Measurements of Enhanced Black Carbon and Brown Carbon Absorption in Biomass Burning Emissions

Emissions from biomass burnings are thought to be responsible for over half of the black carbon aerosol present in the atmosphere. Black carbon, or soot, absorbs significant amounts of solar radiation. Organic coatings can significantly increase the absorption of black carbon cores by focusing more light onto the core. Shorter wavelengths of light can also be absorbed by the organic material itself, in which case it is referred to as brown carbon. There remains significant uncertainty concerning the amount of absorption enhancement caused by organic coatings and the importance of brown carbon in biomass burning emissions. We present results from multi-wavelength absorption measurements made during the 2012 FLAME-IV experiment at the Missoula Fire Laboratory that address these uncertainties for a variety of globally important biomass fuels. Absorption was measured with a 3-wavelength (405, 532, 660 nm) Photo-Acoustic Absorption Spectrometer (PAS). Two additional channels at 405 and 660 nm measured particles that had been heated to 250°C and run through a carbon denuder. In this talk we present observations of absorption enhancement, which we define to be the ratio of the magnitude absorption caused by non-altered particles to the magnitude of absorption caused by denuded particles. Results indicate that absorption enhancement from clear coatings is often less significant than expected with absorption enhancements in the 660 nm channel often being near unity. Brown carbon is a significant source of absorption at 405 nm where absorption enhancement values as high as 3.0 are commonly observed.