Also when it's cold outside the "clean" engines are really dirty as the catalytic converter do almost nothing at low temperatures.
"One of its biggest shortcomings is that it only works at a fairly high temperature. When you start your car cold, the catalytic converter does almost nothing to reduce the pollution in your exhaust." [1]
And when it's cold the car usually starts at someone home around where other people live and that's where the pollution ends up.
For diesel engines that operate at lower temperatures this is even worse.
"One of its biggest shortcomings is that it only works at a fairly high temperature. When you start your car cold, the catalytic converter does almost nothing to reduce the pollution in your exhaust." [1]
And when it's cold the car usually starts at someone home around where other people live and that's where the pollution ends up.
For diesel engines that operate at lower temperatures this is even worse.
[1] https://auto.howstuffworks.com/catalytic-converter3.htm