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The heterogeneous oxidation reaction of single aqueous ascorbic acid (AH$_2$)aerosol particles with gas-phase ozone was investigated in this study utilizingaerosol optical tweezers with Raman spectroscopy. The measured liquid-phasebimolecular rate coefficients of the AH$_2$ + O$_3$ reaction exhibit asignificant pH dependence, and the corresponding values at ionic strength 0.2 Mare $(3.1 \pm 2.0) \times 10^5$ M$^{-1}$s$^{-1}$ and $(1.2 \pm 0.6) \times10^7$ M$^{-1}$s$^{-1}$ for pH $\approx$ 2 and 6, respectively. These resultsmeasured in micron-sized droplets agree with those from previous bulkmeasurements, indicating that the observed aerosol reaction kinetics can besolely explained by liquid phase diffusion and AH$_2$ + O$_3$ reaction.Furthermore, the results indicate that high ionic strengths could enhance theliquid-phase rate coefficients of the AH$_2$ + O$_3$ reaction. The results alsoexhibit a negative ozone pressure dependence that can be rationalized in termsof a Langmuir-Hinshelwood type mechanism for the heterogeneous oxidation ofAH$_2$ aerosol particles by gas-phase ozone. The results of the present workimply that in acidified airway-lining fluids the antioxidant ability of AH$_2$against atmospheric ozone will be significantly suppressed.

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