Long-term changes in lower tropospheric baseline ozone concentrations at northern mid-latitudes

Abstract

Changes in baseline (here understood as representative of continental to hemispheric scales) tropospheric O<sub>3</sub> concentrations that have occurred at northern mid-latitudes over the past six decades are quantified from available measurement records with the goal of providing benchmarks to which retrospective model calculations of the global O<sub>3</sub> distribution can be compared. Eleven data sets (ten ground-based and one airborne) including six European (beginning in the 1950's and before), three North American (beginning in 1984) and two Asian (beginning in 1991) are analyzed. When the full time periods of the data records are considered a consistent picture emerges; O<sub>3</sub> has increased at all sites in all seasons at approximately 1% yr<sup>−1</sup> relative to the site's 2000 yr mixing ratio in each season. For perspective, this rate of increase sustained from 1950 to 2000 corresponds to an approximate doubling. There is little if any evidence for statistically significant differences in average rates of increase among the sites, regardless of varying length of data records. At most sites (most definitively at the European sites) the rate of increase has slowed over the last decade (possibly longer), to the extent that at present O<sub>3</sub> is decreasing at some sites in some seasons, particularly in summer. The average rate of increase before 2000 shows significant seasonal differences (1.08 ± 0.09, 0.89 ± 0.10, 0.85 ± 0.11 and 1.21 ± 0.12% yr<sup>−1</sup> in spring, summer, autumn and winter, respectively, over North America and Europe)

Lower tropospheric ozone at northern midlatitudes: Changing seasonal cycle

“…For the four sites included in Figure 1, the seasonal maximum occurred 10 to 48 days earlier during the later 5 year period compared to the earlier period; however, only at the two sites with the longest data records, Zugspitze and Hohenpeissenberg, are the differences significant at the 95% confidence level. As expected, significant increases in annual average O 3 are also evident at all sites, reflecting the general O 3 increase that has occurred at northern midlatitudes [e.g., Parrish et al, 2012].…”

“…In this latter application, the y o parameter in equation (1) is obtained from the average of the seasonal averages over the 5 year period. The seasonal O 3 averages for each year are directly available from the quadratic fits to the seasonal O 3 trends presented by Parrish et al [2012]. This reduces the least-squares regression fit from three to two parameters.…”

“…Monthly averages at Lassen NP were calculated from hourly average data provided by the U.S. National Park Service, Air Resources Division (available at http:// www.nature.nps.gov/air/monitoring/network.cfm). Parrish et al [2012] give more details of these data sets and sites, including a map of site locations. For all sites, the full data records are included in the analyses.…”

“…[5] To quantify long-term changes in lower tropospheric O 3 concentrations at northern midlatitudes, Parrish et al [2012] examined 11 data sets that included measurements in Europe, North America, and Asia. Here we examine five of those data sets that are most appropriate (those with longest continuous data records from single sites with yearround coverage) for quantifying the evolution of the O 3 seasonal cycle over decadal time scales.…”

“…For example, at Hohenpeissenberg in Germany, where O 3 has been continuously measured since 1971, summer was the season of maximum O 3 throughout at least the first three decades of the data record. However, in recent years O 3 concentrations in spring have been comparable to those in summer [see Parrish et al, 2012, Figure 2]. Here we quantitatively examine how the seasonal cycle in tropospheric O 3 at northern midlatitudes has evolved over the past four decades, possibly in response to changing anthropogenic emissions, natural variability, and changing global climate.…”