ABSTRACTLong-term synergetic fine and coarse mode aerosol observations were analyzed at 1-h intervals at Fukuoka, Japan, from January to June 2015. The GEOS-Chem chemical transport model, including dust and sea-salt acid uptake processes, was used for detailed analysis of observation data. Several Asian dust events and long-range anthropogenic aerosol transport events were observed during our analysis period, and the numerical model generally explained the observed time variation for both fine and coarse mode aerosols. We found that (i) the majority of fine mode NO3– can be considered as long-range transport (LRT) outside of Japan during the cold season, and (ii) the peak timing of fine mode NO3– coincided with that of SO4–, indicating that both aerosols are controlled by LRT. Also, an observed mass concentration ratio of NO3–/SO42– > 0.9 occurred during the cold season, indicating the importance of NO3– as a major contributor to the PM2.5 mass fraction. Finally, we clearly showed that large-scale dust-nitrate outflow from China to Fukuoka was confirmed in all cases of dust events, indicating that the anthropogenic NOx is converted to dust-nitrate and transported to Japan with dust. These results demonstrate the importance of anthropogenic NO3– LRT during the cold season and dust-nitrate LRT for all dust events (even in June).

Two regional chemical transport models were applied to simulate high concentrations of particulate matters (PM) observed in East Asia in January 2015; the first model is the Nested Air Quality Prediction Modeling System (NAQPMS) and the second is the Community Multi-scale Air Quality Model (CMAQ). The variation of PM2.5 in both models showed well agreement with measurements over both eastern China and western Japan. Based on the model results and the aerosol compositions observed in Fukuoka in western Japan, three types of PM long-range transport (LRT) were identified: N-, S-, and D-type. The N episode showed higher fine-mode nitrate (fNO3−) concentrations than fine-mode sulfate (fSO42−), indicating the importance of NO3− LRT. The S episode showed the highest fSO42− concentrations (28.9 µg m−3), which were 3.4-fold higher than fNO3−, due to high relative humidity. During the D episode, dust stagnated in Fukuoka for three days, due to the influence of low- and high-pressure systems; thus, dust LRT is also important in winter besides spring. Both models reasonable explained variations in aerosol components during both N and S episodes; however, both underestimated fSO42− especially during D episode, suggesting that they may miss certain emissions or chemical mechanisms. High coarse-mode NO3− (cNO3−) concentrations (maximum: 6.3 µg m−3), and high cNO3−/fNO3− ratios (maximum: 1.2) were observed during D episode. NAQPMS successfully captured this cNO3− peak after including heterogeneous reactions on dust. Our results emphasize the importance of such heterogeneous processes for understanding the LRT of dust and anthropogenic pollutants over East Asia.

The severe aerosol pollution in East Asia has been a focus of much research. In Japan, the environmental quality standard (EQS) for PM _2.5 was established in 2009 (daily average, 35 μ g m ^−3 ; annual average, 15 μ g m ^−3 ), and its achievement rate was below 50% during the early 2010s. Then, the PM _2.5 concentration gradually decreased, the achievement rate improved, and the EQS for PM _2.5 was finally achieved (100%) in fiscal year (FY) 2021. Because transboundary aerosol pollution is an important factor in Japanese air quality, here we analysed the long-term dataset of the satellite-measured fine-mode aerosol optical depth (AOD _f ) over the East Asian ocean to reveal the changes in the transboundary aerosol over East Asia. Overall, a decrease in AOD _f was seen over the entire East Asian ocean during the period analysed. A gradual declining trend in AOD _f was measured (−4% to −5%/year over the adjacent ocean around Japan) and corresponded well to the trend in PM _2.5 concentration observed in Japan (−5.3%/year) during FY2010–FY2021. Due to the domestic contribution in Japan, the negative trend was slightly greater for Japanese PM _2.5 concentration than for AOD _f over the adjacent ocean around Japan, and we concluded that the main reason for the dramatic air quality improvement in PM _2.5 in Japan was driven by the improvement of transboundary aerosol pollution over East Asia. In addition, the 12 year analysis period (FY2010 to FY2021) was divided into three parts: stagnation (FY2010 to FY2014), in which PM _2.5 and AOD _f remained the same as they were in FY2010; improvement (FY2015 to FY2018), in which PM _2.5 and AOD _f declined dramatically; and achievement (FY2019 to FY2021), in which PM _2.5 and AOD _f declined further.

