Sf. Marshall et al., RELATIONSHIP BETWEEN ASYMMETRY PARAMETER AND HEMISPHERIC BACKSCATTER RATIO - IMPLICATIONS FOR CLIMATE FORCING BY AEROSOLS, Applied optics, 34(27), 1995, pp. 6306-6311
Calculations of direct climate forcing by anthropogenic aerosols commo
nly use radiative transfer parameters, including asymmetry parameter g
. One method of obtaining the asymmetry parameter of a particle popula
tion is to convert measured values of the hemispheric-to-total-scatter
ratio (backscatter ratio b) into their corresponding g values. We com
pare a conversion derived from Mie calculations with one derived from
the Henyey-Greenstein (HG) phase function to show that the HG method s
ystematically overestimates g for typical size distributions of accumu
lation-mode aerosols. A delta-Eddington radiative transfer calculation
is used to show that a 10% overestimation of g can systematically red
uce climate forcing as a result of aerosols by 12% or more. Mie comput
ations are used to derive an empirical relationship between backscatte
r ratio and asymmetry parameter for log-normal accumulation-mode aeros
ols. 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 o
f the breadth sigma(g) of the size distribution.