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.
