The most reliable tool for estimating air quality change for large regions, both past and future, are regional air quality models such as the Regional Acid Deposition Model (RADM). The aggregation method is based on the premise that at any given location, ambient air concentrations of fine particulate are governed by a finite number of different, though recurring meteorological meteorological regimes. If a series of concentration patterns representative of these different meteorological regimes can be identified, they can be aggregated to produce resonable estimates of annual averages.