Characteristics of PM_2.5-bound metals in Japan over six years: Spatial distribution, health risk, and source analysis

Air sampling campaigns were conducted at 100 survey sites across Japan from April 2014 to February 2020, and a comprehensive database of atmospheric particles was obtained. In this study, the characteristics of PM_2.5 and 26 metals were investigated in depth. Spatially, the concentration of PM_2.5 gradually increased from the northeast to the southwest of Japan. The pollution in Kitakyushu City was the most serious, reaching 19.8 μg m^-3. As an important particle component, metals did not show obviously spatial variation in Japan, with a sum concentration of 0.4 μg m^-3. Anthropogenic metals only accounted for about 8% of the total metals, but they could pose a serious threat to public health. For children, the non-carcinogenic risk and carcinogenic risk due to exposure to anthropogenic metals could not be neglected in Japan; the corresponding HI and CR values at 100 survey sites ranged from 2.7 to 15.0 and 4.1 × 10^-5 to 3.4 × 10^-4, respectively. Adults faced lower health risks than children, with HI values ranging from 0.2 to 2.0 and CR values ranging from 2.0 × 10^-5 to 1.6 × 10^-4. The integrated health risk assessment results showed that the coastal region of the Seto Inland Sea and the north Tohoku Region were the most heavily polluted areas of Japan; in this study, 20 survey sites were finally determined to be high-risk sites, among which pollution control for Niihama City, Kitakyushu City, Hachinohe City, and Shimonoseki City were of first priority. With further combination with a positive matrix factorization model, it can be known that these four cities mainly had five to seven metal sources, and their heavy pollution was mainly caused by ship emissions, industrial emissions, biomass burning, and coal combustion. Overall, our study comprehensively revealed the regional patterns of PM_2.5-bound metal pollution across Japan, which can help in making cost-effective risk management policies with limited national/local budgets.