UC San Diego

This dissertation investigates the formation mechanisms of nitrous acid (HONO) in the atmosphere, a significant source of hydroxyl and nitric oxide radicals upon photodissociation at wavelengths within the solar spectrum. It is pertinent to investigate HONO sources to better understand hydroxyl radical sources as the global budget of hydroxyl radicals, the most potent oxidizer in the atmosphere remains poorly understood. Also, nitric oxide radical sources are crucial to investigate as it is part of the nitrogen oxide atmospheric cycle, impacting photochemical smog and acid rain. The enhancement of HONO formation in the marine boundary layer from nitrate photochemistry in the presence of marine relevant organics is explored here. The dissertation presented in this Ph.D. dissertation explores the enhancement of HONO from nitrate photolysis in the presence of marine-relevant organic compounds. The use of marine dissolved organic matter (m-DOM) from the Pacific Ocean, containing aliphatic and chromophoric compounds, serves as a model system to investigate HONO enhancement from both compound types, as both have been found to enhance HONO formation. Molecular proxies, such as ethylene glycol and 4-benzoylbenzoic acid, are also employed to gain mechanistic insights. To measure HONO and nitrogen dioxide (NO2), an in lab made Incoherent Broadband Cavity Enhanced Absorption Spectrometer (IBBCEAS) was optimized for use. This instrument allows simultaneous detection of trace gases, enabling the investigation of HONO enhancement mechanisms. The overarching objective of the dissertation is to bridge knowledge gaps regarding the role of marine-relevant organics in enhancing photolytic HONO formation in the marine boundary layer. The dissertation is structured into chapters, covering first the theoretical background, instrumental setup, data analysis, and validation of the IBBCEAS. Also presented are experimental results that reveal m-DOM as the most efficient enhancer of HONO compared to other marine relevant organics, and its concentration dependence is explored. Lastly, the synergistic enhancement of HONO by aliphatic and chromophoric compounds is investigated.