Assessing Co-Benefits of Air Pollution and Carbon Reduction in a Low-Carbon Pilot City: Six-Year Observations of Atmospheric CO and Its Ratios to CO₂ and CH₄

Long-term, high-precision observations of atmospheric carbon monoxide (CO) and its associated tracers are essential for evaluating the efficacy of synergistic air pollution and carbon reduction policies, yet such data remain scarce in rapidly developing regions of central China. Here we present continuous measurements of atmospheric CO, CO₂, CH₄, and PM_2.5 from December 2018 to February 2025 at Nanchang, a provincial capital and one of China’s inaugural low-carbon pilot cities. We quantified combustion efficiency using CO₂/CO enhancement ratios (Rco₂/co), estimated anthropogenic CO₂ (CO_2,A) via the ΔCO × Rco₂/co method, identified potential source regions using weighted potential source contribution function (WPSCF) analysis, and examined CH₄/CO ratios to trace methane sources. Observed CO mixing ratios exhibit a significant downward trend (−23.5×10^-9 yr^-1, 25.2% reduction from 2019 to 2024), closely tracking provincial oil consumption and COVID-19 lockdown impacts. Wintertime Rco₂/co has more than doubled from 40.2 to 81.4, indicating substantial improvement in regional combustion efficiency, consistent with Jiangxi’s coal-to-gas energy transition (coal share declined from 64.4% to 57.1%). Monthly anthropogenic CO₂ (CO_2,A) and ΔCO are strongly correlated with PM_2.5 (r > 0.7), demonstrating measurable co-benefits of emission reductions. Summer CH₄/CO ratios increase at 5.7% yr^-1, driven by rapid CO decline, while the extreme drought-induced wildfires in autumn 2022 trigger a distinct CH₄-CO co-emission event (R² = 0.61, CH₄/CO = 0.45). Our findings provide quantitative observational evidence that energy structure optimization in low-carbon pilot cities can achieve synergistic reductions in both air pollutants and greenhouse gases. The results highlight the critical role of regional transport (contributing > 50% to winter CO enhancements) in shaping local air quality, underscoring the need for inter-provincial coordinated governance. The 2022 wildfire episode further reveals how climate extremes can perturb atmospheric composition, with implications for monitoring feedbacks under future climate change.