To improve the predictability of concentrations of atmospheric particulate matter, a data assimilation (DA) system using ensemble square root filter (EnSRF) has been developed for the Community Multiscale Air Quality (CMAQ) model. The EnSRF DA method is a deterministic variant of the ensemble Kalman filter (EnKF) method, which means that unlike the EnKF method, it does not add random noise to the observations. To compare the performances of the EnSRF with those of other DA methods, such as EnKF and 3DVAR (three-dimensional variational), these three methods were applied to the same CMAQ model simulations with identical experimental settings. This is the first attempt in the field of chemical DA to compare the EnKF and EnSRF methods. An identical set of surface fine particulate matter (PM2.5) were assimilated every 6 h by all the DA methods over a CMAQ domain of East Asia, during the period from 01 May to 11 June 2016. In parallel with 'reanalysis experiments', we also carried out '48 h prediction experiments' using the optimized initial conditions produced by the three DA methods. Detailed analyses among the three DA methods were then carried out by comparing both the reanalysis and the prediction outputs with the observed surface PM2.5 over four regions (i.e., South Korea, the Beijing-Tianjin-Hebei (BTH) region, Shandong province, and Liaoning province). The comparison results revealed that the EnSRF produced the best reanalysis and prediction fields in terms of several statistical metrics. For example, when the 3DVAR, EnKF, and EnSRF methods were used, averaged normalized mean biases (NMBs) decreased by (57.6, 85.6, and 91.8) % in reanalyses and (39.7, 87.6, and 91.5) % in first-day predictions, compared to the CMAQ control experiment (i.e., without DA) over South Korea, respectively. Also, over the three Chinese regions, the EnSRF method outperformed the EnKF and 3DVAR methods.

Changes in the aerosol composition of sulfate (SO42-) and nitrate (NO3-) from 2012 to 2019 have been captured as a paradigm shift in the region downwind of China. Specifically, SO42- dramatically decreased and NO3- dramatically increased over downwind locations such as western Japan due to the faster reduction of SO2 emissions than NOx emissions and the almost constant trend of NH3 emissions from China. Emissions from China sharply decreased during COVID-19 lockdowns in February-March 2020, after which China's economic situation seemed to recover going into 2021. Given this substantial change in Chinese emissions, it is necessary to clarify the impact of long-range PM2.5 transport into the leeward of East Asia. In this study, ground-based aerosol compositions observed at three sites in western Japan were analysed. The concentrations of PM2.5, SO42- and NO3- decreased in 2020 (during COVID-19) compared with 2018-2019 (before COVID-19). In 2021 (after COVID-19), PM2.5 and NO3- increased and SO42- was unchanged. This suggests the returning long-range PM2.5 transport in 2021. From numerical simulations, the status of Chinese emissions during COVID-19 did not explain this returning impact in 2021. This study shows that the status of Chinese emissions in 2021 recovered to that before COVID-19.

In this study, we developed a data assimilation (DA) system for chemical transport model (CTM) simulations using an ensemble Kalman filter (EnKF) technique. This DA technique is easy to implement in an existing system without seriously modifying the original CTM and can provide flow-dependent corrections based on error covariance by short-term ensemble propagations. First, the PM2.5 observations at ground stations were assimilated in this DA system every 6 h over South Korea for the period of the KORUS–AQ campaign from 1 May to 12 June 2016. The DA performances with the EnKF were then compared to a control run (CTR) without DA and a run with three-dimensional variational (3D-Var) DA. Consistent improvements owing to the initial conditions (ICs) assimilated with the EnKF were found in the DA experiments at a 6 h interval compared to the CTR run and to the run with 3D-Var. In addition, we attempted to assimilate the ground observations from China to examine the impacts of improved boundary conditions (BCs) on the PM2.5 predictability over South Korea. The contributions of the ICs and BCs to improvements in the PM2.5 predictability were also quantified. For example, the relative reductions in terms of the normalized mean bias (NMB) were found to be approximately 27.2 % for the 6 h reanalysis run. A series of 24 h PM2.5 predictions were additionally conducted each day at 00:00 UTC with the optimized ICs. The relative reduction of the NMB was 17.3 % for the 24 h prediction run when the updated ICs were applied at 00:00 UTC. This means that after the application of the updated BCs, an additional 9.0 % reduction in the NMB was achieved for 24 h PM2.5 predictions in South Korea.

