入江 仁士

Abstract In this study, we assessed air quality (AQ) and urban climate during the mobility restrictions implemented in the Greater Tokyo Area, Japan, the world’s most populated region, in response to the COVID-19 pandemic. Observations from dense surface networks were analyzed using an interpretable machine learning approach. In parallel with a ∼50% reduction in mobility and an altered lifestyle of the population, we found limited reductions in nitrogen dioxide; decreases in fine particulate matter not entirely driven by local mobility; minor variations in ozone, with a positive (negative) tendency in areas with high (low) emissions; a decrease in air temperature consistent with mobility; and pollution levels and air temperature changes with well-defined, common spatiotemporal patterns. Specifically, cooling mainly occurred in urbanized areas with an improved AQ. Overall, although reductions in mobility were moderately effective in improving the typical indicators of urban AQ, including those known to negatively impact human health, the reductions in waste heat had a stronger impact on Tokyo’s urban heat island, suggestive of a strategy to minimize exposure to heat stress. These findings can help guide urban planning strategies and policies aimed at addressing climate change.

Abstract. A horizontally pointing lidar is planned for deployment with other instruments in Fukushima, Japan, to continuously monitor and characterize the optical properties of radioactive aerosols and dust in an uninhabited area. Prior to installation, the performance of the lidar is tested at Chiba University. Data from the continuous operation of the lidar from August 2021 to February 2022 are analyzed for extinction and depolarization ratio. These are compared with the weather sensor and particulate matter (PM2.5) measurements to quantify the relationship between atmospheric conditions and optical properties of near-ground aerosols. The results show that lidar data’s extinction coefficient and depolarization ratio can have a quantifiable relationship with relative humidity (RH), absolute humidity, rain rate, wind speed, wind direction, and PM2.5 concentration. Analysis of the seven-month data shows that the optical properties of aerosol and dust depend on the combined effects of the weather parameters. An increase in RH or PM2.5 concentration does not imply an increase in radioactive aerosols. The average extinction coefficient and depolarization ratio of aerosols and dust originating from the land and ocean show different values and opposing trends which can aid in determining the occurrence of ground-based radioactive dust and aerosols. The information obtained from analyzing the interrelationship among lidar, weather parameters, and PM2.5 concentration is essential in assessing the occurrence of radioactive aerosols and characterizing local aerosol-weather relationships in a radioactive area. This result provides essential information in describing radioactive aerosols in Fukushima.

Abstract. This study investigated the spatiotemporal variabilitiesin nitrogen dioxide (NO2), formaldehyde (HCHO), ozone (O3), andlight-absorbing aerosols within the Greater Tokyo Area, Japan, which is the mostpopulous metropolitan area in the world. The analysis is based on totaltropospheric column, partial tropospheric column (within the boundarylayer), and in situ observations retrieved from multiple platforms as well as additionalinformation obtained from reanalysis and box model simulations. This studymainly covers the 2013–2020 period, focusing on 2020 when air quality wasinfluenced by the coronavirus 2019 (COVID-19) pandemic. Although total andpartial tropospheric NO2 columns were reduced by an average of about10 % in 2020, reductions exceeding 40 % occurred in some areas duringthe pandemic state of emergency. Light-absorbing aerosol levels within theboundary layer were also reduced for most of 2020, while smallerfluctuations in HCHO and O3 were observed. The significantly enhanceddegree of weekly cycling of NO2, HCHO, and light-absorbing aerosolfound in urban areas during 2020 suggests that, in contrast to othercountries, mobility in Japan also dropped on weekends. We conclude that,despite the lack of strict mobility restrictions in Japan, widespreadadherence to recommendations designed to limit the COVID-19 spread resultedin unique air quality improvements.