Ozonesonde profiles from the West Pacific Warm Pool

We present a series of ozonesonde profiles measured from Manus Island, Papua New Guinea, during Febru-ary 2014, with new insights on the calibration of ozoneson-des for measurements in the tropical troposphere. The experiment formed a part of a wider airborne campaign involving three aircraft based in Guam, to characterise the atmospheric composition above the tropical West Pacific in unprecedented detail. Thirty-nine ozonesondes were launched between 2 and 25 February of which 34 gave good ozone profiles. Particular attention was paid to evaluating the background current of the ozonesondes, as this can amount to half the measured signal in the tropical tropopause layer (TTL). An unexpected contamination event affected the measurements and required a departure from standard operating procedures for the ozonesondes. The most significant departure was not exposing the sondes to ozone during preparation , which meant that the background current remained stable before launch. Comparison with aircraft measurements allows validation of the measured ozone profiles and confirms that for well-characterized sondes (background current ∼ 50 nA) a constant background current could be assumed throughout the profile, equal to the minimum value measured during preparation just before launch. From this set of 34 ozonesondes, the minimum reproducible ozone concentration measured in the TTL was 12–13 ppbv; no examples of ozone concentrations < 5 ppbv, as reported by other recent papers, were measured. The lowest ozone concentrations coincided with outflow from extensive deep convection to the east of Manus, consistent with uplift of ozone-poor air from the boundary layer. However, these minima were lower than the ozone concentration measured through most of the boundary layer, and were matched only by measurements at the surface in Manus.