East Asian oceans are possibly affected by a high nitrogen (N) burden because of the intense anthropogenic emissions in this region. Based on high-resolution regional chemical transport modeling with horizontal grid scales of 36 and 12 km, we investigated the N burden into East Asian oceans via atmospheric deposition in 2010. We found a high N burden of 2-9 kg N ha(-1) yr(-1) over the Yellow Sea, East China Sea (ECS), and Sea of Japan. Emissions over East Asia were dominated by ammonia (NH3) over land and nitrogen oxides (NOx) over oceans, and N deposition was dominated by reduced N over most land and open ocean, whereas it was dominated by oxidized N over marginal seas and desert areas. The verified numerical modeling identified that the following processes were quantitatively important over East Asian oceans: the dry deposition of nitric acid (HNO3), NH3, and coarse-mode (aerodynamic diameter greater than 2.5 mu m) NO3-, and wet deposition of fine-mode (aerodynamic diameter less than 2.5 mu m) NO3- and NH4+. The relative importance of the dry deposition of coarse-mode NO3- was higher over open ocean. The estimated N deposition to the whole ECS was 390 Gg N yr(-1); this is comparable to the discharge from the Yangtze River to the ECS, indicating the significant contribution of atmospheric deposition. Based on the high-resolution modeling over the ECS, a tendency of high deposition in the western ECS and low deposition in the eastern ECS was found, and a variety of deposition processes were estimated. The dry deposition of coarse-mode NO3- and wet deposition of fine-mode NH4+ were the main factors, and the wet deposition of fine-mode NO3- over the northeastern ECS and wet deposition of coarse-mode NO3- over the southeastern ECS were also found to be significant processes determining N deposition over the ECS.

Precipitation chemistry was observed at the remote background site at Ryori, Ofunato in northeastern Japan, from 1976 to 2011 as the part of World Meteorological Organization-Global Atmospheric Watch (WMO-GAW). During this period, anthropogenic emissions in East Asia varied widely, which affected the precipitation chemistry. The precipitation amount also affects the wet deposition amount. Thus, to eliminate the effect of the precipitation amount, we used the ratio of nitrate (NO3−) to non-sea-salt sulfate (nss-SO42−) concentration in precipitation on an equivalent basis (Ratio). The historical trend of NO3− and nss-SO42− concentrations in precipitation was not clear; however, Ratio showed a strong increase in the 1990s, a sudden drop in 2001, and an increase again after 2007. Based on the analysis of the historical emission dataset, the increases in Ratio during the 1990s and after 2007 were attributed to anthropogenic emission changes in Japan and China. The drop in Ratio in 2001 was explained by the massive SO2 release from Miyakejima volcano from mid-August 2000, rather than by anthropogenic emissions. The 36-year historical record analysis of precipitation chemistry at Ryori detected both anthropogenic and natural emission changes and indicated the transition from local to transboundary air pollution in Japan.

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.

There has been little study on the effect of Asian dust exposure on respiratory symptoms among childrenwho are vulnerable to environmental factors. In this panel study, we investigated the effect of Asian dust on respiratory symptoms among children with and without asthma, and their sensitivity. Children attending two elementary schools (137 total), and 23 children with asthma from cooperating medical institutions in Fukuoka prefecture were recruited. Subjects measured peak expiratory flow rate (PEF), and recorded asthma-like symptoms, cough, nasal symptoms and use of medication in a diary from April 1, 2013 to June 30, 2013. To assess exposure to Asian dust, we used Light Detection and Ranging (LIDAR) data. For the analysis of the association between Asian dust and respiratory symptoms, the case-crossover design and generalized estimating equation (GEE) models were used. Taking individual sensitivity to respiratory aggravation into consideration, the subjects were classified into three groups: children without asthma, children with asthma who do not use long-term preventive medication (CA) and children with asthma who use long-term preventive medication (CA-LTM). For CA, Asian dust exposure was significantly associated with asthma-like symptoms, with a hazard ratio of 5.17 (95%CI: 1.02=26.12) at Lag0, and the change in %maxPEF, -1.65% (95%CI:-2.82, -0.48) at Lag0. For children without asthma, a statistically significant association was found between Asian dust exposure and the change in%maxPEF, -0.56% (95%CI:-1.31,-0.08) at Lag1. However, no adverse effects were observed in CA-LTM. Temperature had significant effects on %maxPEF for three groups. Asian dust, photochemical oxidant and pollen caused simultaneously additive adverse effects on nasal symptoms for children without asthma. This study suggests the possibility that long-term preventive medication to manage asthma may suppress aggravation of respiratory symptoms due to Asian dust and may be an effective prevention. (C) 2020 Elsevier B.V. All rights reserved.

