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

TL;DR

This study investigates how pH and ionic strength affect the reaction rate of ascorbic acid with ozone in tiny water droplets, revealing pH-dependent kinetics and ionic strength effects relevant to atmospheric and biological contexts.

Contribution

It provides the first detailed measurement of pH and ionic strength effects on ascorbic acid ozone reactions in micro-droplets, confirming liquid-phase diffusion as the main mechanism.

Findings

Reaction rates are pH-dependent, increasing with higher pH.

High ionic strength enhances the reaction rate.

Ozone pressure dependence suggests a Langmuir-Hinshelwood mechanism.

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.