Revealing the chemical characteristics of Arctic low-level cloud residuals – in situ observations from a mountain site
Authors: Gramlich Yvette , Siegel Karolina , Haslett Sophie L. , Freitas Gabriel , Krejci Radovan , Zieger Paul , Mohr Claudia 
Published: December 2022

Abstract. The role aerosol chemical composition plays in Arctic low-level cloud formation is still poorly understood. In this study we address this issue by combining in situ observations of the chemical characteristics of cloud residuals (dried liquid cloud droplets or ice crystals) and aerosol particles from the Zeppelin Observatory in Ny-Ålesund, Svalbard (approx. 480 m a. s. l.). These measurements were part of the one-year long Ny-Ålesund Aerosol and Cloud Experiment 2019–2020 (NASCENT). To obtain the chemical composition of cloud residuals at molecular level, we deployed a Filter Inlet for Gases and AEROsols coupled to a Chemical Ionization Mass Spectrometer (FIGAERO-CIMS) with iodide as the reagent ion behind a Ground-based Counterflow Virtual Impactor (GCVI). The station was enshrouded in clouds roughly 15 % of the time during NASCENT, out of which we analyzed 14 cloud events between December 2019 and December 2020. During the entire year, the composition of the cloud residuals shows contributions from oxygenated organic compounds, including organonitrates, and traces of the biomass burning tracer levoglucosan. In summer, methanesulfonic acid (MSA), an oxidation product of dimethyl sulfide (DMS), shows large contributions to the sampled mass, indicating marine natural sources of cloud condensation nuclei (CCN) and ice nucleating particles (INP) mass during the sunlit part of the year. In addition, we also find contributions of the inorganic acids nitric and sulfuric acid, with outstanding high absolute signals of sulfuric acid in one cloud residual sample in spring and one in late summer (May 21 and September 12, 2020), probably caused by high anthropogenic sulfur emissions near the Barents and Kara Sea. During one particular cloud event, on May 18, 2020, the air mass origin did not change from before to during and after the cloud. We therefore chose it as a case study to investigate cloud impact on aerosol physicochemical properties. We show that the overall chemical composition of the organic aerosol particles is similar before, during, and after the cloud, indicating that the particles have already undergone one or several cycles of cloud processing before being measured as residuals at Zeppelin, and/or that on the timescales of the observed cloud event, cloud processing of the organic fraction can be neglected. Meanwhile there are on average fewer particles, but relatively more in the accumulation mode after the cloud. Comparing the signal of sulfur-containing compounds of cloud residuals with aerosols during cloud-free conditions, we find that sulfuric acid has a higher relative contribution to the cloud residuals compared to aerosols during cloud-free conditions, but we did not observe an increase in particulate MSA due to the cloud. Overall, the chemical composition, especially of the organic fraction of the Arctic cloud residuals, reflects the overall composition of the general aerosol population well. Our results thus suggest that most aerosols can serve as seeds for low-level clouds in the Arctic.