AMTAtmospheric Measurement TechniquesAMTAtmos. Meas. Tech.1867-8548Copernicus PublicationsGöttingen, Germany10.5194/amt-9-195-2016A re-evaluated Canadian ozonesonde record: measurements of the vertical
distribution of ozone over Canada from 1966 to 2013TarasickD. W.david.tarasick@canada.caDaviesJ.SmitH. G. J.OltmansS. J.https://orcid.org/0000-0002-7390-2553Environment Canada, 4905 Dufferin Street, Downsview, Toronto, ON, M3H 5T4
CanadaInstitute for Energy and Climate Research: Troposphere (IEK-8),
Research Centre Juelich (FZJ), Juelich, GermanyGlobal Monitoring Division, Earth System Research Laboratory, National
Oceanic and Atmospheric Administration, Boulder, Colorado, USAD. W. Tarasick (david.tarasick@canada.ca)25January20169119521425February201521May20154December201515December2015This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://amt.copernicus.org/articles/9/195/2016/amt-9-195-2016.htmlThe full text article is available as a PDF file from https://amt.copernicus.org/articles/9/195/2016/amt-9-195-2016.pdf
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/IO3C/IGACO-O3/NDACC
(SI2N) 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 altitude-dependent 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.
Introduction
Ozone plays a major role in the chemical and thermal balance of the
atmosphere. It controls the oxidizing capacity of the lower atmosphere via
its photochemical link to the OH radical and also acts as an important
short-lived climate forcer. Ozone changes in the stratosphere, as well as
strongly affecting surface UV radiation, may also affect future climate
(IPCC, 2013, and references therein). In addition to the information they
provide on the vertical distribution of ozone in the lower stratosphere,
ozone soundings are the major source, worldwide, of information on ozone
amounts in the free troposphere.
The Canadian ozonesonde network. Soundings are weekly (generally
Wednesdays), with extra releases during special campaigns (i.e. MATCH,
TOPSE, IONS, BORTAS). Regular ozone soundings have been made at Resolute
since January 1966.
StationLocationAltitude (m)Start of sonde recordEdmonton53.6∘ N, 114.1∘ W766Brewer–Mast (1970); ECC (1979)Goose Bay53.3∘ N, 60.3∘ W44Brewer–Mast (1969); ECC (1980)Churchill58.8∘ N, 94.1∘ W35Brewer–Mast (1973); ECC (1979)Resolute74.7∘ N, 95.0∘ W64Brewer–Mast (1966); ECC (1979)Eureka80.1∘ N, 86.4∘ W10ECC (1992)Alert82.5∘ N, 62.3∘ W62ECC (1987)Kelowna49.9∘ N, 119.4∘ W456ECC (2003)Bratt's Lake50.2∘ N, 104.7∘ W580ECC (2003–2011)Egbert44.2∘ N, 79.8∘ W251ECC (2003–2011)Yarmouth43.9∘ N, 66.1∘ W9ECC (2003)
Vertical distribution information is particularly important for ozone
transport studies, as motion in the atmosphere is predominantly horizontal.
The global ozonesonde record is therefore increasingly important for
understanding long-term changes in both tropospheric and stratospheric ozone,
as each may be affected by changes in long-range quasi-horizontal transport,
as well as by vertical exchange/mixing between layers. For example,
ozonesonde measurements show impact on near-surface ozone concentrations
of intrusions of ozone from the lower stratosphere (e.g. He et al., 2011;
Hocking et al., 2007) and the inter-continental transport of tropospheric
ozone and its precursor species (Oltmans et al., 2006, 2010). Canadian
ozonesondes have also provided essential information on the nature of Arctic
stratospheric ozone loss (Manney et al., 2011; Fioletov et al., 1997; Kerr
et al., 1993), of Arctic surface depletion events (Tarasick and Bottenheim,
2002; Bottenheim et al., 2002), and of the global circulation of ozone (Lin
et al., 2015; Bönisch et al., 2011; Pan et al., 2009), as well as of
tropospheric sources and budgets (Emmons et al., 2015; Parrington et al.,
2012; Walker et al., 2010, 2012; Macdonald et al., 2011; Thompson et
al., 2007; Tarasick et al., 2007).
The time series of ozone soundings from Canadian stations comprise some of
the longest records of vertical ozone profile measurement that exist, as
well as the only time series of measurements in the free troposphere over
Canada. Following some initial ozone soundings conducted in cooperation with
the US Air Force Cambridge Research Laboratories (AFGL) from 1963 to 1965 at
Goose Bay and Churchill, employing chemiluminescent (Regener, 1960) sondes
(Hering, 1964; Hering and Borden, 1964, 1965, 1967), regular ozone soundings using electrochemical
Brewer–Mast sondes (Brewer and Milford, 1960) began at Resolute in January 1966. Table 1
describes the locations of Canadian ozonesonde stations and their data
records.
Preparation procedures for the Brewer–Mast sondes are described in Tarasick et al. (2002)
but essentially followed Mueller (1976). In 1980 the Canadian network switched to
electrochemical concentration cell (ECC) sondes (Komhyr, 1969). ECC sonde
preparation and launch procedures are as described in Tarasick et al. (2005). Although
these procedures were not changed at any time in the Canadian record, the
change of sonde type, as well as minor changes in the design of the ECC
sonde over the past 3 decades, may have introduced biases in the
measurement time series that could affect trends (Table 2). The associated
radiosonde has also changed, which could influence the ozone profile by
introducing altitude shifts, primarily above 25 hPa (25 km), due to
temperature or pressure biases.
Changes in ozonesondes and associated radiosondes in the Canadian
network.
