Abstract. Aerosol hygroscopicity strongly governs particle size, mixing state, and radiative effects, yet remains poorly constrained for organic aerosols due to their chemical complexity and limited observations. Here, we present laboratory-measured size-segregated hygroscopic properties of 22 organic compounds, including carboxylic acids, amino acids, sugars, and alcohols, using a hygroscopic tandem differential mobility analyzer (HTDMA) combined with chemical characterization by Aerosol Mass Spectrometry (AMS). Our results extend previous studies by resolving hygroscopic behaviour across the submicrometer size range most relevant to atmospheric processes and by systematically linking organic hygroscopicity (κorg) across functional groups, as measured by AMS, with physicochemical properties. Structurally similar compounds may exhibit markedly different hygroscopic behavior, underscoring the role of molecular interactions. Similar to carbon chains, increased functionalization generally enhances hygroscopicity and induces a pronounced size dependence. Functional-group-based classifications from the AMS provide a useful approximation for estimating κorg, but may not capture this complexity. Leveraging these laboratory constraints, we use a simple but extensible machine-learning framework that integrates laboratory-derived κorg with ambient aerosol observations. The application of this hybrid approach to urban and rural environments demonstrates substantial improvements in predicting ambient hygroscopicity, with R² values increasing from 0.82 to 0.96 at the Paris suburban site SIRTA (France) and from 0.60 to 0.94 at the rural background site Goldlauter (Germany), compared to conventional composition-based models. By bridging controlled laboratory measurements with data-driven ambient analysis, this study provides a rigorous pathway to improve the representation of the direct aerosol radiative effect in atmospheric and climate models.

Low-level clouds play a crucial role in the Arctic climate system, for example by contributing to surface warming. Although many efforts have been made to investigate low-level clouds, there is still a significant in-situ data gap within the atmospheric boundary layer (ABL) and the lower troposphere. While long-term ground-based observatories provide valuable continuous measurements, they cannot resolve the vertical structure of aerosols and clouds.To address this data gap, five uncrewed aerial systems (UAS) were deployed during two intensive measurement campaigns at the Pallas Atmosphere-Ecosystem Supersite in northern Finland in spring (4–12 April 2025) and autumn (16–30 September 2025). Fixed-wing, vertical take-off and landing (VTOL), and multirotor platforms were operated jointly by the Finnish Meteorological Institute (FMI) and the Technische Universität Braunschweig. In total, 246 measurement flights were conducted, reaching altitudes of up to 2 km above ground level and conducting over 80 hours of in-situ sampling.The UAS were equipped with different sensors to measure aerosols, including two condensation particle counters with different cut-offs to measure the aerosol particle number concentration, a Partector 2 Pro to measure the size distribution between 10 and 300 nm and a POPS to measure the size distribution between 115 and 3370 nm. In addition meteorological parameters, and cloud droplet properties were also measured. This enables a detailed characterization of the vertical distribution of aerosols and their interaction with the ABL and low-level clouds. These measurements were compared to long-term observations from the nearby ground-based observatory Sammaltunturi. This study demonstrates the value of combining ground-based measurements with UAS profiling when investigating aerosol-cloud interactions.Preliminary results indicate pronounced seasonal differences. Spring conditions were dominated by new particle formation events associated with long-range air mass transport from the central Arctic. In contrast, autumn measurements were strongly influenced by low-level cloud formation and local aerosol sources. Overall, this campaign demonstrates the added value of UAS observations in improving the understanding of aerosol-cloud interactions in the sub-Arctic and enhancing the interpretability of existing ground-based datasets.

