Finding dirty diesels.

Diesel-fueled vehicles have gained notoriety for their oily carbon emissions. However, there's wide variability in how much of this soot any car emits. A new Dutch study finds that just 5 percent of cars--mostly diesel-fueled vehicles--account for 43 percent of tailpipe-soot releases.

Andy Kurniawan and Andreas Schmidt-Ott of Delft University of Technology analyzed soot emissions from more than 1,250 cars by using a device set up on the shoulder of the road. As long as cars were spaced at least 8 seconds apart, the device could suck in and determine a single vehicle's exhaust. In an upcoming Environmental Science & Technology, the researchers describe how the device works. By irradiating the carbon particles with ultraviolet light, the device imparts a positive electric charge to them. It then measures the charge to quantify the soot, Schmidt-Ott explains.

Policy makers looking to reduce pollution may get the biggest bang for the buck by focusing on cars badly in need of a tune-up, Schmidt-Ott maintains. The chemical engineer recommends that municipalities begin scouting for "superpolluters" with devices such as the one that his group used.--J.R.