Paradigm shift in aerosol chemical composition over regions downwind of China

Abstract

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 These plans include the phasing out of outdated industrial capacity, small high-emission factories, and small coal-fired industrial boilers, as well as the strengthening of emission standards for power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...

30 Years of Air Quality Trends in Japan

“…The ratio of PM 2.5 concentration in 2019 to that in 2011 is 0.43 in Beijing, 0.69 in Shanghai, and 0.54 in Guangzhou, indicating that the PM 2.5 concentrations have decreased significantly, which is likely due to an effect of the emission control measures in China. This indicates that the impact of transboundary PM 2.5 on Japan is also decreasing, as pointed out by Uno et al (2020) [14]. PM 2.5 concentrations have also shown a decreasing trend since 2015, at the three sites in South Korea.…”

Cytotoxic Effects of Water-Soluble Extracts of Coarse and Fine Atmospheric Particulate Matter on Mast Cell Lines

“…The PM 2.5 concentrations in Fukuoka have been reported to be influenced by cross-border pollution from China. 29) As the concentration of PM 2.5 in China decreased, the concentration in Fukuoka was found to decrease from 2013 to 2016 29) and from 2014 to 2019. 30) Therefore, one of the causes of the high cytotoxicity levels in FY2016 may have been due to the effect of cross-border pollution from China.…”

The Impacts of Changes in Anthropogenic Emissions Over China on PM2.5 Concentrations in South Korea and Japan During 2013–2017

“…Over 2013-2017, anthropogenic emissions of nitrate precursors of NO x and NH 3 were reduced by 20.7 and 3.4% in China, respectively, and the concentrations of nitrate were reduced in China. However, the changes in anthropogenic emissions over China led to an increasing trend of nitrate in South Korea and Japan, which can be explained by that the reductions in SO 2 emissions changed the balance of the ammonia-nitric acid-sulfuric acid-water system, creating free NH 3 that reacted with HNO 3 to form NH 4 NO 3 (Bae et al, 2021;Uno et al, 2020).…”

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