UNCERTAINTY ANALYSIS OF DIRECT RADIATIVE FORCING BY ANTHROPOGENIC SULFATE AEROSOLS

Uncertainty in the direct radiative forcing by anthropogenic sulfate a erosols is analyzed for four different aerosol model structures with 1 3 uncertain parameters using a second-order probabilistic collocation method. The resulting probability density functions agree well with th ose determined here by a computationally much more expensive 10,000-po int Monte Carlo method. The structural difference, measured by the ran ge of the mean responses in models with different approximations, is - 0.28 to -1.3 W m(-2), and the parametric uncertainty, induced by the u ncertainties in the model parameters, is -0.1 to -4.2 W m(-2), both wi th 95% confidence. This implies that refining uncertain input paramete rs may be more important than improving models in order to minimize th e overall uncertainty in the direct radiative forcing by anthropogenic sulfate aerosols in these four models. The variance analysis indicate s that the parametric uncertainty comes mainly from sulfate yield, sul fate lifetime, and ambient relative humidity. Variance contributions f rom aerosol size parameters are much smaller, and this finding agrees with the sensitivity analysis by Boucher and Anderson [1995]. Note tha t the conclusions reached here are dependent on the chosen model struc tures and parameter distributions. This study presents a computational ly efficient framework for assessing uncertainty in aerosol radiative forcing which could be used to address an even wider range of structur es and parameters in the future.