Composition Domains in Monoterpene Secondary Organic Aerosol

The composition and structure of freshly formed oligomers in alpha- and beta- pinene SOA are studied with high performance mass spectrometry to provide insight into the SOA formation mechanism. Van Krevelen plots (H:C ratio vs O:C ratio) are interpreted in the context of distinct structural domains that correspond to separate oligomer formation routes. The domain containing most of the signal intensity encompasses elemental formulas that correspond to oligomerization reactions of intermediates and/or stable molecule monomers produced by ozonolysis of the precursor. While oligomers involving reactive intermediates from the hydroperoxide channel dominate the product distribution, products are also observed that uniquely map to the stable Criegee intermediate and/ or combinations of stable molecule monomers. A second domain encompasses molecules having lower H:C ratios but similar O:C ratios to the first domain. Many of the products observed in this domain have double bond equivalents greater than the maximum number possible when forming dimers by standard reaction mechanisms and are interpreted in the context of repeated self-reactions of alkoxy/peroxy radicals. A third domain encompasses molecules having very high H:C and O:C ratios consistent with polymerization of formaldehyde and/or acetaldehyde. These domains remain distinguishable from experiment to experiment and among different extraction solvents (50/50 methanol-water, 50/50 acetonitrile-water,100% water).