Characterizing long-term changes in surface ozone levels in the United States (1980–2005)

Using statistical trending on a site-by-site basis of the (1) health-based annual 2nd highest 1-h average concentration and annual 4th highest daily maximum 8-h average concentration and (2) vegetation-based annual seasonally corrected 24-h W126 cumulative exposure index, we have investigated temporal and spatial statistically significant changes that occurred in surface O3 in the United States for the periods 1980–2005 and 1990–2005 and explored whether differences in trending occur depending upon the selection of the exposure metric. Using the trending results, the analyses quantitatively explore the evidence for the higher hourly average O3 concentrations decreasing faster than the mid- and lower-values. Most of the monitoring sites analyzed in our study experienced decreasing or no trends. Few monitoring sites experienced increasing trends. For those monitoring sites with declining O3 levels, an initial pattern of rapid decrease in the higher hourly average concentrations, followed by a much slower decrease in mid-level concentrations was observed. In some cases, we observed shifts from the lower hourly average O3 concentrations to the mid-level values. On a site-by-site basis, the majority of monitoring sites (1) changed from negative trend to no trend, (2) continued a negative trend, or (3) remained in the no trend status, when comparing trends for the 1980–2005 to the 1990–2005 time periods. For all three exposure metrics, approximately 60% of the monitoring sites shifted from negative trending to no trending status. It appeared that all regions of the United States were equally affected by the shift in status. The greatest statistically significant decreases in the 2nd highest 1-h average concentrations and the annual 4th highest daily maximum 8-h average concentration for the two temporal periods occurred in southern California. Monitoring sites in other portions of the United States experienced lesser decreases than this geographic area. In contrast to the two exposure indices, the vegetation-based 24-h W126 O3 cumulative index for 1980–2005 experienced significant declines in the midwestern states and the northeastern United States as well as in southern California. For the 1990–2005 period, monitoring sites in southern California and the northeastern United States experienced the greatest decreases in the W126 exposure metric. Testing for statistically significant changes in the number of hourly average concentrations within specified concentration intervals identified specific months that experienced shifts in the distribution of the hourly average concentrations. We observed that a statistically significant trend at a specific monitoring site, using one exposure index, did not necessarily result in a similar trend using the other two indices. Because different trending patterns were observed when applying the various exposure indices, a careful selection of O3 exposure metrics is required when assessing trends for specific purposes, such as human health, vegetation, and climate change effects.