According to the Diesel Technology Forum, heavy-duty on-road diesel engine NOx and particulate emissions have been reduced by 99 percent and currently account for less than 6 percent of all particulate emissions. A North Carolina State University study released in April 2012 concluded trucks in compliance with newer standards showed a 98-percent decrease in NOx and 94 percent reduction in PM emissions.
In a University of California Riverside study funded by the South Coast Air Quality Management District and the San Joaquin Valley Air Pollution Control District that focused on commercial charbroilers, some amusing results came out: Particulate inventory from commercial cooking is more than double that from heavy-duty diesel trucks and, according to Bill Welch, the principle engineer on the UCR study, you'd have to drive a new 18-wheeler more than 140 miles to generate the same mass of particulates as a single charbroiled hamburger patty.
Allen Schaeffer of the Diesel Technology Forum noted this was "an extremely unusual comparison. Generally, clean diesels are matched up against natural gas, hybrids, or electric vehicles for emissions or fuel-efficiency tests. This is the first time we've gone head to head against fast food."