Abstract:

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; τ) and the Ångström wavelength exponent (α). The continents surrounding the AS produce natural and anthropogenic aerosols that have distinctive influences on α and its spectral distribution. The α 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 α and yields accurate estimates of the coefficients (α1 and α2). The coarse-mode (positive curvature in the ln λ vs. ln λ) 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 α340-1020 is 0.90±0.19. The α340-870 exhibits similar values (0.92±0.18), while significant differences revealed for the constant terms of the polynomial fit (α1 and α2) 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. © Author(s) 2010.

Notes:

Cited By :33Export Date: 2 November 2015Correspondence Address: Kalapureddy, M. C. R.; Physical Meteorology and Aerology Div., Indian Institute of Tropical Meteorology, Pashan, Pune 411008, India; email: kalapureddy1@gmail.comReferences: Ångström, A.K., Techniques of determining the Turbidity of the Atmosphere (1961) Tellus, 13, pp. 214-223;Babu, S.S., Moorthy, K.K., Satheesh, S.K., Aerosol black carbon over Arabian sea during inter monsoon and summer monsoon seasons (2004) Geophys. Res. Lett., 31, pp. L06104. , doi:10.1029/2003GL018716; Babu, S.S., Nair, V.S., Moorthy, K.K., Seasonal changes in aerosol characteristics over Arabian Sea and their consequence on aerosol short-wave radiative forcing: Results from ARMEX field campaign (2008) J. Atmos. Solar Terr. 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