Quantifying the impacts of inter-city transport on air quality in the Yangtze River Delta urban agglomeration, China: Implications for regional cooperative controls of PM2.5 and O3

Highlights: • Inter-city transport among 41 YRD cities is conducted for O₃ and PM_2.5 during the EXPLORE-YRD campaign. • Inter-city transport has the largest contribution to PM_2.5 and O₃ in most cities. • Transport from nearby cities becomes more important on pollution days. • The cities within a distance of 184 km and 94 km contribute 60% of the PM_2.5 and O₃, respectively. • Reducing secondary inorganic PM_2.5 in YRD requires cooperative emission controls on a much larger regional scale. The Yangtze River Delta (YRD) urban agglomeration is one of the most developed regions in China. During recent decades, this region has experienced severe regional haze and photochemical smog pollution problems. In this study, we used a source-oriented chemical transport model to quantitatively estimate the effects of inter-city transport on fine particulate matter (PM_2.5) and ozone (O₃) among the 41 cities in the YRD urban agglomeration during the EXPLORE-YRD (EXPeriment on the eLucidation of the atmospheric Oxidation capacity and aerosol foRmation, and their Effects in the Yangtze River Delta) campaign (May 17 to June 17, 2018). The results show that inter-city transport is very significant in the YRD region. On average, the emissions from the local city, the other YRD cities, and the regions outside of the YRD contribute 25.3%, 49.9%, and 24.8% to the PM_2.5, respectively, and they contribute 33.7%, 46.8%, and 19.5% of the non-background O₃, respectively. On PM_2.5 or O₃ pollution days, the transport contribution from the non-local YRD cities becomes much more important, while the local emissions and the transport from non-YRD emissions become less important. The results also suggest that the cities within a distance of 184 km and 94 km contribute 60% of the PM_2.5 and O₃, respectively. Therefore, we recommend that regional cooperative control programs in the YRD consider emission controls over cities within these ranges. The range for primary PM_2.5 (92 km) is very different from that for secondary PM_2.5 (515 km)_. Cooperative emission controls of SO₂ and NO_x on a much larger regional scale are required to reduce the secondary PM_2.5 in the YRD.