The effect of chemical composition and size distribution of aerosol particles on droplet formation and albedo of stratocumulus clouds

A numerical model was applied to simulate the evolution of water droplets on aerosol particles with different sizes and chemical compositions. The detailed microphysical model was incorporated into a one-dimensional parcel model to investigate the indirect effects of aerosol particles on climate. The calculations were initiated with different aerosol size distributions observed during the field measurements in 1973–1976 and at the end of the 20th century in rural Hungary. The effect of changes in the characteristics of aerosol particles during this time period on the number concentration of water droplets formed was calculated at different cloud base updraft velocities. It was found that about 10% of sulfate particles and about 1% of organic aerosol particles with radius larger than 0.025 μm take part in drop formation if the updraft velocity is weak (0.1 m s−1). Mostly haze particles form on this type of aerosol particles in this case: the majority of cloud droplets come into being on sulfate particles. If the updraft velocity is 1 m s−1 at the cloud base, a larger fraction (≈30%) of the organic aerosol particles become activated. On the basis of our calculations, it is concluded that the observed modification of aerosol content resulted in a decrease in albedo of stratocumulus clouds of about 0.02 in the Central European region. If only the sulfate particles are considered, the corresponding albedo decrease is higher, about 0.03. The presence of soot in the aerosol particles, to some extent, mitigates the decrease of the albedo caused by the decrease of sulfate content.