Laboratory investigations of the response of Brewer-Mast

ozonesondes to tropospheric ozone

David W. Tarasick, Jonathan Davies, Kurt Anlauf, and Maurice WattEnvironment Canada, Downsview, Ontario, Canada

Wolfgang Steinbrecht and Hans J. ClaudeMeteorological Observatory Hohenpeissenberg, German Weather Service, Hohenpeissenberg, Germany

Received 30 July 2001; revised 24 November 2001; accepted 21 December 2001; published 28 August 2002.

[1] The Brewer-Mast ozonesonde was used at Canadian stations from 1966 until 1980,when the Canadian network switched to the electrochemical concentration cell sonde.While the sondes appear to agree relatively well in the stratosphere, there is an evidentdiscrepancy of 10–20% in tropospheric measurements [e.g., Tarasick et al., 1995, Figure4]. Comparison of Brewer-Mast sondes with a calibrated ozone source yields someinteresting insight into this discrepancy. Sonde response is strongly dependent on thepreflight preparation procedures employed. Although sondes prepared via proceduresintroduced in the 1980s [Claude et al., 1987] perform quite well, when prepared accordingto the procedures used in Canada in the 1970s, Brewer-Mast sondes indicate 10–30%lower ozone than the calibrator. The following points are noted in particular: (1) a newBrewer-Mast sonde shows a large (�15%) increase in sensitivity between successiveexperiments; (2) especially at low (<100 ppb) O3 levels, the response even of previouslyflown sondes increases slowly with time; and (3) sondes show an additional slow increaseof response with time that is apparently caused by ozone reactions with the phosphatebuffer. The overall response curve indicated by 1, 2, and 3 implies that after correction tothe observed total ozone, the earlier part of a flight would yield values that are too low,while the latter part would be too high. By applying a varying ozone input, simulating thetypical variation in absolute ozone concentration experienced by a sonde in flight, weshow that this can explain both the average correction factor (1.255) for the CanadianBrewer-Mast record and the 10–20% discrepancy in tropospheric measurements.INDEX TERMS: 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry;

0394 Atmospheric Composition and Structure: Instruments and techniques; 0340 Atmospheric

Composition and Structure: Middle atmosphere—composition and chemistry; 1610 Global Change:

Atmosphere (0315, 0325); KEYWORDS: ozone, ozonesonde, Brewer-Mast, troposphere, calibration,

response

1. Introduction

[2] The longest continuous record (since January 1966)of vertical profile measurements of ozone is for ResoluteBay, Canada (74.7�N, 95.0�W). However, the Brewer-Mastozonesonde was used at Canadian stations until 1980, whenthe Canadian network switched to the electrochemicalconcentration cell (ECC) sonde. Three other Canadianstations have Brewer-Mast records of significant length aswell: Edmonton (53.6�N, 114.1�W), Goose Bay (53.3�N,60.3�W), and Churchill (58.8�N, 94.1�W). While thesondes appear to agree relatively well in the stratosphere,attempts to combine the two data sets into a homogeneousrecord are hampered by a discrepancy of 10–20% in thetropospheric values measured by the two types. This appa-rent difference in tropospheric response between the ECC

and Brewer-Mast sonde results in discontinuities in themean values for these four stations at 1980, which can givea positive trend for the combined record even when bothdata sets individually have negative trends [e.g., Tarasicket al., 1995, Figure 4]. (Tarasick et al.’s Figure 4 isreproduced here as the top panel of Figure 8; see also Wanget al. [1993] and London and Liu [1992].) If the newBrewer-Mast pump correction of Steinbrecht et al. [1998]is applied to these data, then the tropospheric valuesdecrease by a further 5–7%, increasing this discrepancy.[3] This difference in tropospheric response is consistent

with the majority of ozonesonde intercomparisons [Att-mannspacher and Dutsch, 1970, 1981; Hilsenrath et al.,1986; Beekman et al., 1994, 1995; Smit et al., 1996] but notwith the Vanscoy 1991 intercomparison [Kerr et al., 1994],nor with that of DeBacker et al. [1998]. In general, however,the Brewer-Mast sondes used in these intercomparisonswere prepared differently from those used in the Canadianprogram.

