A systematic review was conducted to evaluate the effects of long-term exposure to particulate matter with diameters of less than 2.5 μm (PM_2.5) on mortality. We searched PubMed for epidemiological studies investigating the association of long-term exposure to PM_2.5 with mortality published between January 1990 and December 2015. We applied a random-effects model to calculate the combined effect estimates for PM_2.5 mass. Twenty-four articles were identified (3 from Asia, 6 from Europe, and 15 from North America). It was common to assign the PM_2.5 observations monitored at a fixed-site station to all residents in the corresponding area in the study published during 1990–2000. Recently, PM_2.5 exposure assessment has utilized dispersion models, land use regression models, and satellite remote sensing. The mean or median concentration of PM_2.5 for each study area varied from 8.1 to 35.3 μg/m³. The combined risk ratio was 1.07 (95% confidence interval: 1.04, 1.19) per 10 μg/m³ increase in the PM_2.5 concentration. There was a significant heterogeneity. Further studies are needed in the area where the PM_2.5 level is high.

To elucidate the dry deposition mechanisms of PM_2.5, we measured the dry deposition fluxes of sulfate and nitrate in PM_2.5 above a forest in suburban Tokyo using the relaxed eddy accumulation method (REA). We carried out experiments four times in the leafy seasons from September 2014 to August 2015. The REA samplings of PM_2.5 fluxes were continuously done in the daytime and nighttime during about one week for each experiment. Concentration gradients above the forest were also measured. The mean deposition velocities of nitrate were higher than those of sulfate in the four experiments. The median deposition velocities of all the valid data were 0.19 cm/s (sulfate) and 1.4 cm/s (nitrate). The concentration gradients were in accordance with the REA results. The nitrate deposition velocity increased with the ratio of temperature increasing from the air to the forest surface. That is probably caused by the volatilization of NH₄NO₃ just above the high temperature surface. This process possibly enhances the nitrate deposition velocity.