Clean diesel will play a leading role in helping cities and states meet strict new air quality goals set by the U.S. Environmental Protection Agency. The diesel industry is committed to research and development of new technologies and is on-track for the on-time delivery of the cleanest diesel ever beginning in 2007.
Starting with Trucks and Buses…
New trucks and buses will be the first class of equipment to benefit from clean diesel. While today’s trucks and buses already produce only one-eighth the tailpipe exhaust compared to those built in 1990, new engines will be even cleaner. It would take 60 trucks built in 2007 to equal the soot emissions of one truck
sold in 1988.

Source: U.S. Environmental Protection Agency
On-Highway Heavy Duty Diesel Emissions Reductions
The EPA predicts that these new clean diesel trucks - once they fully replace the existing fleet - will reduce emissions of smog-forming gases by 2.6 million tons each year and cut soot emissions by 110,000 tons annually.
(http://www.dieselforum.org/meet-clean-d ... an-diesel/)