Changes in concentration and size distribution of aerosols during fog over the south Indian Ocean |
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| Authors: | Vimlesh Pant C G Deshpande A K Kamra |
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| Institution: | 1.Indian Institute of Tropical Meteorology,Pune,India |
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| 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. |
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