Formation and Degradation of Secondary Organic Aerosol Material

Atmospheric aerosols have strong influences on climate and air quality, yet many open questions about the extent of their impact remain. Secondary organic aerosol (SOA) is of particular interest because it accounts for a significant portion of fine particulate mass. Furthermore SOA formation mechanisms, composition, optical properties, and lifetimes are not fully understood. This is due in part to the numerous and diverse organic precursors in the atmosphere. SOA formation and growth involves complex and varied chemical and physical processes, and despite substantial progress over the past decade, the SOA budget has not been closed. This work explores three distinct projects to address the sources and sinks of SOA in aqueous aerosol particles: 1) the extent of photoactivator chemistry under ambient conditions, a potential source of organic aerosol mass via direct oxidation of volatile organic compounds or generation of oxidants in the particle phase; 2) the speciation and formation rates of isoprene epoxydiol (IEPOX)-derived SOA using a multiphase, photochemical model, GAMMA; and 3) the effect of bacterial metabolism on organic aerosol content. This body of work yields an improved understanding of atmospheric aerosol chemistry, with implications for where best to apply future research efforts. A photochemical chamber was constructed to carry out photosensitization experiments at longer timescales than can typically be achieved with aerosol flow tube equipment in order to mimic conditions of the ambient atmosphere. Light-absorbing humic acid aerosols were exposed to gas-phase limonene in the presence of ultraviolet light. Contrary to previous experimental results, no particle growth was observed. This is explained by the difference in light intensity and limonene concentrations between the two setups. Calculations based on our experimental results under ambient conditions suggest that the photosensitizer chemistry of humic-like substances is not expected to form substantial aerosol mass. Modeling studies of IEPOX SOA formation and aging are conducted to gain insights into the reaction mechanism. Recent instrument development has shown that previous product distributions and corresponding mechanisms were significantly biased by thermal degradation from the measurement techniques. We utilize the current state of knowledge surrounding IEPOX SOA formation in an attempt to elucidate a unifying mechanism. However, model results suggest that significant gaps remain in our understanding of formation and aging processes, especially oligomerization. Finally, we consider microbial consumption of aerosol organics in the atmosphere. Observations of culturable cells in aqueous aerosols and cloud water suggest that they may be actively metabolizing aqueous media while they are airborne, which could have significant impacts on aerosol and cloud properties. Metabolic rates of cells cultured from atmospheric samples are incorporated into GAMMA. While there is a substantial decrease in the concentration of organic species for particles in which cells reside, the overall effect on populations of particles is negligible, and bacterial metabolism is not expected to measurably alter the organic content of the atmosphere.