Brown carbon absorption in the red and near infrared spectral region

Abstract. Black carbon aerosols have been conventionally assumed to be the only light-absorbing carbonaceous particles in the red and near-infrared spectral regions of solar radiation in the atmosphere. Here we report that contrary to the conventional belief tar balls (a specific type of organic aerosol particles from biomass burning) do absorb red and near infrared radiation significantly. Tar balls were produced in a laboratory experiment and their chemical and optical properties were measured. The absorption of these particles in the range between 470 and 950 nm was measured with an aethalometer, which is widely used to measure aerosol absorption in the field. We find that the absorption coefficient of tar balls at 880 nm exceeds 10 % of that at 470 nm. This substantial absorption of red and infrared light is also evident from a relatively low Ångström coefficient (and a significant mass absorption coefficient) of tar balls between 470 and 950 nm. Retrievals of aerosol column optical properties from a global network of surface stations over vast tropical areas dominated by biomass burning suggest that tar balls are the predominant light-absorbing species of organic aerosols over acetone/methanol-soluble BrC or HULIS. Our results also infer that the role of BC (including Diesel soot) in global climate forcing has likely been overestimated at the expense of brown carbon (BrC) from biomass burning.