Lockdowns have improved air quality in major cities around the world, per nitrogen dioxide measurements mapped here by Steven Bernard. Most cities, anyway.
Data source: ESA Sentinel 5
Data was processed by Descartes Labs showing average pollution levels from Mar 1 to Apr 5 2020, compared with the same period last year.
Data was brought into QGIS and styled and then further design work was done in Adobe Illustrator
You can read the full article hereon how Covid-19 has impacted climate change for good and bad
I wonder if these maps expose a lack of social distancing, continued work travel, etc., that could be correlated to hidden prevalence of the disease. For example, if little has changed in Teheran's and Moscow's NO₂ output, is that evidence that there's no lockdown and a lot more people are getting it (and dying of it) than they're admitting?
Another unusual pattern that differs from other cities: London, where the country/exurbs are markedly less polluted than usual but a small dense area in the center hasn't changed much at all.
I made a crude diffed version (below), showing the change in output for each city. Black means no improvement (bear in mind rural areas with little pollution to begin with), green means a little improvement, red means a lot of improvement
P.S. Had no idea what a choked hellhole Milan is.