Review of recent literature on the light absorption properties of black carbon: Refractive index, mass absorption cross section, and absorption function

Knowledge of the optical properties of soot black carbon (BC) is required for the predictionof the radiative effects of freshly-emitted and aged BC particles. Here we review BC massabsorption cross section (MAC) and absorption function E(m) measurements, focusing onfreshly-emitted BC. First, we review recently reported MACs at 550 nm wavelength asobtained from direct measurements of particulate absorption and mass concentration; wefind an average of 8.0 ± 0.7 m2 /g from ten measurements, not significantly higher (p > 0.26)than the widely used MAC of 7.5 ± 1.2 m2/g recommended by Bond and Bergstrom [Bond, T.C., and R. W. Bergstrom. 2006. Light absorption by carbonaceous particles: An investigativereview. Aerosol Sci. Technol. 40(1):27–67]. Second, we review recently reported E(m), whoseretrieval is more complex due to the need to combine measurements with numerical mod-els to estimate the contribution of scattering to extinction. Third, we review recent numer-ical studies that have aimed to predict the BC MAC using various complex refractive indices(m ¼ n þ ik). Most of these studies have used m ¼ 1.95 þ 0.79i recommended by Bond andBergstrom (2006), yet failed to predict a MAC as high as 7.5 or 8.0 m2 /g at 550 nm wave-length. Fourth, we summarize a selected range of alternative values of m that has beenreported by recent studies and place them in the context of measurements using a contourplot of E(m) on the n–k plane. We show that the widely used m ¼ 1.95 þ 0.79i correspondsto an E(m) that is too low to be consistent with the measured MAC values. We concludethat the E(m) of BC in the visible and near infrared should be greater than 0.32, and thatthe commonly used BC models or the refractive index, or both, are still in need ofimprovement.