Direct and global solar radiation measurements carried out in Kathmandu valley (N, 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 (β) is found high in the morning and show decreasing trends from morning to late afternoon on average. The wavelength exponent (α) 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.
Multiband filter radiometers (MBFRs) are extensively used in national networks for UV climate monitoring and information to the public about the potential risk of solar UV exposure. In order to provide an international, uniform expression of the Global UV index measurements, a harmonized calibration scale is needed. In this paper we present the results of the first international intercomparison of MBFRs held in Oslo in 2005. The purposes are to evaluate the UV-index scale of different radiometers and to provide a harmonized UV-index scale based on the radiometers individual directional and absolute spectral response functions. In total 43 MBFR radiometers and 4 high resolution spectroradiometers were assembled, representing UV-monitoring networks operated by institutions in US, Spain, Greece, Poland, Belgium, UK, Austria, Norway, Sweden and Finland. The radiometers are operating worldwide, with stations in the Antarctica and Arctic, North- and South-America, Africa, Europe, Middle-East and Nepal. All sky conditions were realized during the campaign period. The agreement between the users' own processed UVI and the reference is generally very good; within ±5% for 22 out of 26 data sets (75%) and ±10% for 23 out of 26 (88%). Solar zenith dependent discrepancies and drift in the users' UVI scales is seen, but the performance of most radiometers is generally very good. All the objectives planned for the intercomparison were fulfilled and the campaign considered a success.