Introduction
The Tropical Tropopause Layer (TTL) is the region of the tropical atmosphere between the top of the main convective outflow and the base of the stratosphere (approximately 13-17 km altitude) (Holton et al., 1995;Highwood and Hoskins, 1998;Gettelman and a transition layer between the convectively-dominated mid-troposphere beneath and the statically-stable (and convection-free) stratosphere above, whose composition depends both on convective uplift and large-scale transport. Since it is the main source region for air entering the stratosphere in the Brewer-Dobson circulation, the concentration of source gases in the TTL determine the stratospheric burden of ozone-destroying 5 radicals such as Cl x and Br x . Furthermore, the temperature of the cold point determines the concentration of water vapour in the stratosphere, and clouds in the TTL, especially near the cold point, affect the radiation budget. The TTL is therefore a region of considerable importance both for global stratospheric chemistry and for climate.
The region of the tropics from the Maritime Continent to the International Date Line 10 is known as the Tropical Warm Pool, where very warm sea surface temperatures (> 28 • C) support widespread deep convection (Wang and Mehta, 2008). The tropopause is higher and colder here than in other regions of the tropics, especially in Northern Hemisphere winter, making this region of particular importance for the dehydration of air as it enters the stratosphere (Fueglistaler et al., 2009). The West Pacific region is 15 also noted for very low ozone concentrations. Satellite measurements of total ozone show a zonal wave-one structure in the tropics with a maximum over the Atlantic sector and minimum over the West Pacific (Thompson, 2003;Takashima and Shiotani, 2007). This pattern is not restricted to the stratosphere: tropospheric ozone concentrations are also a minimum in the same region, generally attributed to photochemical destruction 20 of ozone in the very clean marine boundary layer followed by rapid vertical mixing by deep convection (Thompson, 2003). Folkins et al. (2002) noted that tropical ozone profiles typically exhibit an "S" shape with height, with a minimum concentration in the boundary layer (where ozone is destroyed photochemically), a maximum in the mid-troposphere due to long-range trans-25 port, and a further minimum at around 11 km before increasing into the stratosphere. They argued that this is consistent with the effect of deep convection lifting air from the boundary layer to the outflow region. Closer examination of this process however suggests a more complex explanation. Heyes et al. (2009) analysed a series of ozoneson-16657 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | des launched from Darwin, Australia as part of the ACTIVE campaign (Vaughan et al., 2008) and concluded that the lowest TTL concentrations of ozone occurred above the level of convective outflow. Back-trajectories suggested that the origin of this air lay to the north-east of Darwin, to the east and north-east of New Guinea. Uplift of air in large convective complexes over the warm ocean in this region was proposed as the 5 source region for the lowest ozone concentrations measured over Darwin. This suggests that there may be preferred locations or "hot spots" for lifting material to the TTL, a hypothesis that we further examine in this paper.
A controversial question regarding ozone measurements in the TTL is whether the concentrations can fall to near-zero values in the outflow of deep convection. 10 Ozonesonde observations during the CEPEX cruise over the central Pacific frequently measured ozone concentrations less than 10 ppbv, and occasionally close to zero in the TTL (Kley et al., 1996). The authors suggested that lifting of near-surface air (where ozone is often strongly depleted in the tropics) essentially unmodified to the outflow of the convection could explain these observations, but they also pointed out that near-15 zero ozone in the TTL was encountered more frequently than near the surface during the cruise, and postulated that there may be a hitherto-unknown mechanism to destroy ozone in clouds. Model simulations by Lawrence et al. (1999) showed that minima in ozone concentration in the TTL over the West Pacific result from convective uplift, but could not replicate the very low ozone concentrations found by  zero ozone values in ozonesonde profiles were also reported by Solomon et al. (2005) and Rex et al. (2014), again in the West Pacific region.
Doubts about the validity of these very low ozone concentrations were raised by Vömel and Diaz (2010), who examined in detail how the ozonesonde measurement is made. In particular they examined the background current -an interfering signal that 25 must be corrected for when deriving ozone concentrations from the raw data. Vömel and Diaz (2010) pointed out that the ozonesondes in Kley et al. (1996) and Solomon et al. (2005) measuring the lowest TTL ozone concentrations also had the highest background current. A re-examination of the ozonesonde profiles of Heyes et al. (2009) 16658 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | shows that the same issue may have arisen there with the minimum value of 4 ppbv occurring in a sonde with a higher background current than the others in that series (Sect. 3). We discuss the issue of the background current in detail in Sect. 2, but there is clearly uncertainty in the literature on the best way to account for it when calculating ozone profiles from the raw ozonesonde data. One of the aims of this paper is to shed 5 light on this uncertainty.
We present a series of ozonesonde profiles measured from Manus Island, Papua New Guinea ( (Fig. 1). The ground campaign took place at the Atmospheric Radiation Measurement (ARM) site next to the 15 airport on Manus, and comprised an ozonesonde campaign with supporting groundlevel observations from a TECO-49C UV photometric ozone monitor, a Picarro G-2401 cavity ring-down spectrometer to measure CO 2 , CH 4 and CO, and a home-built gas chromatograph to measure halogenated compounds (Gostlow et al., 2010). Support with both logistics and meteorological data were provided by ARM and the Papua New 20 Guinea Meteorological Service. The ground-based dataset was collected between 1 and 25 February 2014, with 39 ozonesonde ascents (34 of which gave good data) between 2 and 25 February. As we show in this paper, overflights of the NCAR Gulfstream V provided an opportunity to validate ozonesonde measurements in the TTL during conditions of low ozone concentration.

