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.

To evaluate the impact of PM_2.5 transported from north areas (suburban areas) on the air quality in south areas (urban areas), time-resolved organic tracer components were measured in December 2016 both in south areas and north areas of the Nobi Plain. During PM_2.5 high value events, such as December 12–13 and 26–27 2016, the concentrations of PM_2.5, dicarboxylic acids and tracers of biomass burning such as levoglucosan, were high values and these temporal variations were synchronized. Moreover, the northerly wind prevailed during the observation period and the concentration of levoglucosan firstly showed a maximum value in north areas. These results demonstrated that high concentration of PM_2.5 air masses originating from biomass combustion were transported from the north areas of the Nobi Plain to the south area. The characteristics of high PM_2.5 events in the scatter plots between levoglucosan and optical black carbon were also supported that those air mass were transported. In addition, terephthalic acid produced by plastic combustion also showed similar behavior with levoglucosan. Our findings indicate that biomass combustion in suburban areas could significantly affect the PM_2.5 in downwind urban areas in Nobi Plain.

Sakurajima erupts frequently and brings a fall of ash in the neighboring area. It is important to know the plume containing volcanic ash is diluted and transported after the eruption. By applying image processing technologies, we have developed an algorism to capture plume of volcanic ash and their temporal evolution using satellite images of Himawari-8. Because the information of both cloud and the volcanic ashes was included in a satellite image, we picked candidate domain of volcanic ashes area by applying Random Forest. The temporal evolution of the plume was traced by combining the Random Forest and 3D labeling process to distinguish misrecognized areas. Some case studies demonstrated that our algorism could capture the plume of volcanic ashes more clearly in comparison with a conventional technique using Dust RGB, especially in the nighttime.