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".

[0] https://www.bizjournals.com/denver/news/2017/10/30/here-are-...

[1] https://www.denverpost.com/2020/01/19/colorado-air-pollution...

[2] https://publications.iarc.fr/_publications/media/download/37...

[3] https://www.epa.gov/sites/default/files/2020-10/documents/nf...