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. D16, 4308, 10.1029/2001JD001167, 2002

Copyright 2002 by the American Geophysical Union.0148-0227/02/2001JD001167$09.00

ACH 14 - 1

[4] The Brewer-Mast sonde [Brewer and Milford, 1960]is a galvanic cell consisting of a platinum cathode and asilver anode in a buffered 0.1% KI solution. It requires anexternal applied potential of 0.41 V to balance its internalemf. The operating principle is the well-known reaction ofpotassium iodide with ozone:

2KIþ O3 þ H2O ! 2KOHþ I2 þ O2 ð1Þ

followed by

I2 þ 2e� ! 2I�: ð2Þ

Thus for each molecule of ozone, two electrons areproduced and an equivalent amount of current flowsthrough the external circuit. The measurement is therefore,in principle, absolute; however, there may be losses ofozone in the pump and of ozone or iodine to the walls of thesensor chamber (the chamber is constructed of acrylicplastic, and the pump piston is steel and, more importantly,is oiled). In addition, there appear to be slow side reactionswith the phosphate buffer that can increase the stoichio-metry of reaction (1) [Saltzman and Gilbert, 1959; Flamm,1977].[5] We have previously reported work comparing the

response of both new and older models of Brewer-Mast andECC sondes [Tarasick et al., 1996], which found noevidence of a change in response of either sonde. Noevidence was found to suggest that ECC sondes, of eitherolder or newer designs, might have overestimated tropo-spheric ozone, but Brewer-Mast sondes did appear to haveunderestimated it. An important issue is the preflightpreparation of the sonde. Brewer-Mast sondes flown inCanada were prepared according to procedures describedby Mueller [1976] (which are still distributed by themanufacturer). Currently operational Brewer-Mast stationsemploy some version of the procedures developed byClaude et al. [1987], which are generally acknowledgedto be superior but were not published until after theCanadian program had switched to ECC sondes. Thesignificant difference is that Claude et al. recommenddisassembly and a complex cleaning of the sonde beforeuse. In addition, more recent practice at both Payerne andHohenpeissenberg is to charge the sonde with sensingsolution at least 24 hours before use.[6] Unfortunately, Canada did not participate in either of

the two World Meteorological Organization internationalozonesonde intercomparisons that took place in the 1970–1980 period [Attmannspacher and Dutsch, 1970, 1981].However, the original documentation of the preparationprocedures for Brewer-Mast sondes exists (and is essen-tially Mueller [1976]); so it is possible to examine in thelaboratory the response to ozone of sondes prepared in thisway.

2. Methodology

[7] Experiments were conducted at laboratory temper-ature and pressure, using either a National Institute ofStandards and Technology (NIST)-traceable TEI ozonecalibrator (model 49C-PS) or a NIST-traceable Dasibiozone calibrator (model 1008-PS). The expected precision

of either instrument was 1 ppb. The air output from theozone calibrator was allowed to flow through Teflon tubingwhich vented to the laboratory, while the sondes sampledair from the tube. Tubing lengths were kept short (2 m) inorder to avoid ozone loss. Except as otherwise specified,sondes were prepared according to standard proceduresused in Canada [Mueller, 1976]: that is to say, the sondeswere not disassembled and cleaned, nor was the pump re-oiled; after removal from the plastic shipping bag, the pumprate was measured, then the sonde was ozonized byexposure to a high-ozone source, and then, followingseveral minutes of exposure to ozone-free air, filled withsensing solution.[8] The electronics supplied with the sonde were not

used. Instead, the required 0.41-V bias was supplied by abattery and resistive voltage divider; a variable resistorallowed this to be adjusted. Bias voltage was adjustedbefore each experimental run and checked occasionallyduring the run. A simple operational amplifier electro-meter circuit was used to convert current to voltage,which was read by an HP34401A digital multimeter. Boththe ozone calibrator reading and the voltmeter output wererecorded digitally via RS-232 interfaces to a PC. Ozonepartial pressure was then calculated by using the standardformula

