Multi-year dual-isotope fingerprinting at South Asian receptor sites constrain carbon monoxide sources and enhanced oxidation

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 68–74% of South Asia regional CO (including 34–37% from biomass burning) with secondary atmospheric oxidation contributing 26–32% (dominated by oxidation of non-methane volatile organic compounds NMVOCs at 21–26% with methane oxidation contributing 5.5–6.4%). 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 (IGP) to the northern Indian Ocean, indicating enhanced oxidation near source regions, while the relative contribution of secondary CO increases. Observation-model comparison suggests that continental transport dominates CO over adjacent oceanic regions, while local production is minor. During the COVID-19 pandemic, combustion-derived CO fell sharply, NMVOC-derived CO rose, and CH4-derived CO remained stable, suggesting enhanced oxidation from reduced competition among precursors. Our results reveal a far greater contribution of CO from atmospheric oxidation in South Asia than in current model estimates, highlighting the need for sustained emission controls to deliver concurrent climate and health benefits.