Recent studies show that warm and moist air intrusions are major sources of aerosol particles in the Arctic, affecting local radiative impacts by supplying Cloud Condensation Nuclei (CCN). However, their influence on aerosol size modes, CCN, and cloud droplet number concentrations remains poorly constrained. Here, we use long-term aerosol observations from five Arctic observatories to quantify intrusion impacts. We find that intrusions strongly perturb Arctic CCN, especially in summer, when accumulation-mode and CCN concentrations increase markedly at all sites. In winter and spring, two regimes emerge: intrusions reduce number concentrations at sites near 0° longitude (Zeppelin, Villum, Alert) but enhance them near 180° (Tiksi, Utqiaġvik/Barrow), consistent with competing effects of pollution sources and wet scavenging along trajectories. Intrusions also systematically modify cloud droplet number concentration (Nd): Nd increases at all sites in summer, while in winter it increases at Tiksi and Utqiaġvik/Barrow but decreases at Zeppelin, Villum, and Alert. Overall, intrusions are a key regulator of Arctic aerosol and cloud properties and an important component of the evolving Arctic climate system.Beyond aerosol–cloud number effects, it is unclear how intrusion events modulate cloud optical depth, liquid water content, and precipitation across Arctic sites and seasons. To address this, we will combine cloud and precipitation observations from the year-long Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition with reanalysis data to quantify systematic intrusion-driven changes in liquid water content and precipitation occurrence. Finally, we will use the non-hydrostatic mesoscale Weather Research and Forecasting (WRF) model to examine the sensitivity of mixed-phase cloud lifetime and associated precipitation to intrusion occurrence, providing process-level constraints on how intrusions shape Arctic mixed-phase cloud persistence and hydrometeor production.

The formation of cloud condensation nuclei is a critical but uncertain factor in Arctic climate dynamics. A major nuclei contributor is new particle formation, yet the geographical variations in activity and the factors driving it remain poorly understood. Here, we present a nine-year (2010–2018) analysis of atmospheric particle number size distributions from Tiksi, Russia, integrated with air mass trajectory modelling and ocean remote sensing. We show that aerosol formation rates are significantly enhanced—particle formation rates increase by 300

Abstract. Black carbon (BC) aerosol particles strongly absorb solar radiation and heat the atmosphere. BC aerosols also deposit on snow and ice, lowering the surface albedo and accelerate heating of the Arctic. Because these BC radiative effects are size-dependent, an improved understanding of BC size distributions is indispensable for radiative transfer modelling to estimate the aerosol climate effects. We measured BC size distributions at Pallas, northern Finland, for the first time throughout the whole year, to fully capture its seasonal variability connected with the BC atmospheric processing during transport. The shape of the size distribution was very stable, with little seasonal variation. Comparison to previous seasonal observations at Ny-Ålesund in Svalbard, Norway, and Alert in Canada confirmed very similar size distribution shapes at all three sites, suggesting minor spatial variability. Strong temporal variations were observed in the total mass concentration of BC, but not in the shape of the BC core size distributions. The results were additionally used for validation of the state-of-the-art global climate model CAM5-ATRAS monthly BC size at Pallas. Overall, our observational results provide useful constraints for estimating the effects of BC on climate by model simulations, especially in the Arctic, where the measurements were conducted.

Understanding new particle formation (NPF) and the fate of nanoparticles is crucial because of their close links to air quality, cloud formation, and climate. These effects vary spatially and temporally owing to diverse aerosol sources and their relatively short atmospheric lifetime. Here, we present a comprehensive analysis of long-term trends in NPF-associated nucleation-mode particles and cloud condensation nuclei (CCN) concentrations across diverse observation environments using quality-controlled particle number size distribution (PNSD) and CCN data from 37 sites, primarily from Global Atmosphere Watch (GAW) stations. We identify declining decadal trends in both NPF occurrences and nucleated particle concentrations across most site types, with the strongest declines in urban areas. We observe simultaneous reductions in both CCN concentrations and nucleation-mode particles, suggesting that newly formed particles are a potential source of CCN. This, in turn, suggests that cloud microphysical properties and radiative effects can be indirectly influenced through aerosol-cloud interactions that modify cloud droplet formation. These findings indicate that decreasing anthropogenic emissions could influence the climate forcing potential of aerosol-cloud interactions, with important implications for future climate projections.

