Aerosol climatology in Kathmandu using Sun Photometry

Direct and global solar radiation measurements carried out in Kathmandu valley (27.7 degrees N, 85.5 degrees E 1350 masl) have been analyzed in this study. Observations were made using a microprocessor controlled portable aerosol meter Microtops II working in ultraviolet (340, 380 nm) and visible range (440, 500, 675 nm) under cloudless conditions. Aerosol optical depth at 380 nm was found to be 0.33, 0.48, 0.21 and 0.45 in summer, autumn, spring and winter season respectively. Monitoring of global solar radiation was done with a ground based ultraviolet radiometer (GUV). From the aerosol optical depth from Sun Photometer, Angstrom turbidity parameters were estimated by means of the least square fitting technique. The aerosol optical depth is also utilized to make a model fit for UV index and compare the GUV data. A decrease in humidity by about 70% is found to increase the UV index by about 22%. The atmospheric turbidity parameter (beta) is found high in the morning and show decreasing trends from morning to late afternoon on average. The wavelength exponent (alpha) becomes smaller during noon indicating existence of higher proportion of tiny particles in lower atmosphere in day time. Finally an inversion of spectral optical depth measurements technique allowed estimating aerosol size distribution changing between unimodal to bimodal functions yearly.