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In Sect. 2, experimental details of the ozonesonde campaign are presented, including the procedure to correct for the background current. Section 3 presents the aircraft measurements used to validate the ozonesonde profiles. Section 4 presents a sum-potassium iodide (Reaction R1), followed by half-cell reactions in the anode (Reaction R2) and cathode (Reaction R3) (Komhyr, 1972).
The anode half-cell contains a saturated potassium iodide solution and the cathode an unsaturated KI solution; as the ozonesonde ascends a teflon pump bubbles air through the cathode cell. The current produced is proportional to the flow of ozone through the cathode cell, with each ozone molecule assumed to generate two electrons (Vömel and Diaz, 2010). However, this is not the only reaction that produces 15 a current within the ozonesonde: other reactants produce a residual background current (Thornton and Niazy, 1982), which increases the measured signal and which must be accounted for when calculating the ozone concentration.
The background current is of particular importance in the TTL where it can be a substantial fraction of the total current measured by the sonde. The best way to correct for 20 the background current is the subject of much debate (e.g. Komhyr and Harris, 1971;Thornton andNiazy, 1982, 1983;Reid et al., 1996;Smit and Sträter, 2000;Smit et al., 2007), and the two main manufacturers of ozonesondes, Droplet Measurement Technologies and Science Pump Corporation, recommend two different methods: either a constant value measured before launch or a value that scales linearly with ambient pressure. The practice of using a pressure-dependent correction arises from early suggestions that the ozonesonde reacts with oxygen (Komhyr and Harris, 1971), but later studies ruled out this mechanism and suggested that the background current should be taken as constant with altitude, at least in the troposphere (Thornton and Niazy, 1982;Reid et al., 1996). Nevertheless, Johnson et al. (2002) found a background reaction 5 with the phosphate buffers of a standard electrolyte solution, leading to a pressuredependence.
This confusion led Vömel and Diaz (2010) to examine in detail again the issue of background current. In the normal preparation of an ozonesonde, the background current is measured as the sonde is drawing in ozone-free air before and after the sonde 10 is exposed to ozone to check that it is responding normally. Reid et al. (1996) recommended that the first of these measurements be adopted as the background current and removed (as a constant value) from current measurements in flight. However the standard procedure for ozonesonde preparation uses a value measured at some point (usually around 10 min) after exposure to ozone. Vömel and Diaz (2010) found that the 15 background current continues to decrease after exposure to ozone, even for periods of hours -suggesting that a value measured 10 min after exposure to ozone will be an overestimate by the time a sonde reaches the TTL, leading to an underestimate of the ambient ozone concentration when subtracted from the measured current. They recommended the use of a background current I bg = 0.09I + 0.014 µA for the cell solutions 20 used in this paper, regardless of the measurements made during sonde preparation; the dependence of I bg on the current I suggesting that the assumption of two electrons per ozone molecule passing through the cathode cell is not correct. Reprocessing past soundings with this formula for background current was shown to remove all the cases of near-zero ozone -not surprising as the background current of ∼ 0.025 µA that this 25 gives in the TTL is well below the 0.065 µA used for example in the original analysis of the CEPEX data. Independent verification of Vömel and Diaz (2010) has however not been performed to date, and we examine below the application of this recommendation to the Manus dataset and the comparison with aircraft data. Introduction

Ozonesonde preparation
The ozonesondes used here were EnSci Model Z sondes supplied by Droplet Measurement Technologies, coupled to Väisälä RS92G radiosondes which provided pressure, temperature, humidity and wind profiles. All were from the same batch of sondes supplied just before the campaign. The cathode solution comprised 1 % KI with 25 g L −1 of 5 KBr, 5 g L −1 Na 2 HPO 4 · 12H 2 O and 1.25 g L −1 NaH 2 PO 4 · H 2 O as buffers. Standard procedures for preparing ozonesondes follow a two-stage process aimed at reducing the background current to less than 50 nA at the time of launch and measuring the sonde's pump flow rate. In this work the background current was obtained by drawing air into the sonde through a charcoal filter in an air-conditioned cabin where RH < 50 % at all 10 times. The current was measured with a Keithley 6485 picoammeter, and the pump flow rate F (in mL min −1 ) with a Sensodyne Gilibrator unit. Repeated measurements of pump flow rate generally agree to 1-2 %. The ozone partial pressure p O 3 (in mPa) was derived from the measured sonde current as: where T box was measured by taping a thermistor to the inlet tube as it entered the ozonesonde pump. In this equation I and I bg are both measured in µA and T box in K. The ozonesonde preparation procedures normally involve, at different stages, purging the electrochemical cell and/or the pump with high concentrations of ozone, characterising the cell response to expected atmospheric concentrations of ozone and draw-20 ing ozone-free air through the cell. For this a Science Pump TSC01 ozone calibration unit was available. Normally, each ozonesonde would be first prepared 3-5 days before flight, in a three-step process: (i) passing high ozone through a new cell to remove organic traces; (ii) filling the anode and cathode cells and waiting for the current to fall to 0.5 µA while drawing in ozone-free air; (iii) exposing the cell to atmospheric concentrations of ozone to verify its response; (iv) again drawing ozone-free air, measuring the time response of the cell and the background current after ∼ 10 min. Then, on the 16662 Introduction

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Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | day of flight, a second preparation would follow basically the same steps except that high ozone was only passed through the pump rather than the cathode cell. Standard ozonesonde procedures specify a change of solution once, at the beginning of the second preparation. We found in Manus, however, that repeated changes of solution were needed to reduce the background current to an acceptable value (the number of 5 changes varied from sonde to sonde according to its requirements). The background current was measured both at the beginning of the second preparation and as the minimum value recorded by the Väisälä software after the sonde package was finally assembled (but before taking it out of the air-conditioned environment -in the humid tropical atmosphere outside the cabin the charcoal destruction filter does not work cor-10 rectly).