YearChangePossible effect1979ECC 3A introduced∼ 15 % increase in tropospheric response relative to BM sondes. Sonde T measured via rod thermistor.1984ECC 4A introducedRedesigned pump; maximum change < 1 %, at 50–20 hPa. Sonde “box” T measured; new rod thermistor.1993ECC 5A introducedNew pump correction; maximum change ∼ 1 %, at 100 hPa.1993Vaisala RS-80, RSA-11 introducedOlder VIZ sonde: warm bias in daytime; pressure errors. May introduce altitude shifts in profile; ozone increases of up to ∼ 2 % at 20 hPa.1996ECC 6ANo differences below about 20–25 km (Smit et al., 2007).2000ENSCI 1Z design changeHigh bias with 1 % KI solution (Smit et al., 2007).20043cc solution (new sites)Better ozone capture in troposphere.2006Vaisala RS-92 introducedRS80s low by ∼ 20 m in the troposphere, high by 100 m at 10 hPa (Steinbrecht et al., 2008).2007Thermistor in ECC pumpMore accurate measurement of air volume.
As part of the SPARC/IO3C/IGACO-O3/NDACC (SI2N) initiative,
the Ozonesonde Data Quality Assessment (O3S-DQA) was initiated in order to
resolve inhomogeneities in the global long-term ozone sounding record. The
effects of many of the changes listed in Table 2 have been characterized by
recent laboratory and field work and can now be corrected. The uncertainty
of ozonesonde profile measurements can now also be described with a degree
of confidence that was not available in the past. These developments are
described in a recent report (Smit et al., 2012), and the re-evaluation of the Canadian
record described here follows those recommendations.
Corrections to the sounding data
The operating principle of electrochemical ozonesondes is the well-known reaction of
potassium iodide with ozone:
2KI+O3+H2O⟶2KOH+I2+O2
followed by
I2+2e-⟶2I-.
Thus for each molecule of ozone two electrons are produced and an equivalent
amount of current flows through the external circuit. The measurement is
therefore, in principle, absolute; however, there may be losses of ozone
and/or of iodine, and there may be side reactions that also convert iodide
to iodine. Ozone partial pressure is calculated using the ideal gas law,
noting from Reaction (R2) that the number of moles per second of ozone passing
through the sonde is equal to half the current divided by the Faraday
constant. This gives (e.g. Komhyr, 1986)
PO3=k(i-iB)Tt,
where i is the measured cell current, iB is the background current, T is the
temperature of the air in the pump (often approximated by the sonde box
temperature), and t is the measured time in seconds for the sonde to pump 100 mL of air. k is a constant, equal to 0.0004307 for current in microamperes,
T is in kelvins, and ozone partial pressure is in millipascals. Errors or bias changes in
the temperature or background current measurement or the pump rate (or its
change with ambient pressure during flight) can therefore affect the
ozonesonde measurement.
Total ozone normalization
In practice ECC ozonesondes have a precision of 3–5 % and a total
(random + systematic) uncertainty of about 10 % throughout most of the profile
below ∼ 28 km (Smit et al., 2007; Kerr et al., 1994; Deshler et al., 2008a; Liu et al., 2009). The precision of
the older Brewer–Mast sonde is somewhat poorer, at about 5–10 % (Kerr et al., 1994;
Smit et al., 1996). The Brewer–Mast soundings required normalizing, or “correcting”,
by linearly scaling the entire ozone profile (plus an estimate of the
residual above the balloon burst altitude) to a total ozone measurement.
This was because they showed a typical response equivalent to about 80 %
of the actual ozone amount when prepared according to the manufacturer's
instructions (the Canadian practice) and so needed to be scaled, by what is
traditionally referred to as the “correction factor”, to give a more
accurate result. Although the ECC sonde response is much closer to 100 %,
normalizing to a coincident Brewer or Dobson spectrophotometer measurement
has continued to be the Canadian practice because it demonstrably reduces
uncertainties in ozonesonde data, at least in the stratosphere (e.g. Kerr et al., 1994;
Smit et al., 1996; Beekmann et al., 1994, 1995). Averaged over the profile, uncertainties are 7–10 %
for non-normalized data and 5–7 % for normalized data (Fioletov et al., 2006). This
improvement is because of the greater accuracy of total ozone measurements:
for well-calibrated total ozone instruments the standard uncertainty of
direct sun measurements is less than 3 % (Basher, 1982).
The Canadian total ozone record has been extensively revised, but these
revisions had not, until now, been carried through to the older ozonesonde
records. This meant that the total ozone value in the sonde record (used for
calculating the normalization factor) was frequently not the same as the
revised value in the World Ozone and Ultraviolet Radiation Data Centre
(WOUDC). We found occasional cases of surprisingly large differences
(∼ 35 %). In some cases, particularly in the older Dobson
record, a total ozone value for the previous day appears to have been used.
In addition, historical practice in Canada for estimating the residual ozone
amount above the profile top has been to simply assume constant ozone mixing
ratio above the balloon burst altitude. Much better knowledge now exists for
the distribution of ozone at higher altitudes, and so the use of a
climatological estimate is preferred. We have used the climatology of
McPeters and Labow (2012) to renormalize the Canadian data. The total ozone normalization is
applied only after all other corrections have been applied (to the
non-normalized data; that is, any previous normalization is first removed).
Normalization is not applied to flights that fail to reach 32 hPa. This is
also a change from previous practice, which required flights to reach 17 hPa
for total ozone normalization to be applied.
