Assessing regional and local exposures based upon the 8-hour ozone standard (80 ppb)
G. Foley (US EPA NERL);
P.J. Lioy, P.G. Georgopoulos (EOHSI, UMDNJ - R. W. Johnson Medical School and Rutgers University)
With the recent Supreme Court decision to uphold the 1997 change in the ozone NAAQS to 80 ppb for an 8-hour average value, it has become apparent that existing approaches used to assess population exposures of concern must be extended and modified. It is in fact, necessary to clearly define the spatial and temporal attributes of potential population exposures to ozone in order to identify appropriate precursor reduction strategies. A flexible metric for the comparative evaluation of such strategies for a given region should provide information on the number of people potentially exposed to an ambient level of concern within this region and for various averaging time periods. This work presents the results of analyses that employ EOHSI's MENTOR (Modeling Environment for Total Risk) Framework and EPA's AIRS database and the CMAQ (Community Multiscale Air Quality Model), a component of Models-3, to study ozone and airborne PM patterns in the Eastern United States for selected summer episodes that occurred between 1995 and 2000. Each case study considers regional patterns, as well as, localized ones, downwind and around urban areas. The simulation results are combined with demographic data to estimate the numbers of people potentially exposed in representative locations, such as the Northeast Corridor (urban), and the entire Eastern US (rural/suburban/rural). The analyses focus on overlapping 8-hour periods and provide estimates of the number of people experiencing single or multiple hours of ambient ozone above 80 ppb. Using this metric, the effectiveness of various scenarios of HC/NOx reductions is calculated and compared. Extending previous evaluations (OTAG), this metric is used to examine the impact of HC/NOx reductions on the number of people exposed to 8-hour ozone averages above 80 ppb. This approach reflects better the association of the 8-hour ozone standard with the regional distribution of ozone, as opposed to the former approach, where exceedances of the 1-hour standard were typically associated with the peak concentrations occurring downwind of urban centers. The density of population in the Eastern US vary significantly; thus, the study places separate focus on regional changes versus urban changes in populations exposed. Preliminary analyses show that population exposures to 8-hour average ozone concentrations are most effectively reduced by NOx-control intensive strategies, whereas they are weakly affected by HC controls; this is typically in contrast to strategies for peak ozone concentration reductions. The overall implications of using population-based metrics of potential (outdoor) exposure versus only examining reductions in ambient concentrations are discussed. Finally, the spatial and temporal patterns of co-occurrence of ozone and fine airborne particulate matter, during the summer episodes considered, are also discussed, providing additional perspective to the dynamics of population exposures to the complex mixture of gas and particulate phase contaminants associated with summer air pollution episodes across the Eastern United States.