Paradigm shift in aerosol chemical composition over regions downwind of China

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.