Not sure this gubmint solution here is really the answer.
(Taking off my tinfoil hat for a second...... what I'm on about is unintended consequences. Ex. - here (ON, Canada) the safety police recommended plastic mushroom caps on vertical rebar on construction sites - theory was that they were safer if someone fell on them. In real world use, they found that the problem they created was that in anything but a short fall, these "safety" caps ended up punching silver dollar sized holes through people, instead of pinkie sized holes. Intent was good (preventing injury), but the reality was that bigger problems were created.)
Ever been near a DEF loaded truck when it goes into Regen ? Smells pretty noxious, and I'm told it has been restricted or banned in parts of Europe, as their studies (been used longer over there) are indicating that this urea based "solution" (to the problem) is carcinogenic.
I sort of get the concept for heavy line haul trucks here - park at a truck stop while it regens, go inside and have a meal. Human near field exposure to the regen exhaust is limited, in this scenario.
BUT, if you are driving down a road, or worse, stuck in traffic and the light truck beside you goes into Regen - guess what, you are stuck breathing this urea based soup, regardless of how good you think your vehicle AC filter is.
Talk with heavy diesel mechanics - ask them what a major regen on a heavy truck looks like - does a pretty good job of smoking out a large industrial park - also ask them how much fuel gets burned for the regen.
DEF as a net health benefit ? Time will tell - and let's hope the final analysis allows for the increased fuel consumption of the newer diesel engines.
I wish I knew enough to do the calculation myself; I wouldn't be surprised if global air quality would be much better served by tariffing most Chinese factories out of existence, instead of us messing about with DEF.
D.