干旱区生态环境知识资源中心

Ship-borne sunphotometer measurements obtained in the Arabian Sea (AS) in the pre-monsoon season (18 April-10 May 2006) during a cruise campaign (ICARB) have been used to retrieve the Aerosol Optical Depth (AOD; tau) and the Angstrom wavelength exponent (alpha). The continents surrounding the AS produce natural and anthropogenic aerosols that have distinctive influences on alpha and its spectral distribution. The alpha values were estimated by means of the least-squares method over the spectral bands 340-1020 nm and 340-870 nm. The spectral distribution of AOD in logarithmic co-ordinates could be fit using a 2nd order polynomial with higher accuracy in the wavelength band 340-1020 nm than in the 340-870 nm band. A polynomial fit analytically parameterizes the observed wavelength dependencies of AOD with least errors in spectral variation of alpha and yields accurate estimates of the coefficients (a(1) and a(2)). The coarse-mode (positive curvature in the ln tau(lambda) vs. ln lambda) aerosols are mainly depicted in the Northern part of the AS closely associated with the nearby arid areas while fine-mode aerosols are mainly observed over the far and coastal AS regions. In the study period the mean AOD at 500 nm is 0.25 +/- 0.11 and the alpha(340-1020) is 0.90 +/- 0.19. The alpha(340-870) exhibits similar values (0.92 +/- 0.18), while significant differences revealed for the constant terms of the polynomial fit (a(1) and a(2)) proportionally to the wavelength band used for their determination. Observed day-to-day variability in the aerosol load and optical properties are direct consequence of the local winds and air-mass trajectories along with the position of the ship.