PO3¼ 0:004307iTBt; ð3Þ

where i is the measured current in microamperes, TB(normally the sonde box temperature) is the laboratorytemperature in kelvins, and t is the time in seconds to pump100 mL of air, measured via the standard soap bubbleapparatus. Partial pressures were converted to mixing ratioby dividing by the measured atmospheric pressure.[9] As the ozone calibrator output is in ppb, it was only

necessary to correct for the zero offset (measured before eachrun) and for the measured calibration correction to the NISTtransfer standard. Although in our experiments the sondesproduced a measurable background current in the greatmajority of cases, this does not appear to have been the casein early experiments [Brewer and Milford, 1960; Griggs,1961], and standard practice is not to subtract it. This is ofsome significance to the application of our results, and isdiscussed below. In all of the results that follow, backgroundcurrent was subtracted except where it was negligible (lessthan 1 ppb).

3. Sonde Response

[10] Figure 1 shows the results of a typical experiment, inwhich a new Brewer-Mast sonde, prepared via Mueller[1976], was exposed to ozone repeatedly, at intervals ofseveral hours. It illustrates several features of the response ofBrewer-Mast sondes. First, and most evident, is that thesondes always indicate less ozone than the calibrator, typi-cally by about 15–20%. Although response improved withrepeated calibrations, as described below, it never exceeded90% in the tests described here, even for sondes prepared viaClaude et al. [1987]. This less-than-integral response is wellknown and is the reason that ozone soundings with Brewer-Mast sondes are corrected to agree (after integration) withthe total ozone measured by a Dobson or Brewer spectro-

ACH 14 - 2 TARASICK ET AL.: RESPONSE OF BREWER-MAST OZONESONDES

photometer. Typical correction factors for well-preparedsondes at Hohenpeissenberg average about 1.1 [Claude etal., 1996] (implying an average response of 90%), while forthe Canadian Brewer-Mast record they average 1.25 (imply-ing an average response of 80%).[11] Perhaps surprisingly, the reason for this does not

appear to be incomplete absorption of ozone from the airstream. It is a simple experiment to vary the depth ofsolution in a sensor cell and observe the ozonesonderesponse: no change, even for depth variations of 50%,was observed, as long as the solution completely coveredthe cathode gauze. However, replacing the sensing solutionwith ECC (1% KI) solution immediately increased theresponse to 100%. This was investigated more thoroughlyby Powell and Simmons [1969], who found an increase ofas much as 30% when the solution strength was variedbetween 0.1% and 1.0% KI. Unfortunately, it is not clearwhether the buffer concentration was also varied (see thediscussion below).[12] These observations indicate that all the ozone in the

air stream is being absorbed into the solution, as otherwisethe stronger KI solutions would also give a less-than-integral response. Since reactions (1) and (2) are independ-ent of KI concentration, the difference (for well-conditionedsondes) is probably due to evaporation of iodine from theweaker KI solution. Consideration of the reaction

I� þ I 2ðaqÞ $ I�3 ; ð4Þ

which has an equilibrium constant of 768 M�1 [Davies andGwynne, 1952], shows that in a 1.0% KI solution, 98% of

the iodine will be in the form of tri-iodide, while in a 0.1%KI solution, only 82% will be in the ionic form. Thepossibility of iodine evaporation was also noted by Brewerand Milford [1960].[13] A second feature that is present in all the response

curves is a slow increase in response with time, even after20 min on a constant ozone input. This also resulted in alarge (5–10 ppb) residual signal after the ozone input wasdecreased to zero. This residual decayed to zero with a timeconstant of about 20 min. However, it disappeared imme-diately if the solution was changed. A similar effect isobserved with ECC sondes and is believed to be caused bythe reaction of ozone either with, or catalyzed by, thephosphate buffer. It appears to be well known in theliterature about the reaction of ozone with neutral bufferedKI [Saltzman and Gilbert, 1959; Flamm, 1977] and mayhave been responsible for at least some of the controversy inthe 1970s about the stoichiometry of the neutral buffered KImethod [see Pitts et al., 1976; Bergshoeff et al., 1980, andreferences therein]. In the experiments illustrated in Figures1–3, the second reaction was a small effect, but clearlyevident, accounting for typically 4–6% of the totalresponse.[14] More evidently from Figure 1, there is a large

increase in response (�10%) between the first and secondtrials and much less between the second and third. Innumerous calibrations of new Brewer-Mast sondes preparedaccording to Mueller [1976], we found this effect to betypical: it was observed with each sonde tested and varied inmagnitude from about 5% to more than 20%.

