Effect of photolysis on secondary organic aerosol (SOA) formation and lifetime

Abstract:

We investigate the effect of potentially important, but hitherto unexplored, gas- and particle-phase photolysis reactions on the lifetime and budgets of SOA. Recent laboratory studies showed that freshly formed SOA from e.g. monoterpenes have a lifetime of a few hours when exposed to UV radiation both in dry particles and aqueous solutions, suggesting that photolytic processing of secondary organic vapors and particles could be removing aerosols from the troposphere on timescales comparable to those of wet deposition. Photolytic reactions lead to fragmentation of molecules, changes in their properties (e.g. volatility, solubility, photo-bleaching) and thus their ability to form SOA or remain in the particle phase. These reactions are currently not included in 3D models, and as we are using more and more explicit SOA mechanisms in 3D models this issue comes to the forefront. Here, we use an explicit chemical model to estimate the effect of gas-phase photolysis on SOA formation for various precursors (biogenic and anthropogenic mixtures) and environments (low-, high-NOx). By comparison with laboratory studies, we estimate the relative importance of gas- vs. particle phase photolysis. This understanding is then parameterized within a global chemistry model to assess the potential effect on the SOA global budgets. The results confirm that photolytic reactions are an important loss process, which competes with the wet removal, but has however very different temporal and spatial patterns.