Role of Water in the Formation, Transformation and Fate of Secondary Organic Aerosol

Abstract

Particle-phase water is the most abundant atmospheric aerosol constituent, yet the importance of water for the formation, transformation and fate of secondary organic aerosol (SOA) remains not fully characterized. In order to address this knowledge gap, this thesis explores the role of water in SOA formation, chemical aging and fate as cloud condensation nuclei. The formation of SOA from the photooxidation of isoprene in the presence of various sulfate seed particles was investigated using a flow tube reactor. Under constant environmental conditions, particle-phase water was found to have the largest effect on the amount of SOA formed where this additional organic material was highly oxidized, likely arising from enhanced uptake of organic acids due to their high water solubility. The amount of high molecular weight compounds increases with acidity, suggesting the role of acidity in governing organic composition. The relative humidity (RH) dependence of SOA aging by photolysis was examined using particles containing water-soluble α-pinene SOA material in an environmental chamber at three RH conditions (5, 45 and 85 %). Photolysis led to substantial mass loss where the rate of mass loss increased with increasing RH, suggesting that moisture-induced changes in SOA phase have implications to particle reactivity. Aging of ambient SOA sampled at Whistler, British Columbia found that aging by both gas and aqueous-phase OH increased the degree of oxygenation and CCN activity of the organic material, confirming the hypothesis that there is a simple relationship between the hygroscopicity of organic aerosol and its oxygen-to-carbon ratio. Addition of various types of organic material onto sulfate particles resulted in the suppression of water uptake during droplet growth. Experiments using sulfate particles with different acidity suggest that high molecular weight compounds, formed via acid-catalyzed condensed phase reactions, are the species affecting water uptake kinetics.