SULFATE PRODUCTION IN SUMMER CLOUD OVER ONTARIO, CANADA

The contribution of in-cloud oxidation of S(IV) to the cloudwater SO4= concentration is examined using airborne measurements made in and aro und summer clouds over central Ontario. The measurements include SO2, H2O2, aerosol and cloud droplet size distribution, cloud liquid water content, HNO3, NH3, particulate SO4= and NH4+, and cloudwater SO4=, NH 4+ and H+. The quantity of SO4= produced in cloud is determined by sub tracting the SO4= due to nucleation scavenging from the total cloudwat er SO4=. The nucleation scavenged SO4= is estimated by 3 methods: (1) using NH4+ as a tracer of particulate SO4=; (2) using the number of ac cumulation mode particles (generally the SO4= bearing particles) activ ated in cloud; (3) using the volume of the accumulation mode particles activated in cloud. The NH4+ method gives detectable in-cloud SO4= pr oduction for 20 of 36 data points, ranging from 8 % to 83 % of the tot al, with a median of 27 %. The aerosol number approach yields detectab le production for 6 of the 36 data points, ranging from 39 % to 71 %, with a median of 55 %. The aerosol volume method gives no detectable p roduction. The ammonium and aerosol number method results are consiste nt with the observations of the precursors for S(IV) oxidation. The di fference between the ammonium and aerosol number methods is reasonable in so much as the number method results are considered overestimates because the nucleation-scavenged SO4+ mass may be underestimated, whil e the ammonium method results may underestimate the production due to the scavenging of NH3 by the acidic cloudwater. The lack of detectable sulphate production provided by the aerosol volume method is explaine d if the aerosol sulphate is preferentially distributed among the part icles of sizes smaller than the accumulation mode mass modal size.