Particle linear depolarization ratio is a widely used parameter in lidar research to distinguish different aerosol types and the thermodynamic phase of water. It is most frequently measured at ultraviolet and visible wavelengths (355 and 532 nm), yet multi-wavelength observations suggest that this parameter can vary substantially with wavelength. In this work, we assessed particle linear depolarization ratios at 1565 nm using Halo Photonics StreamLine Doppler lidars. We examined the depolarization ratio through three case studies featuring extremely fresh and aged smoke, and volcanic ash aerosol particles in the troposphere. Both fresh and aged smoke aerosol particles induced low values. Specifically, aerosol layers dominated by extremely fresh smoke showed a depolarization ratio of 0.017 ± 0.004, whereas aged long-range transported smoke particles exhibited marginally higher values. Volcanic aerosol layers induced high depolarization ratios with layer mean values of 0.45 ± 0.01. For the extremely fresh smoke case, we further estimated the smoke mass concentration using the lidar observations at 1565 nm and found good agreement with the in situ observations. These results demonstrate that Halo Doppler lidars operating at 1565 nm wavelength are capable of distinguishing several key aerosol types, enabling a comprehensive characterization of atmospheric conditions by simultaneously observing aerosol properties and wind dynamics.

We compared aerosol light absorption measurements by a photothermal interferometer (PTI) and by extinction minus scattering (EMS) in a laboratory setting using test aerosols with single scattering albedo (SSA) in the range from about 0.1 to 1. The EMS method reported higher light absorption coefficients, b abs, by 25% up to a factor of 2 compared to PTI, with the deviation increasing when coating fresh soot with secondary organic matter. In a second step, the attenuation measured by numerous filter-based absorption photometers and in-situ-measuring instruments was calibrated against PTI at the wavelengths of 450 and 808 nm to determine reliable calibration factors for each instrument. The attenuation coefficient measured by the filter-based instruments, b atn, was larger by a factor of 4 - 9 than the reference b abs measurements of PTI depending on the instrument model and filter tape. We determined similar multiple-scattering correction factors C for the aethalometer AE33 to those reported by recent studies using PTI as a reference method, but almost two times higher than those reported by older studies referenced to the Multi-Angle Absorption Photometer (MAAP). In-situ-measuring instruments showed a reasonable agreement with PTI despite the low aerosol absorption coefficients which were close to the limit of detection for some of the photoacoustic instruments. In view of the revised Air Quality Directive of the European Union, our study highlights the need for traceable aerosol light absorption measurements to harmonize calibration procedures across Europe.

We present a DNA extraction protocol for atmospheric bioaerosol samples collected on glass-fiber filters widely used in air quality monitoring. The protocol produces high-quality molecules suitable for third-generation sequencing and other applications. The initial protocol was developed and applied in a Bioaerosol campaign performed in Finland and Lithuania in 2021 using low-volume air samplers, which posed stringent requirements to the method sensitivity. The protocol included a phenol-chloroform step for DNA purification, thus involving aggressive reagents; it was also quite time consuming and laborious. The present study advances this protocol to exclude the use of hazardous chemicals by using the SPRI paramagnetic bead technology for DNA purification and compares it to several commercial extraction methods. Despite trailing in efficiency to the initial method, the new development proved to be more efficient than several column-based commercial kits. The updated protocol was effective for a relatively high mass ratio of biological material to filter material: 70 nanograms of potential DNA on the filter to one milligram of filter fiber, as detected with the initial phenol-chloroform-based method. However, the new approach was not effective for a mass ratio lower than 15 nanograms of potential DNA per milligram of the filter material. The applicability of the new protocol for preparation of samples for the 3rd generation sequencing was confirmed by subsequent processing of the samples with the Oxford Nanopore (ONT) GridION sequencer.

