Changes in tropospheric nitrogen dioxide vertical column densities over Japan and Korea during the COVID-19 using Pandora and MAX-DOAS

We investigated the impact of COVID-19 on the tropospheric nitrogen dioxide vertical column density (NO2 TropVCD) at two sites in Korea (Gwangju and Busan) and two sites in Japan (Yokosuka and Cape Hedo) from Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) and Pandora. Compared to monthly mean NO2 TropVCD that from 2015 to 2018 and in 2019, that in 2020 was lower due to social distancing in Korea and Japan and lockdown in China. High negative relative changes were observed from May to September (30% to 18%) at the three urban sites; Cape Hedo, a remote site, did not show a significant difference in relative changes between previous years and 2020, suggesting that only anthropogenic emission sources dramatically decreased. In the case of Yokosuka, the 15-day moving average of the NO2 TropVCD exhibited a good relationship with transportation and industry mobility data. In contrast, the NO2 TropVCD at Korean sites showed a moderate to low correlation with the industrial sector and insignificant correlations with transportation. The differences in correlations might be caused by the different social distancing policies in Korea (voluntary) and Japan (mandatory). By applying generalized boosted models to exclude meteorological and seasonal effects associated with the variation in NO2 TropVCD, we revealed that the decreasing trend from 2019 to 2020 was much steeper than that from 2015 to 2020 (a factor by two), and a significant change identified in January 2020, when the first cases of COVID-19 were observed in both Korea and Japan.