Effects of molecular structure on aerosol yields from OH radical-initiated reactions of linear, branched, and cyclic alkanes in the presence of NOx.

The effect of hydrocarbon molecular structure on the measured yield and volatility of secondary organic aerosol (SOA) formed from OH radical-initiated reactions of linear, branched, and cyclic alkanes in the presence of NOx was investigated in an environmental chamber. SOA yields from reactions of homologous series of linear and cyclic alkanes increased monotonically with increasing carbon number due to the decreasing volatility of the parent alkanes and thus the reaction products. For a given carbon number, yields followed the order cyclic > linear > branched, a trend that appears to be determined primarily by the extent to which alkoxy radical intermediates decompose and the nature ofthe resulting products, with parent alkane volatility being of secondary importance. The trend was investigated quantitatively by correlating SOA yields with the fraction of OH radical reactions that lead to alkoxy radical decomposition (the remainder isomerize), calculated using structure-reactivity relationships. For alkoxy radicals with branched or strained cyclic structures, decomposition can compete with isomerization, whereas for those with linear structures it cannot Branched alkoxy radicals fragment to form pairs of smaller, more volatile products, whereas cyclic alkoxy radicals undergo ring opening to form products similar to those formed from reactions of linear alkanes, but with an additional aldehyde group. The lower volatility of multifunctional aldehydes, and their tendency to form oligomers, appears to enhance SOA yields.