Chemically enhanced transboundary ozone pollution suppresses city-level emission control benefits

Severe ozone pollution persists during summertime in 60 cities in the North China Plain and Fenwei Plain, which requires a fundamental change in the current mitigation strategy. Herein, we investigate how city-level ozone pollution would be affected by self- and collaborative mitigation actions among and beyond the 60 cities, by a modeling analysis of the daily maximum 8 h average ozone for summer (June–July–August). We find that a local uniform 20% cut in anthropogenic emissions would decrease ozone by a mere 2.6% and even worsen ozone over two cities. Due to cross-city ozone transport, the implementation of a range of city-specific emission cuts from 10% to 30% resulted in ozone changes that were essentially the same as those obtained from a uniform 20% cut. By contrast, a 20% emission cut across the entire country would decrease ozone in the 60 cities by 4.5% with no ozone deterioration in any city. Furthermore, owing to the transitioned ozone chemical regime and extended ozone chemical lifetime, the transboundary ozone from outside the two plains would be enhanced by emission reductions in the 60 cities (e.g. an increase by 68% with complete removal of emissions), leading to a significant suppression (about 23%) on the expected benefit. Nationwide collaborative action is essential for more effective city-level ozone mitigation.