Retrieval of black carbon and specific absorption over Kanpur city, northern India during 2001–2003 using AERONET data

Column-integrated aerosol black carbon (BC) concentration, [BC] has been retrieved over Kanpur (an industrial city in the Gangetic basin, northern India) during 2001–2003 for the first time. [BC] is derived from BC volume fraction and AERONET-retrieved size distribution using Maxwell–Garnett and Bruggeman mixing rules in a mixture of water, BC and (NH4)2SO4. In addition partly absorbing components like organic carbon (OC) and dust are added to the Maxwell–Garnett mixture depending on the season to investigate their role in the retrieval of [BC]. The volume fraction of each component is retrieved by matching the mixture refractive index (real, n(λ) and imaginary, k(λ  )) with AERONET-retrieved refractive index. [BC] shows seasonal variations with high values (>10mgm-2) observed during the post-monsoon and winter seasons and low values (<6mgm-2) during the monsoon season. Specific absorption cross-section (αa) decreases non-linearly with the increase in [BC], however, the decrease becomes linear when other absorbing components are present. Yearly averaged [BC] and αa are 9.99±1.95, 5.52±1.07, 7.9±1.53 mg m−2 and 7.9±1.83 and 9.67±3.45, 12.74±2.92 m2 g−1 for 2001, 2002 and 2003, respectively, using Maxwell–Garnett mixing, which differ by ∼15% from those using Bruggeman mixing. [BC] shows diurnal variation with morning and afternoon peaks and mid-day minimum. The amplitude is subdued as it represents the total column, which is more influenced by anthropogenic activities than by boundary layer dynamics. In order to estimate [BC] accurately, OC has to be considered when the absolute difference between k(0.44 μm) and k(1.02 μm) becomes higher than 0.0015. The sensitivity of [BC] due to dust n(λ) becomes important during the intense dust loading period. It was found that [BC] is more sensitive to dust k(λ) than dust n(λ), as [BC] increases ∼10–13% for 10% rise in k(λ). Retrieved [BC] shows good agreement with the in situ measurements. Therefore our retrieval can be used as an alternate method to infer BC and OC specific absorption globally.