Temporal characterization of gridded continuous emissions: when and where should one expect to find changes in air quality attributable to reductions in point source NOX emissions

The NOX initiative is a major effort by the USEPA to assess the impact of new NOX regulation by tying changes in emissions to changes in air quality. Among the new rules are the Reasonably Available Control Technology (RACT) rule, Clean Air Act Amendments (CAAA) Title IV, and the NOX State Implementation Plan (SIP) call. The effectiveness of the new rules needs to be demonstrated through analysis of modeling outputs and to track progress in air quality through observations. A unique source of information about NOX emissions is the Continuous Emission database (CEM), which holds ten years of hourly observations of NOX emission rates (mass/energy production) and energy production rates from more than 1,700 electrical generating units (EGU's) and other point sources. NOX emissions represented by the CEM database are one of the few components of the emission inventory used in air quality models that is actually observed. In addition, many of the sources represented by the CEM database have undergone dramatic reductions in NOX emission rates. Time series of average emission rates and daily emissions were created for map grids in the Eastern U.S. and displayed using time series decomposition techniques. A thorough evaluation of the temporal characteristics of gridded CEM time series shows that changes in emissions do not occur contemporaneously nor uniformly at all locations. Differences among the point sources in implementation schedules, energy demand, and the NOX budget trading program paint a complex picture of NOX point source emissions. When combined with supporting information (air quality model outputs, meteorological time series), the question of where and when one might best link policy with improvements in air quality is brought into focus.