Contamination
A complication encountered during this experiment was the sudden appearance of a contamination source inside the TSC01 which produced a large signal from the ozonesonde. This badly affected the first two sondes, rendering their data unusable.

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These sondes were extensively exposed to air drawn through the TSC01 during their second preparation (the first having been completed normally before the contamination appeared). Contamination also rendered the calibration cell on the TSC01 unusable. Sondes 3 and 4 were again clean on first preparation but were briefly exposed to the TSC01 on second preparation, after an initial measurement of the background current. 20 This contamination necessitated a change to the standard operating procedures described above. The remaining sondes were not exposed to the TSC01 at all during the second preparation -the sonde's response to ozone was assumed to be normal and the background current was measured by drawing air through an external charcoal filter. Sondes 5-14 were briefly exposed to the TSC01 on first preparation and were 25 subsequently found to have elevated background currents. Sondes 15 onwards were not exposed at all to the TSC01 and the background currents from these sondes were around 50 nA before launch.  Figure 2 shows how the background currents measured for each sonde varied during the campaign, compared to the minimum current measured by the cell in the TTL (taken from the Väisälä raw data telemetry). During the latter part of the campaign the background current was around half the minimum measured in the TTL, but during the early part the minimum current is close to or even lower than the background -imply- 5 ing an impossible negative ozone. Clearly the contamination did not remain constant during a flight.
On return from Manus a series of laboratory experiments were conducted to ascertain the properties of the contamination. These are summarised in the Appendix, but the salient result is that for lightly contaminated sondes (such as 3-15) the effects of the 10 contamination tended to disappear over a similar timescale -∼ 1 h -to that taken by a sonde to reach the TTL. Based on this, and the evidence in Fig. 2 that the minimum ozonesonde current in the TTL was remarkably stable over the campaign, we assume that in flight the contamination disappeared and the background current returned to a value of 50 nA, consistent with the uncontaminated sondes. A hybrid background 15 current correction was thus devised: where I meas bg was the measured background current before launch, p the pressure and p 0 the surface pressure. Note that an error of 10 nA in the background current equates to an error of 3.4 ppbv in ozone at 100 hPa pressure. The variation in measured back-20 ground current from sonde to sonde is of the order of 10 nA (0.01 µA, Fig. 2, sondes 15 onwards), with a similar difference between the values measured at the beginning and the end of the preparation, so it is reasonable to assume that an accuracy of greater than ±3 ppbv cannot be claimed for TTL ozone measurements due to the uncertainty in measuring I bg . We now consider whether the assumption of a constant background 25 current with height for the uncontaminated sondes is correct.

Validation
One of the aims of the CAST and CONTRAST campaigns was to investigate the accuracy of ozonesonde measurements in the TTL by comparing them with near-coincident aircraft measurements from the NCAR Gulfstream V. Ozone measurements on the Gulfstream V were made using the NCAR chemiluminescence instrument. The tech-5 nique is based on the chemiluminescent reaction of NO and O 3 to produce excited NO 2 , a fraction of which decays by emitting a photon (Ridley et al., 1992). A small flow of pure NO is added to a flow of ambient air and the resulting photons are counted using a dry-ice cooled photomultiplier tube. The instrument is periodically calibrated against a Thermo Scientific 49i-PS primary ozone standard on non-flight days. The 10 overall uncertainty is 5 %, or 1 ppbv at 20 ppbv. The precision of the measurements at 20 ppbv is 0.1 ppbv (0.5 %), or better, for the 10 s averages used here. On 5 February Gulfstream V flight RF09 flew to the west of Manus Island and profiled from the surface to ∼ 11 km. Figure 3 shows the path of ozonesonde 6 and the Gulfstream V flight segment near Manus, and Fig. 4 compares their ozone profiles.

