Ozonolysis of oleic acid particles: evidence for a surface reaction and secondary reactions involving Criegee intermediates.

The heterogeneous reactions of ozone with monodisperse oleic acid and methyl oleate particles were studied by monitoring the loss of the condensed-phase species using an aerosol chemical ionization mass spectrometer (Aerosol CIMS). The reaction of ozone with oleic acid was determined to occur at the surface of the particles despite previous assumptions that it reacts in the bulk. The ozonolysis of methyl oleate particles, on the other hand, was found to be limited by the diffusion of ozone and does react in the bulk. The difference in reaction mechanisms is attributed to the larger degree of order in oleic acid known to result from the formation of hydrogen-bonded dimers. The reactive uptake coefficients, gamma, calculated from the rate of loss of the particle species are gamma = (1.38 +/- 0.06) x 10(-3) for oleic acid and gamma = (1.23 +/- 0.10) x 10(-3) for methyl oleate. However, it is found that secondary reactions between Criegee intermediates and the carboxylic acid moiety in oleic acid are responsible for 36% (+/-4%) of its observed loss. Hence, the rate of loss of ozone cannot be equated to the rate of loss of oleic acid. Accounting for this additional reaction the uptake coefficient for ozone on oleic acid particles is calculated to be gamma = (8.8 +/- 0.5) x 10(-4). The magnitude of these secondary reactions quantitatively reconciles discrepancies between previous coated flow tube and particle-based studies, and it illustrates the need to include additional loss mechanisms when calculating uptake coefficients from the rates of loss of particle species. Implications of reactions with Criegee intermediates in atmospheric particles are discussed.