EPA has expanded what they consider pollutants so they can regulate them. (Read compel, fine, mandate).

It is largely about sulfur in diesel being targeted by EPA for reduction. The biggest and most practical change was reformulating diesel at the refinery level. All the vehicle based mandates are because EPA can, not because there is a cost benefit tradeoff.

cites:

http://www3.epa.gov/airtoxics/orig189.html

http://publicaccess.supportportal.c...rity-Pollutants

http://www.enviroprolabs.com/epa-pr...pollutant-list/