## 3 Results
### Satellite Observations for Long-Range Transport  
To identify the external influx into the study area, we looked over the results of satellite observations provided in aerosol optical depth (AOD), ERA wind field during the event period, back-trajectory analysis, and Sentinel for CO/CO\({}_{2}\). Because AOD provides information on the relative weights of pollutants existing in the atmospheric column, it is useful to investigate changes in spatial contributions for particulates. AOD uses MOD04/MYD04 data from TERRA, a representative satellite, to observe the Earth. MODIS sensor observation data are loaded on the Aqua satellite. All data that corresponded to the event period were rearranged into 0.25-degree grids after obtaining arithmetic means. However, we could not obtain all data during the entire event period due to orbit limitations of the Terra and Aqua satellites, and thus the results were calculated using data from 12 p.m. to 2 p.m. on 30 January, 10 a.m. to 2 p.m. on 1 February, and 11 a.m. to 4 p.m. on 6 February. Figure 3 shows the observation data for AOD. As shown in this graph, AOD had a wide range of 0.7-1.4 from West China centered in Shanghai, China, to the study area. Figure 3: Levels of aerosols in the area at the average of the event periods in AOD satellite observation. Data from Sentinel-5 Precursor (S-5 P) observed during the study period were confirmed. S-5 P is a satellite with a single fade is used in GMES (Global Monitoring for Environment and Security) that is operated by the EU community and the European Space Agency for common goals [18]. To provide high-quality data, information, service, and knowledge in time, GMES aims to support European goals for sustainable development and environmental global governance. S-5 P provides daily information for air quality and concentrations of trace gases and aerosols that are important for climate forcing and ozone. In detail, S-5 P observes air quality using TROPOMI (tropospheric monitoring instrument, co-developed by the Netherlands and European Space Agency) and a spectrometer having spectrum bands of UV, visible light (VIS), near-infrared (NIR), and short wavelength infrared (SWIR). Major atmospheric components such as O\({}_{3}\), NO\({}_{2}\), CO, SO\({}_{2}\), methane (CH\({}_{4}\)), formaldehyde (CH\({}_{2}\)O), aerosols, and clouds can be observed using selected wavelength ranges in TROPOMI [43]. Thus, analyzed CO from data from S-5 P observed during the period of this study. Figure 4 indicates the concentration gradient of CO observed by S-5 P. As shown in the figure, CO appeared in high concentrations in China. High levels of CO from the study area to Japan were also confirmed. Figure 5 shows data from the ERA wind field and meteorological field. We illustrated the ERA wind field and meteorological field on the same map of the back-trajectory graph using the U and V components of ERA5 data provided by the European Centre for Medium-Range Weather Forecasts (ECMWF). These data confirmed that the study area was influenced by the west wind from China on 30 January, 1-2 February, and 6-7 February. A back-trajectory analysis was performed using the ratio of CO/CO\({}_{2}\) to confirm the external influx in the study area. The reason why the ratio of CO/CO\({}_{2}\) was used for the back-trajectory analysis is because CO can be used as an inert precursor for long-range transport. In addition, it is a gaseous material that showed a high concentration during the event period, which can be estimated during the period of external influx. Its concentration in the study area only showed a 1.01 times difference between the event period and other periods. Thus, the ratio of CO/CO\({}_{2}\) can be used as a marker for external influxes. In this study, the residence time-weighted concentration (RTWC) model was used for the back-trajectory analysis.