Ionic-strength and pH dependent reactivities of ascorbic acid toward ozone in aqueous micro-droplets studied by aerosol optical tweezers

Abstract

The heterogeneous oxidation reaction of single aqueous ascorbic acid (AH${}_2$) aerosol particles with gas-phase ozone was investigated in this study utilizing aerosol optical tweezers with Raman spectroscopy. The measured liquid-phase bimolecular rate coefficients of the AH${}_2$ + O${}_3$ reaction exhibit a significant pH dependence, and the corresponding values at ionic strength 0.2 M are $(3.1\pm 2.0)\times 10^5$ M${}^{-1}$s${}^{-1}$ and $(1.2\pm 0.6)\times 10^7$ M${}^{-1}$s${}^{-1}$ for pH $\approx$ 2 and 6, respectively. These results measured in micron-sized droplets agree with those from previous bulk measurements, indicating that the observed aerosol reaction kinetics can be solely explained by liquid phase diffusion and AH${}_2$ + O${}_3$ reaction. Furthermore, the results indicate that high ionic strengths could enhance the liquid-phase rate coefficients of the AH${}_2$ + O${}_3$ reaction. The results also exhibit a negative ozone pressure dependence that can be rationalized in terms of a Langmuir-Hinshelwood type mechanism for the heterogeneous oxidation of AH${}_2$ aerosol particles by gas-phase ozone. The results of the present work imply that in acidified airway-lining fluids the antioxidant ability of AH${}_2$ against atmospheric ozone will be significantly suppressed.