CHANGES IN DIURNAL PATTERNS RELATED TO CHANGES IN OZONE LEVELS

Ozone is an ubiquitous air pollutant that affects both human health an d vegetation. There is concern about the number of hours human populat ions in nonattainment areas in the United States are exposed to levels of O3 at which effects have been observed. As improvement in air qual ity is achieved, it is possible that O3 control strategies may produce distributions of 1-h O3 concentrations that result in different diurn al profiles that produce greater potential exposures to O3 at known ef fects levels for multiple hours of the day. These concerns have prompt ed new analysis of aerometric data. In this analysis, the change in th e seasonally averaged diurnal pattern was investigated as changes in O 3 levels occurred. For the data used in this analysis, 25 ot the 36 si tes that changed compliance status across years showed no statisticall y significant change in the shape of the average diurnal profile (aver aged by O3 season). For 71 percent (10 out of 14) ot the sites in sout hern California and Dallas-FortWorth, Texas, that showed improvement i n O3 levels (i.e., reductions in the number of exceedances over the ye ars), but still remained in nonattainment, a statistically significant change in the shape of the seasonally averaged diurnal profile occurr ed. Based on the results obtained in this study, the evaluation of diu rnal patterns may be useful for identifying the influence of changes i n emission levels versus meteorological variation on attainment status . Using data from the southern California and Dallas-Fort Worth sites, which showed improvements in O3 levels, changes were observed in the seasonally averaged diurnal profiles. On the other hand, tor the sites moving between attainment and nonattainment status, such a change in shape was generally not observed and it was possible that meteorology played a more important role than changes in emission levels relative to attainment status.