Isn't it a little hypocritical to call for law and order and condone vigilantism in the same sentence?

From a European perspective pretty much any diesel car here that has done over 200,000km will have had the DPF removed. In most countries it's illegal, but it's also not tested as part of the emissions tests (particulate matter in general is not tested, other than 'is there black or white smoke? nope, you pass').

Above this mileage it will become clogged and need to be replaced. If it isn't it will increase fuel consumption, reduce power, and damage the engine if left long enough. They are not off-the-shelf parts you can buy (not that they couldn't be, but nobody buys them), so you need to get a 'reconditioned' one which isn't going to last anywhere near as long. For €2500 or more for parts and labour it doesn't make financial sense, especially when your car is probably valued at a similar amount.

I live in a city center next to a relatively busy road and whenever I replace the filters in my apartment ventilation system every 3-6 months they are always thick black. We don't really have any heavy industry in this city, so I assume most of that is from vehicles.

OKay, how many trucks are rolling coal in SF and Oalkand?