Ozone Regimes in or near Forests of the Western United States: I. Regional Patterns

Similarities in ozone regimes at sites in or near forests of the western United States were investigated using hierarchical cluster analysis on a (63 × 17) matrix of frequencies of occurrence of 17 characteristic diurnal patterns in hourly ozone concentrations. Eight groups of sites with similar ozone patterns were identified. Sites within these groups showed geographical consistency at a local scale, but more complex spatial relationships exist regionally. Unlike kriging analyses of ozone concentrations over the United States as a whole and California in particular, our analysis does not identify large areas of gradation in ozone levels. Instead, hotspots linked to populated valleys were identified with gradations in ozone exposure occurring from these hotspot areas. Considering the complex nature of terrain across the West and the restrictive role that such topography plays in atmospheric transport, hotspots in ozone concentrations are more likely than regional gradation from large urban centers. Comparisons of group memberships determined from common univariate statistics (SUM06, SUM08, 7-hour and 24-hour) with our results lead to the conclusion that regional analyses of ozone concentrations using univariate statistics are insensitive to important differences in ozone exposure regimes. Of particular concern is the inability of univariate statistics to consistently discriminate between sites with urban and non-urban characteristics.