Kinetics study of heterogeneous reactions of ozone with unsaturated fatty acid single droplets using micro-FTIR spectroscopy

Ozone initiated heterogeneous oxidation of micron-sized oleic acid (OA), linoleic acid (LA), and linolenic acid (LOA) single droplets was investigated using a gas-flow system combined with microscopic Fourier transform infrared (micro-FTIR) spectrometer. The pseudo-first-order rate constant (k_app) and the overall uptake coefficient (γ) are obtained by quantitatively estimating the changes in absorbance area of the C═O stretching band at 1710 cm⁻¹, which is assigned to the carboxyl group of the reactant. The overall kinetics is dominated by surface reaction. And the effect of surface adsorption, which is derived from the ozone concentration and particle size effects on reaction kinetics, plays an important role during the reaction. Comparison of the k_app values corresponding to OA, LA and LOA shows the positive correlation between double bonds and reaction rate. In the view of RH effect, both k_app and γ are strongly enhanced by over a factor of three for the LOA/O₃ reaction system as the relative humidity (RH) increases from ∼0% to 83%. The LA/O₃ reaction system exhibits a weaker RH dependence. In contrast, the k_app and γ of the OA/O₃ reaction system are independent of the RH changes. Moreover, the various hygroscopicities of the three acids and corresponding products lead to different reactivities.