  Tropospheric nitrogen dioxide (NO$_2$) concentrations are strongly affected
by anthropogenic activities. Using space-based measurements of tropospheric
NO$_2$, here we investigate the responses of tropospheric NO$_2$ to the 2019
novel coronavirus (COVID-19) over China, South Korea, and Italy. We find
noticeable reductions of tropospheric NO$_2$ columns due to the COVID-19
controls by more than 40% over E. China, South Korea, and N. Italy. The 40%
reductions of tropospheric NO$_2$ are coincident with intensive lockdown events
as well as up to 20% reductions in anthropogenic nitrogen oxides (NO$_x$)
emissions. The perturbations in tropospheric NO$_2$ diminished accompanied with
the mitigation of COVID-19 pandemic, and finally disappeared within around
50-70 days after the starts of control measures over all three nations,
providing indications for the start, maximum, and mitigation of intensive
controls. This work exhibits significant influences of lockdown measures on
atmospheric environment, highlighting the importance of satellite observations
to monitor anthropogenic activity changes.
