AAAR 35th Annual Conference October 17 - October 21, 2016 Oregon Convention Center Portland, Oregon, USA
Abstract View
Scattering Directionality Parameters of Fractal Black Carbon Aerosols
APOORVA PANDEY, Rajan Chakrabarty, Washington University in St Louis
Abstract Number: 710 Working Group: Aerosol Physics
Abstract The magnitude of aerosol direct radiative forcing effect is significantly affected by the angular distribution of aerosol light scattering. In radiative transfer schemes, this distribution is typically condensed into single-valued parameters. An analytical expression known as the Henyey-Greenstein (HG) phase function is widely used to connect three single parameter representations of aerosol scattering directionality: the hemispherical upscatter fraction (β), the backscatter fraction (b), and the asymmetry parameter (g). The HG phase function does not take particle morphology into account, which could lead to significant errors for non-spherical particles. We calculated these single parameters for fractal black carbon (BC) aerosols using the numerically exact superposition T-matrix method. Here, we illustrate the effect of particle size and morphology on β, g and b for single particles. We developed empirical equations to capture the relationships between these parameters. For a known value of b, which can be obtained through direct measurement, the errors in β and g estimated using the HG phase function could be up to 35%. However, the large mass absorption cross-section of BC aerosols dominates their radiative forcing effect. Therefore, the rather large errors in scattering directionality result in ≤8% error in BC direct forcing efficiency at 550 nm wavelength.