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Black carbon (BC), the most efficient atmospheric aerosol for absorbing light in the visible spectrum, exerts a warming effect on a region undergoing unprecedented climatic changes. Here, BC is studied indirectly using filter-based methods to ascertain aerosol light absorption parameters. We investigated long-term changes using a harmonised 21-year data set of light absorption measurements, in conjunction with air mass source analysis. The measurements were performed at Zeppelin Observatory (ZEP), Svalbard, from 2002 to 2022. We report a statistically significant (s.s.) decreasing long-term trend for the light absorption coefficient, measured at the site for the entirety of the data set. However, the last 7 years, 2016–2022, showed a slightly increasing s.s. trend for the haze season. In addition, we observed an increasing trend in the single scattering albedo from 2002 to 2022. Five distinct source regions were identified; the trends involving air masses from the five regions showed decreasing absorption coefficients, except for the air masses influenced by emissions from Eurasia. We show that the changes in the occurrences of each transport pathway cannot explain the reductions in the absorption coefficient observed at the Zeppelin station; an increase in contributions of air masses from more marine regions, with lower absorption coefficients, is compensated by the influence from high-emission regions. Along with aerosol optical properties, we also show an increasing trend in accumulated surface precipitation experienced by air masses en route to the Zeppelin Observatory. We argue that rainfall, as a sink of aerosol, plays a role in the long-term trends in the absorption coefficient, explaining approximately a quarter of the overall trend. A decreasing trend in the scavenging ratio further suggests an increase in the aerosol removal processes. We note that there is an increasing potential influence from active forest fires, particularly in the last few summers (i.e. 2015–2022). Active fires have been shown to have a significant impact on the mean seasonal absorption coefficient especially during northern hemispheric summer. However, no noticeable alteration in annual long-term trends can be observed. [ABSTRACT FROM AUTHOR]

In this study we introduce an in-cloud wet deposition scheme for liquid and ice phase clouds for global aerosol–climate models which use a size-segregated aerosol description. For in-cloud nucleation scavenging, the scheme uses cloud droplet activation and ice nucleation rates obtained from the host model. For in-cloud impaction scavenging, we used a method where the removal rate depends on the wet aerosol size and cloud droplet radii. We used the latest release version of ECHAM-HAMMOZ (ECHAM6.3-HAM2.3-MOZ1.0) with the Sectional Aerosol module for Large Scale Applications version 2.0 (SALSA) microphysics package to test and compare our scheme. The scheme was compared to a scheme that uses fixed scavenging coefficients. The comparison included vertical profiles and mass and number distributions of wet deposition fluxes of different aerosol compounds and for different latitude bands. Using the scheme presented here, mass concentrations for black carbon, organic carbon, sulfate, and the number concentration of particles with diameters larger than 100 nm are higher than using fixed scavenging coefficients, with the largest differences in the vertical profiles in the Arctic. On the other hand, the number concentrations of particles smaller than 100 nm in diameter show a decrease, especially in the Arctic region. These results could indicate that, compared to fixed scavenging coefficients, nucleation scavenging is less efficient, resulting in an increase in the number concentration of particles larger than 100 nm. In addition, changes in rates of impaction scavenging and new particle formation (NPF) can be the main cause of reduction in the number concentrations of particles smaller than 100 nm. Without further adjustments in the host model, our wet deposition scheme produced unrealistically high aerosol concentrations, especially at high altitudes. This also leads to a spuriously long lifetime of black carbon aerosol. To find a better setup for simulating aerosol vertical profiles and transport, sensitivity simulations were conducted where aerosol emission distribution and hygroscopicity were altered. Vertical profiles of aerosol species simulated with the scheme which uses fixed scavenging rates and the abovementioned sensitivity simulations were evaluated against vertical profiles from aircraft observations. The lifetimes of different aerosol compounds were also evaluated against the ensemble mean of models involved in the Aerosol Comparisons between Observations and Models (AEROCOM) project. The best comparison between the observations and the model was achieved with our wet deposition scheme when black carbon was emitted internally mixed with soluble compounds instead of keeping it externally mixed. This also produced atmospheric lifetimes for the other species which were comparable to the AEROCOM model means. [ABSTRACT FROM AUTHOR]