Chamber Studies of Secondary Organic Aerosol Formation

Atmospheric oxidation of volatile organic compounds leads to the formation of secondary organic aerosol (SOA). Laboratory chambers provide a controlled environment for investigating aerosol formation and evolution. This thesis presents results on aerosol formation from a wide range of parent organic compounds under a variety of experimental conditions. The effect of particle-phase acidity on aerosol formation is explored in a series of alkene ozonolysis experiments. Oligomeric species are detected regardless of the particle-phase acidity, indicating the ubiquitous existence of particle-phase reactions. As acidity increases, larger oligomers are formed more abundantly and aerosol yields also increase. Volatile organic compounds generally not considered to be SOA precursors, including isoprene and glyoxal, have been shown to lead to aerosol formation. Uptake of glyoxal into particles is evidence that small molecules can potentially produce aerosol via reactive absorption. Although there is strong evidence that heterogeneous reactions play an important role in SOA formation, the detailed mechanisms remain poorly understood. In a comprehensive study on aerosol formation from biogenic hydrocarbons, it is found that data on aerosol growth as a function of the amount of hydrocarbon reacted provide important insights into the general aerosol formation mechanisms by identifying rate-determining steps and whether SOA is formed from first- or second-generation products. The mechanism of aerosol formation by isoprene is specifically investigated over a range of NOx concentrations. Aerosol yields are found to decrease substantially with increasing NOx. The same NOx dependence is observed for monoterpenes ([alpha]-pinene), as well as aromatic hydrocarbons (m-xylene, toluene, and benzene). It is suggested that peroxy radical chemistry plays the central role in the observed NOx dependence. The NOx dependence for larger compounds is, however, different from that of isoprene, monoterpenes, and aromatics. For sesquiterpenes such as longifolene and aromadendrene, aerosol yields increase with increasing NOx concentration. The reversal of the NOx dependence of SOA formation for the sesquiterpenes appears to be the result of formation of relatively nonvolatile organic nitrates, and/or the isomerization of large alkoxy radicals that leads to less volatile products.