The Times notes that "coal plants and diesel engines have begun the work of reducing soot" but left out how much has been accomplished.
According to the California Air Resources Board, particulate-matter emissions from heavy-duty diesel trucks declined from 7.5% of all such emissions in the entire state in 1990 to 3.8% in 2008 and will be just 1.6% in 2020. The diesel truck share of particulate-matter emissions in the South Coast Air Basin decreased from 7% in 2005 to 3% in 2011.
It would take 60 of today's trucks to equal the particulate emissions of one truck made before 1988. In fact, a UC Riverside study found more particle emissions come from charbroiling a single hamburger than driving a new diesel big rig 142 miles.