The importance of phase in the radical-initiated oxidation of model organic aerosols: reactions of solid and liquid brassidic acid particles

Using a flow tube reactor coupled to a chemical ionization mass spectrometer, the Cl-initiated oxidation of solid and supercooled liquid brassidic acid (BA, trans-13-docosenoic acid) particles was investigated at 293 K. For the first time radical-initiated oxidation reactions of liquid and solid organic particles of identical chemical composition were performed making it possible to probe the effect of phase. Despite the fact that solubility and/or diffusion in the solid particles is expected to be reduced dramatically, it was observed that the BA in those particles still reacted at 70% of the rate in liquid droplets. The lack of significant slowing upon solidification suggests that the surface is continuously renewed, perhaps by evaporation of volatile products or mixing of underlying solid BA at a surface melt layer. The initial oxidation products were found to be the keto-acid and the alcohol-acid for both solid and liquid, and they account for as much as two thirds of the reacted BA. The distribution of other products, however, was found to be quite different in the two phases. For equivalent Cl concentrations and reaction times more multiply-oxidized species as well as low-molecular-weight species were created from the oxidation of solid particles. Furthermore, the mean mobility diameter of both liquid and solid particles, as determined from a scanning mobility particle sizer, decreased after reaction with larger decreases for the solid particles. These observations are consistent with a loss of mass through evaporation of small, volatile oxidation products. The findings from this study suggest that slower diffusion of the oxidation products in solid particles confines them to the surface where they continue to react with Cl radicals producing more-highly-functionalized products which may decompose more readily. Thus, the solid particles react nearly as efficiently as the liquid ones, but the manner in which they “age” chemically is substantially different. These experiments with this model system indicate that particle phase could be important in determining how organic aerosols evolve chemically through radical-initiated oxidation in the atmosphere, and future work will try to assess how general the effect of phase is.