Observationally Constrained Wintertime Emission Fluxes and Atmospheric Lifetime of Black Carbon in South Asia

Abstract

Black carbon (BC) aerosols perturb the climate and affect air quality/human health. In the highly populated and heavily polluted South Asian region, the wintertime modeled atmospheric abundance of BC has remained underestimated relative to surface observations. We hypothesize this is linked to underestimated (i) atmospheric lifetime (τ_BC) and/or (ii) regional emission fluxes of BC. To address this hypothesis, we developed a novel inversion framework combining multiwinter (2018–2020) hourly resolved BC and carbon monoxide (CO) measurements from a wide footprint site in the North Indian Ocean, intercepting wintertime South Asian outflow. The average ΔBC/ΔCO ratio in this continental outflow of 14 ± 5 ng m^–3 ppb^–1 was 2–3 times higher than in East Asian outflow and shows a profound regional wintertime presence of BC. The empirically derived τ_BC of 8 ± 0.5 days was higher than global-mean τ_BC of 5.5 days employed in climate models and suggests greater regional longevity of wintertime BC. The ΔBC/ΔCO inversion-estimated ‘top-down’ BC emission flux of ∼200 Gg/month was in fact higher by a factor of ∼1.5 than wintertime monthly BC emission flux from scaled ‘bottom-up’ emission inventory (∼125 Gg/month). Taken together, assimilating higher BC emissions with greater longevity seems promising to reconcile the model-observation offset of wintertime BC abundance for South Asia.