Background Ozone in the Planetary Boundary Layer Over the United States

Reliable estimates of background O3 in the planetary boundary layer are needed as part of the current review by the U.S. EPA of O3 health and welfare criteria and of the National Ambient Air Quality Standards for O3. Such estimates are especially necessary for comparing O3 concentrations at which vegetation effects occur to O3 concentrations reported to represent background levels. Some vegetation researchers have used the seasonal average of the daily 7-h (0900- 1559 h) average as the exposure parameter in exposure-response models. The 7-h (0900-1559 h) seasonal mean reference point for O3 was assumed to be 0.025 ppm. Ozone aerometric data are presented from the monitoring sites in the United States which experience some of the lowest maximum hourly average concentrations, as identified in the U.S. EPA AIRS database. Criteria are enumerated and discussed for determining whether O3 concentrations at a given site can be considered to be "background" O3. The paper also suggests statistical techniques for estimating background O3 concentrations. Linear regression techniques yield valuable information about O3 concentration data from the literature. Coupled with other criteria, such analyses can be used to select sites that represent "background" sites for O3. Selection of such sites thus allows estimations of background O3 in different areas of the country, at different elevations, and for different averaging times. Using several techniques, the current O3 background at inland sites in the United States and Canada for the daylight 7-h (0900-1559 h) seasonal (April-October) average concentrations usually occurred within the range of 35 ± 10 ppb. For coastal sites, the corresponding O3 concentrations were somewhat lower, occurring within the range of 25 ± 10 ppb for locations in the northern hemisphere, but with most O3 concentrations at the coastal sites in the range of 30 ± 5 ppb. The 50th percentile concentrations range from 16 ppb to 45 ppb at inland sites and range from 10 ppb to 33 ppb at coastal sites. The maximum hourly concentrations range from 50 ppb to 98 ppb at inland sites and range from 44 ppb to 80 ppb at coastal sites. We believe that the maximum hourly concentration of 98 ppb experienced in 1988 was influenced by the massive fires in Yellowstone National Park. These ranges suggest that the background O3 is somewhat dependent on a number of conditions such as the nature of upwind flow, lack of pollution sources, and terrain conditions including deposition with respect to forest or agricultural areas.