The physical and chemical properties of ammonium sulfite individual atmospheric particles determine their impact on climate. Hygroscopic inorganic salt particles mixed with trace amounts of organic material are expected to be in a liquid state under typical tropospheric conditions in the summer Arctic. However, we unexpectedly observed a significant increase in the concentration of solid particles consisting of ammonium sulfate with an organic coating under conditions of high relative humidity and low temperature. These particle properties are consistent with the formation and growth of new particles derived from marine organisms, assuming that particle collisions result in a solid phase. Such particle sources are expected to be of increasing relevance in the context of reduced Arctic sea ice and increased open water, with effects on clouds and thus climate.
Abstract

The ability of atmospheric aerosols to affect climate through water absorption and cloud formation depends fundamentally on the size, composition and phase (liquid, semi-solid or solid) of individual particles. The particle phase depends on atmospheric conditions (relative humidity and temperature) and chemical composition, and importantly, solid particles inhibit the uptake of water and other trace gases even under humid conditions. Presumably, the granules, consisting mostly of ammonium sulfate, are liquid at the relative humidity (67% to 98%) and temperature (-2 to 4°C) of Arctic summer. Under these atmospheric conditions, we report observations of solid organic-coated ammonium sulfate particles in the critical size range (0.2 µm) for cloud activation from Utqiagvik marine air masses in the Arctic Ocean, accounting for 30% of the particles by number. %, AK. The composition and size of the observed particles are consistent with recent Arctic models and observations, suggesting that new particle formation and growth result from oxidation of dimethyl sulfide to sulfuric acid, reaction with ammonia, and condensation of marine biosulfate and highly oxygenated organic molecules. Hydrous sulfate pellets typically weather and solidify at less than 34% relative humidity. Therefore, it is hypothesized that the observed solid phase is caused by contact weathering during the collision of newly formed Aitken mode sulfate particles with organically coated ammonium sulfate particles. This source of particles is projected to increase as open water and marine biogenic emissions increase as sea ice decreases in a warming Arctic.