ACPD Atmospheric Chemistry and Physics Discussions ACPD 1680-7375 Copernicus GmbH Göttingen, Germany 10.5194/acpd-9-21111-2009 Background ozone over 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 07 10 2009 9 5 21111 21164 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-discuss.net/9/21111/2009/acpd-9-21111-2009.html The full text article is available as a PDF file from http://www.atmos-chem-phys-discuss.net/9/21111/2009/acpd-9-21111-2009.pdf

Planetary boundary layer (PBL) ozone temporal variations were investigated on diurnal, seasonal and decadal scales in various regions across Canada and the United States for the period 1997–2006. Background ozone is difficult to quantify and define through observations. In light of the importance of its estimates for achievable policy targets, evaluation of health impacts and relationship with climate, background ozone mixing ratios were estimated. Principal Component Analyses (PCA) were performed using 97 non-urban ozone sites for each season to define contiguous regions. Backward air parcel trajectories were used to systematically select the <i>cleanest</i> background air cluster associated with the lowest May–September 95th percentile for each site. Decadal ozone trends were estimated by season for each PCA-derived region using a~generalized linear mixed model (GLMM). <br><br> Background ozone mixing ratios were variable geographically and seasonally. For example, the mixing ratios annually ranged from 21 to 38, and 23 to 38 ppb for the continental Eastern Canada and Eastern US. The Pacific and Atlantic coastal regions typically had relatively low background levels ranging from 14 to 24, and 17 to 36 ppb, respectively. On the decadal scale, the direction and magnitude of trends are different in all seasons across the regions (&minus;1.56 to +0.93 ppb/a). Trends increased in the Pacific region for all seasons. Background ozone decadal changes are shown to be masked by the much stronger regional signals in areas that have seen substantial reductions of ozone precursors since the early 2000s.

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