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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 (PM 2.5 ) 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 PM 2.5 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., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier Ltd. All rights reserved.)

Changes in the aerosol composition of sulfate (SO 4 2- ) and nitrate (NO 3 - ) from 2012 to 2019 have been captured as a paradigm shift in the region downwind of China. Specifically, SO 4 2- dramatically decreased and NO 3 - dramatically increased over downwind locations such as western Japan due to the faster reduction of SO 2 emissions than NO x emissions and the almost constant trend of NH 3 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 PM 2.5 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 PM 2.5 , SO 4 2- and NO 3 - decreased in 2020 (during COVID-19) compared with 2018-2019 (before COVID-19). In 2021 (after COVID-19), PM 2.5 and NO 3 - increased and SO 4 2- was unchanged. This suggests the returning long-range PM 2.5 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., (© 2022. The Author(s).)

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 (NH 3 ) over land and nitrogen oxides (NO x ) 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 (HNO 3 ), NH 3 , and coarse-mode (aerodynamic diameter greater than 2.5 μm) NO 3 - , and wet deposition of fine-mode (aerodynamic diameter less than 2.5 μm) NO 3 - and NH 4 + . The relative importance of the dry deposition of coarse-mode NO 3 - 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 NO 3 - and wet deposition of fine-mode NH 4 + were the main factors, and the wet deposition of fine-mode NO 3 - over the northeastern ECS and wet deposition of coarse-mode NO 3 - over the southeastern ECS were also found to be significant processes determining N deposition over the ECS., (Copyright © 2021 The Authors. Published by Elsevier Ltd.. All rights reserved.)

There has been little study on the effect of Asian dust exposure on respiratory symptoms among children who 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., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 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 NO 2 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 NO 2 concentrations decreased by 42% ± 8% and 26% ± 9% over China in February and March 2020, respectively. The tropospheric NO 2 VCDs based on both OMI and high quality (quality assurance value (QA) ≥ 0.75) TROPOMI showed similar results as the surface NO 2 concentrations. The daily variations of atmospheric NO 2 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 NO 2 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 NO 2 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 NO 2 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., Competing Interests: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (© 2020 Elsevier Ltd. All rights reserved.)

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 PM 10 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., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020. Published by Elsevier B.V.)

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.

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.

The Taklimakan Desert is known to be one of the world's major sources of aeolian dust particles. Continuous images with 10-min temporal and 2-km spatial resolutions from a new-generation geostationary meteorological satellite captured the lifecycle (generation, evolution and outflow) of a previously unrecognized type of Taklimakan dust storm. The dust storm showed an anti-clockwise spiral structure and a clear core and behaved like a "dust vortex". From image analysis, the horizontal scale and temporal lifetime of the dust vortex were estimated to be 600 km and 36 hours, respectively. We found that a strong pressure trough (cut-off low), along with a cold air mass located on the northwestern side of the Taklimakan Desert and the high mountains surrounding the Taklimakan Desert, played important roles in the formation and evolution of the dust vortex.

Mass concentrations of chemical compounds in both PM 2.5 (particle aerodynamic diameter, Dp < 2.5 μm) and PM 2.5-10 (2.5 < Dp < 10 μm), and acidity of aerosol particles were measured at an urban site in western Japan using a continuous dichotomous Aerosol Chemical Speciation Analyzer (ACSA-12) throughout 2014. Mass concentrations of both PM 2.5 and sulfate had distinct seasonal variabilities with maxima in spring and winter, mostly due to long-range transport with the prevailing westerly wind. Mass concentration of nitrate in PM 2.5 (fNO 3 ) showed an obvious warm-season-low and cold-season-high pattern as a result of both gas-aerosol phase equilibrium processes under high temperature conditions as well as transport. Nitrate in PM 2.5-10 (cNO 3 ) increased during long-range transport of dust, implying the great importance of heterogeneous processes at the surface of coarse mode particles. In this study, Δ[H + ] (derived from the difference in pH of extract liquid with/without sampling) was used to indicate the acidity of particles. We found that acidity of particles in PM 2.5 (fΔH) was mostly positive with a maximum in August because of the large fraction of nitrate and sulfate. Acidity of particles in PM 2.5-10 (cΔH) was negative in winter and spring due to presence of alkaline matter from crustal sources. This study highlights the great importance of anthropogenic pollutants on the acidity of particles in the western Pacific Ocean and further impact on the marine environment and climate., (Copyright © 2017 The Authors. Published by Elsevier Ltd.. All rights reserved.)

