Spectral aerosol optical depths over Bay of Bengal and Chennai: I - measurements

A cruise experiment to study the aerosol optical characteristics was conducted in February-March 2001 over the Bay of Bengal, a data void region. An analysis of aerosol optical depths (AODs) measured onboard Saar Kanya shows that the AODs are higher when compared to those measured over the west coast of India and the Arabian Sea. The mean AODs at 0.5 mum over the Bay of Bengal are in the range of 0.2-0.7, and are found to show a variation of about 40-50% in the wavelength region of 0.4-0.85 mum. The mean wavelength exponent alpha over the Bay of Bengal is found to be 1.80, higher than the Arabian Sea value of 1.46, indicating a relatively higher concentration of submicron size particles. The mean Angstrom coefficient beta, which represents the columnar aerosol loading, over the Bay of Bengal is found to be 0.10. Measurements of AODs, made before and after the cruise in Chennai, an urban station located on the eastern coast of India, show higher values compared to the Bay of Bengal data. The mean alpha for Chennai is found to be 1.53, which is lower than the Bay of Bengal value while the mean beta is higher at 0.18. While a higher alpha value indicates the dominance of smaller size particles over Bay of Bengal, a higher beta and a higher AOD at all wavelengths indicate the dominance of both bigger and smaller particles over Chennai. A comparison of AODs obtained over a coastal station Trivandrum, located on the southwest coast of India, during March 2001 showed that in the smaller wavelength range the Chennai AODs are higher while above 0.6 mum, the AODs are comparable. The day-to-day variations of AODs measured at Chennai are less significant when compared to Bay of Bengal and are below 10%. As Chennai is an urban, industrial and a well-populated city, and is a constant source of aerosol particles, there are lesser day-to-day variations in the AOD, while over the Bay of Bengal the air masses come from different source regions carrying aerosols of different chemical and physical properties which are responsible for a higher spatial and temporal variations in the observed AOD spectra. (C) 2003 Elsevier Science Ltd. All rights reserved.