Data and Codes from Sustained efforts at emission controls have restrained Chinese ozone pollution

We evaluate the effectiveness of the national air pollution control program, including phase one (2013-2017) and phase two (2018-2021), in reducing the ozone level over China, using an optimized machine learning approach, high-resolution emission estimates and an improved air quality model. We find that while emission changes in phase one increased the ozone level over the five highly developed regions, further reductions of nitrogen oxide (NOX) emissions in phase two have generally constrained the ozone pollution. The changing effect of emission controls on ozone pollution is due to the shift in the prevailing regime for ozone formation and the weakened effects of aerosol declines, as emission reductions continue.