Abstract Aerosol hygroscopicity is a critical parameter for predicting radiative forcing and climate sensitivity, particularly under sub-saturated regimes where it drives complex aerosol–water interactions. Here, we show that externally mixed aerosols exert a stronger influence on direct radiative forcing than is currently represented in models. Incorporating our findings into radiative forcing calculations indicates a stronger aerosol cooling effect, especially at suburban sites, highlighting the importance of representing regional differences in mixing state. The conventional bulk-chemistry approach, which assumes volume-based mixing with limited spatial variability, exhibits low predictive performance for aerosol hygroscopicity (R² ≈ 0.61) at urban and suburban sites. Using an interpretable machine learning framework trained on geographically diverse, region-specific datasets can capture this variability with higher accuracy (R² ≈ 0.97), identifying key chemical compositional and mixing-state drivers.

Post-winter haze events in Delhi, India, comprise great air quality challenges, yet remain poorly understood due to limited measurements of vertical profiles of particulate matter (PM) concentrations. This study employs a drone-mounted PM low-cost sensor (PM-LCS) with an optimized sampling system to capture vertical PM2.5 profiles during March 2021. Elevated PM2.5 concentrations (160 µg/m3) were observed at an altitude of 100 m, being 60% higher than ground level. Vertical profiles of the PM1/PM2.5 ratio under humid conditions (RH > 70%), showed that haze formation is likely driven by hygroscopic inorganic aerosols. Comparison with model simulations showed significant underestimation of PM2.5 (−52.6 ± 5.5%) during morning haze episodes, coinciding with a dry bias in modeled RH (−30.1 ± 8.3%). During non-hazy episodes, PM2.5 underestimation decreased to 10.8 ± 1.2% with a minimal RH bias. This suggests that the dry bias of the model limits its ability to simulate aerosol hygroscopic growth. Overall, our findings demonstrate that drone-mounted PM-LCS provides a valuable vertical air quality assessment tool.

Marine combined carbohydrates in aerosol particles (CCHOaer) have the potential to influence cloud formation and properties, but it remains unclear to what extent they reach altitudes relevant for cloud processes. Balloon-borne measurements of major sea spray aerosol (SSA) constituents, including sodium (Na-aer(+)) and CCHOaer, were conducted in autumn 2021 and spring 2022 in Ny-& Aring;lesund (Svalbard). Total suspended particles were collected at 321-1112 m, covering both the marine boundary layer and the free troposphere, with Na-aer(+) ranging 23-850 ng m(-3) and CCHOaer 3.8-274 ng m(-3). The chemical composition of balloon-borne aerosol samples was compared with synchronized ground level measurements at the balloon's winch (Na-aer(+): 35-3710 ng m(-3); CCHOaer: 1.9-194 ng m(-3)), and at the Old Pier (Na-aer(+): 140-1470 ng m(-3); CCHOaer: 1.6-10.0 ng m(-3)), where freshly emitted SSA particles were sampled. Surface seawater from the Kongsfjorden was analyzed to evaluate the sea-air transfer of marine CCHO. Air mass histories, atmospheric mixing, and cloud conditions were evaluated for three selected cases to explain vertical concentration patterns. A strong correlation (R=0.78, p<0.001) between combined xylose (< 0.2-14.1 ng m(-3)) in CCHOaer and oxalate(aer) (< 1-67 ng m(-3)) across all altitudes, suggests either coproduction or a connection through atmospheric processing. These results provide a first comprehensive picture of how local primary sea-air transfer of marine combined carbohydrates, long-range transport, in-situ formation, and atmospheric processing together shape their distribution.

