Baseline levels and trends of ground level ozone in Canada and the United States

Abstract

Abstract.A statistical method was developed to extract baseline levels of ground level ozone in Canada and the US, and to quantify the temporal changes of baseline 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. Baseline ozone is defined here as ozone measured at a given site in the absence of strong local influences. The quantification of baseline 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 baseline 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).Baseline ozone mixing ratios determined by this method were found to vary geographically and seasonally. For the 1997-2006 period, baseline mixing ratios were calculated for annual and seasonal periods in seven regions of North America based on multi-site multi-year averages of the baseline data sets. The annual average (±1 standard deviation) baseline mixing ratios for the regions are as follows: Continental Eastern Canada=30±9 ppb, Continental Eastern US=30±10 ppb, Coastal Eastern Canada=27±9 ppb, Coastal Western Canada=19±10 ppb; Coastal Western US=39±10 ppb, Continental Western Canada=28±10 ppb and Continental Western US=46±7 ppb. Trends in the baseline mixing ratios were also found to vary by season and by geographical region. On a decadal scale, increasing baseline 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 Correspondence to: E. Chan (elton.chan@ec.gc.ca) were found with a rate of increase of 0.28±0.26, 0.72±0.55, and 0.93±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 baseline 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.

Multi-year objective analyses of warm season ground-level ozone and PM<sub>2.5</sub> over North America using real-time observations and Canadian operational air quality models

“…Some local exceptions to the overall downward trend in high percentile ozone are found in the northwestern part of North America (northwest US and Alberta). The results presented in this study are comparable with other studies that have examined long-term trends in ozone (Vautard et al, 2006;Chan and Vet, 2010;Cooper et al, 2010Cooper et al, , 2012EPA, 2010EPA, , 2012. The decreasing trends in the high percentiles of ozone and PM 2.5 strongly suggest that domestic emissions reduction has been effective; this is especially obvious for the eastern parts of North America.…”

“…A review of reported ozone trends by Chan and Vet (2010) found mean positive trends ranging between 0.3-1.0 ppbv yr −1 for Canada and the US. A study by Vautard et al (2006) for Europe also established a positive trend in ozone levels of 0.65 ppbv yr −1 , which is within the same range of values.…”

Interannual variability of ozone and carbon monoxide at the Whistler high elevation site: 2002–2006

“…These are 15-20 ppbv higher than the annual medians (23-27 ppbv) reported at a regional coastal California (1995California ( -2001 were 38-43 ppbv (Vingarzen, 2004). This range of values is also consistent with annual baseline values determined for the Canadian coastal sites (19 ppbv) and US coastal sites (39 ppbv), by Chan and Vet (2010) who acknowledged that site elevation was a major difference between the two groups.…”

“…Parrish et al (2009) have found a similar trend from looking at west coast marine boundary layer sites (0.34 ppbv/year). Chan and Vet (2010) used a muti-site cluster analysis and also found a significant upward trend in background ozone along the west coast of North America. Similar trends have been identified in free tropospheric measurements through a comprehensive integration of data from multiple years and west coast studies (Cooper et al, 2010).…”

Free tropospheric peroxyacetyl nitrate (PAN) and ozone at Mount Bachelor: potential causes of variability and timescale for trend detection

“…There are multiple lines of evidence that springtime O 3 concentrations are increasing over western North America Chan and Vet, 2010;Parrish et al, 2009;Jaffe et al, 2003;Jaffe and Ray, 2007;NRC, 2010;Parrish et al, 2004). The most recent analysis by Cooper et al (2010) showed a strong increase in lower free tropospheric (FT) springtime O 3 mixing ratios from 1995 to 2008.…”