured 3D cloud fraction and DMS climatology, we infer that HPMTF cloud loss is the dominant sink of HPMTF in the eastern North Atlantic during all seasons and occurs on timescales faster than what is prescribed in global chemical transport models. Accurately resolving this rapid loss of HPMTF to clouds has important implications for constraining drivers of MBL new-particle formation.

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.

The susceptibility of cloud droplet number to cloud condensation nuclei number is one of the major factors controlling the highly uncertain change in the amount of solar radiation reflected by clouds when aerosol emissions are perturbed (the radiative forcing due to aerosol–cloud interactions). We investigate this susceptibility in low-level stratiform clouds using long-term (3–10-yr) in situ observations of aerosols and clouds at three high-latitude locations. The in situ observations show higher susceptibility for low-level stratiform clouds than values reported for satellite data. We estimate −1.16 W m−2 for the aerosol indirect radiative forcing on the basis of our observations, which is at the higher end of satellite-derived forcing estimates and the uncertainty range of the most recent Intergovernmental Panel on Climate Change report. We evaluate four Earth system models against the observations and find large inter-model variability in the susceptibility. Our results demonstrate that, even if the susceptibility in some of the models is relatively close to observations, the underlying physics in the models is unrealistic when compared with observations. We show that the inter-model variability is driven by differences in sub-grid-scale updraught velocities and aerosol size distributions, raising a need to improve these aspects in models.

Ice nucleating particles (INPs) initiate ice formation, affecting the liquid versus ice distribution and radiative properties of clouds. INPs have been measured around the Arctic, but few INP concentration measurements have been reported for air during movement south out of central Arctic pack ice regions during cold air outbreaks (CAOs). We analyzed cases of transports connecting the Central Arctic location of the Multidisciplinary Drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition to the near sea ice edge in Svalbard and across ice-free ocean to the Cold-air Outbreaks in the Marine Boundary Layer Experiment (COMBLE) site at Andenes, Norway, during the 2019–2020 Arctic winter. Aerosol surface area concentration measurements during CAOs indicate a switch from primarily accumulation mode at MOSAiC toward marine coarse mode (from sea spray emissions) at COMBLE. INP concentrations were independent of aerosol surface area or volume over the pack ice in MOSAiC in winter. At Svalbard, INPs related best to supermicron aerosol surface area and supermicron volume. At the COMBLE site, INPs related best with total aerosol surface area and total aerosol volume. In 5 of 6 case studies analyzed, INP concentrations increased in association with the transition to a dominance of sea spray aerosols. The INPs at COMBLE had a unique INP concentration mode near −18°C and higher ice nucleation active site densities (e.g., INPs per surface area) compared to those previously reported for other open ocean regions dominated by marine aerosols. While the INP sources in this case appear to be from oceanic emissions from shallower oceans under turbid water conditions, attribution solely to sea spray aerosols versus mixing down of free tropospheric aerosols by CAO clouds remains as a future topic. These studies provide a basis for parameterization of INPs for numerical modeling studies of CAO cloud systems.

Aerosols play a critical role in the Arctic's radiative balance, influencing solar radiation and cloud formation. Limited observations in the central Arctic leave gaps in understanding aerosol dynamics year-round, affecting model predictions of climate-relevant aerosol properties. Here, we present the first annual high-time-resolution observations of submicron aerosol chemical composition in the central Arctic during the Arctic Ocean 2018 (AO2018) and the 2019–2020 Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expeditions. Seasonal variations in the aerosol mass concentrations and chemical composition in the central Arctic were found to be driven by typical Arctic seasonal regimes and resemble those of pan-Arctic land-based stations. Organic aerosols dominated the pristine summer, while anthropogenic sulfate prevailed in autumn and spring under haze conditions. Ammonium, which impacts aerosol acidity, was consistently less abundant, relative to sulfate, in the central Arctic compared to lower latitudes of the Arctic. Cyclonic (storm) activity was found to have a significant influence on aerosol variability by enhancing emissions from local sources and the transport of remote aerosol. Local wind-generated particles contributed up to 80 % (20 %) of the cloud condensation nuclei population in autumn (spring). While the analysis presented herein provides the current central Arctic aerosol baseline, which will serve to improve climate model predictions in the region, it also underscores the importance of integrating short-timescale processes, such as seasonal wind-driven aerosol sources from blowing snow and open leads/ocean in model simulations. This is particularly important, given the decline in mid-latitude anthropogenic emissions and the increase in local ones.

The Southern Ocean, wintertime cities, the upper troposphere, the Arctic and Antarctica, and alpine mountains are places where atmospheric chemistry impacts human health, air quality, climate, or geochemical cycles and that are characterized by low temperatures where ice or snow can be present. The atmospheric impact is evident from the role of polar biogenic sulphur emissions on aerosol formation, multiphase nitrogen and sulphur chemistry on wintertime haze, and industrial emissions in snow-covered areas on the ozone budget. The Cryosphere and ATmospheric CHemistry community (CATCH) addresses the environmental processes within these coupled cryosphere–atmosphere systems, and here we present open research questions specific to the cold environments, focusing on the unique interplay of chemistry and physics. These research needs call for interdisciplinary approaches to address atmospheric science in a warming climate with changing human impact in Earth's cold regions.

