Ammonium addition (and aerosol pH) has a dramatic impact on the volatility and yield of glyoxal secondary organic aerosol.

Glyoxal is an important precursor to secondary organic aerosol (SOA) formed through aqueous chemistry in clouds, fogs, and wet aerosols, yet the gas-particle partitioning of the resulting mixture is not well understood. This work characterizes the volatility behavior of the glyoxal precursor/product mix formed after aqueous hydroxyl radical oxidation and droplet evaporation under cloud-relevant conditions for 10 min, thus aiding the prediction of SOA via this pathway (SOACld). This work uses kinetic modeling for droplet composition, droplet evaporation experiments and temperature-programmed desorption aerosol-chemical ionization mass spectrometer analysis of gas-particle partitioning. An effective vapor pressure (p'L,eff) of ∼10(-7) atm and an enthalpy of vaporization (ΔHvap,eff) of ∼70 kJ/mol were estimated for this mixture. These estimates are similar to those of oxalic acid, which is a major product. Addition of ammonium until the pH reached 7 (with ammonium hydroxide) reduced the p'L,eff to <10(-9) atm and increased the ΔHvap,eff to >80 kJ/mol, at least in part via the formation of ammonium oxalate. pH 7 samples behaved like ammonium oxalate, which has a vapor pressure of ∼10(-11) atm. We conclude that ammonium addition has a large effect on the gas-particle partitioning of the mixture, substantially enhancing the yield of SOACld from glyoxal.

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.