| This dissertation provides evidence supporting the hypothesis that secondary organic aerosol (SOA) is formed in the atmosphere through aqueous-phase reactions in clouds. Accurate prediction of SOA formation is critical because organic aerosol adversely affects health, visibility and climate. If in-cloud SOA formation is significant, then current models incorrectly predict the concentrations, atmospheric distributions, properties, behavior and effects of atmospheric organic aerosol. During cloud processing, water-soluble gas-phase oxidation products of reactive organic gases (e.g., aromatics and alkenes including, isoprene) partition into cloud droplets where they react further during regional transport to form low volatility compounds (e.g., carboxylic acids and oligomers) that remain, in part, in the particle phase upon droplet evaporation, adding to the atmospheric particulate matter (PM) burden.; Batch photochemical reactions of glyoxal, methylglyoxal and pyruvic acid with hydrogen peroxide were conducted to validate and improve proposed in-cloud SOA formation pathways. This research verifies that aqueous photooxidation of these compounds yields oxalic acid and other compounds (e.g., oligomes) likely to contribute to SOA. Electro-spray ionization-mass spectrometry (ESI-MS) analysis provided evidence for oligomer formation. This work resolved a discrepancy in the literature regarding the aqueous-phase fate of methylglyoxal and pyruvic acid, providing a link between isoprene, a biogenic compound with a large (∼500 Tg yr-1) world-wide emission flux, and SOA. Experimental time series product concentrations were compared to predictions using proposed pathways and reaction rate constants from the literature. Not all products were predicted by the models and expected products did not match observed time profiles. Additional oxidation pathways for glyoxal and methylglyoxal were identified and modified aqueous-phase oxidation mechanisms were proposed. Modification led to substantially improved agreement between experimental measurements and model predictions.; The improved mechanistic understanding regarding the SOA in-cloud formation pathway provided by this dissertation is needed to improve predictive air quality and climate models and to develop more effective air quality management plans. While the climactic importance of secondary sulfate production via cloud processing is well known, and organic aerosol is known to play a role in global climate, the potential importance of in-cloud SOA formation is only just beginning to be considered. |
