How Do Particle Chemical Properties Affect Aerosol Chemical Reactions?

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The oxidation kinetics and mechanisms of organic molecules incorporated as minor constituents inside particles representative of atmospheric particles will be studied. The overall goal is to understand how the condensed-phase matrix affects the products, rates, and mechanisms of chemical reactions. This information is essential to understanding and quantifying the aging processes of atmospheric particles.Recent studies of lifetimes of organic molecules in atmospheric particles against attack by ozone and hydroxyl radical have resulted in a discrepancy between laboratory studies and field measurements, the latter indicating significantly longer lifetimes of the chemical constituents. The hypothesis of this project is that matrix effects are responsible for the apparent discrepancies. Oleic acid incorporated in secondary organic aerosol (SOA) is, for example, hypothesized to react slowly with gas phase ozone because the ozone must overcome a diffusion barrier in the SOA matrix to reach the reactive double bonds of oleic acid. Moreover, the reaction mechanisms and hence products formed are hypothesized to depend on the nature of the reaction matrix.The experiments to be conducted require the generation of aerosol particles of different matrix compositions and the incorporation of reactive organic molecules as minor constituents. For this purpose, SOA of several compositions will be generated in the new Harvard Smog Chamber, and target organic molecules will be incorporated in the SOA. SOA of different diffusivity, viscosity and acidity will be produced. The effect of inorganic matrices will also be explored by variation of seed particle composition among ammonium sulfate, ammonium nitrate, and sodium chloride. In all cases, the role of water content will be analyzed by varying relative humidity. The reactive organic molecules will include oleic acid, methyl oleate, linoleic acid, vaccenic acid, petroselinic acid, and their ammonium salts. The experiments will employ a newly acquired Aerodyne Time-of-Flight Aerosol Mass Spectrometer (TOF-AMS) as a real-time monitor of the reactive organic molecules during ozonolysis. Particles will also be collected for off-line product analysis by chromatography.This work will provide important information on atmospheric aerosols that will improve our basic understanding of cloud nucleating properties, as well as potential health effects of particulate matter. The award will provide training opportunities for graduate and undergraduate students.