Just how clean is the latest clean diesel technology? Put it this way: A study by the University of California-Riverside found commercially cooked hamburgers emit more particulate matter than diesel trucks meeting EPA 2007 and EPA 2010 emissions standards. The UC-Riverside study was funded by the South Coast Air Quality Management District and the San Joaquin Valley Air Pollution Control District.

"While the primary focus of this new study was on emissions from commercial charbroilers, this comparison clearly illustrates the significant improvements from clean diesel technology on California's air quality," said Allen Schaeffer, the executive director of the Diesel Technology Forum. "In fact, the study also found that the particulate matter inventory from commercial cooking is more than double the inventory from heavy-duty diesel trucks."

Schaeffer notes that his 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," Schaeffer said.

"But more of these kinds of comparisons are likely, especially In California, where clean diesel technology has been such a success story. Today, in California the majority of particulate emissions come from brake and tire wear, with diesel emissions making up small and declining fraction."

According to the Diesel Technology Forum, in the United States, emissions from heavy-duty diesel trucks and buses have been reduced by 99% for nitrogen oxides (NOx) - an ozone precursor - and particulate emissions. A key part of reducing emissions has been the shift to ultra-low sulfur diesel fuel that has been available since 2006.

This change in fuel specification reduced sulfur emissions by 97% - from 500 PM to 15 PM - and enables the use of advanced emissions control technologies. Similar advancements are taking place in off-road engines and machines.

"Across the U.S. emissions of particulate matter from diesel engines are declining and make up less than 6% of all particulate emissions," Shaeffer said. "Because of the investments in new technology, it now takes 60 of today's technology trucks to emit the same level of PM emissions as one truck built in 1988."