The remote central Arctic during summertime has a pristine atmosphere with very low aerosol particle concentrations. As the region becomes increasingly ice-free during summer, enhanced ocean-atmosphere fluxes of aerosol particles and precursor gases may therefore have impacts on the climate. However, large knowledge gaps remain regarding the sources and physicochemical properties of aerosols in this region. Here, we present insights into the molecular composition of semi-volatile aerosol components collected in September 2018 during the MOCCHA (Microbiology-Ocean-Cloud-Coupling in the High Arctic) campaign as part of the Arctic Ocean 2018 expedition with the Swedish Icebreaker Oden. Analysis was performed offline in the laboratory using an iodide High Resolution Time-of-Flight Chemical Ionization Mass Spectrometer with a Filter Inlet for Gases and AEROsols (FIGAERO-HRToF-CIMS). Our analysis revealed significant signal from organic and sulfur-containing compounds, indicative of marine aerosol sources, with a wide range of carbon numbers and O : C ratios. Several of the sulfur-containing compounds are oxidation products of dimethyl sulfide (DMS), a gas released by phytoplankton and ice algae. Comparison of the time series of particulate and gas-phase DMS oxidation products did not reveal a significant correlation, indicative of the different lifetimes of precursor and oxidation products in the different phases. This is the first time the FIGAERO-HRToF-CIMS was used to investigate the composition of aerosols in the central Arctic. The detailed information on the molecular composition of Arctic aerosols presented here can be used for the assessment of aerosol solubility and volatility, which is relevant for understanding aerosol-cloud interactions.

Predictions of cloud droplet activation in the late summertime (September) central Arctic Ocean are made using κ-Köhler theory with novel observations of the aerosol chemical composition from a high-resolution time-of-flight chemical ionization mass spectrometer with a filter inlet for gases and aerosols (FIGAERO-CIMS) and an aerosol mass spectrometer (AMS), deployed during the Arctic Ocean 2018 expedition onboard the Swedish icebreaker Oden. We find that the hygroscopicity parameter κ of the total aerosol is 0.39 ± 0.19 (mean ± std). The predicted activation diameter of ∼25 to 130 nm particles is overestimated by 5%, leading to an underestimation of the cloud condensation nuclei (CCN) number concentration by 4–8%. From this, we conclude that the aerosol in the High Arctic late summer is acidic and therefore highly cloud active, with a substantial CCN contribution from Aitken mode particles. Variability in the predicted activation diameter is addressed mainly as a result of uncertainties in the aerosol size distribution measurements. The organic κ was on average 0.13, close to the commonly assumed κ of 0.1, and therefore did not significantly influence the predictions. These conclusions are supported by laboratory experiments of the activation potential of seven organic compounds selected as representative of the measured aerosol. 

Dimethyl sulfide (DMS), a gas produced by phytoplankton blooms, is the largest source of atmospheric sulfur over marine areas. DMS undergoes photochemical oxidation in the atmosphere to form a range of oxidation products, out of which methanesulfonic acid (MSA, CH₃SO₃H) and sulfuric acid (SA, H₂SO₄) are well-known for participating in the formation and growth of atmospheric aerosol particles, which has implications for cloud formation over oceanic and coastal regions. Recently, a new oxidation product of DMS, hydroperoxymethyl thioformate (HPMTF, C₂H₃OSO₂H) was discovered and later also measured in the atmosphere. Little is still known about the fate of this compound in the atmosphere and its potential to partition to the particle phase. In this study, we present a full year of concurrent gas- and particle phase observations of HPMTF, MSA, SA and other DMS oxidation products at the Zeppelin Observatory, Ny-Ålesund, Svalbard. This is the first time HPMTF has been measured in Svalbard and has been attempted to be observed in the particle phase in the real atmosphere. The results show that HPMTF production largely follows the same pattern as MSA during the sunlit months (April-September), indicating that they are formed via the same oxidation pathway. HPMTF was however not observed in significant amounts in the particle phase, despite high gas-phase levels. Particulate MSA and SA were observed during the sunlit months, although the highest median levels of particulate SA were measured in February, coinciding with the highest gaseous SA levels. We further show that gas- and particle phase MSA and SA are linked in May-September, whereas HPMTF lies outside of this correlation. These results provide more information about the relationship between HPMTF and other DMS oxidation products in a new part of the world, and about HPMTF’s ability to contribute to particle growth and cloud formation.

The role organic aerosols play in Arctic cloud formation is still poorly understood. In this study we address this issue by in-situ observations of cloud residuals at the Zeppelin Observatory in Ny-Ålesund, Svalbard (approx. 480 m a. s. l.) The measurements were part of the one-year long Ny-Ålesund Aerosol and Cloud Experiment 2019-2020 (NASCENT). We deployed a Filter Inlet for Gases and AEROsols coupled to a Chemical Ionization Mass Spectrometer (FIGAERO-CIMS) behind a ground-based counterflow virtual impactor (GCVI) to obtain the chemical composition of cloud residuals at molecular level. Between December 2019 and December 2020, we observed in total 14 cloud events. The compositions of the cloud residuals show a clear signal of methanesulfonic acid in spring, summer and autumn, but not in the winter, suggesting a marine contribution to the aerosol population activating into cloud droplets. In addition, we observe organic compounds in the cloud residuals throughout the entire year, with elevated fractions in summer. The biomass burning tracer levoglucosan was found in the cloud residuals as well, with highest contributions to the cloud residual mass at the end of summer (end of June until mid-September). Inorganic compounds (sulfuric acid and nitric acid) were also detected in the cloud residuals. Sulfuric acid concentrations were especially elevated in two of the cloud residuals (May 21 and September 12, 2020), but followed the overall pattern of the levels of MSA in the cloud residuals during the rest of the year. Overall, the results show the contribution of a marine contribution to the aerosol population able to form clouds in the Arctic environment during the summer months.