Physical and optical properties of aerosols over an urban location in western India: Seasonal variabilities

We present results on various physical and optical properties of aerosols measured over Ahmedabad, an urban location in western India, from 2002 to 2005 and discuss their seasonal and interannual variabilities. Aerosol parameters which have been studied include AOD spectra, aerosol mass concentration, size distribution, BC concentration, wavelength dependency in absorption, scattering coefficient, single scattering albedo and their vertical distribution in the atmosphere. All data have been classified in terms of four major seasons, namely, dry, premonsoon, monsoon and postmonsoon. AODs show an increasing trend over the first half of the year, and this is more consistent at higher wavelengths. Variation of Angstrom parameter a shows dominance of smaller size particles during dry and postmonsoon seasons while increase in coarser particle concentration during premonsoon and monsoon seasons. PM10 mass concentration varied from low values close to 40 mu g/m(3) to highs of about 106 mu g/m(3). Size distribution patterns of near surface aerosols exhibited presence of three distinct modes, all of which could be fitted using three lognormal modes. Highest values of BC mass are obtained during postmonsoon (7.3 +/- 3.7 mg/m(3)) while lowest values are measured during monsoon season (1.5 +/- 0.8 mg/m(3)). Wavelength dependency of aerosol absorption shows signatures of presence of significant amount of absorbing aerosols produced from biofuel/biomass burning in the atmosphere. Single scattering albedo at 0.525 mu m are found to be 0.73 +/- 0.1, 0.84 +/- 0.04, 0.81 +/- 0.03 and 0.73 +/- 0.08 during dry, premonsoon, monsoon and postmonsoon seasons, respectively. Vertical distributions of aerosol for dry and postmonsoon seasons are characterized by high values of extinction coefficients within first few hundred meters from the surface where we find a sharp decrease in the extinction values with increasing altitude. Vertical distribution of aerosols during monsoon season shows presence of a very thick and stable aerosol layer between 0.5 and 2.0 km, contributing significantly to the columnar AODs.