Changes in concentration and size distribution of aerosols during fog over the south Indian Ocean

Abstract

Measurements of the concentration and size distribution of aerosol particles in the size-ranges of 0.5-20 μm and 16-700 nm diameters were made during six fog episodes over the south Indian Ocean. Observations show that concentrations of particles of all sizes start decreasing 1-2 hours before the occurrence of fog. This decrease is more prominent for coarse particles of >1 μm diameter and continues until 10-20 minutes before the onset of fog when particle concentrations in all size ranges rapidly increase by one/two orders of magnitude in ~20 minutes. Thereafter, concentrations of particles of all sizes gradually decrease until the dissipation of fog. After the fog dissipation, concentrations of coarse mode particles rapidly increase and restore to their pre-fog levels but concentrations of the Aitken mode particles decrease slowly and reach their pre-fog levels only after 1-2 hours. The net effect of fog is to change the bimodal size distributions of aerosols with a coarse mode at 1.0 μm and an accumulation mode at 40-60 nm to a power law size distribution. It is proposed that the preferential growth and sedimentation of the coarse mode hygroscopic particles in the initial phase cause a large decrease in the aerosol surface area. As a result, the low vapour pressure gases which were initially being used for the growth of coarse mode particles, now accelerate the growth rates of the accumulation and Aitken mode particles.