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: June 2023

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 1-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
particle (INP) mass during the sunlit part of the year. In addition, we
also find contributions of the inorganic acids nitric acid and sulfuric acid,
with outstanding high absolute signals of sulfuric acid in one cloud
residual sample in spring and one in late summer (21 May and 12 September 2020), probably caused by high anthropogenic sulfur emissions near the
Barents Sea and Kara Sea. During one particular cloud event, on 18 May 2020,
the air mass origin did not change before, during, or 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 was similar before, during, and
after the cloud, indicating that the particles had already undergone one or
several cycles of cloud processing before being measured as residuals at the
Zeppelin Observatory and/or that, on the timescales of the observed cloud event, cloud
processing of the organic fraction can be neglected. Meanwhile, there were on
average fewer particles but relatively more in the accumulation mode after
the cloud. Comparing the signals of sulfur-containing compounds of cloud
residuals with aerosols during cloud-free conditions, we find that sulfuric
acid had a higher relative contribution to the cloud residuals than 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, reflected
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.