Regional ground-level ozone trends in the context of meteorological influences across Canada and the eastern United States from 1997 to 2006

Meteorologically adjusted trends for different ozone averaging metrics (including daily maximum 1 h, daily maximum 8 h average, daily average, daytime average, nighttime average, daily minimum 8 h average, and monthly 5th percentile of ozone mixing ratios) were investigated in different regions of Canada and the United States over the time period 1997-2006. The spatiotemporal variability of the May-September daily ozone mixing ratios from 97 nonurban ozone measurement sites in Canada and the United States was examined to establish regions of common variability using rotated principal component analysis (R-PCA). This was followed by modeling multiple sites within the PCA-derived regions for all months using generalized linear mixed models. Most regions in southeastern Canada and the eastern United States showed statistically significant decreasing trends in the daily maximum 8 h average ranging from 0.53 +/- 0.2 to 2.7 +/- 0.86%/a, whereas significant increasing trends of 0.44 +/- 0.37%/a and 0.98 +/- 0.76%/a were found in Atlantic and Pacific Canada, respectively. In southeastern Canada and the eastern United States, the rates of decrease of the meteorologically adjusted regional trends associated with low ozone levels were slower than those of high levels. However, the rates of increase in the Atlantic and Pacific coastal regions associated with low levels were faster than those of high levels. These results are consistent with decreasing NOx and nonmethane hydrocarbon emissions in southeastern Canada and the eastern United States starting in the early 2000s and a hypothesized widespread increase due to the rising hemispheric background ozone.