Urban and regional air pollution caused by PM_2.5 and other pollutants is a major issue worldwide. Air pollution is especially serious in Asia’s developing countries. Since 2013, when severe air pollution occurred around Beijing, long-range transboundary PM_2.5 air pollution has become a public concern in Japan. Much research on PM_2.5 pollution has been conducted in Japan since then. On the other hand, Asia’s developing countries face more severe air pollution than Japan. PM_2.5 pollution poses not only a major health risk but also impacts ecosystems and influences regional-scale climate change.

This special issue provides comprehensive information on recent progress in PM_2.5 research projects in Japan as well as PM_2.5 pollution and approaches toward its mitigation in Asia. Part 1 provides a review of PM_2.5 pollution research conducted under the Environment Research and Technology Development Fund（ERTDF）of the Ministry of the Environment, Japan（MOEJ）, and a discussion of future issues in PM_2.5 pollution policy needing research. In Part 2, experts in various Asian countries give an overview of the current status of PM_2.5 pollution and its mitigation in their respective countries. In addition, future directions toward ameliorating that pollution are discussed.

Many articles have been provided by researchers in Japan and experts in East and Southeast Asia. We are profoundly grateful for their contribution of valuable scientific knowledge toward clean air and blue skies, and strongly hope that this information will prove useful in actions toward PM_2.5 mitigation in Asia.

Guest Editors

Toshimasa OHARA

Masaji ONO

Keiichi SATO

Urban and regional air pollution caused by PM_2.5 and other air pollutants is a major issue worldwide. Air pollution in the developing counties of Asia is especially serious. In Japan, environmental standards for PM_2.5 were established in 2009. After severe air pollution occurred around Beijing in 2013, long range transboundary air pollution of PM_2.5 became a public concern in Japan. Since then, much research on PM_2.5 pollution has been conducted under the Environment Research and Technology Development Fund（ERTDF）funded by the Ministry of the Environment, Japan（MOEJ）. This paper gives a comprehensive overview of the outcomes of these projects and discusses issues needing research to promote domestic and international PM_2.5 pollution policies. These research efforts need to be further strengthened toward solving issues of PM_2.5 and other air pollutants.

In this study, organic marker compounds in fine particulate matter（PM_2.5）were observed during four seasons at forested（Akagi）, suburban（Maebashi）and urban（Saitama）sites in the Kanto region of Japan, and the source contributions of PM_2.5 were evaluated. Organic compounds were analyzed using an extraction-derivatization GC/MS method. PM_2.5 samples were also analyzed for ion components, organic carbon, elemental carbon and metallic elements. At both urban and suburban sites, the concentrations of levoglucosan（a biomass burning marker） increased in winter and fall, whereas dicarboxylic acids （photo oxidation products） increased during warm seasons. At the forested site, the concentration of 2-methyltetrols（a biogenic secondary organic aerosol（BSOA）marker from isoprene） tended to increase during warm seasons, especially in summer. The levels of organic markers for cooking and vehicle emissions were higher at the urban site than the other sites. As a result of positive matrix factorization analysis using a PM_2.5 component data set, it was possible to apportion 80-90％ of PM_2.5 mass concentration to 12 sources, including sources of organic particles such as biogenic primary/secondary origin, biomass burning and cooking. The contribution ratios of these sources involved in organic aerosols in PM_2.5 were estimated to be 41％at the urban, 39％ at the suburban, and 46％ at the forested site. These results will provide basic data for planning of future PM_2.5 reduction measures in Japan.

Regional air quality modeling is a powerful tool for considering effective strategies to suppress ambient PM_2.5 concentrations. It is particularly suited to evaluating source apportionments─critical information for designing effective strategies─by facilitating consideration of complex photochemical reactions and aerosol formation in the atmosphere. Its performance at reproducing PM_2.5 concentrations, however, needs improvement. It incorporates various factors that could alter simulated pollutant concentrations. It is not an easy task for anyone relying on a single model to get a grasp of the uncertainties arising from these factors overall. The aim of this project, called “Japanʼs Study for Reference Air Quality Modeling”（J-STREAM），is to promote research in Japan that can provide an assemblage of references on air quality modeling. The variabilities and uncertainties in pollutant concentrations simulated by currently available models are being evaluated using model inter-comparisons. The project is also seeking suitable model configurations among the models participating in these inter-comparisons. Such information could serve as a reference for model users in Japan. This project has other sub-themes, as well: contributing to improvement of currently available emission inventories based on the results of model inter-comparisons, solving remaining issues related to secondary inorganic aerosols through specific measurements, and newly investigating three dimensional structures of ozone concentrations. Various findings have already been obtained through the model inter-comparison, to which an ample number of models have submitted their results. Detailed model performance and source apportionments will be evaluated in forthcoming phases of this project.

