Anthropogenic drivers of the accelerated decline in surface NO 2 over China during 2015–2022

Abstract China has experienced accelerated decline in surface NO 2 concentrations under the condition of steady reduction in NO x emissions over the past decade, but the reasons for such a nonlinear response in the changes of surface NO 2 to reduction in NO x emissions remain unexplored. Through integration of measurements, a chemical transport model (GEOS-Chem), and a random forest (RF) model, we comprehensively quantified the contributions of emissions, meteorology, and chemical transformation to the observed trend in surface NO 2 during 2015–2022. After removing meteorological influences using the RF model, the anthropogenically-driven tendency was estimated to be −0.36 ppb y −1 for 2015–2018 (observed tendency: −0.09 ppb y −1 ) and −1.23 ppb y −1 for 2019–2022 (observed tendency: −1.00 ppb y −1 ), confirming the dominant role of anthropogenic drivers. Supported by long-term satellite measurements and GEOS-Chem model sensitivity simulations, we attributed this accelerated decline in surface NO 2 to the shift in emission sectors from power plants to transportation and industry after 2018. In addition to emissions, we demonstrated that the increased atmospheric oxidation capacity (i.e. higher ozone) played a key role in shifting NO 2 production from ozone-limited to NO-limited conditions, thereby making NO 2 concentration more sensitive to NO x emission reductions. Further, measurement interference from oxidized nitrogen species for observing NO 2 can also contribute to the observed decline in surface NO 2 , and its contribution doubled from 2015–2018–2019–2022. Our results elucidate the nonlinear response of the trend in surface NO 2 to reduction in NO x emissions, and highlight the importance of understanding rapid changes in sectoral emissions and the chemical radicals in relation to future NO 2 controls.