Abstract. The absorption coefficient of light absorbing aerosols is difficult to measure with low uncertainty and improvements of measurement procedures based on traceable calibration are needed. Reducing measurement artefacts can be achieved by using in-situ direct measurement methods such as photo-acoustic spectroscopy and photo-thermal interferometry. We developed a traceable calibration method based on monodisperse nigrosin particles. Nebulized nigrosin forms nearly spherical particles for which it is possible to calculate the absorption coefficient in the UV-NIR range using Mie theory. In the presented study, we have experimentally tested size- and mass-selection techniques using a differential mobility analyzer (DMA), a centrifugal particle mass analyzer (CPMA) and a tandem of both in series to compare Mie-calculated absorption coefficient with the measured one using the photo-thermal aerosol absorption monitor PTAAM, traceably calibrated with NO2. We observed that the nigrosin particle density changes with particle size. Because the absorption coefficient depends mainly on particle mass it is preferrable to base the Mie calculation on the measured particle mass instead of the mobility diameter. Calculated versus measured absorption coefficients differed by +5 % to +11 % for the DMA, -2 % to -3 % for the CPMA and +2 % to +8 % for tandem of the CPMA and the DMA. Combined standard uncertainties (coverage factor k=1) for PTAAM calibrated with monodisperse nigrosin particles selected by the DMA, the CPMA, and the CPMA and the DMA tandem are 6.9 %, 5.8 % and 5.2 %, respectively. The optimal classification setup is a tandem of the CPMA and the DMA which avoids the systematic bias of both neutral (in the CPMA) and multiply-charged (in the DMA) particles and provides a high enough absorption signal. Experimentally the simplest selection method is based on the CPMA.

Central Asia is a region with high-altitude mountains, hosting numerous glaciers and widespread seasonal snow cover, both of which play an integral role in the regional hydrological cycle. Despite their importance, observations of cryospheric and atmospheric variables are scarce in this area but are essential to assess the temporal and spatial changes induced by climate change. To address this gap, we present a diverse data set of cryospheric and atmospheric variables from the Zarafshon River Basin and the Hydrographic Party Glacier (GGP) in Tajikistan, spanning 2018–2025. The dataset includes glacier terminus positions and snow conditions, glacier ablation, high resolution aerial photography, meteorological variables, surface reflectance combined with snow chemistry, and atmospheric aerosol concentrations. These observations provide a valuable basis for research on glacier dynamics, snow processes, and atmosphere-cryosphere interactions in a region where monitoring has been sparse. The data are available at the Finnish Meteorological Institute data repository METIS: https://doi.org/10.57707/fmi-b2share.t4vwg-gf542 (Svensson et al., 2026).

This article presents the results of long-term air quality measurements (2015–2024) from the Helsinki Traffic Supersite, including gaseous compounds (NO, NO2, CO, CO2, O3), particulate matter (PM2.5, PM10, particle number (PN) and size distribution), chemical composition of PM (black carbon (BC), organic aerosol (OA), inorganic species), lung-deposited surface area (LDSA), polycyclic aromatic hydrocarbons, volatile organic compounds (VOC), as well as road surface conditions and meteorology. In addition to the long-term observations, large number of targeted short-term measurement campaigns were conducted to investigate emerging phenomena, to further develop supersite measurements and gain new information about sources impacting air quality.The observed air quality improvements at the Traffic Supersite were driven by declining traffic exhaust concentrations (NOx (-8.0%/yr), BC (-7.1%/yr), PN (-3.7%/yr), and anthropogenic VOCs (-3.1 to -8.9%/yr)). The observed emission factors (g/kgfuel) also decreased for NOx (-7.6%/yr), BC (-7.5%/yr), and PN (-4.1%/yr), reflecting fleet renewal. The observed rate of decrease in PN concentration was lower than that of other parameters, particularly in the smallest size classes (< 30 nm). Organic aerosol analysis showed that traffic hydrocarbon tracers (m/z 57) declined faster (-7.9%/yr) than total OA and oxidized OA (m/z 44), which are linked to secondary formation and long-range transport. Long-term results indicate that compliance with the upcoming EU Air Quality Directive limit values for 2030 is already largely achievable, with the most challenging aspect being the exceedances of the PM10 daily limit due to road dust events. However, achieving the EU Zero Pollution target for 2050 remains challenging, and the WHO guideline values for PM10, PM2.5 and NO2 continue to be exceeded. These unique findings highlight the importance of comprehensive, long-term supersite measurements for understanding pollutant sources and trends, and for supporting urban planning and policy development.

