Isotope forensics unravel regional differences in sources of black carbon aerosol in South Asia.

Black carbon (BC) aerosols from incomplete combustion of biomass and fossil fuel exert asignificant influence on the climate system in South Asia, effecting in excess of 1.2 billionpeople. Models—seeking to advise mitigation policy—are constantly challenged inreproducing observations of seasonally varying BC concentrations in the densely populatedregions of South Asia e.g., over the Indo-Gangetic Plain (IGP). The systematicunderestimation of the absorption aerosol optical depth over South Asia by a factor of 2-3in climate models relative to observation-based estimates illustrates the currentlylarge uncertainties. This uncertainty may be related to several different factors,including inaccurate estimates of relative source contributions and emissions ofBC. As part of the South Asian Pollution Experiment 2016 (SAPOEX-16), we investigatedwintertime BC and its dual carbon-isotopic signature (δ13C/Δ14C) - diagnosed sources fromregional and large-scale background receptor sites — the Bangladesh Climate Observatory atBhola Island (BCOB), strategically located in the outflow region of the IGP to intercept theintegrated IGP emission signal and the remote Indian ocean-based Maldives ClimateObservatory at Hanimaadhoo Island (MCOH) to provide an integrated footprint of SouthAsia, respectively. Our results of "top-down" radiocarbon measurements of atmospheric BCfrom two regional-receptor sites show that fossil fuel combustion produces 55±3% (Δ14C:-440 ± 6 per mil) of BC emitted from South Asia. In contrast, the δ13C signalsfor BCOB-BC (∼ -27.2 ± 0.2) are more depleted relative to that of MCOH-BC(∼ −25 ± 0.2). Using a Bayesian statistical approach, we resolve the isotopicsignatures into three source classes namely biomass, coal and liquid fossil fuel.We find the liquid fossil fuel component of BC emissions is 49 ± 5% in the IGPversus the larger South Asian footprint of 23 ± 9%. Our observation is in starkcontrast to previous "Bottom-up" emission inventory-based modelling estimatessuggesting over ∼ 60% BC from biomass burning in the IGP. This finding brings outthe previously unknown sharp contrast in regional emission sources of BC fromanthropogenic combustion in South Asia. Observationally-constrained insights onBC sources allows for a more credible scientific underpinning of policy aimedat mitigating the acuteness and scale of ramifications due to BC in South Asia. [ABSTRACT FROM AUTHOR]