Constraining combustion efficiency and CO to CO2 emission ratio in the Yangtze River Delta region by using high-altitude atmospheric observations.

Carbon monoxide (CO) and carbon dioxide (CO2) are key tracers when evaluating atmospheric pollution and greenhouse gas emissions, with their enhancement ratio (ΔCO:ΔCO2) serving as a widely used metric for assessing combustion efficiency and estimating emissions across major industrial sectors at different spatial scales. However, most previous studies have been conducted based on near-surface observations (i.e. a few meters above ground), limiting the spatial representativeness of emission sources. In contrast, high-altitude mountain observations can provide a more comprehensive perspective for emission assessment at regional scale. In this study, we conducted year-round atmospheric concentration observations of CO and CO2 at a mountain top site (119.00°E, 30.03°N, ∼1500 m a.s.l.) in the Yangtze River Delta (YRD) region, China. By integrating these observations with an atmospheric transport model and prior emissions, we examined regional combustion efficiency and used the observed relationship to evaluate wintertime CO and CO2 emissions for the YRD region. We found that (1) the observed wintertime ΔCO:ΔCO2 ratio was 13.6 ± 0.1 ppb/ppm, lower than earlier values reported in China but still higher than those in many European regions, indicating improved yet incomplete combustion efficiency in the YRD region. (2) The simulated ΔCO:ΔCO2 ratio was 16.1 ± 1.2 ppb/ppm, suggesting that the inventory overestimates CO:CO2 emissions ratio by 18.4(±8.8)%. (3) The prior wintertime CO emissions are underestimated by around 20.0(± 8.0)%, and prior wintertime CO2 emission are underestimated by a much larger ratio of 36.8(±17.6)%. Furthermore, the sectoral characteristics of ΔCO:ΔCO2 suggest that future mitigation efforts should prioritize sources with higher ratios, particularly in the residential and industrial sectors.