There are arguments against normalization of ECC sonde profiles: the process
introduces a degree of uncertainty because the amount of ozone above the
balloon burst height can only be estimated. It is also not clear that a
scaling factor that is constant with altitude is appropriate in all cases.
This is of particular concern for the tropospheric part of the profile;
whether normalization, which is necessarily weighted to the much larger
stratospheric part of the profile, improves tropospheric measurements is an
open question. Normalization also renders the sonde record no longer
independent of the total ozone record, which is an important issue for trend
studies (although to some extent alleviated if there is no trend in scaling
factors) and obviously can introduce a serious bias if the total ozone
instrument calibration is in error. Fortunately, since the scaling is linear
in measured ozone, it can be applied, and as easily removed, in
post-processing or by the data user.
The normalization factor is unquestionably of value as a data quality
control indicator, and we will use it as such in the analysis to follow. We
present here normalized data, for consistency between the Brewer–Mast and
ECC records, and with past trend analyses (e.g. Tarasick et al., 2005).
Correction for Brewer–Mast tropospheric response
Laboratory work (Tarasick et al., 2002) suggests that the response of Brewer–Mast sondes in
the Canadian program was biased low in the troposphere. We have applied a
correction based on simple quadratic fit to the data shown in Fig. 7 of
Tarasick et al. (2002). The correction is consistent with that implied by the WMO-II
intercomparison of 1978 (Attmannspacher and Dütsch, 1981; see also Fig. 10 of Liu et al., 2013) and also
similar to, but somewhat more modest than, that suggested by the WMO-I and
BOIC sonde intercomparison campaigns (Attmannspacher and Dütsch, 1981; Hilsenrath et al., 1986) and the analysis by
Lehmann (2005) of Brewer–Mast data from the Australian program. The Australian
program used similar procedures to those in Canada.
Pump corrections
The efficiency of the ozonesonde pump decreases at low pressures, and a
correction for this is part of normal data reduction. Pump corrections from
Komhyr et al. (1968) were used for Canadian Brewer–Mast sonde data (Mateer, 1977). We have now
applied the more commonly used Komhyr and Harris (1965) pump corrections, recommended by WMO
(Claude et al., 1987), which are larger than the Komhyr et al. (1968) corrections. Significantly larger
pump corrections have been recommended by Steinbrecht et al. (1998), but these may not apply
to older Brewer–Mast sondes (Lehmann and Easson, 2003).
For ECC model 3A sondes, flown in Canada between 1979 and 1982, no change to
the pump correction has been made, but the pump correction table has been
added to the file. The correction is that supplied by the manufacturer but
also similar to that found by Torres (1981).
The ECC model 4A sonde differs significantly from the 3A; the major
difference is a redesigned pump. In the original data reduction the
correction curve supplied in 1983 by the manufacturer was used for all 4A
flights. We have now applied the revised Komhyr (1986) correction curve. This
correction curve was already in use for 5A and all subsequent ECC sonde
models. The pump correction table has been added to the WOUDC file for all
flights.
Solution volume correction
Standard practice in Canada has been to charge ECC sensors with 2.5 mL of
sensing solution rather than the 3.0 mL which is now recommended.
Laboratory and field investigations have shown that with 2.5 mL of sensing
solution only ∼ 96 % of the ozone is captured by the sensing
solution at ground pressure, but at lower pressures the 4 % deficit
vanishes, apparently because of faster gas-diffusion rates in solution
(Davies et al., 2003). We have made a correction for this effect, following Smit et al. (2012).
Use of standard 1 % buffered-KI solution in ENSCI sondes
Two types of ECC ozonesondes have been in use since about 2000: the 2Z model
manufactured by ENSCI Corp. and the 6A model manufactured by Science Pump,
with differences in construction and in recommended concentrations of the
potassium iodide sensing solution and of its phosphate buffer (Smit et al., 2007). Since
the Canadian network has used standard 1 % buffered-KI solution at all
times, where ENSCI sondes have been used a positive bias of about 4 %
below 50 hPa and somewhat larger above is expected (Boyd et al., 1998; Smit et al., 2007; Deshler et al., 2008b).
We have made a correction for this bias, following Smit et al. (2012).
Pump temperature measurement
The measurement of pump temperature is required to accurately measure the
amount of air passing through the pump into the ECC sensor cell. In the past
this has been approximated by a measurement using a rod thermistor at the
base of the electronics unit (3A and 4A sondes) and later a thermistor
suspended in the sonde box. Field and laboratory experiments suggest that
this produced a consistent relationship between the “box” temperature and
the pump body temperature (Komhyr and Harris, 1971). Measurement of the actual pump
temperature only became standard in Canada around 2008. We have made
corrections for temperatures measured by either “rod” or “box”
thermistors following Smit et al. (2012).
Background current
The background current of the ECC sonde is not well understood and may have
several sources. It represents a non-equilibrium condition in the cell,
possibly from residual triiodide in new sensing solution (Thornton and Niazy, 1982, 1983) or
from previous exposure to ozone (Johnson et al., 2002). Canadian practice has been to treat
it as proportional to pressure, but there is no reason now to think that
this is correct, and treating it as approximately constant over the duration
of a flight may be a better approximation and is in fact recommended (Smit and ASOPOS panel, 2011).
Unfortunately to properly recalculate ozone assuming a constant background
current requires knowledge of the pump temperature profile, and this
information has been preserved only for flights after 1999. We have
therefore not attempted to correct the background current but have instead
treated it as an error source (see Sect. 4), a not entirely satisfactory
choice, since although randomly variable in magnitude, it is always a
positive bias.
