From http://www.eia.doe.gov/kids/energyfacts\sources\non-renewable\diesel.html, June 2006?
When diesel fuel is used, carbon dioxide is a byproduct. Carbon dioxide is a greenhouse gas that is linked to global climate change.1 Diesel-powered cars achieve 20-40% better fuel economy than gasoline-powered cars, especially in sport utility vehicles (SUVs) and light trucks, which now make up more than half of all new vehicle sales in the United States. Safety is another advantage of diesel fuel; it is less flammable than gasoline and other alternatives.
Source: Stock photography (copyrighted)
Diesel Emissions
The major disadvantage of diesel fuel is its harmful emissions. Significant progress has been made in reducing emissions from diesel engines. With new clean diesel technologies, today's trucks and buses are eight times cleaner than those built just a dozen years ago. In the future, diesel engines must become even cleaner in order to meet tightening environmental standards.
Sentence above mentions "a dozen years ago." Should be changed to something more evergreen, but I'm not sure what.
New diesel fuels — some of which have lower sulfur content — can also help diesel vehicles achieve lower emissions. Ultra-low sulfur diesel (ULSD) fuel is highly refined for clean, complete combustion, and low emissions. ULSD is necessary for new engine technologies to work properly, and will eventually replace regular diesel fuel. Using low sulfur diesel fuel and adding exhaust control systems can reduce particulate emissions by up to 90% and nitrogen compounds (NOx) by 25-50%.
Even with these advances, diesel still contributes significantly to air pollution in the United States. It will take a long time for the new cleaner-burning diesel vehicles to replace older ones.
1. U.S. Environmental Protection Agency, Climate Change State of Knowledge.