Aerosol optical depth variation observed using sun-photometer over Indore

Columnar aerosol optical depth measurements have been made over Indore (22.7o N, 75.9o E) during May 2001-March 2002 at four spectral bands centred around 400, 499, 668 and 858 nm using a hand-held sun-photometer. The aerosol optical depth (AOD) shows seasonal variation with high values in summer and low values in winter. The summer-increase is found to be due to the high wind speed producing larger amount of wind-derived dust particles. As the summer monsoon sets in at the end of June there is an appreciable decrease in the AOD values. The AOD values decrease further in winter and the decrease is more at higher wavelengths indicating that there is a general reduction in the number of bigger particles. Also during winter months the wind direction changed to southerly and south-easterly which brings air that is more rural to the measurement site. The amplitude of the observed high AOD values in summer and low during winter, is higher for longer wavelengths which shows that the coarse particles contribute more to the observed variation as compared to sub-micron particles. Comparison of the AOD values over Indore with that of Trivandrum and Visakhapatnam shows that the Indore values are comparable to that of Visakhapatnam, but much higher than the Trivandrum values indicating the influence of industrialization and higher population