Figure 2. Relative response of Brewer-Mast sondeP14799 for a constant ozone input. Two experiments afew hours apart. This was a new sonde, but it was filled withsensing solution nine days before the experiment. Thesolution was changed before each run. The ozone source isconstant, at about 75 ppb.

Figure 1. Relative response of a new Brewer-Mast sondeprepared according to Mueller [1976], for a constant ozoneinput, during three experiments over two days. Sensingsolution was changed before each repetition. The ozonesource is constant, at about 75 ppb.

TARASICK ET AL.: RESPONSE OF BREWER-MAST OZONESONDES ACH 14 - 3

[15] In contrast, it was found that older, previously flownBrewer-Mast sondes (taken from a stock of recoveredsondes, flown in the late 1970s) show much smaller ornegligible changes in response between successive sensi-tivity trials. The salient difference between the new sondesand the used sondes is evidently previous exposure tosensing solution and ozone. Figure 2 shows the responseto ozone of a new Brewer-Mast sonde which, after con-ditioning with high ozone in the normal manner, wasexposed to solution for 9 days before its first use in asensitivity experiment. The results in this case are similar tothose for the previously flown sondes, with only a smallincrease in response during the second test.[16] Similarly, no large increase in response between

successive sensitivity trials was seen in new sondes pre-pared via Claude et al. [1987] (where the sondes werecharged with sensing solution at least 24 hours before use,as in current practice at Hohenpeissenberg and Payerne).An exception to this was noted in the case of a sonde thatwas subjected to partial preparation via Claude et al.[1987], that is, with the cleaning of the silver anode viaKCN but without the heating of the cathode or the ultra-sound bath.[17] Taken together, the above observations suggest that

the large increase in response between the first and secondtest of Figure 1 is related to (lack of) iodine conditioning.An iodine conditioning effect has been noted for glass[Bergshoeff et al., 1980; Saltzman and Gilbert, 1959], andiodine conditioning effects on several plastics, including the

acrylic plastic of which the Brewer-Mast sensor is con-structed, were studied by Griggs [1961]. However, thesensitivity change is apparently permanent: it was notobserved with previously used sondes that had been instorage for 20 years. This suggests that it is related to iodineconditioning of the Pt cathode, as both iodine and iodide areknown to adsorb strongly on platinum surfaces, and theadsorption is thermodynamically irreversible; i.e., the iodineis not removed by rinsing with solvent. Saturation coverageis as high as one I per Pt in the surface layer [Lane andHubbard, 1975]. Iodine produced by the reaction of ozonewith KI but adsorbed onto the Pt surface would not acceptelectrons from the cathode via (2) and hence would notcontribute to the sensor output current. An estimate of thecathode surface area yields about 3 1016 atoms of Pt inthe surface layer, while the number of atoms of I producedin the sensor by 60 min of exposure to ozone at 70 ppb isabout 4 1016, so that such an effect could reasonablyaccount for a 10% loss in the latter (at least on arithmeticalgrounds). The heating of the cathode in the Claude et al.pretreatment may render it more susceptible to rapid chem-isorption of iodide from the sensing solution, such that theeffect is minimal after 24 hours of exposure to iodide in thesensing solution.[18] In addition to the initial change just discussed, the

response of sondes over several calibrations was observedto vary systematically in other ways. Figure 3 illustrates thisbehavior: while the second run is actually more than 20%higher than the first (both on 12 January), the third, taken 3days later, is about 4% lower at its midpoint. The fourth,again 3 days later, is slightly lower again (although it shouldbe noted that this is at a lower ozone input), while the fifthrun, on the same day, is almost exactly the same as thesecond run. The sensing solution was changed before eachrun, but ozonization of the dry sonde was performed onlybefore the first run. This behavior was observed frequently;sonde response was generally somewhat lower for a cali-bration repeated after an interval of several days, but wasalways slightly higher for a repeat calibration after aninterval of less than 24 hours. It will be noted that, exceptfor the first, the response curves approach each other closelyafter 30 min. Thus the differences are primarily in terms ofresponse time rather than ultimate response. The differencesalso appear to depend on the amount of ozone input: thefourth run, at 20 ppb, is lower than the third, at 40 ppb,although the time interval since the previous run was thesame.[19] The differences of response time in these curves is

