A multielement tracer technique for the determination of SO2 oxidation in clouds

A technique was developed in this laboratory which uses similarities in the physical and chemical properties of SO4 and Se aerosols to quantitatively resolve SO4 in cloud water derived from the gas and aqueous phase oxidation of SO2. We report here the extension of the technique to include As and Sb as tracers and present experimental data to verify assumptions in our earlier studies regarding the sampling of precloud aerosols. In situ SO4 production in clouds [(SO4in)] is given by [SO4in] = [(SO4/M)(cw) - alpha/(beta,delta,tau) (SO4/M)(aa)].(M)(cw) where M represents Se, As, or Sb; alpha,beta, delta and tau are the scavenging efficiencies of SO4, Se, As, and Sb aerosols, respectively; and cw and aa represent cloud water and ambient precloud aerosols. Field data from three cloud systems are presented, which show that delta/beta = 0.92 +/- 0.12 and tau/beta = 0.95 +/- 0.13. Therefore within experimental uncertainties, the scavenging efficiencies of As and Sb are determined to be equal to that of Se. Results of in-cloud oxidation determined in three clouds using Se, As, and Sb are in excellent agreement. Regression analysis of SO4in by As and Se had a slope of 1.02 +/- 0.05 (r=0.85). Similarly for Sb and Se the slope was 1.06 +/- 0.05 (r=0.85).