It will be impossible to be comprehensive in this brief talk, but I will try to pick topics relevant in answering some questions from the following list: How accurate must one be in characterizing natural versus anthropogenic sources? Which sources are most important? What is the relative importance of characterization of mass versus number of aerosol emissions? Does the answer change with a focus only on direct aerosol forcing, or the indirect effect? How accurate must one be in estimating fire emissions? VOCs? Dust? Sea Salt? How much detail is required to treat organics reasonably? Should one expect to get a reasonable emulation of aerosol cloud interactions in parameterizations that do not include specific treatments of aerosol/cloud/dynamical interactions? Since cloud-aerosol interactions also include cloud effects on aerosol, how sensitive is the forcing to the parameterization of clouds? How can one reconcile the very different response of global and LES models to aerosol perturbations? What characteristics should one require to be consistent in order to be confidence in either type of model?
I will try to highlight our perspective on these questions with example model simulations.