Background Ozone in the Planetary Boundary Layer Over the United States

Abstract 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 techniqu...