Brown carbon in atmospheric outflow from the Indo-Gangetic Plain: Mass absorption efficiency and temporal variability

The simultaneous measurements of brown carbon (BrC) and elemental carbon (EC) are made in ambient aerosols (PM2.5), collected from a site in north-east India during Novemberʹ09–Marchʹ10, 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 M m−1 and exhibits significant linear relationship (P < 0.05) with WSOC concentration (3–29 μg m−3). The angstrom exponent (α: 8.3 ± 2.6, where babs ≈ λ−α.) 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). The mass fraction of WSOC (10–23%) in PM2.5 and mass absorption efficiency of BrC (σabs-BrC: 0.5–1.2 m2 g−1) bring to focus the significance of brown carbon in atmospheric radiative forcing due to anthropogenic aerosols over the Indo-Gangetic Plain.