The relative importance of non-sea-salt sulphate and sea-salt aerosol to the marine cloud condensation nuclei population: An improved multi-componentaerosol-cloud droplet parametrization

The effect of sub-cloud aerosol on cloud droplet concentration was explored over the north Atlantic and east Pacific under a variety of low and high w ind speed conditions. A relationship of the form of D = 197{1 - exp(-6.13 x 10(-3) * A)} was found to fit best the relationship between cloud droplet concentration (D; cm(-3)) and sub-cloud aerosol concentration(A; cm(-3)) un der low to moderate wind conditions. A few noticeable deviations from this relationship were observed which occurred under moderate to high wind speed condition. Under these high wind conditions, sea-salt aerosol provided the primary source of cloud nuclei due to their higher nucleation activity and larger sizes, even under sulphate-rich conditions. Simple model simulation s reveal that the activation of sea-salt nuclei suppresses the peak supersa turation reached in cloud, and thus inhibits the activation of smaller sulp hate nuclei into cloud droplets. A multi-component aerosol-droplet parametr ization for use in general circulation models is developed to allow predict ion of cloud droplet concentration as a function of sea-salt and non-sea-sa lt-(nss) sulphate nuclei. The effects of enhancing an existing nss-sulphate cloud condensation nuclei (CCN) population with sea-salt nuclei are to red uce the number of cloud droplets activated under high (polluted) sulphate c onditions and to increase the cloud droplet concentration under low (clean) sulphate conditions. The presence of sea-salt CCN reduces the influence of nss-sulphate CCN on cloud droplet concentrations, and thus is likely to re duce the predicted effect of nss-sulphate indirect radiative forcing.