Comparisons of techniques for measuring shortwave absorption and black carbon content of aerosols from biomass burning in Brazil

Abstract

Six methods for measuring the shortwave absorption and/or black carbon (BC) content of aerosols from biomass burning were compared during the Smoke, Clouds, and Radiation‐Brazil (SCAR‐B) experiment. The methods were the optical extinction cell (OEC), integrating plate (IP), optical reflectance (OR), particle soot/absorption photometer (PSAP), thermal evolution (TE), and remote sensing (RS). Comparisons were made for individual smoke plumes and for regional hazes dominated by smoke. Taking the OEC as a primary standard, measurements of the absorption coefficient (σa) showed that the OR method had the lowest uncertainty (17%) in σa. The other optical methods had uncertainties ranging from 20 to 40%. However, with sufficient sample size, the values of σa derived from the optical methods converged to within 20% of each other. For biomass burning aerosols in regional hazes over Brazil, this led to systematic differences of ±0.02 in the values of the single‐scattering albedo derived from the various in situ techniques. It was found also that the BC content of the aerosol and σa were poorly correlated. This is likely due to a large uncertainty in the BC content of the aerosol measured by TE, and/or a high variability in the mass absorption efficiency of BC in biomass burning aerosol. Hence there is a high uncertainty in inferring σa from the BC content of smoke aerosol.