Effect of relative humidity on light scattering by polydisperse aerosols

Abstract

Size distribution of aerosols in the mixing region aloft 190 m derived using CW lidar showed a systematic variation with relative humidity (RH). In obtaining the size distribution, the functional form of the aerosol size distribution is taken to be modified power law type. The size index of this distribution is found to decrease from 4.7 to 3.5 as RH increases from 70% to 90%. The effect of hydration on light scattering by atmospheric aerosols is studied using this observed variation in aerosol size distribution. Effect of hydration is also studied for three other size distributions (viz. the Hanel’s power law, the cumulative bimodal size distribution obtained from the multi-wavelength solar radiometer at Trivandrum, and the near surface size distribution obtained using a low pressure impactor at Trivandrum) starting with a dry distribution and model growth rate of particles. The basic forms of all these size distributions are different as they represent different altitude regions. It is seen that the hydration effect on scattering for Hanel’s model compares fairly well with that of the CW lidar derived size distribution.