Radiosonde changes
Errors in radiosonde pressure or temperature will imply corresponding errors
in calculated geopotential heights, causing measured ozone concentrations to
be assigned to incorrect altitudes and pressures. This is potentially an
important issue for the derivation of trends, as radiosonde changes may
therefore introduce vertical shifts in the ozone profile, and apparent
changes in ozone concentration at a given height.
A number of different radiosonde designs have been used in the Canadian
observing network over the last 5 decades. Temperature differences
between the VIZ sonde, used widely in the 1980s and early 1990s, and the
Vaisala RS-80 sonde, adopted subsequently in Canada, are well documented.
The VIZ sonde showed a warm bias in the daytime by as much as
2 ∘C (Richner and Philips, 1981; Luers and Eskridge, 1995; Wang and Young, 2005). From simultaneous
measurements made during a WMO intercomparison in 1985, Schmidlin (1988) estimates
that this bias contributed 17 m at 50 hPa and 71 m at 10 hPa to the difference
in geopotential height estimates from the two sondes. Statistical
comparisons, however, show that the switch from VIZ to Vaisala RS-80 at US
stations introduced a shift of as much as 120 m at 50 hPa in the daytime
(Elliot et al., 2002). This may be in part due to pressure errors, which appear to have a
much larger effect than temperature errors (e.g. Morris et al., 2012; Stauffer et al., 2014): comparisons
with radar measurements of height showed the VIZ high relative to the radar
(and the Vaisala) in daytime by ∼ 150 m at 20 hPa and up to 500 m at
10 hPa (Schmidlin, 1988; Nash and Schmidlin, 1987), while at night both VIZ and Vaisala RS80 calculated
geopotentials were low by ∼ 100 m at 20 hPa and ∼ 150 m at 10 hPa. The daytime differences correspond to ozone differences of
∼ 2 and ∼ 7 % at 20 and 10 hPa
respectively. The effect of pressure errors is most significant at higher
altitudes: a 1hPa offset will introduce a geopotential height error of 63 m
at 100 hPa, 120 m at 50 hPa, and over 300 m at 20 hPa; these correspond to ozone
differences of 0.25, 0.5, and ∼ 4 % respectively.
Pressure errors also seem more variable: local noon flights during
the same intercomparison show much smaller height differences between the
VIZ and Vaisala.
The Vaisala RS-92 has replaced the RS-80 and has been in use in Canada
since 2006. Comparison flights with GPS tracking show that it gives more
accurate heights than the RS80; differences from the GPS are small
(Steinbrecht et al., 2008; Nash et al., 2006). RS80 sondes, however, were found to be low by
∼ 20 m in the troposphere and high by 100 m at 10 hPa
(Steinbrecht et al., 2008; also da Silveira et al., 2006).
Unfortunately, intercomparison experiments do not tell the whole story, as
not all manufacturing changes are advertised by a change in model number.
For example, Steinbrecht et al. (2008) note systematic differences between batches of RS-92
sondes produced before July 2004. Overall, the expected systematic
differences in the ozone profile resulting from radiosonde errors are
probably small below 50 hPa. We do not attempt to correct for radiosonde
errors but do include possible pressure offsets as an error source in the
uncertainty estimation (Sect. 4). Estimated radiosonde errors are largest
for the older VIZ sonde, with the manufacturer quoting a 1 σ
uncertainty in the pressure measurement of 1 hPa.
Effects of the corrections
An analysis of the effects of these corrections is shown in Figs. 1–4 for
the station at Edmonton (Stony Plain). The average change to the ozone
profile has been calculated for the corrections described above, both
individually and collectively. Figure 1 shows the changes for the 1970s when
only Brewer–Mast sondes were flown at Edmonton. The largest change is in the
lowermost troposphere, where the response correction raises ozone values by
about 15 %, although the changes to the normalization make a significant
difference as well. Note that in each case the profile, after one or more
corrections are applied, is normalized, so that the corrected curves all
include the effects of renormalization. This also has the effect of
redistributing the correction over the profile. For example, the “Mateer”
correction, which is about 10 % at the top of Fig. 1, but 0 below 150 hPa, because of this redistribution introduces an additional negative change
of about 1 % in the tropospheric part of the corrected profile, over that
introduced by the renormalization alone.
Average ozone profile at Edmonton before (“None”) and after
corrections to the Brewer–Mast record. Dashed lines show differences from
“None” in percent. “Renormalized” shows the average change introduced by the
use of the McPeters and Labow (2012) climatology (see Sect. 2.1); “Tarasick et al.” shows that
from the response correction (Sect. 2.2); “Mateer” shows the change caused by
the switch to WMO-recommended pump corrections (Sect. 2.3). Note that in each
case the profile is normalized, so that the curves Tarasick et al. and Mateer both
include the effects of renormalization. The largest change is in the
lowermost troposphere, where the response correction (Tarasick et al.)
raises ozone values by about 15 %.
As Fig. 1 but for the first decade of ECC soundings. Dashed
lines show differences from “None” in percent. “Rod thermistor” shows the
average change introduced by the correction to the temperature measured by
the rod thermistor (Sect. 2.6), and “2.5cc solution” that for the
solution volume correction (Sect. 2.4). Note that each curve also includes
the effects of renormalization. The changes to the ECC record in the 1980s
are comparatively minor.
As Fig. 1 but for the 1990s. “Rod/Box/Pump T” shows the
average change introduced by the corrections to the temperature measured by
the different thermistors used during this decade (Sect. 2.6).
As Fig. 1 but for the 2000s. “Deshler” shows the average
change introduced by the correction for the use of standard 1 % KI
solution in ENSCI sondes (Sect. 2.5). Overall changes to the record are
minor.