not in the rapid first-order response, which has a timeconstant of about 20 s [Brewer and Milford, 1960; De Muerand Malcorps, 1984]. Rather, the differences of about 5%after 10 min imply a time constant of the order of a fewminutes. A frequency response of this timescale was alsoobserved by De Muer and Malcorps [1984] (their w3) andinterpreted as a response in the air-sampling system (i.e., theBrewer-Mast pump). The effect illustrated here, however, isclearly related to the history of the sonde being tested.Although this does not rule out wall effects in the air-sampling system, a more plausible explanation is that it isrelated to conditioning of the sensor walls by iodineadsorption: this would also be consistent with the observa-tions of Griggs [1961] and the fact that the effect appears

Figure 3. Relative response of a new Brewer-Mast sondeprepared according to Mueller [1976], for a constant ozoneinput, during five experiments over one week. While thechange in response is greatest between the first and secondexposure to ozone, the response decreases after a three-dayinterval, and improves with a second run on the same day.

ACH 14 - 4 TARASICK ET AL.: RESPONSE OF BREWER-MAST OZONESONDES

more marked at lower ozone input levels, as well as that theeffect disappeared when the sensing solution was replacedwith ECC (1% KI) solution.

4. Background Current

[20] As was noted earlier, in the majority of our experi-ments the sondes produced a measurable background cur-rent, generally of the order of one to several ppb. This wastrue even of sondes that were prepared via Claude et al.[1987]. However, this appears to have not been the case inearly experiments [Brewer and Milford, 1960; Griggs,1961], and standard practice is not to subtract it. Moreimportantly, examination of a representative sample oforiginal chart recordings for which background currentwas measured incidentally as part of the preflight prepara-tion (Figure 4) shows clearly that background currents forCanadian Brewer-Mast soundings were generally negli-gible.[21] We also found low or negligible background currents

for repeat experiments with sondes, as long as the ozoniza-tion step was not repeated. This suggests that ozonization isthe cause of the higher background current, where there isnot sufficient time for it to decay away. The source of highozone used to condition sondes in the 1970s was an ozoneconditioning cabinet containing several UV lamps. Unfortu-nately, none still exist, but it is possible to estimate theconcentration of ozone produced in it as between 200 and500 ppb, about 100 times less than the type of ozone

generator in current use in Canada, which was used formost of the experiments described here. A similar increasein average background current for Brewer-Mast sondes isalso reported by DeBacker [1999] after the Uccle programswitched from an ozone cabinet to a higher-intensity ozoneconditioning source. Further evidence is found in threeexperiments with sondes for which the ozonization stepwas accidentally omitted from the preparation: all showednegligible background current.[22] In the experiments described in the next section, two

sondes (B14804 and B14805) were ozonized using thehigh-ozone source; three (B14794, B14795, and B14796)were ozonized at 500 ppb, and two (B13235 and B13069)were ionized at 200 ppb. Four showed significant back-ground currents (2.2–2.6 ppb), while the remaining three(B14794, B13235, and B13069) showed none. Subtractionof the background current from B14795, B14796, B14804,and B14805 in the simulated flights described below givesresponse curves similar to those for the other three sondes.If this is not done, then the response curves show much lesscoherence in the troposphere, and the average responsebelow 300 hPa varies between 0 and �20%, with anaverage deficiency of 5–10%, rather than the 10–20% wefind below.

5. Simulated Flights

[23] The several time-dependent effects on sonde responsediscussed above are difficult to separate, as they showsignificant sonde-to-sonde variability and they depend onthe ozone input and the sonde history as well as time.However, in the Canadian sonde program, few sondes arerecovered (most are lost in the Arctic or Atlantic Oceans), somost sondes are new instruments. Their ozone exposure isalso somewhat predictable, being quite low (2–4 mPa) forthe first 40 min or so, in the troposphere, then much higher.

Figure 4. Measured background current for 129 flights ofBrewer-Mast sondes in Canada during the 1970s. Mea-surements were made via an ozone destruction filterconnected to the sonde input. The estimated measurementprecision is 0.1 chart recorder divisions, or approximately0.4 ppbv.

