Source quantification of South Asian black carbon aerosols with isotopes and modeling.

Dasari, Sanjeev; Andersson, August; Stohl, Andreas; Evangeliou, Nikolaos; Bikkina, Srinivas; Holmstrand, Henry; Budhavant, Krishnakant; Salam, Abdus; Gustafsson, Örjan

Black carbon (BC) aerosols perturb climate and impoverish air quality/human health-affecting ~1.5 billion people in South Asia. However, a lack of source-diagnostic observations of BC is hindering evaluation of uncertain bottom-up emission inventories (EIs), and thereby also models/policies. Here, we present dual-isotope-based (Δ14C/δ13C) fingerprinting of wintertime BC at two receptor sites of the continental outflow. Our results show a remarkable similarity in contributions of biomass and fossil combustion, both from the site capturing the highly-populated highly-polluted Indo-Gangetic Plain footprint (IGP; Δ14C-fbiomass=50±3%) and the second site in the N. Indian Ocean representing a wider South Asian footprint (52±6%). Yet, both sites reflect distinct δ13C-fingerprints, indicating a distinguishable contribution of C4-biomass burning from peninsular India (PI). Tailored-model-predicted season-averaged BC concentrations (700±440 ng m-3) match observations (740±250 ng m-3), however, unveiling a systematically increasing model-observation bias (+19% to -53%) through winter. Inclusion of BC from open burning alone does not reconcile predictions (fbiomass=44±8%) with observations. Direct source-segregated comparison reveals regional offsets in anthropogenic emission fluxes in EIs, overestimated fossil-BC in the IGP and underestimated biomass-BC in PI, which contributes to the model-observation bias. This ground-truthing pinpoints uncertainties in BC emission sources, which benefit both climate/air-quality modeling and mitigation policies in South Asia.