BACKGROUND OZONE IN THE PLANETARY BOUNDARY-LAYER OVER THE UNITED-STATES

Reliable estimates of background O-3 in the planetary boundary layer a re needed as part of the current review by the U.S. EPA of O-3 health and welfare criteria and of the National Ambient Air Quality Standards for O-3. Such estimates are especially necessary for comparing O-3 co ncentrations at which vegetation effects occur to O-3 concentrations r eported to represent background levels. Some vegetation researchers ha ve used the seasonal average of the daily 7-h (0900-1559 h) average as the exposure parameter in exposure-response models. The 7-h (0900-155 9 h) seasonal mean reference point for O-3 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 aver age concentrations, as identified in the U.S. EPA AIRS database. Crite ria are enumerated and discussed for determining whether O-3 concentra tions at a given site can be considered to be ''background'' O-3. The paper also suggests statistical techniques for estimating background O -3 concentrations. Linear regression techniques yield valuable informa tion about O-3 concentration data from the literature. Coupled with ot her criteria, such analyses can be used to select sites that represent ''background'' sites for O-3. Selection of such sites thus allows est imations of background O-3 in different areas of the country, at diffe rent elevations, and for different averaging times. Using several tech niques, the current O-3 background at inland sites in the United State s and Canada for the daylight 7-h (0900-1559 h) seasonal (April-Octobe r) average concentrations usually occurred within the range of 35 +/- 10 ppb. For coastal sites, the corresponding O-3 concentrations were s omewhat lower, occurring within the range of 25 +/- 10 ppb for locatio ns in the northern hemisphere, but with most O-3 concentrations at the coastal sites in the range of 30 +/- 5 ppb. The 50th percentile conce ntrations range from 16 ppb to 45 ppb at inland sites and range from 1 0 ppb to 33 ppb at coastal sites. The maximum hourly concentrations ra nge 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 O- 3 is somewhat dependent on a number of conditions such as the nature o f upwind flow, lack of pollution sources, and terrain conditions inclu ding deposition with respect to forest or agricultural areas.