Study of atmospheric aerosol loading using the twilight method

A photometer has been developed for monitoring the rate of change of zenith sky illumination during the twilight period. A gelatine filter centered at 700 nm (75 nm Hbw) and RCA 6099 photomultiplier tube were selected for the photometer. The intensity parameter, ( I d h) −1 d l, was determined and its changes attributed to variations in the structure of the atmosphere resulted due to changes in aerosol density. The results indicate that the layer of maximum aerosol drifts at 1.7 km day −1 as sky conditions vary between clear and cloudy. The aerosol loading parameter, obtained from an earlier study at Mt. Abu (Shah, 1970, Tellus 22, 82–93), showed decrease in the layers 8–44 km and 44–87 km as the summer monsoon season approaches. The ground-based observations of Aitken nuclei carried out at Pune (Khemani et al., 1985, Water Air Soil Pollut. 25, 365–376) also supported these results. The relative contributions of condensation nuclei concentration from different sources such as land/ocean, troposphere and stratosphere to cloud formation processes are discussed in the light of the observations.