Secondary organic aerosol formation from acyclic, monocyclic, and polycyclic alkanes.

A large number of organic species emitted into the atmosphere contain cycloalkyl groups. While cyclic species are believed to be important secondary organic aerosol (SOA) precursors, the specific role of cyclic moieties (particularly for species with multiple or fused rings) remains uncertain. Here we examine the yields and composition of SOA formed from the reaction of OH with a series of C10 (cyclo)alkanes, with 0-3 rings, in order to better understand the role of multiple cyclic moieties on aerosol formation pathways. A chamber oxidation technique using high, sustained OH radical concentrations was used to simulate long reaction times in the atmosphere. This aging technique leads to higher yields than in previously reported chamber experiments. Yields were highest for cyclic and polycyclic precursors, though yield exhibited little dependence on number of rings. However, the oxygen-to-carbon ratio of the SOA was highest for the polycyclic precursors. These trends are consistent with aerosol formation requiring two generations of oxidation and 3-4 oxygen-containing functional groups in order to condense. Cyclic, unbranched structures are protected from fragmentation during the first oxidation step, with C-C bond scission instead leading to ring opening, efficient functionalization, and high SOA yields. Fragmentation may occur during subsequent oxidation steps, limiting yields by forming volatile products. Polycyclic structures can undergo multiple ring opening reactions, but do not have markedly higher yields, likely due to enhanced fragmentation in the second oxidation step. By contrast, C-C bond scission for the linear and branched structures leads to fragmentation prior to condensation, resulting in low SOA yields. The results highlight the key roles of multigenerational chemistry and susceptibility to fragmentation in the formation and evolution of SOA.