Initiation of linoleic acid autoxidation with ozone exposure in levitated aerosol particles

Abstract

When atmospheric aerosol particles undergo oxidation, their physico-chemical properties are altered, influencing their environmental impact. Recent work has revealed that ozonolysis and autoxidation both contribute to the decay of linoleic acid. However, the observed linoleic acid decay rates could only be explained if the autoxidation induction period was shorter in experiments with ozone than without ozone. In this study, we investigate whether linoleic acid autoxidation in levitated aerosol particles can be initiated by ozonolysis. Linoleic acid droplets were levitated in an electrodynamic balance, exposed to air with and without ozone, and subsequently analysed by mass spectrometry. Specifically, droplets were first exposed to air containing 39 ppb or 619 ppb ozone for 1 h and thereafter, the gas phase was switched to air without ozone. The observed autoxidation rates were compared with reference experiments without the initial ozonolysis phase, the latter showing an extended induction period. These findings indicate that ozonolysis produces considerable radical concentrations, leading to the observed shortening of the induction period. A comparison of the measurements to a simple one-compartment bulk reaction model was used to estimate the radical contribution from ozonolysis. To explain the observed shortening of the induction period, assuming that radicals are formed from the decomposition of linoleic acid oxidation products, a reaction rate constant for the decay of 5.1 × 10−8 s−1 is obtained. Studies under extreme conditions, such as highly elevated ozone concentrations, may overlook the role of the synergistic effects discussed here, which are only of importance at atmospherically relevant oxidant concentrations.