Multiple wavelength solar radiometer studies at Pune

197-203A multiple wavelength radiometer was developed and used to study the behaviour of aerosol optical depth at Pune during the Indian Middle Atmosphere Programme (IMAP). The radiometer was calibrated using the Langley technique at four selected wavelengths, viz. 558, 630, 690 and 735 nm. The data collected during December 1986-April 1987 was discussed. Application of Langley technique reveals that besides the linear type, two-segment Langley plot is a special feature of the aerosol behaviour at Pune during winter months. Optical depth due to particulates (i.e. the aerosol optical depth, ap) exhibits diurnal variation which is large in February and small in April. Day-to-day variation in ap is small with occasional high values on hazy days. In the monthly data, ap values are high in February and April. While the April high is the expected summer high, February high is related to the large difference in the morning and afternoon values of relative humidity (RH) observed at Pune. This effect gets further strengthened during the temperature inversion episodes occurring in the atmospheric boundary layer at Pune. Aerosol optical depth increases in the annual plot from 1983 to 1987. The spectral variation of ap exhibits a peak around 630 nm. The corresponding particle size distribution has logarithmic Gausian form. The size spectrum is unimodal with a mode radius of 0.378 m. This points to a single source of particles