A long-lasting high particulate matter (PM) concentration episode persisted over East Asia from May 24 to June 3, 2014. The Nested Air Quality Prediction Model System (NAQPMS) was used to investigate the mixing of dust and anthropogenic pollutants during this episode. Comparison of observations revealed that the NAQPMS successfully reproduced the time series PM 2.5 and PM 10 concentrations, as well as the nitrate and sulfate concentrations in fine (aerodynamic diameter ≤ 2.5 μm) and coarse mode (2.5 μm < aerodynamic diameter ≤ 10 μm). This episode originated from two dust events that occurred in the inland desert areas of Mongolia and China, and then the long-range transported dust and anthropogenic pollutants were trapped over the downwind region of East Asia for more than one week due to the blocked north Pacific subtropical high-pressure system over the east of Japan. The model results showed that mineral dust accounted for 53-83% of PM 10 , and 39-67% of PM 2.5 over five cities in East Asia during this episode. Sensitivity analysis indicated that the Qingdao and Seoul regions experienced dust and pollution twice, by direct transport from the dust source region and from dust detoured over the Shanghai area. The results of the NAQPMS model confirmed the importance of dust heterogeneous reactions (HRs) over East Asia. Simulated dust NO 3 - concentrations accounted for 75% and 84% of total NO 3 - in fine and coarse mode, respectively, in Fukuoka, Japan. The horizontal distribution of model results revealed that the ratio of dust NO 3 - /dust concentration increased from about 1% over the Chinese land mass to a maximum of 8% and 6% respectively in fine and coarse mode over the ocean to the southeast of Japan, indicating that dust NO 3 - was mainly formed over the Yellow Sea and the East China Sea before reaching Japan., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

Natural mineral dust and heavy anthropogenic pollution and its complex interactions cause significant environmental problems in East Asia. Due to restrictions of observing technique, real-time morphological change in Asian dust particles owing to coating process of anthropogenic pollutants is still statistically unclear. Here, we first used a newly developed, single-particle polarization detector and quantitatively investigate the evolution of the polarization property of backscattering light reflected from dust particle as they were mixing with anthropogenic pollutants in North China. The decrease in observed depolarization ratio is mainly attributed to the decrease of aspect ratio of the dust particles as a result of continuous coating processes. Hygroscopic growth of Calcium nitrate (Ca(NO 3 ) 2 ) on the surface of the dust particles played a vital role, particularly when they are stagnant in the polluted region with high RH conditions. Reliable statistics highlight the significant importance of internally mixed, 'quasi-spherical' Asian dust particles, which markedly act as cloud condensation nuclei and exert regional climate change.

