Effect of Ozone Concentration and Relative Humidity on the Heterogeneous Oxidation of Linoleic Acid Particles by Ozone : An Insight into the Interchangeability of Ozone Concentration and Time

Ozonolysis and autoxidation have been realized as important pathways for the heterogeneous ozone oxidation of polyunsaturated fatty acids, but the relative significance of these two pathways under various environmental conditions (e.g., ozone concentration ([O₃]) and relative humidity (RH)) remains largely unclear. In this work, the heterogeneous oxidation of linoleic acid particles under a wide range of [O₃] and at different RHs was investigated using an aerosol flow cell reactor coupled with in situ micro-Raman spectroscopy. Linoleic acid autoxidation products containing conjugated diene structures accumulated effectively at [O₃] ≤ 100 ppb, while they were ultimately consumed at [O₃] ≥ 250 ppb via ozonolysis. Increased RH (60%) exhibited a retarding effect on the ozonolysis, likely due to the competition between ozone and water molecules for adsorption on the particle surface. The formation of autoxidation products containing conjugated diene structures at higher RH was also less effective, partly because the active Criegee intermediates have higher tendency to react with water than to produce free radicals to initiate the autoxidation process. The interchangeability between [O₃] and time in exposure was qualitatively examined. It holds at ozone exposure ≤ 500 ppb h and [O₃] ≤ 100 ppb for the formation of autoxidation products containing conjugated diene structures but does not hold for the decay of parent linoleic acid or at other levels of [O₃]. This work suggests that the extrapolation of experimental results of the oxidation of linoleic acid particles by ozone under high [O₃] conditions to ambient levels should be carried out with caution.