Regional source attribution of tropospheric ozone to NOx and volatile organic compounds in the Beijing-Tianjin-Hebei region using the WRF-chem model.

As one of China's densely populated and economically developed regions, the Beijing-Tianjin-Hebei (BTH) area faces severe ozone (O3) pollution, largely driven by nitrogen oxides (NOx) and volatile organic compounds (VOCs) through complex chemical reactions. A detailed quantification of the specific contributions of various NOx and VOCs sources to O3 levels under meteorological and boundary-layer dynamics is crucial for effective pollution control in the BTH region. In this study, we developed and implemented an explicitly tagging approach and process analysis method in the WRF-Chem model to attribute O3 formation separately to NOx and VOCs from various sources, thereby quantifying the contributions of anthropogenic and natural emissions in O3 budget in the BTH region. The results reveal that NOx emissions primarily contribute to O3 through anthropogenic sources (63.5%), while for VOCs, the contributions from anthropogenic emissions (29.3%), background methane (15.8%), and biogenic sources (11.8%) are comparable. Notably, regional transport significantly contributes to O3 levels in the BTH region through both VOCs (80.4%) and NOx (71.2%), with emissions from Shandong, Henan, northeast China, and the Yangtze River Delta, identified as key anthropogenic sources regions of the interprovincial transport. The northeastern low-pressure system and the western Pacific subtropical high drive interprovincial pollutant transport through the free troposphere, with subsequent downward mixing of O3 formed in the free troposphere, contributing significantly to regional transport's impact on O3 pollution in the BTH. This study provides a comprehensive assessment of ozone source attribution in the BTH area, emphasizing the importance of coordinated regional strategies for O3 mitigation, with particular attention to synoptic influences and planetary boundary layer processes.Regional pollutant transport strongly affects BTH ozone levels, particularly from Shandong, Henan, and the Yangtze River Delta. Large-scale weather systems further enhance long-range ozone transport through vertical mixing. These findings highlight the need for coordinated regional and cross-sector emission control strategies to effectively reduce ozone pollution in northern China.