Appropriate policies to improve air quality by reducing anthropogenic emissions are urgently needed. This is typified by the particulate matter (PM) problem and it is well known that one type of PM, sulfate aerosol (SO 4 2- ), has a large-scale impact due to long range transport. In this study we evaluate the source-receptor relationships of SO 4 2- over East Asia for 2005, when anthropogenic sulfur dioxide (SO 2 ) emissions from China peaked. SO 2 emissions from China have been declining since 2005-2006, so the possible maximum impact of Chinese contributions of SO 4 2- is evaluated. This kind of information provides a foundation for policy making and the estimation of control effects. The tagged tracer method was applied to estimate the source apportionment of SO 4 2- for 31 Chinese province-scale regions. In addition, overall one-year source apportionments were evaluated to clarify the seasonal dependency. Model performance was confirmed by comparing with ground-based observations over mainland China, Taiwan, Korea, and Japan, and the model results fully satisfied the performance goal for PM. We found the following results. Shandong and Hebei provinces, which were the largest and second largest SO 2 sources in China, had the greatest impact over the whole of East Asia with apportionments of around 10-30% locally and around 5-15% in downwind receptor regions during the year. Despite large SO 2 emissions, the impact of south China (e.g., Guizhou, Guangdong, and Sichuan provinces) was limited to local impact. These results suggest that the reduction policy in south China contributes to improving the local air quality, whereas policies in north and central China are beneficial for both the whole of China and downwind regions. Over Taiwan, Korea, and Japan, the impact of China was dominant; however, local contributions were important during summer., (Copyright © 2016 The Authors. Published by Elsevier Ltd.. All rights reserved.)

Carbon monoxide (CO) emissions in China in 2005-2010 were estimated by inversion, using the Green's function method from vertical CO profiles derived from MOPITT Version 5 satellite data and a tagged CO simulation, and validated with independent in situ observations from the World Data Centre for Greenhouse Gases. Modeling with a posteriori emission successfully reproduced CO outflow from the continent to the East China Sea, Sea of Japan, and Japanese islands during winter and spring, and compensated for underestimates in central and eastern China in summer. A posteriori emissions showed large seasonal variations in which December and March emissions were on average 23% larger than August emissions, consistent with other studies. Estimated Chinese CO emissions were 184.4, 173.1, 184.6, 158.4, 157.4, and 157.3 Tg/year for 2005-2010, respectively. The decrease after 2007 is partly attributed to Chinese socioeconomic conditions and improved combustion efficiency., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

We applied the decoupled direct method (DDM), a sensitivity analysis technique for computing sensitivities accurately and efficiently, to determine the source-receptor relationships of anthropogenic SO(2) emissions to sulfate aerosol over East Asia. We assessed source contributions from East Asia being transported to Oki Island downwind from China and Korea during two air pollution episodes that occurred in July 2005. The contribution from China, particularly that from central eastern China (CEC), was found to dominate the sulfate aerosols. To study these contributions in more detail, CEC was divided into three regions, and the contributions from each region were examined. Source contributions exhibited both temporal and vertical variability, largely due to transport patterns imposed by the Asian summer monsoon. Our results are consistent with backward trajectory analyses. We found that anthropogenic SO(2) emissions from China produce significant quantities of summertime sulfate aerosols downwind of source areas. We used a parametric scaling method for estimating anthropogenic SO(2) emissions in China. Using column amounts of SO(2) derived from satellite data, and relationships between the column amounts of SO(2) and anthropogenic emissions, 2009 emissions were diagnosed. The results showed that 2009 emissions of SO(2) from China were equivalent to 2004 levels.

An intensive field survey, with 6-h measurement intervals, of concentrations of chemical species in particulate matter and gaseous compounds was carried out at coastal sites on the Sea of Japan during winter. The concentration variation of SO(2)(g) and HNO(3)(g) were well correlated, whereas the NH(3)(g) concentration variation had no correlation with those of SO(2)(g) and HNO(3)(g). The NH(4) (+) (p)/non-sea-salt- (nss-)SO(4) (2 -)(p) ratio in particulate matter was mainly affected by the location of the sampling site. One or more concentration peaks of nss-Ca(2 +) for survey period were observed. Backward trajectories analyses for the highest nss-Ca(2 +) concentration peaks showed some inconsistency in pathways. We consider that insufficient mixing of the atmosphere and/or insufficient time for the transported air pollutants to react with those discharged locally are the most likely explanations for the discrepancies between the measured products [HNO(3)][NH(3)] and the calculated values.