Urban Soot Particles May Warm Climate Less Than Believed

Studies of urban soot particles indicate that scientists may be overestimating the climate-warming effect of such aerosols (Science, DOI: 10.1126/science.1223447). Combustion of fossil fuels or plant material emits soot, also called black carbon, to the atmosphere, where the particles can absorb sunlight. Researchers believed that subsequent chemical reactions in the air added coatings that could act as a lens to increase radiation absorption as much as twofold. But when a research group led by Christopher D. Cappa of the University of California, Davis, and Timothy B. Onasch of Massachusetts-based Aerodyne Research looked at black carbon particles in urban air in California, they found that air-based chemical processes only increased radiation absorption by about 6%. The findings contradict other theoretical and laboratory work but are consistent with earlier single-particle microscopy studies that show that black carbon resides at the surface of these types of particles—not at the core wrapped in a coating. The results suggest that climate models may overestimate warming from black carbon, although the researchers caution that soot emitted from biomass burning may behave differently than urban particles.