In Fig. 2, the changes to the ECC record in the 1980s are comparatively
minor, although again the largest change is in the lowermost troposphere,
where the solution volume correction raises ozone values by as much as
4 %. The new normalization also increases ozone values through the entire
profile by ∼ 1 %. In the 1990s (Fig. 3) the shifts are
larger: up to 2–3 % throughout the stratosphere. Most of this appears to
be due to the change of temperature measurement, from the rod thermistor at
the base of the electronics unit, to the “box” temperature, and in a few
cases in 1999, pump temperature measurements. In the 2000s (Fig. 4) the
“Deshler” correction for the change to ENSCI sondes seems to almost
cancel that for the change of temperature measurement, so that the overall
correction is close to 0, except at the top of the profile and in the
lower troposphere.
With the exception of the Brewer–Mast data in the troposphere, the overall
effect of the corrections is generally modest. They can be summarized as
tropospheric changes: increases of up to 5 % after 1979 and up to 20 %
before 1980 (Brewer–Mast sondes), declining with altitude;
stratospheric changes: decreases of up to 4 % before 1980 at 25 km, with
smaller decreases above and below, increases of ∼ 1 % in the
1980s and ∼ 2–3 % in the 1990s, and little change in the 2000s.
An examination of the revised record shows that the removal of these
artifacts from it has indeed reduced uncertainty, as measured by the changes
in the comparison to the total ozone record. Table 3 describes these
differences. The normalization factors are closer to 1, and their variance
is reduced, for both Brewer–Mast and ECC sondes. A trend in the
normalization factors for the Brewer–Mast sondes is reduced, and that for
ECC sondes (cf. Tarasick et al., 2005) is effectively removed (no longer statistically
significant).
Cumulative effects of corrections to ozonesonde data for the
record at Edmonton (Stony Plain), as indicated by changes in the comparison
of the integrated profile to a coincident spectrophotometric total ozone
measurement.
Mean ratio (normalization factor)Standard deviationTrend in normalization factorsBM data (up to 1979) Original1.270.3035.5 % decade-1Renormalized1.200.284All corrections1.030.2574.5 % decade-1ECC data (1980–2013) Original0.970.110-2.6 ± 0.7 % decade-1All corrections0.990.0990.7 ± 0.6 % decade-1Uncertainty analysis
An important goal of the Ozonesonde Data Quality Assessment (O3S-DQA) is to
produce an uncertainty analysis for ozonesonde data. There have been only a
few published efforts to quantify the uncertainty in ozonesonde profile
measurements, either from an analysis of error sources (Komhyr et al., 1995) or
empirically from field or laboratory intercomparisons (Smit et al., 2007; Kerr et al., 1994;
Deshler et al., 2008a; Barnes et al., 1985; Smit and ASOPOS panel, 2011) or via statistical data analysis (Liu et al., 2009). Here we
attempt a “bottom-up” approach similar to that of Komhyr et al. (1995).
Sources of ozonesonde profile error considered in this analysis and
their estimated magnitudes. See text for details.
Table 4 lists the error sources considered in this analysis. The first five
lines refer to errors that are assumed constant throughout the profile.
Stoichiometry
Although the stoichiometry of the neutral buffered-KI method for measuring
ozone was the subject of some controversy in the 1970s (e.g. Boyd et al., 1970; Pitts et al.,
1976),
most workers have found a stoichiometry of 1.0 within experimental error
(Hodgeson et al., 1971; Kopczynski and Bufalini, 1971; Dietz et al., 1973), especially when potassium bromide is added (Lanting, 1979; Bergshoeff et al.,
1980), as is the case in ozonesondes, and provided that slow side reactions
with the phosphate buffer are excluded (Saltzman and Gilbert, 1959; Flamm, 1977; Johnson et al., 2002). We have
allowed a modest (1 %) uncertainty for the reaction stoichiometry in both
types of ozonesonde.
Temperature measurement
The Brewer–Mast sonde did not have a measurement of the instrument
temperature, and so the processing assumes a constant temperature of 300 K.
Measurements of the actual temperature made by Dütsch (1966) and Steinbrecht et al. (1998) suggest
that it varies over a range of 10–20 K (3–6 %) over a flight, with a
standard deviation of 1–3 %. We have represented this as a 3 %
uncertainty. For the ECC sondes, the box temperature measurement in the 3A
and 4A models was less accurate than the pump measurement used with later
models; we have assumed a standard error of 0.5 K for the latter and 1.0 K for
the former.
Pump flow measurement
An examination of pre-flight volumetric pump flow measurement data from
several sites shows that standard deviations of 0.1–0.3 % in this
measurement (performed the day before launch) are typical. However,
differences between this measurement and the corresponding flow rate
determination made at the manufacturer's facility are larger, with standard
deviations of about 1 %. Torres (1981) found a 1 σ variation in the speed
of individual model 3A pump motors of 0.5 %. We have assumed a calibration
uncertainty of 0.5 % for all types of sonde.
Relative humidity error
For ECC sondes an additional error source is present, as during the pump
flow measurement the pump draws relatively dry air from the room and expels
water-saturated air into the graduated cylinder. The measured volume is
larger than the actual volume pumped by an amount proportional to the ratio
of the saturation vapour pressure to the room pressure times the relative
humidity change. Assuming a typical indoor humidity range of 40–70 %
(1 σ) gives an uncertainty of ±0.5 %.
Correction for use of standard 1 % buffered-KI solution in ENSCI sondes
A bias correction of about 4 % below 50 hPa and somewhat larger above has
been made to ENSCI sondes flown with 1 % KI solution (Deshler et al., 2008b). We have
allowed an additional uncertainty of ±0.5 %, representing the
standard error of the Deshler et al. (2008b) measurements, where this correction was made.
