Scavenging efficiencies of aerosol particles in marine stratocumulus and cumulus clouds

Scavenging efficiencies of aerosol particles in marine stratocumulus and cumulus clouds obtained using Counterflow Virtual Impactors mounted on aircraft are presented as a function of pollution level. The greatest accumulation-mode particle number and mass-scavenging fractions were found for clean conditions, and are consistent with (but slightly lower than) previous studies. Vertical gradients in the concentrations of coarse-mode particles observed in shallow and decoupled marine boundary layers reduced the influence of sea-salt particles, in terms of broadening the cloud droplet spectra and as a possible mechanism to oxidize SO2 to non-sea-salt sulphate in the marine stratiform cloud layer. For supersaturations typical for marine stratocumulus clouds, particles smaller than 0.1 mum in diameter dominated the formation of cloud droplets for several of the cases analysed here. For larger supersaturations, in stratocumulus clouds inside an area of cumulus penetration, the concentration of residual particles smaller than 0.1 mum in diameter was nearly a factor of three higher than the accumulation-mode number. Poor correlation was found between accumulation-mode residual particle number and mass with cloud droplet number for the clean-condition category. This suggests that the particles determining the cloud droplet number populations in clean conditions and for polluted situations with relatively large updraughts were below the accumulation-mode size range. Additionally, it seems that the droplet numbers were insensitive to changes in the accumulation-mode mass for these conditions.