Unexpected shift in air pollution explained for regions downwind of China

While reduced pollution emissions are a victory for the environment overall, new research from Kyushu University indicates that they can also have unexpected consequences for areas downwind. Using new modeling and observations, researchers have now shown that a change in the balance of emissions has actually led to an increase in nitrogen-containing pollutants called nitrates long distances from China despite a reduction at the source, which could put a new burden on East Asia and the surrounding oceanic region.

As the concentration of pollution particles less than 2.5 micrometers in diameter—referred to as PM_2.5—in China has rapidly dropped in response to strong emission control policies from 2012 to 2017, total concentrations observed in Japan have also greatly decreased. However, reductions for different chemical components have not followed the same trends in China and downwind regions, and these differences have been poorly understood to date.

Researchers led by Itsushi Uno, professor of Kyushu University’s Research Institute for Applied Mechanics, have now successfully revealed the origin of these differences based on advanced modeling using source/receptor analysis with a 3D chemical transport model (GEOS-Chem model).

While the modeling verified that the rapid emissions reductions improved overall PM_2.5 concentrations both in China and its downwind regions, it could also explain the increase in nitrates found downwind despite the reduction of their starting components at the source based on a chemical imbalance in something called the ammonia–nitric acid–sulfuric acid–water system.

The researchers found that emission of sulfur-containing and nitrogen-containing pollutants named SO₂ and NO_x, respectively, were both reduced in China, but the reduction in SO₂ was much larger than that of NO_x. While SO₂ often reacts with ammonia to form other chemicals, the greater reduction in SO₂ led to an excess of unreacted ammonia that can then combine with NO_x to form chemicals able to travel much longer distances than NO_x can alone.

Including these consideration in their modeling, the researchers could explain this increase in nitrates downwind based on this chemical imbalance, and observations on a clean remote island and numerical modeling confirmed that this paradigm shift has occurred since 2014–2015.

These new findings indicate the importance for continuing to carefully track changes in air pollution both locally and downwind and will help guide the understanding of new environmental challenges that may have to be faced as the composition of emissions shifts.

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For more information about this research, see “Paradigm shift in aerosol chemical composition over regions downwind of China,” Itsushi Uno, Zhe Wang, Syuichi Itahashi, Keiya Yumimoto, Yuki Yamamura, Ayako Yoshino, Akinori Takami, Masamitsu Hayasaki, and Byung-Gon Kim, Scientific Reports 10, 6450 (2020). https://doi.org/10.1038/s41598-020-63592-6

Research-related inquiries

Itsushi Uno, Professor
Research Institute for Applied Mechanics