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The effectiveness of national policies for air pollution control has been demonstrated, but the relative effectiveness of short-term emission reduction measures in comparison with national policies has not. Here we show that short-term abatement measures during important international events substantially reduced PM2.5 concentrations, but air quality rebounded to pre-event levels after the measures ceased. Long-term adherence to strict emission reduction policies led to successful decreases of 54% in PM2.5 concentrations in Beijing, and 23% in atmospheric nitrogen deposition in China from 2012 to 2020. Incentivized by "blue skies" type campaigns, economic development and reactive nitrogen pollution are quickly decoupled, showing that a combination of inspiring but aggressive short-term measures and effective but durable long-term policies delivers sustainable air quality improvement. However, increased ammonia concentrations, transboundary pollutant flows, and the complexity to achieving reduction targets under climate change scenarios, underscore the need for the synergistic control of multiple pollutants and inter-regional action. In response to the rebound of air pollution during large events in China, the authors propose that a combination of ambitious short-term measures and enduring long-term pollution control policies is a sustainable solution for improving air quality. [ABSTRACT FROM AUTHOR]

In order to cope with heavy haze pollution in Beijing, the Air Pollution Prevention and Control Action Plan was issued in 2013. In this study, we took January observations from 2014 to 2018 as a case study to assess wintertime air quality improvement in Beijing after five years of continuous emission control measures and to investigate the causes of haze formation. SO 2 and NO 2 concentrations in January 2018 decreased by 84% and 41% from January 2014, while NH 3 concentrations remained relatively stable during these years with an average of 7.28 μg m−3. The mean concentrations of PM 2.5 and water-soluble inorganic ions declined over the study period, while compared with 2014, decreased SO 4 2− ratio and increased NO 3 − contribution in PM 2.5 were observed, highlighting the importance of reactive nitrogen (Nr) pollution control. High concentrations of NH 3 , NO 2 , as well as NH 4 + and NO 3 − in PM 2.5 at roadside monitoring sites, indicated that vehicle sources played a vital role in local urban Nr emissions. Finally, we found most NH 3 existed in the gas phase in the Beijing atmosphere, even on polluted winter days, suggesting a considerable risk for additional secondary aerosol formation. Our findings demonstrate the effectiveness of the emissions control in alleviating Beijing's air pollution. Significant reductions of precursor emissions are needed to achieve the goals set by air quality standards in the future, especially for NH 3. It is especially crucial to jointly consider and control multiple pollution sources to effectively reduce the high levels of air pollution that remain in Beijing. • Significant improvements in Beijing air quality were observed during January 2014 to 2018. • The concentrations of SO 2 , NO 2 in air and SO 4 2−, NO 3 −, NH 4 + in PM 2.5 significantly decreased. • NH 3 concentrations remained stable during these 5 years. • Vehicle sources are vital contributors to NH 3 and NO 2 emissions. • The Beijing atmosphere is NH 3 -rich. [ABSTRACT FROM AUTHOR]