Ozone Regimes in or near Forests of the Western United States: Part 2. Factors Influencing Regional Patterns

Abstract

Ozone levels in or near forests of the western United States resulted from transport of ozone from urban areas, photochemical formation of ozone in nonurban areas from either natural or manmade precursors, and downward mixing of ozone from the tropospheric reservoir. Similarities in ozone exposure regimes were clearly associated with ozone characteristics, such as the shape of the diurnal curve in hourly ozone concentrations and the magnitudes of ozone levels. No single site characteristic dominated across the region. Eight classes of ozone regimes were identified: largeurban, medium-urban, small-urban, urban downwind-urban, urban-transport, rural, rural-remote, and remote. Sites in the western United States can be classified according to these classes. Of the sites included in the analysis, Olympic National Park, Colorado National Monument, Redwood National Park, Grand Canyon, and Crook County in Oregon showed the lowest impact from urban-generated ozone. The greatest impact of manmade ozone was found in forests of southern and central California, and to some extent in Rocky Mountain National Park and along the western slopes of the Cascades in Washington.

The eight groups of sites showing similar ozone exposure regimes as identified by Böhm et al.¹ corresponded closely with the eight classes of ozone regimes, indicating that regional similarities in ozone exposure regimes can be linked to ozone formation and transport processes despite a lack of geographic cohesion. Sixty-four percent of variance in ozone exposure regimes can be expressed in a twodimensional space called diurnal-curve space. Group membership is more cohesive in diurnal-curve than geographic space. Applications of diurnal-curve space include (1) a sensitive regional analysis of the nature and magnitudes of ozone exposure regimes in or near western forests; (2) a pro-active means for identifying sites with changing ozone exposure regimes; and (3) guidelines from which biologists can develop realistic experimental fumigation protocols. It is suggested that univariate statistics, such as 7-hour mean, 24-hour mean, SUM06, and SUM08, cannot identify up to 39% of the variance in ozone exposure regimes among sites in or near forests of the western United States.