The interactions between aerosols and clouds are still one of the largest sources of uncertainty in quantifying anthropogenic radiative forcing. To reduce this uncertainty, we must first determine the baseline natural aerosol loading for different environments. In the pristine and hardly accessible polar regions, the exact nature of local aerosol sources remains poorly understood. It is unclear how oceans, including sea ice, control the aerosol budget, influence cloud formation, and determine the cloud phase. One critical question relates to the abundance and characteristics of biological aerosol particles that are important for the formation and microphysical properties of Arctic mixed-phase clouds. Within this work, we conducted a comprehensive analysis of various potential local sources of natural aerosols in the high Arctic over the pack ice during the ARTofMELT expedition in May–June 2023. Samples of snow, sea ice, seawater, and the sea surface microlayer (SML) were analysed for their microphysical, chemical, and fluorescent properties immediately after collection. Accompanied analyses of ice nucleating properties and biological cell quantification were performed at a later stage. We found that increased biological activity in seawater and the SML during the late Arctic spring led to higher emissions of fluorescent primary biological aerosol particles (fPBAPs) and other highly fluorescent particles (OHFPs, here organic-coated sea salt particles). Surprisingly, the concentrations of ice nucleating particles (INPs) in the corresponding liquid samples did not follow this trend. Gradients in OHFPs, fPBAPs, and black carbon indicated an anthropogenic pollution signal in surface samples especially in snow but also in the top layer of the sea ice core and SML samples. Salinity did not affect the aerosolisation of fPBAPs or sample ice nucleating activity. Compared to seawater, INP and fPBAP concentrations were enriched in sea ice samples. All samples showed distinct differences in their biological, chemical, and physical properties, which can be used in future work for an improved source apportionment of natural Arctic aerosol to reduce uncertainties associated with their representation in models and impacts on Arctic mixed-phase clouds.

The aerosol particles serving as cloud condensation and ice nuclei contribute to key cloud processes associated with cold-air outbreak (CAO) events but are poorly constrained in climate models due to sparse observations. Here we retrieve aerosol number size distribution modes from measurements at Andenes, Norway, during the Cold-Air Outbreaks in the Marine Boundary Layer Experiment (COMBLE) and at Zeppelin Observatory, approximately 1000 km upwind from Andenes at Svalbard. During CAO events at Andenes, the sea-spray-mode number concentration is correlated with strong over-ocean winds with a mean of 8±4 cm-3 that is 71 % higher than during non-CAO conditions. Additionally, during CAO events at Andenes, the mean Hoppel minimum diameter is 6 nm smaller than during non-CAO conditions, though the estimated supersaturation is lower, and the mean number concentration of particles that likely activated in-cloud is 109±61 cm-3 with no statistically significant difference from the non-CAO mean of 99±66 cm-3. For CAO trajectories between Zeppelin Observatory and Andenes, the upwind-to-downwind change in number concentration is the largest for the accumulation mode with a mean decrease of 93±95 cm-3, likely attributable primarily to precipitation scavenging. These characteristic properties of aerosol number size distributions during CAO events provide guidance for evaluating CAO aerosol-cloud interaction processes in models.

Studying primary biological aerosol particles in the Arctic is crucial to understanding their role in cloud formation and climate regulation at high latitudes. During the Arctic Ocean 2018 expedition, fluorescent primary biological aerosol particles (fPBAPs) were observed, using a multiparameter bioaerosol spectrometer, near the North Pole during the transition from summer to early fall. The fPBAPs showed a strong correlation with the occurrence of ice nucleating particles (INPs) and had similar concentration levels during the first half of the expedition. This relationship highlights the potential importance of biological sources of INPs in the formation of mixed-phase clouds during the central Arctic’s summer and early fall seasons.

Our analysis shows that the observed fPBAPs were independent of local wind speed and the co-occurrence of other coarse mode particles, suggesting sources other than local sea spray from leads, melt ponds, re-suspension of particles from the surface, or other wind-driven processes within the pack ice. In contrast, other fluorescent particles were correlated with wind speed and coarse mode particle concentration.

A multi-day event of high concentrations of fPBAPs was observed at the North Pole, during which the contribution of fPBAPs to the total concentration of coarse mode aerosol increased dramatically from less than 0.1% up to 55%. Analysis of chemical composition and particle size suggested a marine origin for these fPBAPs, a hypothesis further supported by additional evidence. Air parcel trajectory analysis coupled with ocean productivity reanalysis data, as well as analysis of large-scale meteorological conditions, all linked the high concentrations of fPBAPs to biologically active, ice-free areas of the Arctic Ocean.