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

We applied the global 3-D chemical transport model GEOS-Chem to examine the effects of changes in anthropogenic emissions over China on PM2.5 concentrations in South Korea and Japan over 2013–2017 during which the Chinese governmental “Action Plan on the Prevention and Control of Air Pollution” was implemented. The reductions in anthropogenic emissions in China alleviated PM2.5 concentrations in South Korea and Japan. The reductions in anthropogenic emissions in China alone led to reductions in annual mean PM2.5 concentrations by 13.7 (19.7%), 2.7 (7.7%), and 1.0 (7.9%) μg m−3 over 2013–2017 in eastern China, South Korea, and Japan, respectively. The reductions in anthropogenic emissions in China also changed the components of PM2.5 in South Korea and Japan. The concentrations of sulfate, ammonium, black carbon and organic carbon were simulated to decrease and those of nitrate were simulated to increase in both South Korea and Japan. With the changes in anthropogenic emissions and the changes in meteorology (BASE simulation), the proportions of sulfate, nitrate, and OC in PM2.5 in South Korea (Japan) were, respectively, 17.1, 44.4, and 15.8% (27.2, 30.5, and 20.0%) in 2013, and 14.3, 46.0, and 17.8% (23.7, 33.2, and 21.6%) in 2017. Model results shown here are helpful for understanding the temporal evolution in PM2.5 on different timescales in the downwind regions of China.


