The effect of stratospheric aerosols due to the volcanic activity is reflected with substitution as the solar irradiance reduction at the modelfs top of atmosphere (by changing the solar constant). The magnitude of the reduction is linearly scaled so that the global averaged radiative forcing fits to the estimation by Sato et al. (1993) (Figure below). The data is obtained from http://www.giss.nasa.gov/data/strataer/ .