Some current numbers:
https://www.standard.co.uk/news/london/sadiq-khan-london-air-quality-coronavirus-lockdown-pollution-a4422531.html
I'm afraid I don't think it will last, at all.
Nope, the rebound is going to be even more people in cars as everyone is scared of public transport. Although we'll get a few people who are like "wow I didn't realise everything was so close by bike/walking" that will continue.
@hippy started
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Some current numbers:
https://www.standard.co.uk/news/london/sadiq-khan-london-air-quality-coronavirus-lockdown-pollution-a4422531.html
I'm afraid I don't think it will last, at all.