(160d) Condensed-Phase Photolysis Processes in Secondary Organic Aerosol
Exposing secondary organic aerosol (SOA) to ultra-violet (UV) radiation at atmospherically relevant wavelengths leads to changes in aerosol composition and volatility. The carbonyls, peroxides, nitrates, and other multifunctional compounds typically present in SOA undergo direct photolysis when exposed to UV radiation. These direct photolysis rates can be competitive with atmospheric oxidation by OH and other gas-phase oxidants. Much of the previous research aimed at studying the photolysis of SOA involved exposing both particles and their surrounding vapors to UV light. These photolabile vapors may produce semi-volatile products with vapor pressures that induce condensation onto adjacent particles. These bulk SOA photolysis studies, typically performed in reaction chambers, do not permit for the decoupling of gas-phase and condensed-phase photochemistry. We have designed and built a system where SOA mixtures are stripped of their vapors after passing through three, one-meter denuders and then photolyzed in a UV-transparent flow cell. The denuder system leads to an order of magnitude reduction in vapor concentration. We employ an Aerodyne HR-ToF-AMS to monitor changes in particle composition during photolysis. Experiments designed to age SOA formed from biogenic precursors will reveal how aerosol oxidation state influences the rate of particle photolysis. Additional experiments with SOA seeds of varying compositions will illustrate the dependence of photolysis rate on seed composition.