Figure 5. Idealized ozone profiles used in the simulatedflight experiments.

TARASICK ET AL.: RESPONSE OF BREWER-MAST OZONESONDES ACH 14 - 5

This suggests the possibility of measuring an overallresponse curve that will be a function of time, a combinationof the time-dependent effects described above. As theindividual time dependencies each induce an increase ofresponse with time, their combined effect will certainly be toproduce a lower response in the troposphere than later in theflight.[24] Experiments were therefore conducted, at surface

pressure, to investigate the response of Brewer-Mastsondes during simulated ‘‘flights.’’ Sondes were preparedaccording to standard procedures used at Canadian stationsin the 1970s. Specifically, after removal from the plasticshipping bag, the pump rate was measured, then the sondewas ozonized by exposure to a high-ozone source for30 min, and then, following several minutes of exposureto ozone-free air, filled with 2.5 cm3 of sensing solution.During the next hour the sonde was run intermittently, as adecay test was performed and the background current wasmeasured. After 1 hour the ‘‘flight’’ was begun by settingthe ozone calibrator to produce the amount of ozone ap-propriate to the tropospheric part of the profile. The flightwas simulated by changing the ozone input at the appro-priate time (i.e., using a hypothetical ascent rate for thesonde), as indicated in the idealized profiles shown inFigure 5. Five of the sondes in the experiments shown inFigures 6 and 7 were new instruments, while two (B13235and B13069) had been previously exposed to sensingsolution and ozone.[25] Evidently this method does not simulate the pressure

and temperature changes of an actual balloon flight. The

temperature change experienced by the sonde is fairly small[Claude et al., 1987]. Pressure should produce importantdifferences above 100 hPa (where pump efficiency becomesan issue) but may also have unknown effects on thebubbling action and hence on ozone absorption efficiencyand on I2 transport to the cathode (J. Davies et al., paper inpreparation, 2002). This is also suggested by intercompar-ison results [DeBacker et al., 1998], indicating pressure-dependent differences in response between Brewer-Mastand ECC sondes at pressures below 100 hPa.[26] However, to the extent that these effects can be

neglected, the response curves shown in Figure 6 shouldbe representative of those achieved during actual flights inCanada in the 1970s. Two quite different profiles were used,in order to verify that the sonde response was not stronglydependent on the details of the ozone profile. The firstprofile in Figure 5 is an idealization of the ‘‘standardmidlatitude profile’’ used in the JOSIE 1996 intercompar-ison [Smit et al., 1996], while the second is taken from anactual flight at Resolute on 26 February 2000. Differentballoon ascent rates were used as well, resulting in some-what different time intervals for the ozone input steps. Forfour of the sondes, using the standard midlatitude profile,pressure profiles and ascent rates typical of Edmonton wereused (14794, 14796, 14804, and 14805), while for the fifth(14795) the pressure profile and ascent rate for the actualResolute flight was used. Despite these differences, theresults (Figure 7) show considerable similarity.[27] Using these pressure profiles, it was possible to

integrate both the calibrator output and that of the sonde

Figure 6. Response as a function of time for the simulated flight experiments. In each case there is asignificant increase with time. Spikes in the response curves occur when the ozone setting of thecalibrator is changed, and are artifacts of the stepped nature of the ozone ‘‘profile.’’

ACH 14 - 6 TARASICK ET AL.: RESPONSE OF BREWER-MAST OZONESONDES

and so calculate a correction factor for the ‘‘flight’’ (Table 1).This also requires an assumption about the time intervalbetween starting the sonde pump (so that the sonde beginsto respond to the surface ozone concentration) and actualballoon release. This was estimated on the basis of currentpractice, and so the first 10 min, representing the largest

variation in response in Figure 6, has been omitted. Theestimate of 10 min may be excessive, as the interval mayhave been as little as 3 min; in this case, the troposphericresponse would be even lower. Figure 7 shows the result ofapplying the derived correction factors to the responsecurves of Figure 6 (plotted against the assumed pressure

Figure 7. The same data as the previous figure, but multiplied by the correction factor calculated forthis ‘‘flight’’ (Table 1), and displayed as a function of pressure for the hypothetical pressure profile andballoon ascent rate used in each case. It was assumed that the sonde was operated for ten minutes on theground before launch. Spikes in the data have been removed. Background current has been subtractedwhere it was non-negligible. Also shown, for comparison, is the average response found for Brewer-Mastsondes prepared via Claude et al. [1987] and for Canadian ECC sondes, to the standard midlatitudeprofile in the JOSIE 1996 intercomparison [Smit et al., 1996] (similar to our Profile 1).

