JANUARY 2004: Variability in surface ozone background over the United States: Implications for air quality policy
When setting the U.S. national ozone air quality standard, the Environmental Protection Agency (EPA) accounts for a background ozone level above which risk to human health is assessed. The EPA is currently reviewing the scientific basis for its ozone standard, and in this review process has defined background ozone to be those concentrations in surface air that would exist if North American anthropogenic emissions were turned off. In the past, the EPA used observational statistics to define a 25-45 ppbv range of surface ozone background during the U.S. ozone pollution season. A 40 ppbv level was then adopted for use in risk assessments. The figure below shows our best estimate for the U.S. surface ozone background (green diamonds) that would exist if North American emissions were turned off, simulated with the GEOS-CHEM model.
We classified ozone data from the Clean Air Status and Trends Network (CASTNet) into surface sites (generally below 1.5 km) and elevated sites (above 1.5 km). All elevated sites are in the west. We then aggregated our results to construct the cumulative probability distributions shown in the figure, for the 58 surface sites and the 12 elevated sites, for the three seasons, for the observations (black asterisks) and the model (red triangles). The corresponding distribution of background ozone concentrations is shown as green diamonds. The figure indicates that an appropriate background for use in risk assessment should vary as a function of season, altitude, and total ozone level. In particular, the depletion of the background during high-ozone events should be taken into account; the current 40 ppbv value used by the EPA will underestimate the risk posed by ozone concentrations above the background under these circumstances. The highest observed ozone concentrations at all altitudes in all seasons are associated with pollution from North American anthropogenic emissions, as seen by the difference between the green diamonds and red triangles. The background ozone concentrations shown here include the contribution from hemispheric pollution and would be even lower if international emission controls could be considered as part of a broader strategy to improve U.S. air quality. For a full account of this work, please see Fiore et al., 2003