A rapid decrease in pM 2.5 concentrations in china has been observed in response to the enactment of strong emission control policies. From 2012 to 2017, total emissions of SO 2 and no x from china decreased by approximately 63% and 24%, respectively. Simultaneously, decreases in the PM 2.5 concentration in Japan have been observed since 2014, and the proportion of stations that satisfy the pM 2.5 environmental standard (daily, 35 µg/m 3 ; annual average, 15 µg/m 3 ) increased from 37.8% in fiscal year (FY) 2014 (April 2014 to March 2015) to 89.9% in FY 2017. However, the quantitative relationship between the pM 2.5 improvement in china and the pM 2.5 concentration in downwind regions is not well understood. Here, we (1) quantitatively evaluate the impacts of Chinese environmental improvements on downwind areas using source/receptor analysis with a chemical transport model, and (2) show that these rapid emissions reductions improved pM 2.5 concentrations both in china and its downwind regions, but the difference between SO 2 and no x reduction rates led to greater production of nitrates (e.g., NH 4 no 3 ) due to a chemical imbalance in the ammonia-nitric acid-sulfuric acid-water system. Observations from a clean remote island in western Japan and numerical modeling confirmed this paradigm shift.The long-range trans-boundary transport behavior of pollutants in East Asia is an important environmental issue due to frequent outflows of heavy pollution. Among pollutants, PM 2.5 (particulate matter less than 2.5 µm in diameter) poses serious human health risks, including lung cancer, respiratory disease, and asthma, particularly over China and its downwind regions 1-5 . Serious PM 2.5 pollution has been observed in the northern China region since the early 2010s. To reduce this pollution, China has implemented active clean air policies in recent years (e.g., the Action Plan for Prevention and Control of Air Pollution, enacted in September 2013). These plans include the phasing out of outdated industrial capacity, small high-emission factories, and small coal-fired industrial boilers, as well as the strengthening of emission standards for power plants, industries, and vehicles, and the replacement of residential coal use with electricity and natural gas 6,7 . These strong emission-reduction policies in China have led to a successful reduction in PM 2.5 concentration (e.g., PM 2.5 concentrations decreased from 102 µg/m 3 in 2013 to 43 µg/m 3 in 2019, as observed at the U.S. embassy in Beijing). Rapid reductions in SO 2 and NO x emissions were also confirmed using environmental satellite data 8 and bottom-up emissions inventory studies 1 . MODIS AOD (aerosol optical depth) data revealed that there has been a consistent trend of year-to-year decreases in China and its downwind regions. The chemical composition of PM 2.5 also changed significantly over China, with especially large decreases in levels of organic matter, mineral components, and sulfate aerosols 9 . Studies have suggested that emission control has a dominant e...