Source Quantification of South Asian Black Carbon Aerosols with Isotopes and Modeling

Abstract

Black carbon (BC) aerosols perturb climate and impoverish air quality/human health—affecting ∼1.5 billion people in South Asia. However, the lack of source-diagnostic observations of BC is hindering the evaluation of uncertain bottom-up emission inventories (EIs) and thereby also models/policies. Here, we present dual-isotope-based (Δ 14 C/δ 13 C) 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; Δ 14 C- f biomass = 50 ± 3%) and the second site in the N. Indian Ocean representing a wider South Asian footprint (52 ± 6%). Yet, both sites reflect distinct δ 13 C-fingerprints, indicating a distinguishable contribution of C 4 -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 ( f biomass = 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.

Observational Evidence of Large Contribution from Primary Sources for Carbon Monoxide in the South Asian Outflow

“… 42 , 43 Hence, in addition to fossil fuel combustion and C3 biomass burning, C4 biomass burning is a plausible primary source in the context of South Asian continental emissions of CO, as has also been recently shown for incomplete combustion-derived black carbon aerosols investigated at the same site. 36 Taken together, based on the observed sharp positive trend in δ 13 C and δ 18 O during polluted air mass regime, we hypothesize that formation of CO is likely driven by a combination of primary sources. For further source-diagnostics the source signatures representative of the dominant air mass regimes is investigated.…”

“…34 , 35 Hence, sampling at the Maldives Climate Observatory at Hanimaadhoo (MCOH; Figure 1 a), located downwind of the continent during winter, allows to intercept and fingerprint the sources as well as trace the geographical emission regions contributing to the CO transported from the continent to the North Indian Ocean. 36 …”

“…The widely varying air mass back trajectories and the corresponding changes in CO also illustrate that the sampling site is not dominated by local emissions and is representative of a wider regional South Asian footprint. 36 To extend our analysis beyond the determination of geographical source origins, we used stable isotopes to deconvolute the emission sources of CO.…”

“…Indeed, the wintertime sensitivity of emissions reaching MCOH is found to be a combination of emissions from the IGP and peninsular India (south of <23.4°N). 36 Crop-residue burning of sugar cane and associated bagasse-based power generation is more widespread in peninsular India than in the IGP during winter. 36 , 41 Such C4 plant burning sources emit 13 C-enriched CO, which likely contribute to the observed enrichment of the isotopic signals sampled downwind over North Indian Ocean.…”

“…Source-resolved observational data reported here ( f primary 79 ± 4%) is expected to offer a way forward toward resolving these issues. 36 Meanwhile, this information is also of direct relevance for policy developments, seeking to minimize the severe air pollution situation in the region.…”