Multiyear dual-isotope fingerprinting at South Asian receptor sites constrains carbon monoxide sources and enhanced...

Carbon monoxide (CO) is an indirect short-lived climate forcer with uncertainties both in sources and its role in atmospheric oxidation. Based on nine winter-long dual-isotope campaigns at two South Asian receptor sites intercepting the continental outflow, we quantified CO source contributions and emission-sink dynamics. Combustion accounts for 68 to 74% of South Asia regional CO (including 34 to 37% from biomass burning) with secondary atmospheric oxidation contributing 26 to 32% (dominated by oxidation of nonmethane volatile organic compounds (NMVOCs) at 21 to 26%, with methane oxidation contributing 5.5 to 6.4%). These isotope-observational constraints suggest a twice higher role for atmospheric oxidation than in model estimates. Spatially, the absolute contributions of both primary and secondary CO decrease from the Indo-Gangetic Plain to the northern Indian Ocean, while the relative contribution of oxidation-derived CO increases, indicating enhanced secondary CO production near source regions. Observation-model comparison suggests that transport from the continental areas dominates CO over adjacent oceanic regions, with minor local production. During the COVID-19 emission perturbation, combustion-derived CO fell sharply in absolute terms, NMVOC-derived CO rose, and CH₄-derived CO remained stable, providing an observational constraint on the sensitivity of atmospheric oxidation to emission changes. Our results reveal a significant contribution of CO from atmospheric oxidation in South Asia, calling for a reassessment of regional emission-oxidation coupling in models and of mitigation strategies.