Why couldn't it have been penetrated with policy? As you note, Europe did this quite successfully with successively-tighter diesel emissions standards for new

Why couldn't it have been penetrated with policy? As you note, Europe did this quite successfully with successively-tighter diesel emissions standards for new vehicles being introduced every few years since 1992. Other countries - China, Brazil, Russia, Mexico, Indonesia, Australia, etc - have either adopted the Euro standards wholesale or tweaked them slightly for their local market conditions. That was backed up by measures such as testing individual vehicles for continued compliance at their annual inspections, banning commercial vehicles that fail to meet recent standards from using public roads, charging private vehicles which fail to meet recent standards when they enter dense urban areas, and scrappage schemes to mop up the remaining non-compliant vehicles. It took a while to produce results, but eventually resulted in pretty dramatic improvements in air quality - and we now have good evidence of improved health outcomes, particularly in children with asthma or other respiratory conditions. Perhaps too late for the US to start now given the switch to EVs, but it ought to be kept in mind for the next time a "we can't possibly do anything about this" issue comes up.