In-cloud oxidation of SO2 by O-3 and H2O2: Cloud chamber measurements and modeling of particle growth

Controlled cloud chamber experiments were conducted to measure particle gro wth resulting from the oxidation of SO2 by O-3 and H2O2 in cloud droplets f ormed on sulfuric acid seed aerosol. Clouds were formed in a 590 m(3) envir onmental chamber with total liquid water contents ranging from 0.3-0.6 g m( 3) and reactant gas concentrations < 10 ppbv for SO2 and H2O2 and < 70 ppbv for O-3. Aerosol growth was measured by comparison of differential mobilit y analyzer size distributions before and after each 3-4 min cloud cycle. Pr edictions of aerosol growth were then made with a full microphysical cloud model used to simulate each individual experimental cloud cycle. Model resu lts of the H2O2 oxidation experiments best fit the experimental data using the third-order rate constant of Maass et al. [1999] (k = 9.1 x 10(7) M-2 s (-1)), with relative aerosol growth agreeing within 3% of measured values, while the rate of Hoffmann and Calvert [1985] produced agreement within 4-9 %, and the rate of Martin and Damschen [ 1981] only within 13-18%. Simulati on results of aerosol growth during the O-3, oxidation experiments were 60- 80% less than the measured values, confirming previous results [Hoppel et a l., 1994b]. Experimental results and analyses presented here show that the SO2 - O-3 rate constants would have to be more than 5 times larger than cur rently accepted values to explain the measured growth. However, unmeasured NH3 contamination present in trace amounts (<0.2 ppb) could explain the dis agreement, but this is speculative and the source of this discrepancy is st ill unknown.