夏季の富士山頂における粒子数濃度の経年変化

It is important to improve the understanding of spatiotemporal distributions of the aerosol concentration over East Asia, a major emission source region, for reliable estimates of a radiative forcing in global models. The long-term measurements of the aerosol number concentration (N) were conducted at the summit of Mt. Fuji (3776 m above sea level) during the summer of 2006-2019, where free tropospheric transport dominates the arriving air-mass in the nighttime. Median N values within 15-149 nm diameter range decreased from 614 cm⁻³ to 227 cm⁻³ during the observation periods and the decreasing ratio of N was the largest in the nucleation mode range whose median N value in 2019 was lower by a factor of about 5 than that in 2006. The decreasing ratio in the nighttime was similar to that in the daytime within the Aitken and accumulation mode ranges. The decrease in the nighttime partly arises from the emission regulation of aerosols and precursor gases throughout East Asia and the decrease in a new particle formation in the free troposphere. The N in the daytime is partly affected by the decrease in the concentrations of the pollutants in the planetary boundary layer and free troposphere. Our long-term N data in the free troposphere is useful to validate climate models.