Relationship between asymmetry parameter and hemispheric backscatter ratio: implications for climate forcing by aerosols.

Calculations of direct climate forcing by anthropogenic aerosols commonly use radiative transfer parameters, including asymmetry parameter g. One method of obtaining the asymmetry parameter of a particle population is to convert measured values of the hemispheric-to-total-scatter ratio (backscatter ratio b) into their corresponding g values. We compare a conversion derived from Mie calculations with one derived from the Henyey-Greenstein (HG) phase function to show that the HG method systematically overestimates g for typical size distributions of accumulation-mode aerosols. A delta-Eddington radiative transfer calculation is used to show that a 10% overestimation of g can systematically reduce climate forcing as a result of aerosols by 12% or more. Mie computations are used to derive an empirical relationship between backscatter ratio and asymmetry parameter for log-normal accumulation-mode aerosols. This relationship can be used to convert the backscatter ratio to the asymmetry parameter, independent of geometric mean diameter D(gv) or complex refractive index m, but the conversion requires knowledge of the breadth σ(g) of the size distribution.