Physical and Chemical Properties of Cloud Droplet Residuals and Aerosol Particles During the Arctic Ocean 2018 Expedition

This is the final version. Available on open access from Wiley via the DOI in this recordData Availability Statement: The data of the Arctic Ocean 2018 expedition is available at the data centre of the Bolin Centre for Climate Research (https://bolin.su.se/data/oden-ao-2018-expedition-1; Leck et al., 2021). The main data used within this study is found here: https://doi.org/10.17043/oden-ao-2018-aerosol-cvi-1 (Karlsson et al., 2022).Detailed knowledge of the physical and chemical properties and sources of particles that form clouds is especially important in pristine areas like the Arctic, where particle concentrations are often low and observations are sparse. Here, we present in situ cloud and aerosol measurements from the central Arctic Ocean in August-September 2018 combined with air parcel source analysis. We provide direct experimental evidence that Aitken mode particles (particles with diameters ≲70 nm) significantly contribute to cloud condensation nuclei (CCN) or cloud droplet residuals, especially after the freeze-up of the sea ice in the transition toward fall. These Aitken mode particles were associated with air that spent more time over the pack ice, while size distributions dominated by accumulation mode particles (particles with diameters ≳70 nm) showed a stronger contribution of oceanic air and slightly different source regions. This was accompanied by changes in the average chemical composition of the accumulation mode aerosol with an increased relative contribution of organic material toward fall. Addition of aerosol mass due to aqueous-phase chemistry during in-cloud processing was probably small over the pack ice given the fact that we observed very similar particle size distributions in both the whole-air and cloud droplet residual data. These aerosol-cloud interaction observations provide valuable insight into the origin and physical and chemical properties of CCN over the pristine central Arctic Ocean.Knut-and- Alice-Wallenberg FoundationBolin Centre for Climate ResearchSwedish Research CouncilSwiss National Science FoundationSwiss Polar InstituteBNP Paribas Swiss FoundationNatural Sciences and Engineering Research Council (NSERC) CanadaOcean Frontier InstituteCanada First Research Excellence FundNatural Environment Research Council (NERC)Canadian Transatlantic Ocean System Science & Technology programFerring PharmaceuticalsEuropean Union Horizon 2020European Research Council (ERC