環境

In order to understand the role of hilly forest of the Tama River basin as scavenging PM2.5 around Tokyo, the dry deposition fluxes of PM2.5 components were measured by means of an experimental tower in a hilly forest of the Field Museum Tamakyuryo. Around the site, the level of PM2.5 exceeded the environmental standard, and the main components of SO42-, NO3- and NH4+ accounted for 89% of the total inorganic ions in PM2.5 in annual mean. Vertical fluxes of PM2.5 components were directly measured by the relaxed eddy accumulation (REA) method during the times from 2 to 7 September 2014, from 27 November to 5 December 2014, from 9 to 17 April 2015, and from 21 July to 1 August 2015. The total fluxes of sulfate and nitrate in PM2.5 showed downward flux (deposition). The deposition velocities of sulfate were in agreement with experimental results of other forests, however those of nitrate were significantly higher than those of sulfate possibly due to the enhancement of deposition by volatilization process of ammonium nitrate. These results indicate that hilly forests of the Tama River basin contribute to scavenge PM2.5 around Tokyo, especially ammonium nitrate in winter.

In general, it is difficult to measure dry deposition of air pollutants directory. There have been few experiments direct measurements of PM2.5 components by relaxed eddy accumulation method, which is one of advanced methods. This study using the method found that sulfate and nitrate in PM2.5 were deposited to hilly forests of the Tama River basin, and their deposition velocities were possibly the same level or higher compared with forests in other regions. These information are valuable for future assessments to improve the serious situation of PM2.5 around Tokyo. Moreover the enhancement of deposition by volatilization process of ammonium nitrate possibly appears in other forests. Therefore the experimental results are also valuable for environmental science.