INTRODUCTION
Air pollution is a serious environmental problem that influences human health, ecosystem, and climate (IPCC, 2021). The Chinese government issued the "Action Plan on the Prevention and Control of Air Pollution" in 2013 to aggressively control anthropogenic emissions thereafter. With the implementation of a series of stringent air pollution control measures, the observed annual mean PM 2.5 concentrations from the China National Environmental Monitoring Center (CNEMC) showed decreases in PM 2.5 concentrations by 34-49% over 2013-2018 in the five megacity clusters (Beijing-Tianjin-Hebei, Yangtze River Delta, Pearl River Delta, Fenwei Plain, and Sichuan Basin) (Zhai et al., 2019). Because of the prevailing westerlies in Northeast Asia, it is of interest to examine how the changes in anthropogenic emissions in China can influence air quality in the downwind areas, especially in Japan and South Korea, through the long-range transport of air pollutants.
A number of observational and modeling studies have demonstrated the influence of long-range transport of aerosols from China on aerosol levels in South Korea and Japan. Lee et al. (2018) selected four long-lasting haze episodes (all in winter) occurred in South Korea during 2009-2014 using aerosol optical depth (AOD) at 550 nm from Terra and Aqua Moderate Resolution Imaging Spectroradiometer (MODIS). The distributions of AODs over northeast Asia during each episode showed that the high-AOD area moved from China to South Korea in the presence of northwestliers or westerlies. Similarly, Lee et al. (2019) examined three representative cases during the Korea-United States Air Quality (KORUS-AQ) campaign from 1 May to 12 June 2016 by using the hourly Geostationary Ocean Colour Imager (GOCI) AOD images. They found that the heavy aerosol plumes generated in central-eastern China (particularly in the Shandong peninsula) could pass across the Yellow Sea, and then reached the middle to northern South Korea. Park et al. (2018) collected PM 2.5 samples in Seoul, South Korea, from September 2013 to May 2015 and in Nagasaki, Japan, from February 2014 to May 2015 to analyze the 72-h backward trajectories of aerosols. For Seoul, 62% of total trajectories originated from the Yellow Sea near the industrial area in Liaoning Province in China and then moved to the southwestern of Seoul. For Nagasaki, 61% of total trajectories originated from northern China (near Beijing) and passed through North Korea and South Korea to Nagasaki. Aikawa et al. (2010) used a 3-dimensional regional chemical transport model to estimate the impact of transboundary transport on sulfate concentrations at 24 sites in urban, suburban, and rural areas of Japan. They performed simulations with and without anthropogenic emissions in China from 1 January 2003 to 31 March 2006, and reported that Chinese emissions contributed 50-70% to modeled 3-year mean sulfate concentrations at 24 sites in Japan, with a maximum of about 65-80% in winter. Lee et al. (2017) examined PM 2.5 concentrations in 10 PM 2.5 episodes (both modeled and measured PM 2.5 concentrations exceeded the daily air quality standard of 50 μg m −3 ) in May of 2009-2013 using the GEOS-Chem model, and found that anthropogenic emissions from Shandong Province in China had the largest modeled influence on PM 2.5 in Seoul in May. Averaged over the 10 episodes, 69% of simulated PM 2.5 in Seoul were from China (39% from the Shandong Province, 16% from Shanghai, and 14% from the Beijing). Yim et al. (2019) used the Weather Research and Forecasting Model (WRF) and Community Multiscale Air Quality Modeling System (CMAQ) to simulate the contributions of anthropogenic emissions in China to PM 2.5 in South Korea and Japan in 2010 by sensitivity studies. Anthropogenic emissions in China were estimated to contribute 54% to annual mean surface concentrations of PM 2.5 in both Japan and South Korea in 2010. In Japan, the highest contribution from China occurred in spring (61.4%), followed by winter (55.1%) and summer (50.5%), and autumn (44.0%). In South Korea, the contributions of Chinese emissions were higher in winter (62.9%) and spring (55.5%), while lower in autumn (51.7%) and summer (43.8%). Kumar et al. (2021) used the GEOS-Chem model to estimate source attribution to PM 2.5 in South Korea from international and domestic emissions in 2015 and 2016 by sensitivity simulations. Their results showed that, on an annual mean basis, anthropogenic emissions in China contributed 45% to PM 2.5 in South Korea in 2016, which was 3-5% lower than the Chinese contribution in 2015 because of emissions reductions in China. Results from these studies indicate that the air quality in South Korea and Japan is closely related to air pollution in China.
The long-range transboundary transport of air pollutants (LRTT) is an important issue in air quality planning. East Asia is located in the warm conveyor belts (WCBs) of mid-latitude storm tracks, where airstreams rapidly transport the atmospheric boundary layer air into the upper troposphere by convection, orographic lifting, frontal lifting and ascension (Stohl et al., 2002;Eckhardt et al., 2004). The analysis of meteorological conditions for heavy pollution episodes in South Korea showed that the anomalous low pressure over the upwind regions lifted air pollutants into the upper troposphere, then the strong persistent westerlies in this region driven the flow of pollutants (Qu et al., 2016). When the outflow of air pollutants arrives at South Korea and Japan, the anomalous high pressure causes the air to descend and leads to the accumulation of pollutants in these regions (Lee et al., 2011;Lee et al., 2018).
There were several studies that examined the effects of changes in outflow from China in recent years on aerosol levels in Japan and South Korea. Bhardwaj et al. (2019) carried out the multiyear trend analysis for observed AOD, CO and SO 2 with a special focus on the Yellow Sea region. They showed that percentage changes in AOD over 2012-2016 were -23.7%, -11.7% and -16.5% over the North China Plain, Yellow Sea, and South Korea, respectively, and the surface CO mixing ratios over 2012-2016 decreased by 8.3, 5.1 and 3.9%, respectively, in these three regions. Kanaya et al. (2020) analyzed the changes in black carbon (BC) and PM 2.5 over 2009-2019 at Fukue island in Japan, and reported that concentrations of BC and PM 2.5 decreased by 48 and 21%, respectively. Uno et al. (2020) reported that the observed surfacelayer PM 2.5 in Japan decreased in recent years, and the proportion of stations that satisfied the PM 2.5 air quality standard increased from 37.8% in 2014 to 89.9% in 2017. Bae et al. (2021) showed that, from 2015 to 2018, the observed PM 2.5 concentrations averaged over 344 monitoring stations across South Korea decreased by 4.3% per year and the MODIS AOD in South Korea decreased by 4.7% per year. They also used the Community Multiscale Air Quality (CMAQ) model to quantify the impacts of the emissions and meteorology on the PM 2.5 concentration over 2015-2018. Their results showed that the changes in emissions and meteorology over 2015-2018 contributed 51 and 49%, respectively, to the reductions in PM 2.5 in South Korea. Previous studies, however, did not quantify the influence of air quality improvement in China on air quality in the downwind regions.
In this study, we examined the effects of reductions in anthropogenic emissions reductions in China over 2013-2017 on PM 2.5 concentrations in downwind areas (South Korea and Japan) through the long-range transport of air pollutants. The scientific goals of this work include: 1) to quantify the influence of