To evaluate the short-term effects of fine particulate matter（PM_2.5）on respiratory health, epidemiological studies were conducted in two regions around the Seto Inland Sea in Japan. In Himeji City, Hyogo Prefecture, weekly data on asthma attacks reported from 46 hospitals and clinics and daily data on primary care visits（PCVs）due to asthma attacks were collected. The mass concentrations and chemical constituents of airborne PM_2.5 were continuously measured at a central site in the city. The weekly numbers of asthma attacks were significantly associated with the average concentration of PM_2.5 in the previous week among subjects aged 15 to 64 years. The concentration of water soluble organic carbon and acidity（hydrogen ions）were also significantly associated with weekly asthma attacks. These associations were marked in spring and summer. By contrast, the associations between daily PCVs for asthma and PM_2.5 were significant in winter. On Yuge Island, Ehime Prefecture, peak expiratory flow（PEF）and the forced expiratory volume in 1 second（FEV₁）were measured every morning among 48 healthy students, for about one month. Among subjects with a history of allergies, outdoor concentration of PM_2.5 was significantly associated with a decrease in FEV₁. The association between indoor concentration of PM_2.5 and PEF was also significant. However, among subjects without such a history, neither outdoor nor indoor concentrations of PM_2.5 were associated with changes in any parameters of pulmonary function. In conclusion, these studies revealed that increases of PM_2.5 have short-term effects on respiratory health. Subjects with allergic diseases were considered to be more susceptible to air pollution.

PM_2.5 air pollution at urban and regional scales is a serious environmental issue in Asia. Developing counties in South and Southeast Asia are facing an especially severe situation with urban PM_2.5 pollution. Also, transboundary air pollution results in high PM_2.5 concentrations over wide regions leeward of the source country. Additionally, long-range transport of Asian dust aerosols in Northeast Asia and biomass burning in Southeast Asia has resulted in regional haze episodes. This paper provides an overview of temporal trends, focusing on PM_2.5 pollution in Asian region, ambient standard levels in each country and regional characteristics. It also discusses the way forward toward PM_2.5 mitigation.

In China, rapid economic development accompanied by complex air pollution issues has resulted in a need for innovative solutions simultaneously addressing these complex pollution issues that lack well developed solutions, since no country has ever faced a similar battle before. To seek innovative solutions, efforts have been made under the Five Sphere Integrated Plan to carry out development under a new concept. In air pollution prevention and control, the Action Plan on Air Pollution Prevention and Control was implemented in 2013–2016. This article provides a brief description of its achievements, with an emphasis on atmospheric quality conditions in 2016. Major air pollutants such as PM_2.5 and PM₁₀ have decreased, achieving the Action Plan’s target in advance. Furthermore, international cooperation at the global, regional and bilateral levels related to supporting air pollution prevention and control was also been introduced. Air pollution prevention and control has entered a crucial stage, with great challenges from development and new emerging issues such as ozone. At this stage, to improve air quality further, the Three Year Plan on Defending the Blue Sky was published, serving as a guide for the upcoming three years. The major strategy and objectives of this Three Year Plan are introduced. To improve air quality further with substantial effects, technology cooperation is recommended for facilitating progress toward achieving air pollution prevention and control targets among countries under the guidance of air-related SDG goals and targets.

PM_2.5 samples were collected in the Seoul, Daejeon, Gwangju, Baengnyeong, Jeju and Ulsan areas in 2014. Water-soluble ions, elements and carbonaceous PM2.5 species were analyzed and the concentration distribution and seasonal characteristics were investigated. The annual mean concentrations of PM_2.5 were 25.9, 28.1, 27.4, 24.1, 15.5, 22.4 µg/m³ in Seoul, Daejeon, Gwangju, Baengnyeong, Jeju and Ulsan, respectively. Water-soluble inorganic ions accounted for 52.9 to 61.3％ of the total PM_2.5 mass concentrations. Sulfate, nitrate and ammonium were the major water-soluble ions in the aerosols. The elemental compositions were dominated by potassium, calcium and iron, and the proportion of total elemental components ranged from 6.3 to 8.2％ in the PM_2.5 mass concentrations. The annual average contribution of TC（Total carbon）to PM_2.5 mass ranged from 18.3 to 26.6％. Iron, lead and zinc constituted the main elements in these fine particles. In the case of ions, NH₄^＋, NO₃^－, and SO₄^2－ exhibited much variation according to season. As for the seasonal characteristics of elements, most regions showed high concentrations of crustal elements in spring. Organic carbon concentrations were generally higher in winter than in summer.

Air pollution is one of the biggest problems in Ulaanbaatar, Mongolia, with the most harmful pollutant being PM_2.5. This paper aims to provide an overview of the current status of PM_2.5 pollution in Ulaanbaatar based on the governmentʼs monitoring data and mitigation policies. During the last several years, PM₁₀ and PM_2.5 concentrations have gradually decreased in Ulaanbaatar, even though in winter the average PM_2.5 concentration has been 2–5 times higher than national Air Quality Standard（AQS）. The Government of Mongolia is implementing measures to reduce air pollution by approving laws, policy documents and strategies for air pollution mitigation in Ulaanbaatar.

Vietnam is experiencing serious fine particulate matter（PM_2.5）pollution as a result of local activities and long-range transport（LRT）pollutants. In this article, we summarize and analyze PM_2.5 data from ground stations and manual measurements showing PM_2.5 status, characteristics and emission sources in the period from 1996 to 2017 in Vietnam. In addition, we provide a brief impact assessment of PM_2.5 pollution on public health regarding diseases and deaths. Conscious of PM_2.5ʼs harmful effects, Vietnam has been taking steps to mitigate PM_2.5 pollution in various forms through efforts by the government, non governmental organizations, media, communities and individuals, and has obtained initial results. This article presents a comprehensive review of current PM_2.5 pollution and its mitigation in Vietnam.