The lockdown measures due to COVID-19 affected the industry, transportation and other human activities within China in early 2020, and subsequently the emissions of air pollutants. The decrease of atmospheric NO2 due to the COVID-19 lockdown and other factors were quantitively analyzed based on the surface concentrations by in-situ observations, the tropospheric vertical column densities (VCDs) by different satellite retrievals including OMI and TROPOMI, and the model simulations by GEOS-Chem. The results indicated that due to the COVID-19 lockdown, the surface NO2 concentrations decreased by 42% ± 8% and 26% ± 9% over China in February and March 2020, respectively. The tropospheric NO2 VCDs based on both OMI and high quality (quality assurance value (QA) ≥ 0.75) TROPOMI showed similar results as the surface NO2 concentrations. The daily variations of atmospheric NO2 during the first quarter (Q1) of 2020 were not only affected by the COVID-19 lockdown, but also by the Spring Festival (SF) holiday (January 24-30, 2020) as well as the meteorology changes due to seasonal transition. The SF holiday effect resulted in a NO2 reduction from 8 days before SF to 21 days after it (i.e. January 17 - February 15), with a maximum of 37%. From the 6 days after SF (January 31) to the end of March, the COVID-19 lockdown played an important role in the NO2 reduction, with a maximum of 51%. The meteorology changes due to seasonal transition resulted in a nearly linear decreasing trend of 25% and 40% reduction over the 90 days for the NO2 concentrations and VCDs, respectively. Comparisons between different datasets indicated that medium quality (QA ≥ 0.5) TROPOMI retrievals might suffer large biases in some periods, and thus attention must be paid when they are used for analyses, data assimilations and emission inversions.

The black carbon (BC) and carbon monoxide (CO) emission ratios were estimated and compiled from long-term, harmonized observations of the ΔBC∕ΔCO ratios under conditions unaffected by wet deposition at four sites in East Asia, including two sites in South Korea (Baengnyeong and Gosan) and two sites in Japan (Noto and Fukuoka). Extended spatio-temporal coverage enabled estimation of the full seasonality and elucidation of the emission ratio in North Korea for the first time. The estimated ratios were used to validate the Regional Emission inventory in ASia (REAS) version 2.1 based on six study domains (“East China”, “North China”, “Northeast China”, South Korea, North Korea, and Japan). We found that the ΔBC∕ΔCO ratios from four sites converged into a narrow range (6.2–7.9 ng m−3 ppb−1), suggesting consistency in the results from independent observations and similarity in source profiles over the regions. The BC∕CO ratios from the REAS emission inventory (7.7 ng m−3 ppb−1 for East China – 23.2 ng m−3 ppb−1 for South Korea) were overestimated by factors of 1.1 for East China to 3.0 for South Korea, whereas the ratio for North Korea (3.7 ng m−3 ppb−1 from REAS) was underestimated by a factor of 2.0, most likely due to inaccurate emissions from the road transportation sector. Seasonal variation in the BC∕CO ratio from REAS was found to be the highest in winter (China and North Korea) or summer (South Korea and Japan), whereas the measured ΔBC∕ΔCO ratio was the highest in spring in all source regions, indicating the need for further characterization of the seasonality when creating a bottom-up emission inventory. At levels of administrative districts, overestimation in Seoul, the southwestern regions of South Korea, and Northeast China was noticeable, and underestimation was mainly observed in the western regions in North Korea, including Pyongyang. These diagnoses are useful for identifying regions where revisions in the inventory are necessary, providing guidance for the refinement of BC and CO emission rate estimates over East Asia.

Atmospheric ammonia (NH3) plays an important role in the formation of secondary inorganic aerosols, the neutralization of acid rain, and the deposition to ecosystems, but has not been well understood yet, especially over East Asia. Based on the GEOS-Chem model results, the IASI satellite retrievals, the in-site surface observations of a nationwide filter pack (FP) network over Japan and the long-term high resolution online NH3 measurements at Fukuoka of western Japan, the spatio-temporal distributions of atmospheric NH3 over East Asia was analyzed comprehensively. A significant seasonal variation with a summer peak was found in all datasets. Comparison between the satellite retrievals and model simulations indicated that the IASI NH3 vertical column density (VCD) showed good consistency with GEOS-Chem results over North and central China, but had large differences over South China due to the effect of clouds. Over the Japan area, GEOS-Chem simulated NH3 concentrations successfully reproduced the spatio-temporal variations compared with in-situ observations, while IASI NH3 VCD retrievals were below or near the detection limit and difficult to obtain a reasonable correlation for with model results. The comprehensive analysis indicated that there were still some differences among different datasets, and more in-situ observations, improved satellite retrievals, and high-resolution model simulations with more accurate emissions are necessary for better understanding the atmospheric NH3 over East Asia.

