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 (AH2) 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 AH2 + O3 reaction exhibit a significant pH dependence, and the corresponding values at ionic strength 0.2 M are (3.1 ± 2.0) × 105 M-1 s-1 and (1.2 ± 0.6) × 107 M-1 s-1 for pH ≈ 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 AH2 + O3 reaction. Furthermore, the results indicate that high ionic strengths could enhance the liquid-phase rate coefficients of the AH2 + O3 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 AH2 aerosol particles by gas-phase ozone. The results of the present work imply that in acidified airway-lining fluids the antioxidant ability of AH2 against atmospheric ozone will be significantly suppressed.

Temperature-Dependent Rate Coefficient for the Reaction of CH3SH with the Simplest Criegee Intermediate.

The kinetics of the reaction of the simplest Criegee intermediate CH2OO with CH3SH was measured with transient IR absorption spectroscopy in a temperature-controlled flow reaction cell, and the bimolecular rate coefficients were measured from 278 to 349 K and at total pressure from 10 to 300 Torr. The measured bimolecular rate coefficient at 298 K and 300 Torr is (1.01 ± 0.17) × 10-12 cm3 s-1. The results exhibit a weak negative temperature dependence: the activation energy Ea ( k = Ae- Ea/ RT) is -1.83 ± 0.05 kcal mol-1, measured at 30 and 100 Torr. Quantum chemistry calculations of the reaction rate coefficient at the QCISD(T)/CBS//B3LYP/6-311+G(2d,2p) level (1.6 × 10-12 cm3 s-1 at 298 K; Ea = - 2.80 kcal mol-1) are in reasonable agreement with the experimental results. The experimental and theoretical results of the reaction of CH2OO with CH3SH are compared to the reactions of CH2OO with methanol and hydrogen sulfide, and the trends in reactivity are discussed. The results of the present work indicate that this reaction has a negligible influence to atmospheric CH2OO or CH3SH.