Abstract. A comprehensive observational data set of the vertical and horizontal distribution of aerosol particles and meteorological parameters is presented. Data were obtained from an Arctic field campaign conducted with three different measurement platforms deployed at distinct locations in Ny-Ålesund, Svalbard, during the transition period from spring to summer between 19 May 2024 and 8 June 2024. The uncrewed aerial system ALADINA was used for vertical and horizontal profiling in the lowermost 930 m above sea level, covering 38 measurement flights including 143 vertical profiles and 62 horizontal flight legs at different constant altitudes. The tethered balloon system BELUGA performed 90 vertical profiles up to a maximum altitude of 1.3 km above sea level. Together, the airborne platforms provide information on particle number concentrations, including ultrafine particles with diameters below 20 nm, size distribution from the nucleation to coarse mode, and meteorological parameters (i.e. temperature, humidity, pressure, wind direction, wind speed, and short-wave irradiance). In addition, a surface flux gradient system was applied for the calculation of turbulent fluxes of sensible heat and vertical motion of particles, covering a sampling time of about 214 h. An illustrative case study is shown for 3 June 2024, highlighting the spatial variability of aerosol particles, which is strongly influenced by the stability of the polar atmospheric boundary layer. The high‐resolution observations enable the study of processes of aerosol–cloud interactions, new particle formation and lead to a generally improved understanding of the spatial distribution of Arctic aerosols. The data of all three measurement platforms are publicly available on the world data centre PANGAEA as described in the data availability section.

Refractory black carbon (rBC) aerosol particles strongly influence Arctic atmospheric radiative transfer, making it essential to understand their microphysical properties and mixing state. However, in-situ investigations on microphysical properties and mixing state of rBC particles over the central Arctic marine boundary layer are scarce. To address this gap, we carried out a comprehensive investigation of rBC particles in the central Arctic onboard the RV Polarstern during the ATWAICE cruise. Our results revealed pronounced spatial and temporal variability in microphysical properties of rBC in the Arctic marine boundary layer, governed by transport pathways and removal mechanisms. Under pristine background conditions, rBC mass concentrations were at their lowest (median similar to 0.4-0.6 ng m(-3)). The mass median diameter of rBC cores was found to increase with latitude, from the lowest value (similar to 156 nm) in lower-latitude regions influenced by higher anthropogenic emissions to similar to 220 nm in the high Arctic, consistent with the persistence of aged aerosols under background conditions. Warm airmass intrusions into the Arctic atmosphere were found to bring polluted anthropogenic aerosols into this pristine environment with an eightfold increase in rBC mass concentrations (median similar to 3.4 ng m(-3), rBC(max) similar to 74 ng m(-3)). A dominant influence of biomass-burning emissions from Eurasia during the warm airmass intrusion, which coincided with a shift toward larger rBC cores (similar to 264 nm) and moderate coating thickness. The light absorption enhancement of rBC estimated using core-shell Mie theory remained low during warm airmass intrusions (similar to 1-1.2) than under background conditions (similar to 1.1-1.6), underscoring a strong dependence of rBC radiative effects in the central Arctic on source regions and aging/processing during long-range transport. This study highlights the complexity of rBC aging and mixing state in the central Arctic, driven by variable source characteristics and summertime processing conditions and will help to increase the accuracy in representing rBC in climate models.

New particle formation (NPF) and subsequent growth are key processes controlling cloud condensation nuclei (CCN) number concentrations, as newly formed particles can grow into the CCN size range and thereby influence cloud properties and climate. In this study, we investigate particle number size distributions, CCN activity, and hygroscopicity during the Cloud–Aerosol Interactions in a Nitrogen Dominated Atmosphere (CAINA) campaign conducted in spring 2025 at a coastal site in the northern Netherlands, using a combination of a Scanning Mobility Particle Sizer (SMPS), a Particle Size Magnifier (PSM), and size-resolved CCN measurements. SMPS measurements covering the size range 6.7–969 nm were conducted between 29 March and 13 May 2025, while PSM measurements (1.19–12.0 nm) were available from 4 April to 9 May 2025. Based on visual classification of particle size distribution evolution, 19 NPF events were identified during the 46-day period (41%), 5 days were classified as undefined (11%), and the remaining 22 days as non-event days (48%). In addition, size-resolved CCN measurements were performed between 12 and 23 April 2025 to investigate in more detail the processes governing new particle formation and their growth towards CCN-relevant sizes. The measurements were carried out using a CCN counter operating at supersaturations (SS) of 0.3% and 1% downstream of a Differential Mobility Analyzer (DMA), covering particle diameters between 40 and 140 nm. The data were used to derive CCN activation fractions, characteristic activation diameters (D50), and the apparent hygroscopicity parameter kappa for the two different supersaturations. Our results show a clear size dependence of particle hygroscopicity, with particles activated at 0.3% SS generally exhibiting higher kappa values than particles activated at 1% SS. Average kappa values are around 0.1–0.2 for larger particles and 0.3–0.4 for smaller particles. A detailed case study of a NPF event shows a higher particle hygroscopocity before and during the start of the event, while the hygroscopicity decreases when the particles grow. These findings provide new insights into the link between NPF, particle chemical properties, and their ability to act as CCN.

