Ambient air concentrations of fine particulate matter are an issue of increasing concern for the U.S. Environmental Protection Agency. Accordingly, the Clean Air Act and the Amendments of 1990, call for an assessment of past and future regulations to protect both health and visibility. Unfortunately, our most reliable tools for assessing long-term air quality change, Eulerian models, challenge the practical limits of current computer resources and require extensive input data. To reduce the resource requirement, an aggregation method, initially developed for RADM (Regional Acid Deposition Model) acid-deposition applications, is currently being applied to a limited number (thirty) of RADM simulations in order to provide estimates of long-term (annual) ambient air concentrations of fine particulate matter. This paper briefly examines this aggregation technique, its application to fine particulate matter, and the suitability of the original thirty RADM simulations.