Winter cloud water and air composition in central Ontario

Time histories of surface air composition and cloud water composition in winter in Central Ontario are compared in order to establish a winter data set for this region, and to identify dominant processes responsible for the incorporation of sulfate, nitrate, and acidity into cloud water. Cloud water comparisons are restricted to low-level collections (median altitude 625 m above ground) within the boundary layer. Surface level measurements during the 6-week period show a variety of conditions, including several episodes of elevated concentrations of SO/sub 2/, NO/sub 3//sup -/, HNO/sub 3/, aerosol SO/sub 4//sup =/, and occasionally aerosol NO/sub 3//sup -/. Nitric acid comprised the majority of soluble acidity at ground level more than half the time, and was relatively more important during the periods of elevated SO/sub 4//sup =/ and NO/sub 3//sup -/ concentrations. Examination of cloud water composition indicated that HNO/sub 3/ was also the dominant contributor to cloud water acidity on the majority of days sampled. Comparisons of air and cloud water absolute concentrations and concentration ratios suggest that aqueous SO/sub 2/ oxidation was of minor importance in determining cloud composition. The conclusion is supported by measurements of hydrogen peroxide in cloud water samples. Cloud water NO/sub 3//sup -/SO/submore » 4//sup =/ ratios were found to exceed (HNO/sub 3/+aerosol NO/sub 3//sup -/)SO/sub 4//sup =/ ratios in air at the surface in most cases, and in a few instances cloud water NO/sub 3//sup -/ concentrations were higher than expected based on the surface air composition data; several possible causes for this observation are discussed. It is concluded that in this region in winter, the composition of low-level clouds can be estimated to within a factor of 2 by measurements of air composition at the surface, and that dissolution of soluble aerosol and gaseous species is primarily responsible for cloud composition.« less