Abstract. The Arctic is warming at a significantly faster rate than the global average, which is affecting local climate processes. Aerosol particles play a central role by influencing the energy balance directly through the scattering and absorption of solar radiation and indirectly by acting as cloud condensation nuclei. However, the processes that control aerosol concentrations, such as the mechanisms governing vertical particle exchange, are not well understood, particularly over sea ice. During the ARTofMELT campaign in spring 2023, five weeks of continuous eddy covariance measurements of turbulent particle fluxes were conducted in the High Arctic to investigate the spatial and temporal variability of particle sources and sinks over three surface types. Overall, net particle deposition dominated, with median deposition fluxes of −0.02 × 106m-2 s-1 over closed ice. This confirms the role of ice surfaces as a particle sink under low to moderate turbulence. Under strong winds, net particle emission fluxes of up to 0.98 × 106m-2 s-1 were observed over closed ice surfaces, which are likely linked to processes involving blowing snow. A mixture of emission and deposition was observed over leads and open water surfaces. These observations highlight how surface heterogeneity and turbulence intensity can influence particle exchange in the High Arctic. As sea ice retreats and the extent of open water increases, local particle sources are expected to become more relevant to Arctic aerosol budgets and cloud processes. The results provide observational constraints on particle fluxes, helping to reduce related uncertainties in Arctic model simulations.

Hygroscopicity strongly influences aerosol properties and multiphase chemistry, which is essential in several atmospheric processes. Although CCN (cloud condensation nuclei) properties are commonly measured, sub-saturated hygroscopicity measurements remain rare. During the ACROSS campaign (Atmospheric ChemistRy Of the Suburban foreSt, conducted in Paris in summer 2022), particles' hygroscopic growth rates at 90 % relative humidity (RH) and chemical composition were measured at the sub-urban site using a Hygroscopicity Tandem Differential Mobility Analyser (HTDMA, scanning at 100, 150, 200, and 250 nm) and an Aerodyne High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS). Growth factor probability density functions (GF-PDFs) revealed two distinct modes, namely hydrophobic and hygroscopic, suggesting a combination of internal and external particle mixing, with the split at GF 1.2. The prevalence of the hygroscopic mode increased with particle size, with mean hygroscopicity (κ) values of 0.23 and 0.38 for 100 and 200 nm particles, respectively. Using the Zdanovskii–Stokes–Robinson (ZSR) mixing rule, the agreement between measured and chemically derived hygroscopicity was approximately 51% for 100 nm particles, which declined for 200 and 250 nm. These emphasise the large effect of external particle mixing and its influence on predicting hygroscopicity. The ZSR approach proves to be unreliable in predicting the wide growth distribution of externally mixed particles. In this measurement, 80 %–90 % of the particles were externally mixed and influenced by fresh emission, which affected the hygroscopicity prediction by a factor of 2. A cluster analysis based on backward trajectories and meteorological conditions gives valuable insights into the chemical composition and variations in the hygroscopicity of different air masses.

