Photodegradation and photosensitization reactions of secondary organic aerosols on environmental surfaces

Organic compounds trapped inside and on surfaces of atmospheric particles are readily accessible to actinic radiation from the sun. The resulting photochemical processes are known to degrade organic compounds, leading to a loss of volatile photochemical products from the particles. In contrast, photosensitized reactions, where a photosensitizer molecule absorbs light at a given wavelength and the excited state of the photosensitizer reacts with a nearby volatile organic compound (VOC), cause an increase in aerosol mass and diameter. The photosensitization and photodegradation oppose each other, and it is not presently known which of these two processes is more important for aerosol particles and semivolatile organics trapped on environmental surfaces. This presentation will describe our ongoing experiments that characterize the photodegradation and photosensitization reactions of secondary organic aerosols (SOA) on environmental surfaces, such as surfaces of vegetation. We generate oxidants that flow over a variety of whole plants and/or model environmental surfaces, and we examine the interactions of VOCs with plants coated with SOA using a suite of instrumental techniques. Special attention is given to the fluxes of VOCs, which can be either emitted by plants in an SOA photodegradation process or taken up by plant surfaces in a photosensitized process.