Isotope-based constraints on sources and processing of black carbon, carbon monoxide, and brown carbon in South Asia

Abstract: The highly populated South Asian region is facing rapid economic growth and urbanization. Here, both climate- and health-affecting atmospheric agents such as light-absorbing aerosols black carbon (BC) and brown carbon (BrC), trace gas carbon monoxide (CO), are often found in relatively high levels compared to in other regions. However, atmospheric chemistry-transport/climate models are unable to fully capture the extent of the abundance of BC, CO, and BrC in the regional atmosphere during winter. The Thesis aims to address potentially important uncertainties that may be contributing to the model-observation offset — uncertainties in the ambient optical properties of BrC, uncertainties in the relative source contributions of BC (biomass burning vs. fossil fuel combustion) and CO (direct emission-derived vs. atmospheric chemical oxidation-derived), uncertainties in the regional lifetime and absolute emission fluxes of BC. For the Thesis work, field sampling was conducted at three sites, megacity Delhi (key source region), the Bangladesh Climate Observatory–Bhola Island (BCOB; receptor site for the highly-polluted Indo-Gangetic Plain) and the Maldives Climate Observatory–Hanimaadhoo Island (MCOH; receptor site for wider South Asia).The light-absorptivity of water-soluble BrC is found to decrease by ~84% during transport of haze from source-to-receptor regions i.e., Delhi-to-BCOB-to-MCOH — much greater than estimated in chamber studies and accounted in models. Atmospheric photochemical oxidation is found to be a likely driver for the loss of water-soluble BrC light-absorption in the S Asian outflow (with an estimated bleaching rate of 0.20±0.05 day−1) (Paper I). Radiocarbon (Δ14C)-based source apportionment of BC aerosols shows a stark similarity in the relative contributions of fossil (~50%) and biomass sources (~50%) at BCOB as well as at MCOH, suggesting a regional homogeneity in BC source contributions. However, a distinct stable isotopic fingerprint (δ13C) of BC in the N Indian Ocean is found to be arising from a small yet significant contribution (upto 10%) from C4-biomass burning in peninsular India (region south of 23.4°N) (Paper II). Comparison of source-segregated observed and emission inventory-driven modeled BC concentrations indicates regional offsets in the anthropogenic emission fluxes of BC in emission inventories—overestimated fossil-BC in the Indo-Gangetic Plain and underestimated biomass-BC in peninsular India (Paper II). Dual-isotope (δ13C, δ18O)-based source apportionment of CO shows a significantly large contribution (~80%) from direct emissions of primary sources (biomass burning and fossil fuel combustion) in South Asia, in contrast to modeled CO budget (Paper III). The BC-to-CO ratio in South Asia is found to be higher, by a factor of 2-3, than in other polluted regions such as in East Asia during winter. The regional lifetime and emission flux of BC are estimated to be 8±0.5 days (higher than values used in models) and ~2.4±1 Tg/yr (significantly higher than estimated in current emission inventories), respectively (Paper IV).Taken together, for convergence between models and observations in wintertime South Asia, i) the ‘dynamic’ nature of BrC light-absorption should be considered in models, ii) improvements in emission information of BC and CO are needed for better-simulating concentrations. Controls on activities such as open burning (such as agricultural crop residue burning, domestic burning of wood and dung as fuel) in South Asia could enable a reduction in BC, CO, and BrC, thereby leading to improved air quality and paving the way for achieving some of the key sustainable development goals outlined by the United Nations.

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