Oxidation of biogenic and water-soluble compounds in aqueous and organic aerosol droplets by ozone: a kinetic and product analysis approach using laser Raman tweezers

Abstract

The results of an experimental study into the oxidative degradation of proxies for atmospheric aerosol are presented. We demonstrate that the laser Raman tweezers method can be used successfully to obtain uptake coefficients for gaseous oxidants on individual aqueous and organic droplets, whilst the size and composition of the droplets is simultaneously followed. A laser tweezers system was used to trap individual droplets containing an unsaturated organic compound in either an aqueous or organic (alkane) solvent. The droplet was exposed to gas-phase ozone and the reaction kinetics and products followed using Raman spectroscopy. The reactions of three different organic compounds with ozone were studied: fumarate anions, benzoate anions and α-pinene. The fumarate and benzoate anions in aqueous solution were used to represent components of humic-like substances, HULIS; α-pinene in an alkane solvent was studied as a proxy for biogenic aerosol. The kinetic analysis shows that for these systems the diffusive transport and mass accommodation of ozone is relatively fast, and that liquid-phase diffusion and reaction are the rate determining steps. Uptake coefficients, γ, were found to be (1.1 ± 0.7) × 10⁻⁵, (1.5 ± 0.7) × 10⁻⁵ and (3.0–7.5) × 10⁻³ for the reactions of ozone with the fumarate, benzoate and α-pinene containing droplets, respectively. Liquid-phase bimolecular rate coefficients for reactions of dissolved ozone molecules with fumarate, benzoate and α-pinene were also obtained: k_fumarate = (2.7 ± 2) × 10⁵, k_benzoate = (3.5 ± 3) × 10⁵ and k_α-pinene = (1–3) × 10⁷ dm³ mol⁻¹ s⁻¹. The droplet size was found to remain stable over the course of the oxidation process for the HULIS-proxies and for the oxidation of α-pinene in pentadecane. The study of the α-pinene/ozone system is the first using organic seed particles to show that the hygroscopicity of the particle does not increase dramatically over the course of the oxidation. No products were detected by Raman spectroscopy for the reaction of benzoate ions with ozone. One product peak, consistent with aqueous carbonate anions, was observed when following the oxidation of fumarate ions by ozone. Product peaks observed in the reaction of ozone with α-pinene suggest the formation of new species containing carbonyl groups.