They can absorb light, or scatter it. They are present in the atmosphere because of the incomplete combustion of fossil fuels. Now they are thought to have a significant impact on global warming. But until just 10 or 15 years ago, the scientific community did not accept that carbonaceous aerosol particles were common in the atmosphere. That they accept it now is because of the work of a research group led by Tihomir Novakov at Lawrence Berkeley National Laboratory, which has been studying these particles since the 1970s.