The effect of the nonsphericity of mineral dust aerosols on its deposition and transport was investigated based on model simulation for a typical dust event over northern China from April 6 to 12, 2018. The settling velocity related to morphological change in dust size was considered in Nested Air Quality Prediction Modeling System (NAQPMS) to simulate the dust spatial distribution. Comparison of these results with observations showed that the model reproduced the temporal variability in the mass concentration of particles along the dust plume pathway. The most frequently reported aspect ratio (λ) was 1.7 ± 0.2 for Asian dust aerosols. Changing the nonsphericity of the particle from typical prolate ellipsoids (λ = 1.7) to spherical ellipsoids (λ = 1) caused an ~3% decrease in the surface dust concentration on average. For particles with diameters >5 μm, nonsphericity caused a change in the surface dust concentration up to 10%, especially at the periphery of the dust source region. The overall effects on the fine dust (<2.5 μm) were not significant. A sensitivity study using a more extreme nonspherical shape (λ = 2) showed that the differences in PM10 concentration were evident, and the surface dust concentration increased by 15 ± 5% as a result of an ~10% decrease in settling velocity. These results confirmed that the effect of the variability in the nonsphericity of Asian dust particles on their regional transport highly depended on synoptical and pollution conditions, and the adoption of a deposition value that changes over time due to this morphological variability could improve the performance of dust modeling and the assessment of climate effects on a global scale, especially for transboundary processes.

In order to prevent the spread of COVID-19, the Chinese government imposed a lockdown During this period, anthropogenic emissions will be reduced;therefore, the trans-boundary air pollution will be changed The analysis of surface observations by the automated aerosol chemical speciation analyzer (ACSA) showed that the dramatic reduction with 3050 of PM 2 5 , sulfate (SO 4 2), and nitrate (NO 3) on FebruaryMarch 2020 compared to 20182019 The results of the chemical transport model suggested that the reduction of SO 4 2 was mostly caused by SO 2 emission reduction whereas that of NO 3 was dominated by the meteorological variability The record high warm winter on 2020 was related to the unfavorable condition to produce NO 3

Atmospheric aerosols over polar regions have attracted considerable attention for their pivotal effects on climate change. In this study, temporospatial variations in single-particle-based depolarization ratios (δ: s-polarized component divided by the total backward scattering intensity) were studied over the Northwest Pacific and the Arctic Ocean using an optical particle counter with a depolarization module. The δ value of aerosols was 0.06 ± 0.01 for the entire observation period, 61 ± 10% lower than the observations for coastal Japan (0.12 ± 0.02) ( Pan et al. Atmos. Chem. Phys. 2016 , 16 , 9863 - 9873 ) and inland China (0.19 ± 0.02) ( Tian et al. Atmos. Chem. Phys. 2018 , 18 , 18203 - 18217 ) in summer. The volume concentration showed two dominant size modes at 0.9 and 2 μm. The supermicrometer particles were mostly related to sea-salt aerosols with a δ value of 0.09 over marine polar areas, ∼22% larger than in the low-latitude region because of differences in chemical composition and dry air conditions. The δ values for fine particles (<1 μm) were 0.05 ± 0.1, 50% lower than inland anthropogenic pollutants, mainly because of the complex mixtures of submicrometer sea salts. High particle concentrations in the Arctic Ocean could mostly be attributed to the strong marine emission of sea salt associated with deep oceanic cyclones, whereas long-range transport pollutants from the continent were among the primary causes of high particle concentrations in the Northwest Pacific region.

Dust aerosol has important climate and environmental effects, which could be changed by internally mixing with anthropogenic aerosol as a result of heterogeneous reactions; however, the importance of these reactions is not fully understood yet. In this study, synergetic observations and an air quality model were used to analyze the transport of a severe dust storm and its impacts on nitrate and sulfate levels over East Asia between 3 and 11 May 2017. The model successfully reproduced the occurrence and transport of the dust storm compared to dust RGB imageries of the Himawari-8 satellite and dust extinction coefficients observed by LIDAR. The model also reasonably simulated the variations of observed nitrate and sulfate concentrations, and the results indicated that the dust heterogeneous reactions were dominant pathways for nitrate formation, but they had limited contribution for sulfate in both fine and coarse mode in Fukuoka, Japan. Dust nitrate formed rapidly after leaving China, and the highest period-averaged concentration of dust nitrate (>5 μg m−3) was shown over the Yellow Sea. Based on model results; we found that the mass ratio of dust nitrate to dust aerosol could reach 10% over the Pacific Ocean. Our results confirmed the importance of heterogeneous reactions on compositions of dust particles.