In the Arctic climate system, the onset of melting is a crucial point, and timing is still difficult to predict. Therefore, the expedition ARTofMELT was dedicated to exploring atmospheric conditions and processes that are involved in triggering the onset of melting.The helicopter borne sensor system HELIPOD was deployed in this expedition to measure the spatial variability of atmospheric dynamics, radiation, aerosols, trace gases and surface properties on a horizontal scale up to 40 km around the icebreaker ODEN. During the ARTofMELT23 expedition, the HELIPOD conducted 12 measurement flights in the FRAM strait around 80° North and the prime meridian between 9 May and 9 June 2023 with 26.5 hours in the air. The flights covered an area of about 20 NM around the location of the icebreaker ODEN and a vertical range from 50 m to 2700 m above sea level. The flight patterns were aligned parallel and perpendicular to dominating directions as the sea ice edge and the wind direction. In one case a cloud layer edge apparently structured the atmospheric situation. The flights covered pre-melt onset conditions, refreezing situations and the melt onset. Synoptic air mass changes were probed as well. The presentation gives an overview of the temporal changes of the ambient conditions during the research flights, and a first assessment of the flights during transient weather situations.

Understanding aerosol particles in the Arctic is crucial due to their impact on the region’s radiative balance and their role in modifying cloud properties. These interactions drive unique feedback mechanisms that enhance Arctic warming and influence global climate systems. Consequently, it is important to identify and quantify Arctic aerosol particle sources and sinks, including their vertical transport, and to characterize their optical properties and resulting effects on cloud formation. Despite the importance of aerosol particles in the Arctic, there is a lack of direct measurements of aerosol particles over the Arctic especially over the Arctic marine boundary layer. In this context, we have conducted aerosol measurements aboard the German research vessel Polarstern during the ATWAICE (Atlantic Water Pathways to the Ice in the Nansen Basin and Fram Strait) expedition from June to August 2022. This study included continuous measurements of physical and chemical aerosol parameters to investigate variations in aerosol properties. On-line measurements of black carbon (BC) and its mixing state were complemented by off-line analyses of seawater and fog water samples to identify transport pathways of BC particles. Additionally, seawater, aerosol filter samples, and fog water samples were analyzed to explore how ice nucleating particles are linked across these compartments. Vertical profiles of aerosol particles were measured above different surface conditions to examine the direction of vertical particle transport. Higher aerosol concentrations were recorded as the ship passed through the outer margin of the marginal ice zone, where marine sources dominate, supported by evidence of significant photochemical ageing processes. The highest values of refractory black carbon (rBC) and light scattering coefficients were measured during the transact from northern Europe to the Arctic circle (between 56°N to 70°N), with average rBC concentrations of approximately 40 ng m-3 and light scattering at 525 nm averaging ~29 Mm-1. During this period, air mass trajectories reflected a nearly equal influence from both continental and marine sources. In contrast, the lowest scattering and absorption values were observed in the central Arctic, when the ship navigated in densely packed ice regions under the influence of north-easterly air masses originating over the Arctic Ocean. A comprehensive analysis of these findings will be presented in this presentation.

Few studies investigated residential particle concentration levels with a full picture of aerosol particles from 10 nm to 10 µm size range with size-resolved information, and none was performed in central Europe in the long-term in multiple homes. To capture representative diurnal and seasonal patterns of exposure to particles, and investigate the driving factors to their variations, measurements were performed in 40 homes for around two weeks each in Leipzig and Berlin, Germany. These over 500 days’ measurements combined PM10 and PM2.5 mass concentrations, particle number concentration and size distribution (PNC and PNSD, 10–800 nm), CO2 concentration, and residential activities diary into a unique dataset. Natural ventilation was dominated, the mean ventilation rate calculated from CO2 measurements was 0.2 h–1 and 3.7 h–1 with closed and opened windows, respectively. The main findings of this study showed that, the residents in German homes were exposed to a significantly higher mass concentration of coarse particles than outdoors, thus indoor exposure to coarse particles cannot be described by outdoors. The median indoor PNC diurnal cycles were generally lower than outdoors (median I/O ratio 0.69). However, indoor exposure to particles was different in the cold and warm season. In the warm season, due to longer opening window periods, indoor sources’ contribution was weakened, which also resulted in the indoor PNC and PNSD being very similar to the outdoors. In the cold season, indoor sources caused strong peaks of indoor PNC that exceeded outdoors, along with the relatively low penetration factor - 0.5 for all size ranges, and indoor particle losses, which was particularly effective in reducing the ultrafine PNC, resulting in a different particle exposure load than outdoors. This study provides a detailed understanding of residential particle exposure in multiple homes, facilitating future studies to assess health effects in residential environments.

