Earlier quoted context omitted.
I live in [redacted], [redacted] and we often have high air pollution warnings and you really can feel it in the air. During lockdown, back in April and May, there was a noticeable difference in the air quality. It felt different. When we went out for a walk you could also tell the difference in noise pollution from there being so few cars on the roads. It was lovely. There is a church nearby that we can sometimes he…
Only minority of cars are responsible for majority of pollution. A single badly maintained diesel car produces as much pollution as thousands of legal new cars. Same goes for trucks -- the low in air pollution you felt was likely not due to less cars but less trucks (and possibly busses, depending on where in Europe you live)
A car without a catalytic converter generates as much toxic pollution as 100 new cars with catalytic converters. A car whose catalytic converter has broken and which has engine trouble produces even more. And diesel produces more than gas. (Though new vehicles of both types are about equal.)
Thus a single badly maintained diesel car can produce as much pollution as thousands of legal new cars. Smog truly is a situation where a tiny majority of vehicles produce the vast majority of the problem.