Microscopic Analysis of Aerosol Chemical Mixing State in Cold Environments

Abstract

Atmospheric aerosol particles impact climate through scattering or absorbing solar radiation, altering surface albedo upon deposition, and aiding in cloud formation. These climate effects depend on the physical and chemical properties of individual particles, including chemical composition, size, phase, and morphology (physicochemical mixing state). In the Arctic, atmospheric particles play a key role in climate processes in the drastically changing region, but Arctic aerosol physicochemical mixing state is not well constrained. Large knowledge gaps remain in understanding the sources and composition of Arctic aerosol, as there is a lack of single particle measurements, particularly across seasons (including fall – winter) and throughout the region (especially the high Arctic). In this dissertation, the physicochemical properties of individual particles were examined at multiple Arctic locations (Bering Strait, Chukchi Sea, coastal Beaufort Sea, and high Arctic Ocean) and during different seasons (winter – spring and summer). The results of this work will increase understanding of Arctic aerosol sources and composition across scales, which is critical to predicting aerosol composition and climate-relevant properties in a New Arctic. Sea spray aerosol (SSA) contributes the largest global flux of particles to the atmosphere, and is therefore an important climate driver, particularly in remote regions, including the Arctic. Laboratory-based sea spray aerosol generation experiments were conducted to evaluate the impacts of seawater temperature, salinity, and marine biology on SSA production and composition in cold environments. These results showed that temperature was as important as biology for controlling SSA production, and organic enrichment was observed in individual SSA particles, indicative of marine organics being transferred to the particle phase. To investigate the role of SSA in the ambient Arctic environment, atmospheric particles were collected during wintertime in the Alaskan Arctic. SSA was a major fraction of the observed aerosol number, demonstrating that sea ice fractures are a source of wintertime aerosol. Further analysis of the wintertime SSA showed a major organic component in the SSA particles, with organic carbon coatings consisting of saccharides, amino acids, and fatty acids. These compounds are derived from exopolymeric substances (EPS) associated with biologically productive sea ice. Greater SSA organic enrichment was observed in winter than in summer, suggesting a unique, previously unidentified source of SSA organics from sea ice algae EPS during winter. Motivated by this work in the wintertime Alaskan Arctic, both ambient and laboratory-generated SSA particles were collected during summer in the high Arctic. SSA production from open leads was observed under high wind conditions, and organic enrichment was observed in both lab-generated and ambient individual SSA particles. Saccharides and fatty acids were identified as the dominant organic compounds present in SSA particles, derived from EPS and marine organics. Atmospheric particle samples were collected in the summertime Alaskan Arctic to investigate the impacts of sea ice loss and increasing development. Samples in the Bering Strait demonstrated great anthropogenic influence. Within the Chukchi Sea, samples were mainly influenced by marine biogenic sources. On the North Slope of Alaska during summer, ammonium sulfate particles demonstrating unique phase and morphology were observed. Organic coatings observed on the particles may play a role in inducing phase changes not previously predicted in the ambient atmosphere. Through these studies, we have gained a greater understanding of the climate-relevant complex atmospheric chemistry and aerosol physicochemical mixing state in the Arctic occurring under changing conditions.