The recent activities in research and developments of PM_2.5 mass monitoring and its chemical speciation were introduced. Major chemical components of recent PM_2.5 in Japan were sulfate, nitrate, ammonium, organics, and elemental carbon. In order to avoid or suppress artifacts derived from their sampling and/or analyses of those chemical components, this paper reviews several sampling methods with an annular-denuder filter-pack and related analysis or high-time resolution monitoring method such as a thermal-optical reflectance carbon analysis and an Aerodyne aerosol mass spectrometer. The advantages and limitations of their methods are discussed.

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Environmental Air Quality Standard or its guideline value for PM_2.5 has been revised or newly set in US, WHO and Japan since 2006 based on epidemiological and toxicological review. Epidemiologic evidence indicates the causal relationship between PM_2.5 exposure and effects on cardiovascular and respiratory systems. It is suggested that exposure to PM_2.5 around 10-15μg/m³ can cause increased mortality from cardiovascular diseases by exposure-response relationship analysis obtained from prospective cohort studies, but such studies were conducted in the Western countries. Effects of short-term exposure to PM_2.5 on total and cardiovascular deaths were also confirmed by multi-city studies with time-series/case-crossover analysis in North America. Contradictory to these results, similar results were not obtained in studies conducted in Japan. Since it is well known that disease pattern as well as distribution pattern of cardiovascular risk factors are quite different in Japan from the Westerns, new epidemiological studies should be done in Japan. Comparability of potential confounders is important for selection of study regions. Another topic for PM_2.5 epidemiology is lung function development of children who are a susceptible population. All these studies should be done carefully, especially to include regions in Japan where PM_2.5 concentration varies from background to high level.

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Atmospheric fine particles (PM_2.5) have an adverse effect not only on human health such as increase in the death rate, but also on quality of life (QOL) by deteriorating level of visibility and spoiling scenery. In addition, negative impact on forestry ecosystem and decrease in harvest of crops as a result of influence given by PM_2.5 are also of a concern. On the other hand, PM_2.5 also gives influence on global climate system directly or indirectly because they change the radiation conditions in the atmosphere, where degree of such influence varies depending on how PM_2.5 is chemically composed and distributed in a three dimensional manner. Because of this variance, it is difficult to have accurate evaluation of the influence given by PM_2.5. It is suspected that synergetic effect caused by gaseous pollutants and fine particulate matter is driving the influence given by PM_2.5, and understanding such actual status in an accurate manner is of great urgency and importance. In Japan, environmental standard of PM_2.5 was established in September, 2010. To understand the background of this standard, the information of PM_2.5 epidemiological study and processes in setting the PM_2.5 environmental standard in the US is helpful. PM_2.5 is not composed of single matter, and is emitted in both anthropogenic and natural manners, thus the mechanism of PM_2.5 generation is widely ranged. In order to conduct effective emission control of PM_2.5, it is important to identify the linkage between its emission source and environmental concentration, where measurement, monitoring and componential analysis of atmospheric environment containing PM_2.5 are required. Under these context, future challenges in measurement, monitoring and componential analysis of PM_2.5 are addressed in this review by explaining overseas trends of PM_2.5 including those of U.S. and Europe, and how discussions on this issue have developed in Japan.

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An air quality standard for airborne particulate matter with diameter less than 2.5 μm (PM_2.5) has been newly constituted in September 2009 in Japan. Although concentration of PM_2.5 has been decreasing as well as suspended particulate matter (SPM, particles < 10 μm) in recent years, achievement of the PM_2.5 air quality standard is not easy especially in urban areas, and effective countermeasures based on the actual situation of PM_2.5 is needed. PM_2.5 consists mainly of combustion generated particles and secondary particles formed from gaseous precursors in the atmosphere. Combustion sources vary widely and their exhaust particles are highly complex mixture. Secondary particle is also complex, especially for organics, and is not fully understood. Recent increase of the transboundary pollution from the Asian Continent makes their understanding more difficult. This review describes the current state of knowledge on the PM_2.5 situation in urban areas in Japan and other countries from two viewpoints of measurement and model. Our recent study including comprehensive filed measurement, receptor analysis, and model simulation in Kanto area is also presented. Studies on transboundary pollution were outlined describing our field and modeling study in Western Japan. Finally, their major tasks ahead are discussed and future research directions toward a solution of PM_2.5 situation are proposed.

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A three-dimensional air quality simulation was conducted to analyze the sensitivities of the emission sources and transboundary transport on PM_2.5 concentrations in three major Japanese urban areas for the year 2005. The sensitivities of the emission sources were derived from the differences in the concentrations in two simulation cases in which the emissions of the respective sources were absent or used. The sensitivity of the foreign anthropogenic sources was high in the Osaka-Hyogo area, but those of the domestic sources including volcanoes, ships, vehicle exhausts, other combustions and NH₃ were higher in 23 Tokyo wards. The seasonal meteorological conditions caused higher sensitivities of volcanoes and ships in summer, foreign anthropogenic sources in spring and autumn, and domestic sources in winter of the Shuto area and 23 Tokyo wards. The sensitivities of volcanoes, ships, and foreign anthropogenic sources to SO₄^2-, vehicle exhausts and other combustion sources to NO₃^-, NH₄⁺, EC and primary organic aerosols (POA), and biogenic and VOC evaporative sources to secondary organic aerosols (SOA) were evident. The NH₃ sources had high and nonlinear sensitivities to NO₃^- and NH₄⁺. However, PM_2.5 concentrations were underestimated in the current simulation by 30-40% for the entire year. The underestimated EC and OC may be the main causes for the PM_2.5 underestimation. The sensitivities of the sources which influence EC and OC in the real atmosphere were inevitably underestimated in this study.

