Discerning Trends in Wintertime Reactive Chlorine Chemistry and Air Quality

Abstract

Reactive chlorine species (Cl*) are crucial to atmospheric chemistry, impacting oxidative cycles, pollutant formation, and air quality. However, their behavior, particularly in urban winter environments, remains underexplored. This thesis investigates the sources, transformations, and impacts of reactive chlorine in urban settings using high-time-resolution measurements, size-resolved aerosol analysis, and advanced modeling techniques. High-time-resolution measurements of hydrogen chloride (HCl) were conducted in coastal (St. John’s, NL) and continental (Toronto, ON) regions. In the coastal environment, HCl variability was influenced by photochemical and acid displacement processes, while in Toronto, direct emissions dominated, with road salting contributing to particulate chloride but not directly to HCl production. Simulations using GEOS-Chem captured coastal variability but significantly underestimated urban HCl levels, indicating the complexity of urban chlorine sources. Size-resolved aerosol analysis during road salt application revealed that chloride levels in urban aerosols were similar to marine environments, with road salt-derived chloride redistributed into finer aerosol modes. These findings suggest that acid displacement and heterogeneous reactions play a critical role in sustaining HCl production over time. Simulations with the E-AIM model underscored the need for improved representation of ammonia and ammonium chloride chemistry to predict HCl partitioning more accurately. Indoor sources, particularly chloramines (NH₂Cl, NHCl₂) from hypochlorite-based cleaning products, were identified as significant outdoor contributors to reactive chlorine budgets. These sources were found to rival smaller industrial emissions. Mobile and vertical gradient measurements quantified spatial variability in HCl emissions, highlighting the influence of local meteorological conditions on HCl fluxes and deposition. These findings advance our understanding of urban chlorine chemistry, providing insights into sources, transformations, and their implications for air quality and atmospheric models.