The latter seven lines in Table 4 refer to errors that vary throughout the
profile, either with pressure or ozone gradient. Errors are calculated for
each point in the profile:
Pump correction error
Pump corrections, and their associated uncertainties, have been measured by
a small number of authors. For Brewer–Mast sondes we have used the estimates
of Komhyr and Harris (1965), and for ECC 3A sondes those of Torres (1981). For ECC 4A and later
models (which have similar pumps), Johnson et al. (2002) provide a table summarizing the
results of very large number of pump tests, primarily at the University of
Wyoming and at the NOAA/CMDL laboratories. Both of these give much larger
uncertainties than those quoted by Komhyr (1986), for a small number of tests. We
have averaged these larger uncertainty values from the Wyoming and NOAA/CMDL
tests. Torres (1981) also notes that his uncertainty estimates are based on a
modest number of sondes from the same manufacturing batch and so may also
be biased low. For each sonde type we have interpolated the measured
uncertainties to other pressures to estimate this error for all points in
each profile.
Solution volume correction
As the ozone loss in sensors charged with only 2.5 mL of KI solution appears
quite variable, a fairly large error of 4 % at 1000 hPa, proportional to
pressure, was assumed.
Background current
As noted above, Canadian practice has been to treat background current as
proportional to pressure, but it is now recommended (Smit and ASOPOS panel, 2011) to treat it as
constant. Here we have treated the difference between the two values as an
uncertainty, although it should be noted that although randomly variable in
magnitude, it is always a positive bias. It is largest in relative terms
just below the tropopause, where absolute amounts of ozone tend to be
lowest. The average magnitude of the difference is largest in the 1980s
and has a modest effect on calculated trends in the upper troposphere
(Tarasick et al., 2005).
Brewer–Mast response correction
The quadratic fit to the data shown in Fig. 7 of Tarasick et al. (2002) has a standard
deviation of ∼ 7 %. We have added this uncertainty, scaled
to the absolute magnitude of the correction, which is largest at 1000 hPa.
The correction is largest (i.e. 7 %) at 1000 hPa and declines
quadratically with log(pressure).
Iodine loss
Brewer–Mast sondes show increasing errors at higher altitudes relative to
ECC sondes (Kerr et al., 1994; Fioletov et al., 2006).
One possibility for this is solution evaporation
and/or iodine loss from the sensing solution. The Brewer–Mast sensor has a
somewhat more open construction that may allow more solution evaporation.
Brewer–Mast sondes also use a much weaker (0.1 %) KI solution, which may
allow significant iodine evaporation (Brewer and Milford, 1960; Tarasick et al., 2002). We have included an
empirical estimate for this uncertainty of 0.6/p, where p is pressure in hPa.
Ascent rate variation
The relatively slow response of ECC sondes causes their response to lag
changes in the ozone concentration as the balloon rises. This implies that
different balloon rise rates will give somewhat differing ozone amounts,
especially in parts of the profile with large ozone gradients. We assumed an
e-1 response time of τ=20 s (Smit and Kley, 1998), so this difference is
proportional to e-Δt/τ∇tO3, where
Δt is the time interval between successive measurements, and ∇tO3 is, in essence, the vertical gradient of ozone
but calculated as the difference in ozone between successive measurements as
the balloon rises. The standard deviation of balloon rise rate at Edmonton
in the 2000s is ∼ 12 %, which yields modest errors
(< 1 %) at the sharp ozone gradients near the tropopause and
mostly insignificant errors elsewhere.
Pressure offset
The error in ozone implied by a pressure offset equal to the manufacturer's
estimated 1 σ uncertainty is calculated for every point in the
profile by multiplying by the measured ozone gradient with respect to
pressure. We have used the values quoted by Richner and Phillips (1981) for the VIZ sonde and
Steinbrecht et al. (2008) for the Vaisala sondes.
The uncertainty profile is calculated for each flight, using the pressure
and ozone partial pressure data for that flight. Figure 5 shows the average
uncertainty profile for the Brewer–Mast flights at Edmonton, along with the
standard deviation of the response of ECC sondes during the Vanscoy and
JOSIE 1996 ozonesonde intercomparison campaigns (Kerr et al., 1994; Smit et al., 2007) and the
standard deviation of the response of Brewer–Mast sondes during the Vanscoy
campaign (Kerr et al., 1994). Several of the individual contributions to the overall
uncertainty are shown. The total uncertainty without the contribution from
radiosonde pressure offsets is also shown, to facilitate comparison with the
JOSIE 1996 and Vanscoy intercomparison uncertainty estimates, which were
referenced to a common pressure measurement. It will be noted that the
uncertainty in the VIZ radiosonde pressure measurement dominates the
calculated uncertainty above about 32 km.
Average estimated uncertainty of Brewer–Mast soundings at
Edmonton, showing contributions from selected sources. The total uncertainty
without the contribution from radiosonde pressure offsets is also shown, to
facilitate comparison with the JOSIE and Vanscoy intercomparison uncertainty
estimates, which were referenced to a common pressure measurement. The
uncertainty in the VIZ radiosonde pressure measurement dominates the
calculated uncertainty above about 32 km.
Average estimated uncertainty of ECC (3A and 4A) soundings in the
1980s at Edmonton, showing contributions from selected sources. The total
uncertainty without the contribution from radiosonde pressure offsets is
also shown, to facilitate comparison with the JOSIE and Vanscoy
intercomparison uncertainty estimates, which were referenced to a common
pressure measurement. As the overall uncertainty is smaller, the uncertainty
in the VIZ radiosonde pressure measurement now dominates the calculated
uncertainty above about 26 km.
