These are the last remaining targets. We started with industrial and home air pollution in the early and mid 20th century, driven by events such as the Great S

These are the last remaining targets. We started with industrial and home air pollution in the early and mid 20th century, driven by events such as the Great Smog of London. Smokestacks mandated, a great deal of process improvements on industrial sites, and natural gas & electric replaced coal in the home. We basically solved most automobile car pollution over a period from the 1970's to the 1990's, with class ratings, better engines, catalytic convertors, and mandatory emissions checks, in jurisdictions with lots of people and not a lot of corruption. That's over. Unless you live in a dysfunctional municipality controlled by the auto industry, or in a rural area where the pollution is so spread out it doesn't make much difference, you have mandatory emissions checks. For the last ten years, and for the next ten years, we've been dealing with the problem in commercial trucks, a major share of interstate road traffic in the US. Progressively lower sulfur diesels implemented, catalytic converters mandated, and a supplement to the catalytic converters, a diesel particulate filter, mandated. Regulations vary locally on the matter of small household engines, but the general trend for decades has been to phase out two-strokes in favor of simple four-strokes. A small two-stroke lawnmower will tend to be dirtier on some metrics than thousands of cars combined, and four-stroke engines have weight issues that may disallow handheld machines, so there is some consumer lament, but it's happening anyway. There's probably also some automotive emissions measures that will migrate to the larger ones. As a result of all this, the road fleet is becoming exceedingly clean. The only things left to clean up are marine (bunker fuel), rail (bunker fuel) and small aviation (leaded avgas) engines, and we're at the beginning of the several-decade process of dealing with those. Fuel sulfur content is both a primary emissions problem, and a signal. The catalysts required to cut particulate & VOC pollution down to negligible levels are apparently destroyed by sulfur, so it's only once you go to ultra-low-sulfur fuels that you get the option of cleaning up the exhaust. I'm not sure where that leaves sour crude suppliers.