Aerosol optical properties over a coastal site in Goa, along the west coast of India

Abstract

Spectral characteristics of the Aerosol optical depths (AODs) measured over a coastal site in Goa (15.46 degrees N and 73.83 degrees E), from a plateau approx. 50 m above mean sea level, for the period 2008-2010, are analyzed to understand the inter-seasonal and intra-seasonal variability and to delineate different aerosol sources. A Microtops-II sunphotometer having five different wavelengths centered at 0.380, 0.440, 0.500, 0.675 and 0.870 mu m was used to estimate AODs in different seasons classified as: winter monsoon season from December to March (WMS), spring inter-monsoon season from April to May (SIMS), summer monsoon season from June to September (SMS) and fall inter-monsoon season from October to November (FIMS). The number of data (AODs) generated in each season is 569 in WMS, 131 in SIMS, 38 in SMS and 256 in FIMS. The highest AOD at 500 nm (AOD500) was recorded in SIMS (0.43 plus or minus 0.18) while the lowest value was observed in SMS (0.32 plus or minus 0.10). The seasonal mean values of Angstrom alpha computed from the least-square method in the wavelength range 0.440-0.870 mu m showed higher values (1.23 plus or minus 0.20) in FIMS than those in SMS (0.75 plus or minus 0.34). The highest Angstrom beta values were noticed in SIMS (0.25 plus or minus 0.10) and lowest in FIMS (0.17 plus or minus 0.06). To make a source appropriation and thus to resolve the complexity of aerosols in the study area, alpha was computed in different wavelength ranges, viz: short wavelengths (0.440-0.500 mu m) and long wavelengths (0.675-0.870 mu m), which revealed differing alpha values for different ranges of wavelengths. To account for the curvature, a second order polynomial fit is introduced. Subsequently, the second-order Angstrom exponent and the coefficient of the second-order polynomial fit are analyzed to understand the dominant aerosol type.