Cloud droplet nucleation and cloud scavenging of aerosol sulphate in polluted atmospheres

Abstract

Aircraft investigations of the microphysical and chemical properties of mildly to moderately convective clouds and stratiform clouds were conducted during the summer of 1982 and the early winter of 1984 over Algonquin Park, Ontario, Canada. Similar studies were carried out during the autumn of 1984 over upper New York state. Varying degrees of polluted conditions were observed. For the situation where the cloud liquid water content was close to the adiabatic value and the particle concentration was <750 cm -3 , the concentration of cloud droplets was directly proportional to the number concentration of cloud-base aerosol particles. For particle concentrations above 750 cm -3 , the cloud droplet concentration did not increase as fast as the particle numbers. An adiabatic model describing the condensational growth of cloud droplets predicted this apparent non-proportionality and indicated it to be the result of increasing competition for water vapour by increasing numbers of cloud condensation nuclei. The best quantitative agreement with the observations was obtained for the accommodation coefficient for condensation of water equal to unity and low values of the updraft speed at cloud base. Comparisons between cloud-averaged number concentrations of cloud droplets and number concentrations of cloud-base aerosol particles behaved in a very similar manner to the comparisons of near adiabatic cloud droplet concentrations with aerosol concentrations. Comparisons of the mass concentrations of cloudwater SO 4 with that of aerosol SO 4 , measured at ground level, appeared to follow a similar behaviour as :he cloud droplet concentrations, but with a much larger uncertainty. DOI: 10.1111/j.1600-0889.1986.tb00258.x