The reaction of sulfur dioxide with ozone in water and its possible atmospheric significance

Aqueous phase oxidation of sulphur dioxide by ozone in cloud droplets

“…This contrasts with the findings of at least two previous studies (Hoppel et al, 1994b;Caffrey et al, 2001), and seems to confirm the suggestion made by those authors that the observed disagreement resulted from the presence of contaminants such as ammonia. Furthermore, we have performed what we believe to be the first laboratory-based measurements of the aqueous phase oxidation of SO 2 in a population of super-cooled cloud droplets, and confirmed that the generally accepted temperature dependence of the oxidation reactions, measured by Erickson et al (1977), is consistent with our experimental results at temperatures of −10 • C.…”

“…Similarly to the present study, they used O 3 mixing ratios of 120 to 150 ppbv, with a wider range of SO 2 mixing ratios (23 to 310 ppbv). They found good agreement between the measured (dual channel ion chromatograph) increase in S(VI) in the cloud water, and values calculated using the oxidation rate constants measured by Erickson et al (1977) in bulk solutions, suggesting that such rate constants are indeed applicable to the chemistry occurring in cloud droplets, at warm temperatures.…”

“…The temperature dependence of this reaction is therefore very important. Measurements performed by Maahs (1983), at 25 and 10 • C, using buffered solutions at pH up to 6.2 suggest a slightly weaker temperature dependence, and therefore greater oxidation rates at low temperatures than suggested by Erickson et al (1977).…”

“…This may point to a slightly too large temperature dependence of the reaction rate constants, and indeed, using the temperature dependence of Maahs (1983) leads to a slight closing of the gap between the −10 • C and the 10 • C modelled points (not shown). However, as the rate constants given in the studies of Erickson et al (1977) and Maahs (1983) were based on measurements at 25 • C, weakening the temperature dependence also increases the modelled growth of aerosol at 10 • C. During the CLOUD8 experiments at −10 • C, the presence of an ice phase was detected from the PPD-2K measurements. The onset of ice formation was also seen in the change of SIMONE forward scattering and depolarisation signals.…”

“…However, to the best of our knowledge, no studies have been performed at temperatures below 0 • C, thus values for sub-zero temperatures are based on extrapolations of the temperature dependence at higher temperatures. The temperature dependence recommended by Hoffmann (1986), and adopted in most modelling studies, is that of Erickson et al (1977), based on measurements at 25 and 16 • C. In that study, experiments were only performed up to a pH of 4.02, where the vast majority (i.e. 0.994) of dissolved S(IV) is present as HSO − 3 .…”