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The concentrations of PM_2.5 at Fukuoka City (population of 1,440,000) and at Fukue Island (population of 40,000, 190 km west of Fukuoka City) in Kyushu, the western part of Japan, were compared during the Japanese fiscal year 2009. The similarity in the concentration and temporal variations of the hourly PM_2.5 at Fukue and Fukuoka seen in April, 2009, described in a previous paper, was also observed in the summer, fall, and winter. PM_2.5 monitoring in Nagasaki City, started in October, 2009, also showed a behavior almost identical to the other two sites. These temporal variations are the result of long-range transport mechanisms driven by synoptic weather systems such as “behind the front” and “circular flow around migrating anticyclones.” In addition, in the rainy season during July, the Meiyu/Baiu stationary front plays a critical role in the occurrence of high concentration events as being the southern border of the continental air mass. As a result, the monthly average PM_2.5 concentrations at these three sites were almost the same throughout the year. The results of the FY2009 monitoring suggest that in the northern Kyushu area, even in a city as large as Fukuoka, the concentration of PM_2.5 was dominated by the long-range transport of air pollution rather than domestic urban air pollution throughout the year.

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In order to investigate the influences of long-range transport on PM_2.5 in Kanto area, observations of PM_2.5 and its inorganic ions were done from 20 February to 22 April, 2010, in Tama area, Tokyo. PM_2.5 was measured by the Tapered Element Oscillating Microbalance method (TEOM) with moisture control. Daily sampling of PM_2.5 was continuously done for the chemical analysis of the inorganic ions. The mean concentration and maximum concentration of the daily means of PM_2.5 were 13 μg m^-3 and 32 μg m^-3, respectively. The ratio of the inorganic ion components in PM_2.5 averaged 49%, and SO₄^2- was highest among the ions. PM_2.5 rapidly increased on 20 March due to a large-scale Asian dust event. At the same time, the maximum concentrations of Ca²⁺ and SO₄^2- appeared, caused by the long-range transport of Asian dusts and sulfate aerosols such as ammonium sulfate. As a result of back trajectory analysis during the entire observation period, PM_2.5, SO₄^2- and NH₄⁺ in the air mass from the Asian continent were significantly higher than those in other air masses. This indicated that the increases were caused by long-range transport from the Asian continent and western Japan rather than air pollution in Tokyo. The long-range transport of NO₃^- was not significant in these observations.

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PM_2.5 continuous monitors equipped with beta radiation attenuation/light scattering photometry detectors have been operated at four sites in Tokyo. The hourly concentrations of SO₄^2- and water-soluble organic carbon (WSOC) were analyzed using the instruments tape filters. The Measurement of the PM_2.5 using low-volume samplers and the chemical analysis including the carbon fraction were also performed at two sites to ensure the values of the continuous monitors and to determine the ratio of the WSOC to organic carbon. The PM_2.5 concentrations in the summer tended to increase when the high concentrations of the oxidants in consecutive days were observed. The SO₄^2- and WSOC concentrations were the major components in the PM_2.5. The WSOC concentrations were higher at the sites in which the O_X concentrations were higher although the differences in the SO₄^2- concentrations between the sites were not very clear. This indicates that the secondary formation of organic particulates occurred in the Tokyo area. The amount of the secondary organic matter may be around ten percent of the PM_2.5 mass. The Correlation of K⁺, which is an indicator of biomass burning, and the WSOC in the summer was not clear, while that in the winter was very clear. This indicates that theWSOC generated in the summer is derived from volatile organic compounds.

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Parallel observations of the PM_2.5 were carried out for one and a half years starting from April, 2009. One is the reference methods defined by the Ministry of the Environment, Japan ( described as Run 2_ref) and the other is a method using a different sampling duration, filter material and conditioning humidity ( described as Run 1 ). Two PM_2.5 samplers, Partisol Plus 2025 ( Thermo Fischer Scientific ) equipped with a WINS impactor made in the United States were colocated. We evaluated 358 daily samples and 51 weekly samples of PM_2.5 from September, 2009 to August, 2010. As the results of the comparison of the mass concentrations and water soluble inorganic ions, the mass concentrations almost corresponded to each other ( Run 1/Run 2_ref is 0.97, s.d = 0.16, n = 51 ). The monthly concentration ratios ( Run 1/Run 2_ref ) from September, 2009 to February, 2010 and April, 2010 were in the range of 1.00±0.05, On the other hand, those in the other months were 0.88 - 0.93. The chloride ions and nitrate ions, which had semi-volatile properties, reasonably increased in the winter and decreased in the summer. The Run 1/Run 2_ref ratio of sulfate ions, which would be stable as ammonium sulfate, were unexpectedly in the range of 0.35 - 0.82. The adsorption of sulfur dioxides as a precursor of sulfate ions on the filters and particles obtained overnight were confirmed by additional experiments. The concentration differences of the total water soluble inorganic ions exceeded those of the mass concentrations in all the seasons. The difference in the filter materials and humidity while filtering caused this phenomenon.

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