Paradigm shift in aerosol chemical composition over regions downwind of China

Itsushi Uno,Syuichi Itahashi,Masamitsu Hayasaki,Yuki Yamamura,Zhe Wang,Byung-Gon Kim,Akinori Takami,Keiya Yumimoto,Ayako Yoshino

doi:10.1038/s41598-020-63592-6

Abstract

A rapid decrease in PM2.5 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 NOx from China decreased by approximately 63% and 24%, respectively. Simultaneously, decreases in the PM2.5 concentration in Japan have been observed since 2014, and the proportion of stations that satisfy the PM2.5 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 PM2.5 improvement in China and the PM2.5 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 PM2.5 concentrations both in China and its downwind regions, but the difference between SO2 and NOx 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.

Highlights

A rapid decrease in PM2.5 concentrations in China has been observed in response to the enactment of strong emission control policies
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 PM2.5 concentrations both in China and its downwind regions, but the difference between SO2 and NOx 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
To quantitatively analyze the increase in NO3−, we modeled an additional four cases of sensitivity experiments, changing the SO2 and NOx emission intensities based on the bottom-up Multi-resolution Emission Inventory for China (MEIC)[1] results (Table 1)

Summary

Introduction

A rapid decrease in PM2.5 concentrations in China has been observed in response to the enactment of strong emission control policies. Focusing on regions downwind of China, the proportion of monitoring stations meeting the Japanese environmental standard for PM2.5 (defined as the achievement ratio) increased rapidly from 37.8% in fiscal year (FY) 2014 to 89.9% in FY 2017 (Ministry of Environment, Japan)[16] During this period, Japanese pollutant emissions exhibited a slight decreasing trend[17]. Improvements in the PM2.5 achievement ratio result from complex interactions between Japanese and Chinese emission-reduction measures, and it is not yet clear which factor has a greater contribution This lack of clarity arises because most PM2.5 studies are focused on an individual country, and quantitative evaluations or correlation analyses of the impacts of environmental improvement on downwind regions (beyond national borders) have rarely been published[18,19,20,21]. These experiments showed that the observed nitrate increase can be explained by changes in the ammonia–nitric acid–sulfuric acid–water system balance due to the greater rate of decrease in SO2 emissions compared to that in NOx emissions

Methods

Results

Conclusion