Model Description
For simulation of aerosols in East Asia, we used the 3-D Goddard Earth Observing System chemical transport model (GEOS-Chem) which includes fully coupled HO x -NO x -VOC-O 3aerosol chemistry with more than 80 species and 300 reactions and online calculations of aerosols including sulfate (Park et al., 2004), nitrate (Pye et al., 2009), ammonium, BC and organic carbon (OC) (Park et al., 2003), mineral dust (Fairlie et al., 2007), and sea salt (Alexander et al., 2005). We simulated concentrations of aerosols using the nested-grid version of the GEOS-Chem model version11-01 (http://acmg.seas.harvard.edu/geos/) driven by Modern-Era Retrospective Analysis for Research and Applications Version (MERRA-2) assimilated meteorological fields from NASA's Global Modeling and Assimilation Office (GMAO) (Gelaro et al., 2017). The nested domain for Asia (60-150°E, 11°S-55°N) has 47 vertical layers up to 0.01 hPa and a horizontal resolution of 0.5°latitude by 0.625°longitude. Both wet deposition of soluble aerosols and gases (Liu et al., 2001) and dry deposition (Wesely, 2007) are included in the model simulation. PM 2.5 concentration is calculated as the sum of sulfate, nitrate, ammonium, BC, and OC from the model. We do not consider sea salt and mineral dust aerosols in PM 2.5 in this study.

Emissions
The NO  Figure S1). The reduction in emissions of SO 2 was the largest, with a decrease of 58.7% over 2013-2017. Over 2013-2017, the emissions of NO x , CO, NH 3 , BC and OC in China were reduced by 20.7, 22.9, 3.4, 28.2 and 32.4%, respectively. In South Korea, over 2013-2017, the annual anthropogenic emissions of BC and NO x decreased, while other anthropogenic emissions increased, especially those of OC. In Japan, the anthropogenic emissions kept decreasing year by year during 2013-2017, the annual anthropogenic emissions of NO x , CO, NMVOCs, SO 2 , NH 3 , BC and OC in Japan decreased by 20.0, 17.7, 12.5, 21.9, 4.9, 30.7 and 12.7%, respectively.

Numerical Experiments
This study will be focused on aerosol concentrations in China, South Korea and Japan. To investigate the influences of anthropogenic emission reductions in China on air quality over South Korea and Japan, we conducted three 5-year simulations with the GEOS-Chem model with nested domain in Asia ( Table 2)

Simulated PM 2.5 Concentration in East
Asia From 2013 to 2017 Figure 2 shows the simulated annual and seasonal mean surface layer PM 2.5 concentrations over East Asia from 2013 to 2017 from the BASE simulation. The simulated annual mean PM 2.5 concentrations in eastern China reached 80-120 μg m −3 during 2013-2017. Seasonally, PM 2.5 concentrations in China were the highest in DJF due to the strengthened anthropogenic emissions as well as the unfavorable weather conditions, and were the lowest in JJA because of the enhanced wet removal from the East Asian summer monsoon (Zhang et al., 2010). Both the annual and seasonal mean PM 2.5 concentrations in China exhibited clear decreasing trends over 2013-2017 because of the stringent emission control measures in China. PM 2.5 concentrations in South Korea were generally higher than those in Japan, the simulated annual mean PM 2.5 concentrations were in the range of 20-40 μg m −3 over South Korea and were 10-20 μg m −3 in Japan. PM 2.5 concentrations over South Korea and Japan had the same seasonal variations, with concentrations higher in DJF and MAM and lower in JJA and SON.
We use the ground-based observations to evaluate the model performance in simulating the surface-layer PM 2.5 for each year. Supplementary Figure S2-4 display the spatial distributions of simulated (from the BASE simulation) and observed annual mean PM 2.5 concentrations in China, South Korea, and Japan.

