Mass absorption efficiency of Brown Carbon and Elemental Carbon in atmospheric outflow from the Indo-Gangetic Plain

The simultaneous measurements of mass absorption efficiency (σabs) of brown and elemental carbon (BrC & EC) are made in ambient aerosols (PM2.5), collected from a site in the north-east India (Kharagpur: 22.02N, 87.11E) during November 2009-March 2010, representing the atmospheric outflow from the Indo-Gangetic Plain (IGP) to the Bay of Bengal (BOB). The absorption coefficient of BrC (babs), assessed from water soluble organic carbon (WSOC) at 365 nm, varies from 2 to 21 Mm-1 and exhibits significant linear relationship (P < 0.05, N = 46) with the concentration of WSOC (3-29 μg m-3). The Angstrom exponent (Åp: 8.3×2.6, where babs ≈ λ- Åp) is consistent with that reported for humic-like substances (HULIS) from biomass burning emissions (BBE). The impact of BBE is also discernible from mass ratios of nss-K+/EC (0.2-1.4) and OC/EC (3.4-11.5). Temporal variability in the mass absorption efficiency of BrC (σabs-BrC: 0.5-1.2 m2 g-1) is not significantly pronounced in contrast to that of EC (σabs-EC: 1.3-7.9 m2 g-1). This is consistent with the variability in nss-SO42-/EC ratio (3.9×2.1) and internal mixing of EC in high sulphate concentration (7-29 μg m-3) over the IGP. The mass fractions of WSOC (10-23 %) and EC (3-11 %) in PM2.5 bring to focus the significance of BrC in atmospheric radiative forcing due to anthropogenic aerosols over the Indo-Gangetic Plain.