Ionic-Strength And Ph Dependent Reactivities Of Ascorbic Acid Toward Ozone In Aqueous Micro-Droplets Studied Using Aerosol Optical Tweezers

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 +/- 2.0) x 10(5) M-1 s(-1) and (1.2 +/- 0.6) x 10(7) M-1 s(-1) for pH approximate to 2 and 6, respectively. These results measured in micron-sized droplets are in agreement 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.