Reactive uptake of ozone by aerosol-associated unsaturated fatty acids: Kinetics, mechanism, and products

The heterogeneous reaction between ozone and oleic and linoleic acids, prevalent components of both marine and urban organic aerosol, were studied in a flow reactor using electron impact and chemical ionization mass spectrometry, Liquids and frozen liquids were used as proxies for atmospheric aerosol. The reactive uptake coefficients, gamma, were determined to be (8.3 +/- 0.2) x 10(-4) and (1.2 +/- 0.2) x 10(-3) for liquid oleic and linoleic acid respectively and (5.2 +/- 0.1) x 10(-5) and (1.4 +/- 0.1) x 10(-4) for frozen oleic and linoleic acid, respectively. Although, the reacto-diffusive length is estimated to be rather small in the liquid experiments, < 10 nm, a clear indication of the participation of subsurface layers in the uptake is observed. This is in contrast to uptake by the frozen acids where the reaction is limited to the surface. Aldehydes were identified as the major volatile reaction products: 1-nonanal was detected following reaction with oleic acid, 2-nonenal, 4-nonenal, and 1-hexanal were detected following reaction with linoleic acid. The aldehyde yield, defined as the amount of the volatile product released relative to the ozone consumed, is dictated by its solubility in the liquid and frozen liquid acids. Azelaic acid was identified as a liquid-phase reaction product following the reaction with oleic acid. The implications regarding the atmospheric aging of aerosols with a fatty acid component are discussed.