Atmospheric aerosol characteristics retrieved using ground based solar extinction studies at Mohal in the Kullu valley of northwestern Himalayan region, India

Aerosol parameters are measured using a ground-based Multi-wavelength Radiometer (MWR) at Mohal (31.90 degrees N, 77.11 degrees E, 1154 m amsl) in the Kullu valley during clear sky days of a seasonal year. The study shows that the values of spectral aerosol optical depths (AODs) at 500 nm and the Angstrom turbidity coefficient 'beta' (a measure of columnar loading in atmosphere) are high (0.41 +/- 0.03, 0.27 +/- 0.01) in summer, moderate (0.30 +/- 0.03, 0.15 +/- 0.03) in monsoon, low (0.19 +/- 0.02, 0.08 +/- 0.01) in winter and lowest (0.18 +/- 0.01, 0.07 +/- 0.01) in autumn, respectively. The Angstrom wavelength exponent 'alpha' (indicator of the fraction of accumulation-mode particles to coarse-mode particles) has an opposite trend having lowest value (0.64 +/- 0.06) in summer, low (0.99 +/- 0.10) in monsoon, moderate (1.20 +/- 0.15) in winter and highest value (1.52 +/- 0.03) in autumn. The annual mean value of AOD at 500 nm, 'alpha' and 'beta' are 0.24 +/- 0.01, 1.06 +/- 0.09 and 0.14 +/- 0.01, respectively. The fractional asymmetry factor is more negative in summer due to enhanced tourists' arrival and also in autumn months due to the month-long International Kullu Dussehra fair. The AOD values given by MWR and satellite-based moderate resolution imaging spectro-radiometer have good correlation of 0.76, 0.92 and 0.97 on diurnal, monthly and seasonal basis, respectively. The AODs at 500 nm as well as 'beta' are found to be highly correlated, while 'alpha' is found to be strongly anti-correlated with temperature and wind speed suggesting high AODs and turbidity but low concentration of fine particles during hot and windy days. With wind direction, the AOD and 'beta' are found to be strongly anti-correlated, while 'alpha' is strongly correlated.