Abstract. The global warming is amplified in the Arctic. To collect data that help to constrain weather and climate models, which often do not realistically represent the enhanced Arctic warming, the HALO-(AC)³ aircraft campaign was conducted in March and April 2022 over the Norwegian and Greenland Seas, the Fram Strait, and the central Arctic Ocean. Observations were made over areas of open ocean, the marginal sea ice zone, and the central Arctic sea ice. Two low-flying and one long-range, high-altitude research aircraft have been employed. Whenever possible, the three aircraft were flown in collocated formation. The campaign focused on one specific challenge posed by the models: The reasonable representation of transformations of air masses during their meridional transport into (northward by moist and warm air intrusions, WAIs) and out of (southward via marine cold air outbreaks, CAOs) the Arctic. To observe the air mass transformations, a quasi-Lagrangian flight strategy using trajectory calculations was realized enabling to sample the moving air mass parcels twice along their trajectories. Eight distinct WAI and 12 CAO cases were probed extensively. From the quasi-Lagrangian measurements, we have derived the diabatic heating and moistening of the moving air masses during CAOs and WAIs, the development of cloud macrophysical and microphysical properties along the southward pathways of the air masses during CAOs, and the moisture budget of WAIs. As an example result, we have obtained typical values of the surface-driven diabatic heating between 1–3 K h-1 and of the near-surface moistening between 0.05–0.3 g kg-1 h-1 within the lowest about 0.5 km. From the observations of WAIs, a weak diabatic cooling of up to 0.4 K h-1 and a moisture loss of up to 0.1 g kg-1 h-1 from the ground to about 5 km altitude were derived. In addition, we discuss the frequency of occurrence of the different thermodynamic phases of Arctic low-level clouds, the interaction of Arctic cirrus with sea ice, water vapor, and aerosol particles, and the characteristic microphysical and chemical properties of Arctic aerosol particles. Finally, we provide proof of a concept to measure mesoscale divergence and subsidence in the Arctic using data from dropsondes released during circular flight patterns.

Global warming is amplified in the Arctic. However, numerical models struggle to represent key processes that determine Arctic weather and climate. To collect data that help to constrain the models, the HALO–(𝒜𝒞)3 aircraft campaign was conducted over the Norwegian and Greenland seas, the Fram Strait, and the central Arctic Ocean in March and April 2022. The campaign focused on one specific challenge posed by the models, namely the reasonable representation of transformations of air masses during their meridional transport into and out of the Arctic via northward moist- and warm-air intrusions (WAIs) and southward marine cold-air outbreaks (CAOs). Observations were made over areas of open ocean, the marginal sea ice zone, and the central Arctic sea ice. Two low-flying and one long-range, high-altitude research aircraft were flown in colocated formation whenever possible. To follow the air mass transformations, a quasi-Lagrangian flight strategy using trajectory calculations was realized, enabling us to sample the same moving-air parcels twice along their trajectories. Seven distinct WAI and 12 CAO cases were probed. From the quasi-Lagrangian measurements, we have quantified the diabatic heating/cooling and moistening/drying of the transported air masses. During CAOs, maximum values of 3 K h−1 warming and 0.3 g kg−1 h−1 moistening were obtained below 1 km altitude. From the observations of WAIs, diabatic cooling rates of up to 0.4 K h−1 and a moisture loss of up to 0.1 g kg−1 h−1 from the ground to about 5.5 km altitude were derived. Furthermore, the development of cloud macrophysical (cloud-top height and horizontal cloud cover) and microphysical (liquid water path, precipitation, and ice index) properties along the southward pathways of the air masses were documented during CAOs, and the moisture budget during a specific WAI event was estimated. In addition, we discuss the statistical frequency of occurrence of the different thermodynamic phases of Arctic low-level clouds, the interaction of Arctic cirrus clouds with sea ice and water vapor, and the characteristics of microphysical and chemical properties of Arctic aerosol particles. Finally, we provide a proof of concept to measure mesoscale divergence and subsidence in the Arctic using data from dropsondes released during the flights.

