Sulphate production in summer cloud over Ontario, Canada

Abstract

The contribution of in-cloud oxidation of S(IV) to the cloudwater SO₄= concentration is examined using airborne measurements made in and around 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=, NH₄+ and H⁺. The quantity of SO₄= produced in cloud is determined by subtracting the SO₄= due to nucleation scavenging from the total cloudwater SO₄=. The nucleation scavenged SO₄= is estimated by 3 methods: (1) using NH₄+ as a tracer of particulate SO₄=; (2) using the number of accumulation mode particles (generally the SO₄= bearing particles) activated in cloud; (3) using the volume of the accumulation mode particles activated in cloud. The NH₄+ method gives detectable in-cloud SO₄= production for 20 of 36 data points, ranging from 8% to 83% of the total, with a median of 27%. The aerosol number approach yields detectable 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 production. The ammonium and aerosol number method results are consistent with the observations of the precursors for S(IV) oxidation. The difference 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, while 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 explained if the aerosol sulphate is preferentially distributed among the particles of sizes smaller than the accumulation mode mass modal size.