CHEMICAL COMPOSITION AND IMMERSION FREEZING ACTIVITIES OF AEROSOL PARTICLES IN THE EUROPEAN ARCTIC

The Arctic region is sensitive to climate change, and aerosols play an important but poorly constrained role in climate in this region. A subset of aerosols that act as ice-nucleating particles (INPs) can impact cloud radiative properties and precipitation, consequently impacting weather, and climate in the Arctic region. However, our understanding of the role of chemical composition on ice formations in the Arctic region is still limited. This study reports the chemical composition of ambient aerosol particles and the INP concentration measured offline for the particles collected on polycarbonate filters at the Gruvebadet observatory in Ny-Ålesund, Svalbard islands, from October 2021 to February 2022. More than 6000 individual particles were analyzed using Computer-Controlled Scanning Electron Microscopy with Energy-Dispersive X-ray (CCSEM-EDX) to get the chemical composition. High-resolution Transmission Electron Microscopy (TEM) and Scanning transmission X-ray microscopy with near-edge X-ray absorption fine structure spectroscopy (STXM-NEXAFS) were used to investigate the coating of the particles. To calculate the atmospheric INP concentration (nINP) and assess the immersion freezing as well as the heat sensitivity of INPs, an offline droplet-freezing instrument West Texas Cryogenic Refrigerator Applied to Freezing Test system (WT-CRAFT) was used. The CCSEM-EDX analysis of ambient particles indicates a variation in size-resolved chemical composition among the samples. The major classes of particles identified include Na-rich, Sulfate, and Carbonaceous. The immersion freezing assay for some particulate samples before and after heating (for 20-min at ≤ 100 °C) shows a substantial decrease in IN activity at ≥ -20 °C, suggesting the presence of a biogenic contribution in the particles. Combining chemical composition and IN results of the same samples, we observed that the heat-sensitive IN population is richer in Na-rich particles. From the results of TEM and STXM-NEXAFS, those particles appear to be coated with organic and possibly originated from the sea surface microbiological activity. We further study the ageing of salt particles by investigating chlorine depletion. This study will help improve the understanding of ice cloud formation over the Arctic.