| Summary: | The Arctic is a harbinger of global change, and is warming at a rate twice the global average. While Arctic warming is driven by increased greenhouse gases, short-lived climate forcers such as tropospheric ozone and aerosol are important drivers of Arctic climate. Aerosol impacts on Arctic climate are insufficiently understood, largely owing to poor constraints on the physical and chemical processes controlling aerosol. This thesis presents observations of sub-micron aerosol in Arctic summer and spring, which capture different regimes of the Arctic aerosol seasonal cycle. Aircraft-based observations of aerosol physical and chemical properties address various aspects of Arctic aerosol sources, removal and chemical processing. Under chronic Arctic Haze conditions in spring, we observed evidence for vertical variations in both aerosol sources and removal mechanisms. We show evidence for sources of partially neutralized aerosol with higher organic aerosol (OA) and black carbon content in the mid-troposphere and sources of acidic sulfate in the lower troposphere. With support from model calculations, we demonstrate that mid-tropospheric air influences the Arctic Boundary Layer (ABL) on ~1-week time scales. We observed evidence for aerosol depletion relative to carbon monoxide, both in the mid-to-upper troposphere and within the ABL. Dry deposition, with low removal efficiency, contributed to aerosol removal in the ABL while precipitation scavenging contributed to efficient aerosol removal during transport at higher altitudes. Under clean Arctic background conditions in summer, we observed evidence for marine influenced secondary OA formation. We demonstrated a relationship between methansulfonic acid and OA with the residence time of air over open water. Sea salt aerosol was externally mixed from a larger number fraction of OA, sulfate and amine-containing particles. High OA fractions coincided with elevated cloud condensation nuclei (CCN) concentrations, suggesting a role for secondary OA formation in growing particles to CCN-active sizes. A case study of aerosol growth over open water supports these observations. This work contributes to our understanding of aerosol vertical variability and its connection to observations made at the surface in Arctic spring and, to our understanding of aerosol sources and composition in Arctic summer. Ph.D.
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