Evolution in the Reactivity of Citric Acid toward Heterogeneous Oxidation by Gas-Phase OH Radicals

Abstract

Oxidation initiated at or near a particle surface by gas-phase oxidants can continuously change the composition and properties of organic particles, which in turn alter the heterogeneous reactivity over time. However, chemical transformation of organic particles by heterogeneous oxidation is typically described by a single kinetic parameter (effective OH uptake coefficient, γeff), which implicitly assumes the reactivity does not change significantly over their atmospheric lifetimes. Using time-resolved particle composition and size data measured in an environmental chamber, it is shown that the heterogeneous reactivity of citric acid toward gas-phase OH radicals continuously decreases over reaction time and slows down by 16% from an initial γeff of 1.61 ± 0.16 to 1.35 ± 0.14 after oxidation equivalent to about 2 days of OH exposure. The decrease in the γeff over time can be explained by consumption of citric acid, and its concentration at the particle surface drops due to OH oxidation and the formation of...