 the clean air act amendments ( caaa ) of 1990 have established selected comprehensive , three - dimensional , photochemical air quality simulation models ( paqsms ) as the required regulatory tools for analyzing the urban and regional problem of high ambient ozone levels across the united states . 
 these models are currently applied to study and establish strategies for meeting the national ambient air quality standard ( naaqs ) for ozone in nonattainment areas ; state implementation plans ( sips ) resulting from these efforts must be submitted to the u.s . 
 environmental protection agency ( u.s . 
 epa ) in november 1994 . 
 the following presentation provides an overview and discussion of the regulatory ozone modeling process and its implications . 
 first , the paqsm - based ozone attainment demonstration process is summarized in the framework of the 1994 sips . 
 then , following a brief overview of the representation of physical and chemical processes in paqsms , the essential attributes of standard modeling systems currently in regulatory use are presented in a nonmathematical , self - contained format , intended to provide a basic understanding of both model capabilities and limitations . 
 the types of air quality , emission , and meteorological data needed for applying and evaluating paqsms are discussed , as well as the sources , availability , and limitations of existing databases . 
 the issue of evaluating a model 's performance in order to accept it as a tool for policy making is discussed , and various methodologies for implementing this objective are summarized . 
 selected interim results from diagnostic analyses , which are performed as a component of the regulatory ozone modeling process for the philadelphia - new jersey region , are also presented to provide some specific examples related to the general issues discussed in this work . finally , research needs related to a ) the evaluation and refinement of regulatory ozone modeling , b ) the characterization of uncertainty in photochemical modeling , and c ) the improvement of the model - based ozone - attainment demonstration process are presented to identify future directions in this area.imagesfigure 7.figure 7.figure 7.figure 8.figure 9 . 
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