Eastern China
In the BASE simulation, from 2013 to 2017, the annual mean PM 2.5 concentrations in eastern China were 69.4, 66.3, 65.4, 58.7, and 56.6 μg m −3 , respectively. The year by year decreases in PM 2.5

South Korea
In the BASE simulation with combined changes in emissions and meteorology, simulated annual mean PM 2. higher than the trends of decreases in South Korea and Japan (Figure 7). These decreasing trends in eastern China were mainly caused by domestic changes in anthropogenic emissions. In South Korea, sulfate had the largest decreasing trend (−0.32 μg m −3 year −1 ), followed by nitrate (−0.16 μg m −3 year −1 ) and ammonium (−0.15 μg m −3 year −1 ), in the BASE simulation. The changes in anthropogenic emissions in China led to large reductions in sulfate and ammonium whereas an increase in nitrate. Changes in meteorological conditions led to a relatively large decreasing trend of nitrate in South Korea.
The trends of aerosol components in the BASE simulation in Japan were the smallest among the three countries. The trends of aerosol concentrations influenced by meteorology were positive for all aerosol species. The changes in anthropogenic emissions over China had similar impacts on aerosol components in Japan as compared to South Korea. Nitrate concentrations increased in Japan from 2013 to 2017 by changes both in anthropogenic emissions in China and in meteorological fields.
To summarize, the changes in anthropogenic emissions over China in 2013-2017 had the largest impacts on sulfate concentrations in South Korea and Japan, leading to large reductions in sulfate. On the contrary, the changes in meteorology had almost no effect on sulfate in South Korea and Japan. 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).

The Changes in Mass Outflow From Eastern China During 2013-2017
In order to have a clearer understanding of the impacts of reductions in anthropogenic emissions in China as well as the changes in meteorological conditions on PM 2.5 in South Korea and Japan, we analyzed the changes in mass outflow from eastern China during 2013-2017. Figure 8 shows the simulated annual and seasonal mass fluxes of PM 2.5 and zonal winds across the selected plane from the surface to 100 hPa altitude and from 25°N to 55°N along 120°E in the BASE simulation. The maximum mass fluxes of PM 2.5 were located at 700-1,000 hPa. On an annual mean basis, a maximum flux was located over 40°N   Frontiers in Environmental Science | www.frontiersin.org February 2022 | Volume 10 | Article 841285 As summarized in Table 3, the seasonal outflow flux of PM 2.5 peaked in MAM of every year as a result of the strong westerlies, although PM 2.5 levels in MAM in China were not the highest (Figure 2). The mass outflow of PM 2.5 was the lowest in JJA, which was caused by the lowest concentrations of PM 2.5 in China as well as the weakest westerlies in this season (Figure 8).  Figure 5). The interannual and seasonal variations in mass flux are mainly driven by the changes in meteorological parameters (Holzer et al., 2005;Yang et al., 2015). With the changes in both emissions and meteorology, from 2013 to 2017, the annual mass fluxes of PM 2.5 from eastern China had a linear increasing trend of 13.1 × 10 6 kg yr −1 in the BASE simulation. However, the trend of flux was −99.4 × 10 6 kg yr −1 over 2013-2017 with changes in anthropogenic emissions in China alone (FIXEM_KJ minus FIXEM_CKJ).

The Impact of Meteorological Conditions on PM 2.5 Concentration in South Korea and Japan From 2013 to 2017
Over 2013-2017, the changes in meteorological conditions have different effects on PM 2.5 concentrations in South Korea and Japan. The changes in meteorological conditions led to a decreasing trend of 0.30 μg m −3 year −1 in PM 2.5 concentration in South Korea, whereas an increasing trend of 0.08 μg m −3 year −1 in Japan ( Figure 5B). Therefore, to further understand the impacts of meteorological fields on PM 2.5 in South Korea and  Japan, we calculated the trends of annual mean temperature, relative humidity (RH) and planetary boundary layer height (PBLH) as well as the trend of annual total precipitation over 2013-2017 (Figure 9). Temperature and RH can affect the thermodynamics of ammonium nitrate formation. In South Korea, the combined effect of increases in temperature and decreases in RH had an effect of reducing nitrate concentrations from 2013 to 2017 (Zhai et al., 2021) (Figure 7B). In South Korea, from 2013 to 2017, PBLH had an increasing trend, which increased the diffusion of pollutants and also reduced PM 2.5 concentrations. Although precipitation decreased in South Korea, the overall impact of meteorological conditions in South Korea was to reduce PM 2.5 over 2013-2017. In Japan, temperature and RH increased whereas PBLH decreased over 2013-2017. The increases in temperature could increase sulfate but reduce nitrate (Aw and Kleeman 2003). Higher RH was reported to increase the formation of secondary aerosols through enhanced chemical reactions (Yin et al., 2016). The decreases in PBLH in Japan restrained vertical mixing and lead to the accumulation of particles (Gao et al., 2015). The spatial distributions of changes in precipitation in Japan were uneven. As a result, the meteorological conditions in Japan led to increases of PM 2.5 .

