Impact of Precipitation on Aerosol Spectral Optical Depth and Retrieved Size Distributions: A Case Study

Abstract

A case study is presented on the impact of two isolated, strong thundershowers during a prevailing dry, sunny season on the spectral optical depths and inferred columnar size characteristics of atmospheric aerosols at a tropical station. Results show a remarkable decrease in the aerosol optical depth and change in the spectral slope after the rain. The scavenging was found to be dependent on the particle size distribution; the larger, supermicron particles were found to be removed faster during the first shower itself, even though it was of only moderate intensity, resulting in about a 64% decrease in the columnar mass loading. In the second shower, which was stronger and more widespread than the former, more of the submicron particles in the optically active submicron size range were removed, but the reduction in mass loading was very small. The effective radius decreased continuously and so too did the columnar mass loading (total aerosol volume). The data are used to estimate the apparent columnar scavenging efficiency. The inferred apparent scavenging efficiencies were ?57% and 68% for the aerosol columnar number density in the optically active submicron size range for the two events, whereas for the coarse aerosols (r > 0.5 ?m), it was ?75% for the first event but insignificant for the subsequent event, in line with the pattern of mass loading. The prevailing synoptic conditions (continental air mass and dry weather) helped the atmosphere to ?recharge? within about 1 week after the events (which removed more than 70% of the aerosol burden), unlike the case with extensive synoptic phenomena, like monsoons, for which the aerosol optical depths remain depleted over the entire season. This recharging is also dependent on the size distribution of aerosols; the fine and accumulation particles are replenished faster than the coarse particles.