Exposure to black carbon (BC) in the residential environment was found to be positively associated with elevated blood pressure and cardiovascular disease. However, BC has been under-measured and under-studied compared to other common indoor gaseous and particulate pollutants. Representative indoor mass concentrations of equivalent black carbon (eBC) and the sources’ contribution from indoors and outdoors in real-life residential environments in 40 German households were evaluated and presented in this work. During the 500 measurement days, the mean indoor eBC mass concentration was 0.6 µg m−3, which is less than half of the outdoor concentration in the urban background in Germany. However, common indoor sources contributed differently to indoor eBC, which also resulted in higher mass concentrations in the cold season than in the warm season. Indoor pollutant measurements are often performed with only a limited number of instruments and pollutant data. To fill in the missing knowledge of indoor BC, a proxy model was developed. This proxy model can predict indoor eBC concentrations based on existing indoor databases or in cases where direct measurements of indoor eBC concentrations are not available. Due to the complex influence of climate and indoor activities, the model separated six scenarios for weather (including warm and cold seasons) and indoor activities (burning, non-burning, and other activities) for typical urban residential environments in Germany. In this study, indoor eBC mass concentrations were found to be best estimated by indoor and outdoor PM1. For different scenarios, the model achieved a satisfactory to good coefficient of determination (0.49 < R 2 < 0.77). With the aid of this model, a more accurate prediction of indoor eBC mass concentration and the resulting exposure and health risk assessment can be achieved for households under similar climatic conditions and activity habits of the occupants, e.g., in Central Europe.

Low-level clouds in the Arctic affect the surface energy budget and vertical transport of heat and moisture. The limited availability of cloud-droplet-forming aerosol particles strongly impacts cloud properties and lifetime. Vertical particle distributions are required to study aerosol–cloud interaction over sea ice comprehensively. This article presents vertically resolved measurements of aerosol particle number concentrations and sizes using tethered balloons. The data were collected during the Multidisciplinary drifting Observatory for the Study of Arctic Climate expedition in the summer of 2020. Thirty-four profiles of aerosol particle number concentration were observed in 2 particle size ranges: 12–150 nm (N12−150) and above 150 nm (N>150). Concurrent balloon-borne meteorological measurements provided context for the continuous profiles through the cloudy atmospheric boundary layer. Radiosoundings, cloud remote sensing data, and 5-day back trajectories supplemented the analysis. The majority of aerosol profiles showed more particles above the lowest temperature inversion, on average, double the number concentration compared to below. Increased N12−150 up to 3,000 cm−3 were observed in the free troposphere above low-level clouds related to secondary particle formation. Long-range transport of pollution increased N>150 to 310 cm−3 in a warm, moist air mass. Droplet activation inside clouds caused reductions of N>150 by up to 100%, while the decrease in N12−150 was less than 50%. When low-level clouds were thermodynamically coupled with the surface, profiles showed 5 times higher values of N12−150 in the free troposphere than below the cloud-capping temperature inversion. Enhanced N12−150 and N>150 interacting with clouds were advected above the lowest inversion from beyond the sea ice edge when clouds were decoupled from the surface. Vertically discontinuous aerosol profiles below decoupled clouds suggest that particles emitted at the surface are not transported to clouds in these conditions. It is concluded that the cloud-surface coupling state and free tropospheric particle abundance are crucial when assessing the aerosol budget for Arctic low-level clouds over sea ice.