The study’s central tool is deceptively simple. Whenever fossil fuels burn, they release both carbon dioxide, the dominant greenhouse gas, and carbon monoxide, a toxic pollutant produced when combustion is incomplete. The relative amounts of the two gases, expressed as the enhancement ratio of CO to CO2 above background levels, act as a diagnostic dial for combustion efficiency. A low ratio means fuels are burning cleanly and completely; a high ratio signals inefficient combustion, whether from aging industrial furnaces, poorly tuned engines, or smoldering residential coal and biomass. Because the ratio can be measured directly in the air, it provides an independent way to test whether emission inventories, which are assembled from fuel statistics and emission factors, are telling the truth.

What makes this study distinctive is where the measurements were taken. Most previous assessments of the CO to CO2 ratio in China have relied on near-surface stations just a few meters above the ground, typically in urban areas. Such sites see a heavily biased slice of the emission landscape: they are dominated by traffic exhaust and local heating plumes, and their readings shift dramatically with street canyons, wind direction and the daily rhythm of city life. A mountain-top observatory, by contrast, samples air that has traveled over an entire region, integrating emissions from power plants, industry, transport and households across hundreds of kilometers. The site used here, located at 119.00 degrees east and 30.03 degrees north in the Yangtze River Delta, sits high enough to catch regional air masses while remaining close enough to the emission heartland to detect their signals clearly.

The team collected continuous hourly concentrations of carbon monoxide and carbon dioxide throughout a full year, then focused on the winter months, when heating demand pushes emissions to their peak and atmospheric conditions make the regional signal easiest to isolate. To translate raw concentrations into emission estimates, the researchers combined their observations with an atmospheric transport model driven by meteorological data from the ERA5 reanalysis dataset of the European Centre for Medium-Range Weather Forecasts. The model traces air parcels backward in time, determining which parts of the delta region contributed the gases arriving at the mountain. By comparing the observed enhancements with the enhancements predicted using prior emission inventories, specifically the EDGAR v8.1 database, the team could calculate how far the official numbers deviated from atmospheric reality.

The headline result concerns combustion efficiency itself. The observed wintertime enhancement ratio came in at 13.6 plus or minus 0.1 parts of CO per million parts of CO2, a figure expressed as ppb per ppm. That number tells a nuanced story. It is lower than ratios reported in earlier studies across China, which is good news: it confirms that a decade of aggressive clean air policies, from phasing out small coal boilers to tightening vehicle standards, has genuinely improved how completely the region burns its fuel. Previous work by some of the same researchers had already documented a decade-long improvement in combustion efficiency across the delta. But the ratio remains higher than values reported for many European regions, a reminder that combustion in the Yangtze River Delta is cleaner than it used to be yet still not as efficient as in the world’s most developed economies.

When the researchers ran the same calculation using the emission inventory and their transport model, the simulated ratio came out at 16.1 plus or minus 1.2 ppb per ppm, noticeably higher than what the mountain actually measured. The gap implies that the inventory overestimates the ratio of CO to CO2 emissions by 18.4 percent, with an uncertainty of plus or minus 8.8 percent. In other words, the official accounting assumes the region’s combustion is dirtier, relative to its carbon dioxide output, than the atmosphere says it is. This kind of discrepancy matters because the CO to CO2 ratio is widely used to attribute emissions to sectors and to scale up estimates from limited measurements, so a biased ratio propagates errors throughout the entire accounting framework.

The absolute emission totals revealed even larger surprises. According to the atmospheric evidence, prior wintertime carbon monoxide emissions in the delta are underestimated by roughly 20 percent, plus or minus 8 percent. Far more striking, wintertime carbon dioxide emissions are underestimated by 36.8 percent, with an uncertainty spanning plus or minus 17.6 percent. If that correction holds broadly, the region’s actual winter carbon footprint is more than a third larger than the inventories suggest, a shortfall with direct consequences for China’s carbon neutrality pledges, which depend on accurate baselines to verify that reductions are real. Underestimating CO2 emissions by such a margin would mean that progress toward peak carbon targets could be overstated even as the climate burden accumulates faster than assumed.

Why would inventories miss the mark so badly in winter? The study points to the sectoral composition of the discrepancy. The characteristics of the observed ratio suggest that sources with high CO to CO2 ratios, particularly residential heating and industrial combustion, are larger contributors to the winter emission mix than the inventories capture. Residential coal and biomass burning in rural and peri-urban areas is notoriously difficult to quantify, since much of it falls outside formal fuel statistics, and industrial boilers operating at suboptimal efficiency during peak demand can emit far more CO than standard emission factors predict. Winter also brings stagnant meteorological conditions that trap pollutants, amplifying the atmospheric signal of these hard-to-count sources. The authors argue that future mitigation efforts should prioritize precisely these high-ratio sources, because reducing them delivers a double dividend: cutting both the climate-warming CO2 and the health-damaging CO and co-emitted pollutants.

The methodological significance of the work extends well beyond the Yangtze River Delta. High-altitude mountain stations have long been used to monitor global background trends in greenhouse gases, but this study demonstrates their power as regional emission auditors when paired with transport modeling. Compared with urban surface sites, a mountain observatory offers spatial representativeness that no street-level network can match; compared with satellites, it provides continuous, high-precision in situ measurements that can validate and calibrate space-based retrievals. As nations race to implement the Paris Agreement and verify their national determined contributions, atmospheric top-down verification of this kind is becoming an essential complement to bottom-up accounting, catching the systematic errors that fuel statistics alone cannot reveal.

For the Yangtze River Delta itself, the message is twofold. The region has made measurable progress in cleaning up its combustion, as shown by a wintertime CO to CO2 ratio that has fallen below earlier Chinese benchmarks, yet it still trails Europe in burning efficiency, and its total carbon output in winter appears substantially larger than official figures admit. Closing that accounting gap will require both better inventories, with sharper treatment of residential and industrial winter emissions, and continued expansion of the atmospheric monitoring infrastructure that makes independent verification possible. The mountain-top sensors that produced these findings will keep watching, and each winter’s data will test whether the region’s emission reductions are as real, and as large, as its climate commitments demand.

Subject of Research: Atmospheric observation-based constraint of combustion efficiency and CO to CO2 emission ratios in the Yangtze River Delta region of China

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

Article References: Constraining combustion efficiency and CO to CO2 emission ratio in the Yangtze River Delta region by using high-altitude atmospheric observations. (n.d.). https://doi.org/10.1007/s10661-026-16040-3

Image Credits: AI Generated

DOI: 10.1007/s10661-026-16040-3

Keywords: carbon monoxide, carbon dioxide, combustion efficiency, emission inventory, Yangtze River Delta, atmospheric transport model, greenhouse gas emissions, mountain-top observatory, air quality, carbon neutrality, EDGAR, atmospheric inversion