Oxidation pathways of linoleic acid revisited with electrodynamic balance-mass spectrometry

Abstract

Unsaturated organic compounds in aerosol particles undergo oxidative ageing via heterogeneous reactions with atmospheric oxidants. Ozonolysis of linoleic acid has been serving as a proxy for this process and the linoleic acid decay has been shown to deviate from a linear dependence on ozone concentration. We use electrodynamic balance-mass spectrometry to measure mass spectra from levitated droplets before and after exposure to specific ozone mixing ratios. We find almost identical reactive uptake coefficients for ozone mixing ratios from 1 to 10 ppm, suggesting that a limitation due to droplet surface coverage with ozone is absent in this regime. However, a strong increase of the uptake coefficient for ozone mixing ratios below approximately 0.2 ppm is observed. Based on measurements using an oxygen atmosphere without ozone, which also show a degradation of linoleic acid, we attribute the apparent increase of the uptake coefficient to the oxidation of linoleic acid with molecular oxygen. These findings are consistent with an autoxidation mechanism, as proposed previously. Our results highlight that strongly elevated oxidant concentrations can mask the impact of autoxidation, which is slow but may dominate under atmospheric conditions.