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 SO2, NO3-, HNO3, aerosol SO4=, and occasionally aerosol NO3-. 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 SO4= and NO3- concentrations. Examination of cloud water composition indicated that HNO3 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 SO2 oxidation was of minor importance in determining cloud composition. The conclusion is supported by measurements of hydrogen peroxide in cloud water samples. Cloud water NO3-/SO4= ratios were found to exceed (HNO3+aerosol NO3-)/SO4= ratios in air at the surface in most cases, and in a few instances cloud water NO3- 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. In-cloud SO2 oxidation is found to be of minor significance, but the data may indicate in-cloud production of HNO3 in some cases. ¿ American Geophysical Union 1988