Figure 6 shows similar calculations for the first decade of ECC soundings
(3A and 4A models). The VIZ radiosonde was used throughout. As the other
sources of uncertainty are smaller, the uncertainty in the VIZ radiosonde
pressure measurement now dominates the calculated uncertainty above about 26 km. Figures 7 and 8 show similar calculations for the 1990s and 2000s
respectively. Notable improvements are reductions in background current and
the reduction of pressure offsets with the introduction of the Vaisala
radiosondes.
Average estimated uncertainty of ECC (4A and 5A) soundings in the
1990s at Edmonton, showing contributions from selected sources. The
uncertainty in the VIZ or (from 1994) RS-80 radiosonde pressure measurement
dominates the calculated uncertainty above about 28 km.
Average estimated uncertainty of ECC (5A and ENSCI) soundings in
the 2000s at Edmonton, showing contributions from selected sources. The
uncertainty in the RS-80 or (from 2006) RS-92 radiosonde pressure
measurement now dominates the calculated uncertainty only above about 31 km.
Percent deviations in average ozone mixing ratio for the surface
and three tropospheric layers, for three midlatitude stations. Monthly
anomalies have been smoothed with a 4-month running average. The overall
station trend lines (up to 45 years in the case of Goose Bay) are shown. The
troposphere and stratosphere have been explicitly separated; that is,
integration for the 400–250 hPa layer is from 400 to 250 hPa or the
tropopause, whichever comes first.
As Fig. 9, for the three Arctic stations. The overall station
trend lines (up to 48 years in the case of Resolute) are shown.
Percent deviations in average ozone mixing ratio for four lower
stratospheric layers, using data from three midlatitude stations. Monthly
anomalies have been smoothed with a 4-month running average. The overall
station trend lines are shown. The troposphere and stratosphere have been
explicitly separated; that is, integration of the 250–158 hPa layer starts
either at 250 hPa or at the tropopause, when the latter is found above 250 hPa.
As Fig. 11, for the three Arctic stations. The overall station
trend lines are shown.
Time series and trend analysis
For this analysis each ozone profile was represented by a surface-level
measurement (the ozone measurement at sonde release) and 11 layers equally
spaced in log pressure (each ∼ 3 km in thickness). Troposphere
and stratosphere have been explicitly separated: that is, integration for
the 400–250 hPa layer is from 400 to 250 hPa or the tropopause,
whichever comes first. Similarly, integration of the 250–158 hPa layer
starts either at 250 hPa or at the tropopause, if the latter is found above
250 hPa. (Cases where the tropopause is below the 400 hPa height or above
158 hPa occur rarely but are dealt with similarly.) The WMO definition of
the tropopause (WMO, 1992) is employed.
Partial ozone columns were integrated within these 11 layers and divided by
the pressure difference across each layer to find average ozone mixing
ratios. These and the ground-level mixing ratio values were deseasonalized
by subtracting the average annual cycle as described in Tarasick et al. (1995). The
deseasonalized time series were also adjusted for the effects of diurnal
variation in ozone concentration. Sondes are generally launched at either
12:00 or 00:00 GMT, which are early morning and mid-afternoon in Kelowna and Edmonton,
and later at other stations. The amount of diurnal shift (a scalar value for
each station at each level) was calculated as the average difference between
values for the two launch times, where both were available in the same year
and month. The difference presumably results from the competing effects of
photochemical production and NO titration, which vary with time of day, but
the sparse (weekly) data make it difficult to draw firm conclusions. The
effect is significant primarily at Edmonton, where it can be as large as
42 % at ground level and 14 % below 700 hPa (Tarasick et al., 2005). However, for
consistency all stations were adjusted at all levels.
Percent deviations in average ozone mixing
ratio for four middle stratospheric layers, using data from three
midlatitude stations. Monthly anomalies have been smoothed with a 4-month
running average. The overall station trend lines are shown.
As Fig. 13, for the three Arctic stations. The overall station
trend lines are shown.
Linear trends in ozone mixing ratio for the overall (48-year)
period at the six Canadian sites with long-term ozonesonde records, for the
surface and 11 layers equally spaced in log pressure (∼ 3 km).
Error bars show 95 % (2 σ) confidence limits. The troposphere and
stratosphere have been explicitly separated; that is, integration of the
250–158 hPa layer starts either at 250 hPa or at the tropopause, when the
latter is found above 250 hPa. Similarly, integration of the 250–158 hPa
layer starts either at 250 hPa or at the tropopause, when the latter is found
above 250 hPa. Trends using only ECC data (from 1980) are shown in red.
Trends from 1980 using ECC data before corrections are applied are shown in
green.
Figures 9 through 14 show time series of percent deviations from the
long-term mean in monthly average ozone mixing ratio for three northern
midlatitude stations (Edmonton, Goose Bay, and Churchill) and for the three
Arctic stations (Resolute, Alert, and Eureka). For ease of visualization, a
4-month running average has been applied to smooth the data.
Figures 9 and 10 show the surface and the three tropospheric layers. The
most notable feature in both cases is that there appears to be no long-term
trend in the troposphere, over the 45-year (midlatitude) or 48-year (Arctic)
record, except at the surface and possibly in the upper troposphere of the
Arctic. In the latter cases these trends are negative. The surface trend at
the northern midlatitude sites may be primarily due to urban development
near Edmonton (Tarasick et al., 2005), although Churchill shows a strong decline at the
surface in recent decades, for unknown reasons. The surface trend at the
Arctic sites may be related to an increase in the frequency of
halogen-induced surface ozone depletions, which appear to correlate with
negative anomalies in the surface ozone record shown in Fig. 10 (Oltmans et al., 2012).
