Insights into China's Air Quality: WRF-Chem and SHERPA Analysis for Effective Air Quality Solutions

Air quality in China has been significantly improved over the past decade. However, many areas still face challenges with air pollutants such as PM2.5 and ozone. More than a quarter of the 339 cities regularly exceed the Chinese air quality standards and all of them exceed the latest World Health Organization guidelines, despite compliance efforts in reducing sulfur dioxide (SO2), nitrogen oxides (NOx), and carbon emissions. Continuous and deep improvements in air quality need a more in-depth quantitative assessment to understand the spatial scales (urban, regional, and national) of air pollution sources and to clarify the sequence of precursors and emission sectors that contribute to these pollutants. Here we combine the WRF-Chem model sensitivity simulations and the Screening for High Emission Reduction Potentials for Air quality (SHERPA) tool to address these challenges, particularly focusing on PM2.5. We first assess the chemical regimes of secondary inorganic aerosols (SIA) formation by simulating emission reduction scenarios for three main precursors (SO2, NOx, NH3). We find that NH3 predominantly controls SIA formation over 60% of China during cold seasons and NOx- or SO2-senesitive grids only dominate in four warm months (April to July). The spatial distributions of the chemical regimes throughout the year show a distinct demarcation between eastern NH3-NOx-controlled area and western NH3-SO2(-NOx)-controlled area. Sichuan Basin and Henan Province, however, are NOx-sensitive throughout the year. We then train the source-receptor relationships in SHERPA using the baseline and sensitivity simulation outputs of WRF-Chem. SHERPA can well reproduce the responses of PM2.5 concentrations to the emission changes of all precursors in China. Our results show that for yearly average PM2.5, local actions of precursor emission reduction at the urban scale are effective for most cities and mitigations in agricultural sector are important. Our study stresses the essential role of NH3 to further PM2.5 mitigation strategies of whole China.