Study of aerosol optical depth and precipitable water vapour content at Rajkot, a tropical semi-arid station

27-32Aerosol optical depth (AOD) at 1020 nm for the period July 2004 - July 2005 and at six different wavelengths ranging from 380 nm to 1020 nm for March-June 2005 is studied. The measurements are being made at the tropical semi-arid location Rajkot (22º18' N, 70º44' E, 142 m above sea level) using a hand-held microprocessor-based sun photometer, MICROTOPS-II under the aegis of ISRO’s Geosphere Biosphere Programme (IGBP). Precipitable water vapour is estimated from the measurements of solar intensity at 936 nm and 1020 nm. The AOD shows seasonal variation with high values (0.41) in summer and low values (0.11) in winter. Angstrom wavelength exponent ⍺ has been found to be high (0.59) during March, indicating relative dominance of accumulation-mode particles. During summer season, low value (0.25) of Angstrom wavelength exponent ⍺ indicates relative dominance of coarse-mode particles. The changes in the column water vapour have been found to be correlated with the changes in AOD. This is supported by the observed increase of AOD with relative humidity at high humidity values