I haven't found exact data for the contribution of ICE vehicles to PM 2.5 levels in Denver, but I've found some evidence of ozone [0] and carbon dioxide [1] contributions in Denver being around 30%, as well as contribution to PM 2.5 in "developed countries" being 25–30% [2].
Later edit: this EPA paper from 25 years ago [3] says "Vehicle exhaust was the largest PM 2.5 carbon contributor, constituting ~85% of PM 2.5 carbon at sites in the Denver metropolitan area".
Yes, I researched this for evaluating my son's previous school - there's a sharp drop off even from the 10-20-30 metre range to the 100-150 metre one, and things like wind directions certainly matter.
AusGov in their infinite wisdom built this primary school right on top of one of the busiest Melbourne streets for east-west through traffic, with long periods of bumper-to-bumper including the heavy trucks which are the worst. (You'd think cars in Aus have bad standards, but then there's the trucks.)
I'm willing to bet no-one even cared, as evidenced by the complete lack of air purifiers in the classrooms.
Yeah, the smell that comes with it in the fall and winter is something else. I like Denver, but the brown cloud is no fun. It’s a downer coming out of the hills on I70 into the city and seeing it hanging there.
The sooner we can transition to EVs, the better.