Over the past two decades, the AQM system has had to grapple increasingly with air pollution phenomena that extend over multistate airsheds—phenomena whose effective control requires coordination across state boundaries and participation by states that may contribute to serious air quality problems but not experience them. For example, for acid rain, Congress chose to prescribe a national rule; for regional haze, Congress attempted to provide EPA with the authority to develop multistate strategies; and for O3, OTC (mandated by the CAA) and OTAG (developed by EPA and the states) were formed to develop suitable multistate strategies.
The current form of the CAA does not provide EPA and states a clear mandate and procedure for regularly analyzing and identifying when and how to implement multistate efforts. With the exception of the OTC, the multistate regulatory approach for O3 and regional haze has been linked to the traditional SIP, requiring that a set of multistate controls be identified first and then incorporated voluntarily into the SIPs of all the states in the given region. This indirect and controversial process has been time consuming and fraught with legal uncertainties. The SIPs of the future will require a more effective mechanism for identifying and linking multistate control strategies with local measures. Of particular importance will be the need for mechanisms to induce states upwind of emission sources to take actions that have little direct benefits for them but that are needed for successful attainment of the NAAQS in states downwind of the sources.
The SIP process provides a reasonable mechanism for state and local agencies to take into account emission controls adopted at the federal and multistate levels and then to choose a suitable suite of additional local emission-control measures to attain the NAAQS.
The existence of federally mandated emission-control measures has eased the burden of state and local authorities in developing attainment SIPS.
The requirement for emission inventories in SIPs has facilitated the development of a uniform methodology for quantifying pollutant emissions in the United States.
The requirement for modeling analysis in SIPs has promoted the development of increasingly sophisticated air quality models that link pollutant emissions to pollutant concentrations in the atmosphere.