The frequency of such events at Resolute has increased by nearly 32 % over
the 1966–2013 period (Tarasick et al., 2014).
The decadal trends (not shown) are much more variable. In general, however,
trends are negative in the 1980s, positive in the 1990s, and small after
2000.
As Fig. 15 but for 1966–1996. Trends using only ECC data (from
1980) are shown in red.
As Fig. 15 but for 1997–2013.
Figures 11 and 12 show the four lower stratospheric layers. Here the
long-term trends are all negative (with the exception of Eureka, whose
record began in 1993). Notable features are the low values in the early
1990s and the high values in the early 2000s, the latter possibly caused by
small changes in the Brewer–Dobson circulation (Bönisch et al., 2011). These high values
cause the lower stratospheric trends for 2000–2013 (which might otherwise be
expected to show signs of recovery from stratospheric ozone depletion with
declining effective chlorine levels over this period) to be negative, both
at midlatitudes and in the Arctic. In the Arctic, particularly above 100 hPa, the springtime negative anomalies in cold vortex years (1996, 1997,
2000, 2005, and 2011) are evident. At these levels the 2011 anomaly (e.g.
Manney et al., 2011) is larger than the 1993 anomaly related to the eruption of Mt.
Pinatubo.
The four middle stratospheric layers (Figs. 13 and 14) show less
variability, and the decadal trends more closely follow the long-term trends
at each level. These long-term linear trends are shown in Figs. 15–17.
Figure 15 shows calculated trends in ozone mixing ratio from ozonesonde data
at six Canadian stations from 1966 to 2013 (for Alert and Eureka from 1987 and
1992 respectively), for the ground level and the 11 layers equally spaced in
log pressure. To calculate these trends the deseasonalized station time
series were averaged by month, and a simple linear regression (without
subtraction of QBO, solar-cycle, or other known influences on ozone) was used
to derive trends. Trends are expressed as percent per decade, relative to
the layer mean. The time series of monthly means show in general significant
autocorrelation both in the stratosphere and the troposphere. Allowance is
made for this in the confidence limits for trends by basing the confidence
limit calculation on a (reduced) effective sample size, neff=n(1-ρ)/(1+ρ), where ρ is the lag-1
autocorrelation coefficient, and the ozone variability is assumed to be an
AR(1) process (Zwiers and von Storch, 1995; Thiébaux and Zwiers, 1984).
Except at the surface, trends in the troposphere are in general
non-significant over this very significant period. Trends in the middle
stratosphere are also non-significant at the 95 % (2 σ) level,
while those in the lower stratosphere are significant and negative. Trends
in the lower stratosphere, however, are as large -5 % per decade over the
48-year record. To gauge the uncertainty introduced by the addition of the
older Brewer–Mast data, we have also calculated trends using only ECC data
(that is, from 1980). The differences are surprisingly modest. We also show
trends from 1980 calculated using ECC data before corrections are applied.
The largest differences are seen at Alert and Eureka. The increases of
2–5 % to the 1990s data (Fig. 3) have a larger effect on trends at these
sites as they lack data from the early 1980s.
For comparison with other analyses in the SI2N initiative (e.g. Harris et al., 2015)
and the WMO Scientific Assessment of Ozone Depletion: 2014 (WMO, 2014), in Figs. 16 and 17 we show trends calculated using
only data prior to 1997 (Fig. 16) and from 1997 to 2013 (Fig. 17). The
trends for 1966–1996 show a similar picture to that of Fig. 15, although
here some of the middle stratospheric layers show positive trends. When the
trends are calculated using only data after 1979 (that is, ECC-only data)
the trend picture is similar. However, trends in the 17-year period
from 1997 to 2013 are almost all non-significant at the 95 % (2 σ)
level, except at the surface, which shows some surprisingly large
variations. This is true even in the Arctic lower stratosphere, despite the
large negative anomaly in 1997 (Fig. 14). Since stratospheric halogen
loading has been decreasing during this period (WMO, 2014), the lack of evident
ozone increases may be due to atmospheric variability (Kiesewetter et al., 2010; Chehade et al., 2014), in
particular the high values in the early 2000s, possibly caused by changes in
the Brewer–Dobson circulation (Bönisch et al., 2011). However, the standard deviations of
the monthly ozone anomalies in the stratosphere at the four long-term
stations for the 17 years prior to 1997 average 8–40 % greater than those
for the 17-year period 1997–2013, which suggests that the stratosphere has
in fact been less variable in the latter period.
Conclusion
As part of the SPARC/IO3C/IGACO-O3/NDACC (SI2N) initiative,
Canada's important record of ozone sounding data has been re-evaluated,
taking into account the estimated effects of changes in the type and design
of ozonesondes used in Canada over the last 5 decades.
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 altitude-dependent estimated
uncertainty is included with each revised profile.
The resulting time series show negative trends in the lower stratosphere of
up to 5 % per decade 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.
Acknowledgements
The authors thank the many observers who, over many years, obtained the
ozonesonde measurements used in this study. Their careful work is gratefully
acknowledged. The ozone sounding data were obtained from the World Ozone and
Ultraviolet Radiation Data Centre (WOUDC, http://www.woudc.org) operated by
Environment Canada, Toronto, Ontario, Canada, under the auspices of the
World Meteorological Organization.
Edited by: C. von Savigny
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