Air quality and urban climate improvements in the world's most populated region during the COVID-19 pandemic

In this study, we assessed air quality 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 air quality. Overall, although reductions in mobility were moderately effective in improving the typical indicators of urban air quality, 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.