Atmospheric and Snowpack Halogen Chemistry in Urban Areas and the Arctic

Abstract

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 behavior, driven by photochemistry, and model simulations predicted the snowpack to be the primary BrCl source. Upon snow melt onset, production of reactive bromine species ceased, further demonstrating the essential role of the snowpack for sustaining reactive halogen chemistry in the Arctic.

Nitryl chloride (ClNO2), formed from the multi-phase reaction of dinitrogen pentoxide (N2O5) and aqueous particulate chloride, is an important reservoir of both Cl radicals and NOx in urban areas. Widespread wintertime road salting was hypothesized as a prominent source of chloride for ClNO2 production in inland areas. For the first time, this source was confirmed through measurements of ClNO2, N2O5, and individual road salt aerosol particles in Ann Arbor, Michigan during winter 2016. Aerosolized road salt was responsible for 80-100% of ClNO2 production. Using these comprehensive measurements, a new parametrization for ClNO2 production was developed, dramatically lowering previous model overestimates of ClNO2 levels by considering the heterogeneity of the aerosol population. The influence of road salt on ClNO2 production in the inland wintertime environment was further investigated in Kalamazoo, Michigan, where the road salt-contaminated snowpack was quantitatively confirmed as a ClNO2 source, highlighting yet another route for the production of photolyzable chlorine.

The results gained from the CIMS field measurements and numerical modeling presented in this dissertation significantly improve our understanding of halogen production mechanisms in cold environments, including the Arctic and wintertime urban environment. This work highlights important connections between the snowpack and atmospheric composition, at a time when fossil fuel extraction and shipping routes are expanding in the Arctic, and road salting practices are being scrutinized for their harmful effects on the environment.