Black carbon (BC) aerosol particles are emitted by the incomplete combustion of carbonaceous fuels. These particles absorb solar radiation and BC-dominated aerosol mixtures with low single scattering albedo have a positive radiative forcing, thus heating the atmosphere. Radiative transfer models make use of the BC mass absorption cross section (MACBC) to derive the radiative forcing of BC given a certain particle mass concentration. Freshly emitted BC has a MAC value of 8 ± 1 m2/g at 550 nm (Bond et al., 2013). However, MAC can increase as aerosols age in the atmosphere due to increase in particle coating. This is the so-called lensing effect, which leads to MACBC observations of up to 15 m2/g at 550 nm (Li et al., 2022; Savadkoohi et al., 2024). The effect of coatings and the evolution of MACBC with ageing have been and still are a matter of intense scientific discussions.The determination of MACBC is carried out in the lab and in the field using various methods for light absorption and BC mass measurement. The most common techniques for absorption measurement include filter-based attenuation measurements, whereas the most common technique for mass measurement is thermo-optical analysis, which quantifies elemental carbon mass (EC; EN 16909:2017). The development of more accurate techniques with operational and scientific advantages for both light absorption and BC mass quantification has led to more reliable MACBC field measurements, allowing researchers to have a clearer picture of how atmospheric ageing and regional conditions affect the optical properties of BC.In this study, we have reviewed 63 publications that provide atmospheric MACBC values and present the results in terms of aerosol type, measurement technique, regional variability, and how interpretation of results using these factors can help the community to use the appropriate MAC in models. We provide guidance and perspectives for future studies and how the literature on MACBC can be exploited and interpreted in order to improve radiative models that include BC.References Bond, T. C., Doherty, S. J., Fahey, D. W., Forster, P. M., Berntsen, T., DeAngelo, B. J., Flanner, M. G., Ghan, S., Kärcher, B., Koch, D., Kinne, S., Kondo, Y., Quinn, P. K., Sarofim, M. C., Schultz, M. G., Schulz, M., Venkataraman, C., Zhang, H., Zhang, S., … Zender, C. S. (2013). Bounding the role of black carbon in the climate system: A scientific assessment. Journal of Geophysical Research: Atmospheres, 118(11), 5380–5552. https://doi.org/10.1002/jgrd.50171Hanyang Li & Andrew A. May (2022) Estimating mass-absorption cross-section of ambient black carbon aerosols: Theoretical, empirical, and machine learning models, Aerosol Science and Technology, 56:11, 980-997, https://doi.org/10.1080/02786826.2022.2114311Savadkoohi, Marjan, Marco Pandolfi, Cristina Reche, Jarkko V. Niemi, Dennis Mooibroek, Gloria Titos, David C. Green, et al. (2023) The Variability of Mass Concentrations and Source Apportionment Analysis of Equivalent Black Carbon across Urban Europe. Environment International 178: 108081. https://doi.org/10.1016/j.envint.2023.108081.

