That black stuff coming out of big rigs and buses has never been pretty, but it does more than darken the skies. Black carbon found in soot contributes to global warming.
Mark Jacobson is a Professor of Civil and Environmental Engineering at Stanford University: "Well, soot is probably the most important in terms of its warming effect. This is the stuff that comes out of the tailpipe of diesel trucks, and buses, and cars. And so these soot particles are causing warming."
Legislation has been introduced, calling on the EPA to study how to improve public health and reduce the global warming impact of black carbon emissions.
Mark Jacobson: "There's two levels of things we can do. One is to try to reduce their emissions from existing vehicles. The other is to replace diesel combustion with some kind of clean electric power source or hydrogen fuel source vehicle."
Technological advances and policy changes like this could have an immediate and positive effect.