Resumen
Observations from a ground based multi-wavelength radiometer (MWR) in the Kullu valley of the North Western Himalayan region from April 2007 to March 2008 show that the spectral aerosol optical depth (AOD) and the Ångstr?m turbidity coefficient (ß) are high during the monsoon season, slightly less in summer, low in winter and lowest during the autumn for clear, hazy and partially clear days while the Ångstr?m wavelength exponent (a) has an opposite trend. Average annual values of the AOD at 500 nm are 0.25 ± 0.01, 0.46 ± 0.02 and 0.28 ± 0.02, for clear, hazy and partially clear sky days, respectively. The corresponding values of the ß are 0.13 ± 0.01, 0.22 ± 0.01 and 0.15 ± 0.01 and those of a are 1.09 ± 0.04, 1.18 ± 0.03 and 0.89 ± 0.05. The a is slightly high, but the ß is considerably higher on hazy days than on clear days, indicating that mountain haze is rich in coarse particles. There is a good agreement between MWR and satellite-based AOD values from MODIS, with annual correlation coefficients of 0.89, 0.70 and 0.81 for clear, hazy and partially clear days, respectively. The correlation of the AOD at 500 nm and the ß coefficient with temperature, wind speed and humidity is significantly positive while that of the a exponent is negative for most of days suggesting high AOD and turbidity but low concentration of fine particles on hot, humid and windy days and vice versa. Also, the correlation of AOD at 500 nm and ß coefficient with wind direction is mostly negative while that of the a exponent is positive, indicating that AOD and turbidity decrease but the concentration of fine particles increases as wind direction veers to become more southwardly at our site. Thus, winds heading toward our site from the Indo-Gangetic plains brought air rich in coarse particles while winds from more southward direction or from the Thar-Desert advected mostly fine particles during the period analyzed here