Heterogeneous Reactions of Linoleic Acid and Linolenic Acid Particles with Ozone:  Reaction Pathways and Changes in Particle Mass, Hygroscopicity, and Morphology

Abstract

In this study, an electrodynamic balance (EDB) and a single particle Raman spectroscopic system were used to investigate the heterogeneous reactions of linoleic acid and linolenic acid with ozone under ambient temperatures (22−24 °C) and dry conditions (RH < 5%). Raman characterizations provide evidence that ozone-induced autoxidation, in addition to direct ozonolysis, is a plausible pathway in the reactions between ozone and linoleic acid and linolenic acid particles. Furthermore, the significance of this specific oxidation pathway depends on the ozone concentrations used in the experiment. A low ozone concentration (∼200−250 ppb) with a longer exposure period (20 h) favors autoxidation but an extremely high ozone concentration (∼10 ppm) favors ozonolysis and forces most unsaturated fatty acids to react with ozone in a relatively short period of time. In the low ozone concentration experiments, the mass of the ozone-processed linoleic acid and linolenic acid particles increased by about 2−3% and 10−13%, respectively. In addition, the mass ratios (particle mass at RH ≈ 85% to particle mass at RH < 5%) of the ozone-processed linoleic acid and linolenic acid particles increased by about 2−3% and 3−4%, respectively. The morphology of the pure and ozone-processed linoleic acid and linolenic acid particles are compared, based on imagining and their light scattering patterns