CONCLUSION
We examined the impacts of reductions in anthropogenic emissions over China and/or changes in meteorological conditions over 2013-2017 on PM 2.5 and its components in South Korea and Japan. The seasonal variation, interannual variation, and trend of PM 2.5 and its components in China, South Korea and Japan were simulated using the GEOS-Chem model driven by the MERRA-2 meteorological fields. The impacts of variations in anthropogenic emissions and in meteorological parameters were imposed separately and together by sensitivity studies.
From 2013-2017, with the changes in both anthropogenic emissions and meteorology (BASE simulation), the trends of simulated annual mean PM 2.5 concentration in eastern China, South Korea, and Japan were −3.33, −0.66, and −0.09 μg m −3 year −1 , respectively, in which the reductions in anthropogenic emissions in China played a dominant role compared to that of meteorology. From 2013 to 2017, the annual mean PM 2.5 concentrations in eastern China, South Korea, and Japan were reduced by 12.8 (18.5%), 3.0 (8.3%), and 0.36 (2.7%) μg m −3 , respectively. The reductions in anthropogenic emissions in China (FIXEM_KJ minus FIXEM_CKJ) over 2013-2017 alone led to reductions in annual mean PM 2.5 concentrations by 13.7 (19.7%), 2.7 (7.7%), and 1.0 (7.9%) μg m −3 in eastern China, South Korea, and Japan, respectively. The changes in meteorological fields (FIXEM_CKJ simulation) were also important for PM 2.5 concentrations in South Korea and Japan. From 2013 to 2017, the changes in meteorological conditions led to reductions in PM 2.5 concentration in South Korea by 4.7% and increases in PM 2.5 concentration in Japan by 2.5%.
From 2013 to 2017, the reductions in anthropogenic emissions in China (FIXEM_KJ minus FIXEM_CKJ) also changed the components of PM 2.5 in South Korea and Japan. The changes in anthropogenic emissions in China led to reductions in sulfate, ammonium, BC and OC concentrations whereas increases of nitrate in both South Korea and Japan. The reductions in SO 2 emissions in China 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 . In South Korea, the proportions of sulfate, nitrate, and OC in PM 2.5 were, respectively, 17. 1, 44.4, and 15.8% in 2013, and 14.3, 46.0, and 17.8% in 2017. In Japan, the fractions of sulfate, nitrate, and OC in PM 2.5 were 27.2, 30.5, and 20.0% in 2013 while 23.7, 33.2, and 21.6% in 2017, respectively. The changes in anthropogenic emissions in China instead of the changes in meteorology dominated this compositional shift.
Model results shown here are helpful for understanding the temporal evolution in the PM 2.5 on different timescales in the downwind regions of China. Conclusions from this work also have important implications for air quality planning, which needs to account for changes in emissions in the upwind countries. It should be noted that our model results may have uncertainties, especially from the uncertainties in emissions inventories. First, the anthropogenic emissions in other regions except Asia were fixed at the 2013 levels in the BASE simulation, which could not account for the effects of changes in emissions outside Asia. Second, the REAS emission inventory is available only up to 2015, so anthropogenic emissions from REAS in 2016 and 2017 were obtained by linear extrapolation of the values over 2012-2015 for each grid. Third, since we were focused on the impact of the Chinese Governmental "Action Plan on the Prevention and Control of Air Pollution," we did not account for the role of natural aerosols such as mineral dust and sea salt in this work. All these aspects can be improved in future studies.

DATA AVAILABILITY STATEMENT
The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.