An accurate assessment of black carbon (BC) climate and health impacts requires knowledge of its mass absorption cross-section (MACBC) – a parameter linking optical and mass measurements. The mean MACBC for freshly emitted soot typically spans a narrow range of 8 ± 1 m2 g⁻1 at 550 nm1,2 but is modified by subsequent atmospheric aging. Determination of MACBC requires simultaneous measurements of aerosol light-absorption coefficient (βabs) and BC mass. Here, we compile 230 measured MACBC values from 80 atmospheric studies and explore the effects of sampling location, study duration, instrumentation, and measurement wavelength. The compiled data set shows a broad variability in MACBC values (a factor of about 200%). We conclude that this variability is attributable to a combination of the above-mentioned effects with additional instrumental uncertainties (e.g., cross-sensitivities and/or inadequate instrument calibration). The current state of knowledge does not support the use of simplistic generalizations or assumptions about MACBC in the atmosphere, motivating a recommendation to further improve and standardize measurement practices.

Primary biological aerosol particles (PBAPs), including fungal spores, bacteria and pollen grains, are widely distributed in the atmosphere. Some PBAPs are highly efficient ice nucleating particles (INPs), but their impact on atmospheric ice formation is currently uncertain. PBAPs have been associated with INPs that are active at high sub-zero temperatures and may contribute disproportionally high in places with little anthropogenic influence, such as the high Arctic [1] and in the boreal forest [2].This study investigates PBAPs and INPs in the pristine Finnish sub-Arctic at the Pallas supersite from September 2022 to September 2023. To study PBAPs, we combine measurements of highly fluorescent aerosol particles (HFAPs) with the Wideband Integrated Bioaerosol Sensor (WIBS) [3], fungal spore counts from a Hirst-type volumetric sampler and eDNA sequence analysis from filter samples. We compare PBAPs to INP measurements over a wide temperature range using the Portable Ice Nucleation Experiment (PINE) [4] and the Ice Nucleation Spectrometer of the Karlsruhe Institute of Technology (INSEKT) [2].We found a strong seasonal trend of a subset of HFAPs with maximum concentrations in summer and an abrupt and strong decrease with snow cover. Together with an exponential relationship with temperature, this suggests locally emitted bioaerosols. The measured bioaerosols show a positive correlation with INPs active over a wide activation temperature range (-31°C - -8°C). An exceptionally high correlation (r=0.94, p

Gaseous ammonia, while influential in atmospheric processes, is critically underrepresented in atmospheric measurements. This limits our understanding of key climate-relevant processes, such as new particle formation, particularly in remote regions. Here, we present highly sensitive, online observations of gaseous ammonia from a coastal site in Antarctica, which allows us to constrain the mechanism of new particle formation in this region in unprecedented detail. Our observations show that penguin colonies are a large source of ammonia in coastal Antarctica, whereas ammonia originating from the Southern Ocean is, in comparison, negligible. In conjunction with sulfur compounds sourced from oceanic microbiology, ammonia initiates new particle formation and is an important source of cloud condensation nuclei. Dimethylamine, likely originating from penguin guano, also participates in the initial steps of particle formation, effectively boosting particle formation rates up to 10000 times. These findings emphasize the importance of ecosystem processes from penguin/bird colonies and oceanic phytoplankton/bacteria on climate-relevant aerosol processes in coastal Antarctica. This demonstrates an important connection between ecosystem and atmospheric processes that impact the Antarctic climate, which is crucial given the current rate of environmental changes in the region.

