Direct solar ultraviolet irradiance over Nainital, India, in the central Himalayas for clear-sky day conditions during December 2004

Abstract

From a high-altitude station, Nainital, India (29.4°N, 79.5°E, 1958 masl), located in the central part of lower Himalayas, the observations made during December 2004 using a pair of Sun photometers (Microtops II) at wavelengths ranging from 305 to 1020 nm are reported. The observed parameters are the direct solar UV irradiance, column ozone, water vapor, and aerosol optical depths (AOD). The results are presented for the full day clear-sky conditions that prevailed for about 16 days during the whole month. It is found that Nainital is a comparatively pristine site with average AOD at 500 nm ranging between 0.03 and 0.09 and Angstrom exponent generally close to 1. The high AOD values occurred on 2 and 25 December due to winds from populated north Indian plains as revealed by the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) backward trajectory model. The total column ozone varies between 251 and 308 DU during the entire period of observations. The maximum diurnal UV irradiance values in the 2.4 nm bandwidth centered at 305.5, 312.5, and 320.0 nm varied between 0.027 and 0.049, 0.15 and 0.20, and 0.29 and 0.37 W m-², respectively. The measured UV irradiances are compared with the Tropospheric Ultraviolet Visible (TUV) radiation model and show a good agreement.