Chemical perspectives on aerosol-cloud interactions in the High Arctic

Abstract: Atmospheric aerosol particles have important yet highly uncertain impacts on the Earth’s climate, with the largest uncertainties residing in the interactions between aerosols and clouds. The extent to which aerosols act as cloud condensation nuclei (CCN) depends on the chemical composition and size of the particles. To make correct predictions of cloud formation and the associated climate forcing, more knowledge on the physicochemical properties of aerosols is needed.This thesis investigates the chemical composition and CCN activity of aerosols in the High Arctic using a Chemical Ionization Mass Spectrometer with a Filter Inlet for Gases and Aerosols (FIGAERO-CIMS). The Arctic is the region on Earth with the current largest increase of mean surface temperature due to global warming and with big knowledge gaps in terms of aerosol-cloud-climate interactions.The first two articles focus on the region within the pack ice and marginal ice zone (MIZ) during Arctic late summer. They introduce new insights into the molecular composition of organic submicron (diameter<1 μm) aerosols and the associated hygroscopicity. The composition is shown to include a wide range of carbon and oxygen numbers, with a clear contribution from dimethyl sulfide (DMS) oxidation products. Together with observations of the inorganic aerosol fraction and CCN, the aerosol is shown to be highly hygroscopic, and the activation diameter and CCN number concentration to be possible to predict using κ-Köhler theory.The last two articles present results from a year-long study in Ny-Ålesund, Svalbard. The third article addresses the seasonality of DMS oxidation products, with a focus on the newly discovered compound hydroperoxymethyl thioformate (HPMTF). The analysis shows that gas-phase HPMTF follows the same development pattern in summer as the well-known oxidation product methylsulfonic acid (MSA), indicating a local source of DMS. HPMTF was however not found in significant amounts in the particle phase in either season. In the fourth article, the chemical composition of cloud residuals (particles remaining after drying of cloud droplets) was shown to be clearly influenced by DMS oxidation products (MSA and sulfuric acid) in summer. The importance of MSA and sulfuric acid for Arctic low-level cloud formation has previously been presumed, but not confirmed by in-situ observations.