Features in wavelength dependence of aerosol absorption observed over central India

Abstract

Measurements on spectral dependence of aerosol light absorption and BC mass concentrations were made at various locations between Ahmedabad (23.03°N, 72.50°E) and Hyderabad (17.47°N, 78.45°E) during February 2004. Average concentration of BC was close to 2 µg/m³ which is about 2 to 5% of the total aerosol mass loading measured at the surface. Absorption Angstrom coefficient estimated using a relation of the form β_abs (λ) = K.λ^-α is found to vary between 1.2 to 2.0 at different locations. Our result shows an excess absorption of up to 30% in lower wavelengths below 880 nm. Deviation from λ^-1 dependence, which is typical of BC from fossil fuel, shows that aerosol mixture contains other particles from biofuel/biomass or mineral dust. As there was no major biomass burning observed in the study region as well as no specific enhancement observed in the coarse size aerosols, we attribute the observed excess absorption to the biofuel combustion.