A re-evaluated Canadian ozonesonde record: measurements of the vertical distribution of ozone over Canada from 1966 to 2013

Abstract

Abstract. In Canada routine ozone soundings have been carried at Resolute Bay since 1966, making this record the longest in the world. Similar measurements started in the 1970s at three other sites, and the network was expanded in stages to 10 sites by 2003. This important record for understanding long-term changes in tropospheric and stratospheric ozone has been re-evaluated as part of the SPARC/IO 3 C/IGACO-O 3 /NDACC (SI 2 N) initiative. The Brewer-Mast sonde, used in the Canadian network until 1980, is different in construction from the electrochemical concentration cell (ECC) sonde, and the ECC sonde itself has also undergone a variety of minor design changes over the period 1980-2013. Corrections have been made for the estimated effects of these changes to produce a more homogeneous data set.The effect of the corrections is generally modest. However, the overall result is entirely positive: the comparison with co-located total ozone spectrometers is improved, in terms of both bias and standard deviation, and trends in the bias have been reduced or eliminated. An uncertainty analysis (including the additional uncertainty from the corrections, where appropriate) has also been conducted, and the altitudedependent estimated uncertainty is included with each revised profile.The resulting time series show negative trends in the lower stratosphere of up to 5 % decade −1 for the period 1966-2013. Most of this decline occurred before 1997, and linear trends for the more recent period are generally not significant. The time series also show large variations from year to year. Some of these anomalies can be related to cold winters (in the Arctic stratosphere) or changes in the Brewer-Dobson circulation, which may thereby be influencing trends.In the troposphere, trends for the 48-year period are small and for the most part not significant. This suggests that ozone levels in the free troposphere over Canada have not changed significantly in nearly 50 years.

Methods to homogenize electrochemical concentration cell (ECC) ozonesonde measurements across changes in sensing solution concentration or ozonesonde manufacturer

“…These changes began in the mid-1990s and played a role in the analysis of ozonesonde data between then and the late 2000s (Mercer et al, 2007;Tarasick et al, 2016). Recognizing that these differences exceeded the accuracy and precision that is possible from ozonesondes (Smit et al, 2007;Deshler et al, 2008) led many investigators to independently explore the differences that occur when the same ozonesonde is operated with differing solution concentrations and when differing ozonesonde types are operated with the same solution concentrations (Johnson et al, 2002;Kivi et al, 2007;Smit et al, 2007;Deshler et al, 2008).…”

“…The absorption efficiency has been measured in the laboratory with two ECC ozonesondes measuring in series. These tests have shown that the absorption of ozone is somewhat dependent on the amount of solution used (e.g., 2.5 mL vs. 3.0 mL; Tarasick et al, 2016;Davies et al, 2003), with the absorption increasing from 0.96 as the amount of solution increases from 2.5 mL. The increase of the gas diffusion rate during balloon ascent due to the pressure decrease should limit this ozone conversion efficiency loss to the lower part of the profile (Davies et al, 2003).…”

On instrumental errors and related correction strategies of ozonesondes: possible effect on calculated ozone trends for the nearby sites Uccle and De Bilt

“…Additionally, this radiosonde change also caused a change in the Vaisala interface card and hence the pump temperature sensor, so that an effect on the recorded pump temperatures cannot be excluded (see Fig. 2 in Van Malderen et al, 2014, which shows a 2 • C decrease at 700 hPa). Since this effect is not quantified, no correction can be applied.…”

“…This exercise should result in an improved ozonesonde record with a reduced uncertainty (from 10-20 % down to 5-10 %), giving more solid information about the atmospheric changes that have occurred, as well as a better dataset for comparison with satellite measurements. The Canadian ozonesonde record has recently been re-evaluated regarding the O3S-DQA principles and the results are presented in Tarasick et al (2016).…”

“…As a consequence, these data should enable us to investigate the impact of different correction strategies on the vertical ozone profiles and the derived trends. This research is a follow-up study of Van Malderen et al (2014), in which the comparison of the ozonesonde data of both stations was used to identify the origin of anomalous high tropospheric ozone in the Uccle data during a 2.5-year period (beginning of 2007 to mid-2009). Moreover, this study is an update of De , who already reported on differences between profiles obtained at Uccle and De Bilt with different types of ozonesondes in the early years of the De Bilt time series.…”