  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 $(1.8 \pm 0.8) \times
10^7$ 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.