Table 1. Ozone Input Profile, Hypothetical Pressure Profile and Balloon Ascent

Rate, Shown Along With Calculated Parameters for Each Simulated Sounding

Sonde Profile AscentRate, m s�1

SondeIntegral, DU

CalibratorIntegral, DU

FlightDuration,

min

CorrectionFactor

14804 1 5.0 287 343 117 1.19514805 1 5.0 281 344 115 1.22414794 1a 4.5 277 321 127 1.15914795 2 3.6 312 408 133 1.30814796 1b 4.5 246 310 127 1.26013235 1a 4.5 217 296 126 1.36413069 2a 3.6 328 400 137 1.220

aExcept that tropospheric part was 2 mPa, rather than as shown in Figure 5.bExcept that tropospheric part was 3 mPa, rather than as shown in Figure 5.

TARASICK ET AL.: RESPONSE OF BREWER-MAST OZONESONDES ACH 14 - 7

profile in each case). Response profiles for Brewer-Mastand ECC soundings during JOSIE 1996 [Smit et al., 1996]are also indicated. For these seven simulated flights theaverage response below 300 hPa is 10–20% too low inabsolute terms, and even more when compared with theJOSIE profiles. The correction factors are similar to thosefound in the Canadian record, averaging 1.25 for the sevensondes.[28] The average response curve in Figure 7 is similar in

form to that deduced by De Muer [1980, 1984] from acomparison of the ascent and descent portions of soundingsmade at Uccle, Belgium. The correction derived by De

Muer is nearly twice as large, however, as he assumed thatthe descent values were correct. In fact, descent values inthe troposphere will be strongly affected by the second,slow reaction discussed above, as the sonde descendsrapidly from the stratosphere where ozone partial pressuresare much greater. If the slow reaction represents 5% of thetotal response at 20 mPa, it will be 20% of the troposphericresponse at 5 mPa.[29] Figure 8 is an example of a preliminary attempt to

correct for these deficiencies in tropospheric response. Thetop panel is reproduced from Tarasick et al. [1995] andshows trends calculated from Brewer-Mast and ECC data

Figure 8. Trends in the middle troposphere at Edmonton from Brewer-Mast and ECC data. The thicklines are the trend lines that are calculated for the data sets individually (shorter, downward-sloping lines)and when combined. Upper panel: The trend calculated from the combined data is evidently erroneous.Lower panel: Each Brewer-Mast sounding has been corrected, in the troposphere, by the averageresponse for 700–500 hPa from Figure 7, and the sounding rescaled to agree with the total ozonemeasurement.

ACH 14 - 8 TARASICK ET AL.: RESPONSE OF BREWER-MAST OZONESONDES

for the 700–500 hPa layer at Edmonton. The data are asrecorded in the World Ozone and Ultraviolet Data Centre;that is, the Brewer-Mast soundings have all been linearlyscaled to agree with a total ozone measurement. The lowerpanel shows the same data, except that each Brewer-Mastsounding has been corrected, in the troposphere, by theaverage response for 700–500 hPa from Figure 7 and thesounding has been rescaled to agree with the total ozonemeasurement. The correction seems to remove the dis-continuity in the tropospheric time series, and the overalltrend is now similar to the trends for the separate datasets.[30] The low level of ozone concentration that we have

inferred above for the ozonization of sondes by the con-ditioning cabinet raises another potential source of poortropospheric response. Three sondes for which the ozoniza-tion step was omitted showed a pattern of increasedresponse with time that was similar to that in Figure 7,except that the overall response was lower: averagingbetween 61% and 77%. Treating these runs as simulatedflights gave correction factors between 1.303 and 1.636,with ‘‘tropospheric’’ responses between 7% and 30% low.This suggests that inadequate ozonization might lead tofurther deterioration of tropospheric response, and exami-nation of the Canadian record shows that there is indeed aweak positive correlation, at all four stations at all levels,between high correction factors and low tropospheric ozonevalues.

