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 SO₂ 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/m³; annual average, 15 µg/m³) 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 SO₂ and NO_x reduction rates led to greater production of nitrates (e.g., NH₄NO₃) 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.