| Summary: | Highly reactive trace halogen radicals in the atmosphere can significantly impact air quality and composition, and alter the fates of pollutants. These radicals are formed from the photolysis of a number of different halogen-containing gases, which vary extensively by location, concentration, and source. However, a paucity of observations currently limits our knowledge of the sources, sinks, and specific chemical mechanisms controlling the abundances of these halogen species, particularly in the urban wintertime environment and the coastal Arctic. In this dissertation, chemical interactions between the snowpack and overlying atmosphere are explored, while demonstrating the far-reaching effects of anthropogenic activity on halogen chemistry in cold environments. Using a field-deployable chemical ionization mass spectrometer (CIMS), several molecular halogens and reactive nitrogen species were measured during three field campaigns: Utqiaġvik, Alaska in March – May 2016, Ann Arbor, Michigan in February – March 2016, and Kalamazoo, Michigan in January – February 2018. These sensitive (ppt-level) in situ trace gas CIMS measurements, coupled with chemical measurements of atmospheric particles and numerical modeling, enabled the evaluation of major halogen production mechanisms and uncovered key sources of halogen radical precursor gases. Using CIMS, we obtained the first Arctic observations of nitryl chloride (ClNO2), dinitrogen pentoxide (N2O5), and peroxynitric acid (HO2NO2), reservoirs of nitrogen oxide (NOx) pollution. Episodic enhancements in chlorine chemistry coincided with periods of NOx pollution influence from the town of Utqiaġvik, as well as the North Slope (Prudhoe Bay) Oilfields, hundreds of kilometers to the southeast. Bromine chloride (BrCl), a potent source of chlorine and bromine radicals, was also observed throughout the campaign, and its production and removal pathways were explored using a numerical model constrained by CIMS measurements of other halogen gases. BrCl exhibited a distinct diel ...
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