Feasibility of Photosensitized Reactions with Secondary Organic Aerosol Particles in the Presence of Volatile Organic Compounds.

The ability of a complex mixture of organic compounds found in secondary organic aerosol (SOA) to act as a photosensitizer in the oxidation of volatile organic compounds (VOCs) was investigated. Different types of SOAs were produced in a smog chamber by oxidation of various biogenic and anthropogenic VOCs. The SOA particles were collected from the chamber onto an inert substrate, and the resulting material was exposed to 365 nm radiation in an air flow containing ∼200 ppbv of limonene vapor. The mixing ratio of limonene and other VOCs in the flow was observed with a proton transfer reaction time-of-flight mass spectrometer (PTR-ToF-MS). The photosensitized uptake of limonene was observed for several SOA materials, with a lower limit for the reactive uptake coefficient on the scale of ∼10-5. The lower limit for the uptake coefficient under conditions of Los Angeles, California on the summer solstice at noon was estimated to be on the order of ∼10-6. Photoproduction of oxygenated VOCs (OVOCs) resulting from photodegradation of the SOA material also occurred in parallel with the photosensitized uptake of limonene. The estimated photosensitized limonene uptake rates by atmospheric SOA particles and vegetation surfaces appear to be too small to compete with the atmospheric oxidation of limonene by the hydroxyl radical or ozone. However, these processes could play a role in the leaf boundary layer where concentrations of oxidants are depleted and concentrations of VOCs are enhanced relative to the free atmosphere.