6. Conclusions

[31] Brewer-Mast sondes were employed in the Canadianozone sounding network during the period 1966–1979.When prepared according to the procedures then in use inCanada, they show an average response of about 80%compared with a NIST-traceable ozone calibrator, which isconsistent with the average total ozone correction factor forozonesonde flights during this period of 1.255. At no timedid any sonde give a reading higher than the calibrator.Possible loss processes include evaporation of iodine fromsolution, losses of ozone in the pump and to the sensorwalls, and of iodine by adsorption to the Pt cathode andpossibly the walls as well. In general, except for iodineevaporation, these loss processes are strongly dependent onthe details of the preflight preparation procedures employed.For sondes prepared according to the procedures introducedin the 1980s [Claude et al., 1987], they are much reduced oreliminated.[32] Two loss processes are observed to diminish with

time, such that the response of the sonde will improveduring a flight. The first, and largest, causes new Brewer-Mast sondes to show a large (�15%) increase in sensitivityafter first exposure to sensing solution and ozone. Thiseffect is much reduced with previously flown sondes and insondes that are conditioned by being exposed to sensingsolution for several days before calibration. It is also quasi-permanent. We suggest, therefore, that it is related toconditioning of the Pt cathode by iodine adsorption. Thesecond process may to be related to iodine conditioning ofthe sensor walls and consequent changes in iodine or ozoneloss there, as it is also reduced by exposure to sensingsolution and ozone, but the improvement is temporary.

[33] Sondes show an additional slow increase of responsewith time that is apparently caused by secondary reactionswith the phosphate buffer.[34] The above mentioned processes, both individually

and in combination, will cause tropospheric values for ozonesoundings to be too low in relation to stratospheric measure-ments and therefore too low overall once the total ozonecorrection factor is applied. By exposing Brewer-Mastsondes, prepared via the procedures used in Canada, to ozoneinputs similar to actual atmospheric profiles, we have shownthat it is possible to reproduce both the low troposphericresponse and the typical total ozone correction factors foundin the Canadian Brewer-Mast record. The average correctionfactor for the sondes in Figure 7 is 1.247, while the averagefor the Canadian Brewer-Mast record is 1.255.[35] While noting that our experiments have been limited

to surface temperature and pressure, we suggest that thecorrection to tropospheric ozone values in the 1970s atCanadian stations that is indicated by Figure 7 is sufficientto explain the low tropospheric values for Brewer-Mastozone soundings in Canada. We intend to continue thesepreliminary experiments with corresponding work in thefield and in a pressure-temperature chamber and to applythese results to a reevaluation of the Canadian Brewer-Mastsounding record, which will then be submitted to the WorldOzone and UV-B Data Centre.[36] Finally, we note that field intercomparisons of sonde

types and of preparation procedures may produce ambig-uous results unless the effects of differences in sondepreparation are properly understood. (This is likely to beequally true for ECC sondes.) For example, such apparentlyunimportant (and generally unrecorded) factors as the lengthof time between filling the sonde with solution and launch,or between calibration and launch, may seriously affect thetropospheric results in particular. The potential biases thatmay result from such recent improvements, or others in thepast, should also be considered in future analyses of trendsderived from ozone sounding data.

[37] Acknowledgments. We are grateful to J. Easson and D. Ander-son of CSIRO, Australia, for having kindly provided some of the sondesused in this study. We are indebted to I. A. Asbridge for assistance ininterpreting the older data records.

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�����������K. Anlauf, J. Davies, D. W. Tarasick, and M. Watt, MSC/Environment

Canada, 4905 Dufferin Street, Downsview, Ontario, Canada M3H 5T4.(david.tarasick@ec.gc.ca)H. J. Claude and W. Steinbrecht, Meteorological Observatory Hohen-

peissenberg, Deutscher Wetterdienst–German Weather Service, Albin-Schwaiger-Weg 10, D-82383, Hohenpeissenberg, Germany.

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