In-cloud oxidation of SO2 by O3 and H2O2:: Cloud chamber measurements and modeling of particle growth

Controlled cloud chamber experiments were conducted to measure particle growth resulting from the oxidation of SO2 by O-3 and H2O2 in cloud droplets formed on sulfuric acid seed aerosol. Clouds were formed in a 590 m(3) environmental 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 mobility analyzer size distributions before and after each 3-4 min cloud cycle. Predictions of aerosol growth were then made with a full microphysical cloud model used to simulate each individual experimental cloud cycle. Model results 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%. Simulation results of aerosol growth during the O-3, oxidation experiments were 60-80% less than the measured values, confirming previous results [Hoppel et al., 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 currently accepted values to explain the measured growth. However, unmeasured NH3 contamination present in trace amounts (<0.2 ppb) could explain the disagreement, but this is speculative and the source of this discrepancy is still unknown.