Identifying the chemical composition of “brown carbon” in the atmosphere



Aerosol particles in the atmosphere can either absorb or scatter incoming solar radiation, thus either heating or cooling the atmosphere. One of the most studied types of aerosols that absorb radiation is black carbon (also called soot), which comes from incomplete combustion of fossil fuels, biofuel, and biomass. Black carbon has been identified as a significant factor contributing to global warming. Somewhat less well studied is brown carbon, which also absorbs solar radiation but does so slightly differently than black carbon: Brown carbon absorbs light most strongly in ultraviolet and short visible wavelengths, giving it a yellowish or brownish appearance. Biomass burning is a major source of brown carbon.