ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-10-8629-2010 Baseline levels and trends of ground level ozone in Canada and the United States Chan E. 1 Vet R. J. 1 Air Quality Research Division, Science and Technology Branch, Environment Canada, 4905 Dufferin Street, Toronto, Ontario, M3H 5T4, Canada 16 09 2010 10 18 8629 8647 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/10/8629/2010/acp-10-8629-2010.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/10/8629/2010/acp-10-8629-2010.pdf

A statistical method was developed to extract <i>baseline</i> levels of ground level ozone in Canada and the US, and to quantify the temporal changes of <i>baseline</i> ozone levels on annual, seasonal, diurnal and decadal scales for the period 1997 to 2006 based on ground-level observations from 97 non-urban monitoring sites. <i>Baseline</i> ozone is defined here as ozone measured at a given site in the absence of strong local influences. The quantification of <i>baseline</i> levels involved using a Principal Component Analyses (PCA) to derive groups of commonly-varying sites in contiguous regions by season, followed by using backward air parcel trajectories to systematically select ozone mixing ratios associated with the <i>baseline</i> condition in each of the PCA-derived regions. Decadal trends were estimated by season for each of the regions using a generalized linear mixed model (GLMM). <br><br> <i>Baseline</i> ozone mixing ratios determined by this method were found to vary geographically and seasonally. For the 1997–2006 period, <i>baseline</i> mixing ratios were calculated for annual and seasonal periods in seven regions of North America based on multi-site multi-year averages of the <i>baseline</i> data sets. The annual average (&plusmn;1 standard deviation) <i>baseline</i> mixing ratios for the regions are as follows: Continental Eastern Canada=30&plusmn;9 ppb, Continental Eastern US=30&plusmn;10 ppb, Coastal Eastern Canada=27&plusmn;9 ppb, Coastal Western Canada=19&plusmn;10 ppb; Coastal Western US=39&plusmn;10 ppb, Continental Western Canada=28&plusmn;10 ppb and Continental Western US=46&plusmn;7 ppb. Trends in the <i>baseline</i> mixing ratios were also found to vary by season and by geographical region. On a decadal scale, increasing <i>baseline</i> ozone trends (temperature-adjusted) were observed in all seasons along the Pacific coasts of Canada and the US, although the trends in California were not statistically significant. In the coastal zone of Pacific Canada, positive trends were found with a rate of increase of 0.28&plusmn;0.26, 0.72&plusmn;0.55, and 0.93&plusmn;0.41 ppb/a in spring (MAM), summer (JJA) and winter (DJF), respectively. In the Atlantic coastal region, the trends were also positive in 3 of the 4 seasons (but only significantly so in MAM). In the high ozone precursor emission areas of the Eastern United States, decadal trends in <i>baseline</i> ozone are, in general, negative in the spring, summer and fall and appear to be controlled by the strong within-region changes induced by decreasing ozone precursor emissions.

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