It is a measure of the preferred scattering direction ( forward or backward ) for the light encountering the aerosol particles.
In radiative transfer studies, asymmetry factor 'g' is equal to the mean value of μ (the cosine of the scattering angle), weighted by the angular scattering phase function P(μ). Phase function is defined as the energy scattered per unit solid angle in a given direction to the average energy in all directions.
The asymmetry factor approaches +1 for scattering strongly peaked in the forward direction and -1 for scattering strongly peaked in the backward direction.
In general, g=0 indicates scattering directions evenly distributed between forward and backward directions, i.e isotropic scattering (e.g scattering from small particles)
g<0 scattering in the backward direction (i.e scattering angle > 90 deg., often backscattering is referred to scattering at 180 deg.
g>0 scattering in the forward direction (i.e scattering angle < 90, often forward-scattering is referred to scattering at 0 deg. For larger size or Mie particles, g is close to +1.