The impact of background ozone on compliance with revised National Ambient Air Quality Standards

The National Ambient Air Quality Standard (NAAQS) for ground-level ozone, previously set as an 8-hr average of 0.08 parts per million (ppm), has been revised by the U.S. Environmental Protection Agency (EPA). Historically, background concentrations originating from non-local sources were not considered to be a major constituent of ground-level ozone. However, previous research has shown that background ozone concentrations often exceed the new 8-hr NAAQS of 0.075 ppm, and that high background concentrations are particularly troublesome in the mid-latitudes. This study measured ozone at ground level and at 210 m above ground level in Tulsa, OK (36 degrees N, 96 degrees W), from June 1 to November 30, 2005. Background ozone concentrations as high as 0.05 ppm were recorded, and substantial variability was observed in conjunction with the polar jet stream and the seasonal influence of large-scale subtropical high pressure at the study location. Additionally, the highest observed background concentrations coincided with maximum photochemical generation at ground level. On the basis of the magnitude and variability of background ozone, a more stringent 8-hr ozone standard will be difficult for local or regional regulatory agencies to meet. A clear understanding of the impacts of background ozone will be required to make and meet new State Implementation Plans (SIPs).