The chemical and physical properties of a supercooled stratus cloud layer and surrounding clear air covering southern Ontario on February 20, 1984, were studied with the objectives of identifying the processes responsible for the cloud water chemical composition. The cloud layer, which extended from circa 650 to 1050 m mean sea level, was bounded by a strong temperature inversion just at cloud top. The air below this inversion was well mixed vertically, as indicated by the near independence of concentrations of various trace gas species with altitude and by conservation of number concentration of particles from the surface to cloud top, with below-cloud aerosol number concentrations (0.2<d<3 μm) approximately equal to the sum of the interstitial aerosol and cloud droplet (2<d<3 μm) number concentrations. Fractional incorporation of aerosol sulfate into cloud water, determined by comparison of interstitial and cloud water sulfate concentrations, was in the range of 85--90%, similar to the result inferred from comparison of interstitial aerosol and cloud droplet number concentrations. Gas phase equivalent concentrations of cloud water species in the below-cloud air. These results suggest that the dominant processes determining cloud water composition in this cloud layer were nucleation scavenging of sulfate aerosol and scavenging of gaseous HNO3, with no significant contribution from reactive scavenging processes. ¿ American Geophysical Union 1987