In order to reduce the uncertainty of aerosol radiative forcing in global climate models, we need to better understand natural aerosol sources which are important to constrain the current and pre-industrial climate. Here, we analyse particle number size distributions (PNSDs) collected during a year (2015) across four coastal and inland Antarctic research bases (Halley, Marambio, Dome C and King Sejong). We utilise k-means cluster analysis to separate the PNSD data into six main categories. “Nucleation” and “bursting” PNSDs occur 28 %–48 % of the time between sites, most commonly at the coastal sites of Marambio and King Sejong where air masses mostly come from the west and travel over extensive regions of sea ice, marginal ice and open ocean and likely arise from new particle formation. “Aitken high”, “Aitken low” and “bimodal” PNSDs occur 37 %–68 % of the time, most commonly at Dome C on the Antarctic Plateau, and likely arise from atmospheric transport and ageing from aerosol originating likely in both the coastal boundary layer and free troposphere. “Pristine” PNSDs with low aerosol concentrations occur 12 %–45 % of the time, most commonly at Halley, located at low altitudes and far from the coastal melting ice and influenced by air masses from the west. Not only the sea spray primary aerosols and gas to particle secondary aerosol sources, but also the different air masses impacting the research stations should be kept in mind when deliberating upon different aerosol precursor sources across research stations. We infer that both primary and secondary components from pelagic and sympagic regions strongly contribute to the annual seasonal cycle of Antarctic aerosols. Our simultaneous aerosol measurements stress the importance of the variation in atmospheric biogeochemistry across the Antarctic region.

Modeling indoor contaminant dispersion is crucial for exposure analysis in fields like occupant safety and infection prevention. Accurate predictions necessitate appropriate computational approaches because numerical solutions to turbulent indoor flow conditions are vulnerable to modeling errors. Large-eddy simulation (LES) is a turbulence-resolving approach with the potential to describe the relevant flow physics governing indoor contaminant dispersion. This work documents a quantitative validation study of the PALM LES model against experimental indoor dispersion measurements. The experiments were conducted in a controlled chamber with a mechanical ventilation system operated at two different ventilation flow rates (2 and 5 air changes per hour). The LES results were obtained using three different resolutions (1, 1.5, and 2 cm), labeled Fine, Medium, and Coarse. The evolution of particle concentration was monitored identically in the chamber and the LES model using a multipoint measurement network. The validation analysis assessed the performance of the PALM LES model in predicting aerosol dispersion using four validation metrics. The results indicated strong performance for the fine model under both ventilation rates. Validation performance declined with reduced resolution, and the coarse model demonstrated evidently lower accuracy due to deficiencies in capturing thermal stratification effects. Sensitivity analysis revealed that the validation results were largely unaffected by changes in thermal boundary conditions. This study highlights the importance of model resolution in predicting indoor contaminant dispersion and cautions against assuming predictive capacity in thermal modeling based on dispersion modeling results.

Primary biological aerosol particles (PBAPs) can influence weather and climate by acting as high-temperature ice nucleating particles (INPs), especially in clean, rural regions like the European sub-Arctic. However, the actual contribution to atmospheric ice nucleation and the exact identity of PBAPs serving as INPs remain poorly understood. Here, we present measurements of INPs and highly fluorescent aerosol particles (HFAPs) over the course of 1 year at the Pallas Atmosphere-Ecosystem Supersite in the Finnish sub-Arctic, aiming to determine whether PBAPs significantly contribute to atmospheric INPs and to identify which types do so. Our findings indicate that certain HFAPs are strongly influenced by meteorological variables, with high concentrations occurring when the station is within the atmospheric mixing layer, suggesting local biological sources. These HFAPs are the main contributors to high-temperature INPs, with an exceptionally strong correlation (r = 0.94, p < 0.0001) between HFAP concentrations and INPs active at -13.5 degrees C. For the first time, to the best of our knowledge, we combine INP and HFAP data with direct fungal spore counts and environmental DNA (eDNA) analysis to determine the biological origins of HFAPs and INPs. The results suggest that most high-temperature INPs are likely fungal spores. eDNA analysis further reveals that airborne fungi are dominated by Basidiomycota and that only a small fraction of the detected fungal genera have, to date, been tested for ice nucleation activity (INA) according to the literature. Among those reported in the literature, most exhibit very low or no INA. This underscores the significant knowledge gap in our understanding of biological ice nucleation in the atmosphere.