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Ozonesonde 6 was affected by contamination with a high background current (143 nA). In Fig. 4, four ozonesonde profiles are shown -one using a constant background current correction (black dashed line), one using a pressure-dependent correction (blue dashed line), one using the recommendation of Vömel and Diaz (2010) (green dashed line) and the fourth using the hybrid correction (solid black line). It is clear that the hy-20 brid correction fits the Gulfstream V measurements (red solid line) very well, while the constant correction gives artificially low (and in this case negative) ozone in the TTL similar to the profiles reported by Kley et al. (1996) and Rex et al. (2014). This is strong evidence that the method used to correct for the background current in this case is valid. Introduction aircraft altitude as the aircraft made closest approach to Manus. Ozonesonde 34, coincident with the outbound leg, was launched at 01:31 UTC (11:31 LT) and reached the Gulfstream V cruising altitude of 13.5 km at 02:11 UTC. In the flight-path map in Fig. 3, the red line is the outbound leg of RF14. Figure 5 shows the ozone profiles from ozonesonde 34 and the co-located measurements of RF14. Likewise ozonesonde  Figure 6 shows the profiles from ozonesonde 35 and the co-located measurements from RF14.
Ozonesondes 34 and 35 were uncontaminated, so constant background currents of 61 and 54 nA respectively were used in the data analysis. In both cases, the agreement between the ozonesonde and the aircraft data is within 3 ppbv -consistent with the uncertainty in the background currents. By contrast, the pressure-dependent correction and that recommended by Vömel and Diaz (2010) clearly overestimate the ozone con-15 centration. We therefore conclude that for a well-conditioned ozonesonde not exposed to ozone at all in the pre-flight preparation, where the background current at the end of the preparation is around 50 nA or less, subtraction of this constant background produces an ozone measurement in the TTL within a few ppbv of the correct value. We also conclude that our method of applying a hybrid correction produces good results 20 for the contaminated sondes.
What we cannot be sure of is whether the hybrid method applies only to this particular batch of sondes, or whether it can be applied more generally to sondes where the background current in the preparation is substantially larger than 50 nA. The hybrid correction is basically a way of allowing the background current to decrease with time, We have therefore applied the following background current correction to the Manus dataset, after discarding the first two profiles: 10 for sondes 3 and 4, a hybrid correction was applied using I bg measured at the beginning of the second preparation, before exposure to the TSC01 ozoniser. This value was considerably smaller that that measured after exposure to the ozoniser, but higher than the ∼ 50 nA typical of the uncontaminated sondes.
for sondes 5 to 14, a hybrid correction was applied using I bg measured just before 15 launch.
for sondes 15 on, a constant value of I bg was applied equal to that measured just before launch.
Note that for sondes 15 onwards I bg measured at the beginning and end of the second preparation were very similar (Fig. 2).

Results
We present here an overview of the measurements made at Manus during CAST. The campaign experienced two distinct weather regimes -a dry period from around 1-10 February with little precipitation (Fig. 7)  of Manus, and a wetter period from 11 February on, with two particularly wet periods around 13-15 February and 20-23 February. During the latter period in particular widespread deep convection occurred around and to the east of Manus (Fig. 9), providing the conditions needed to examine the ozone concentration in fresh convective outflow.

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The two meteorological regimes are reflected in the time series of tropopause (cold point) temperature and potential temperature from the ozonesondes (Fig. 8), with θ generally around 370 K from 1-12 February and rather lower, around 364 K, from 13 February onwards. Tropopause heights and pressures for the whole campaign (not shown) ranged from 15.7 to 17.2 km, and 89 to 115 hPa respectively. Double This jet was confined to the troposphere -by 1.5 km above the tropopause the wind had backed round to westerly, and remained westerly between 18 and 26 km. A cor-20 responding minimum in wind speed (of ≤ 2 m s −1 in most cases) was measured 700-1200 m above the tropopause from 16 February onwards. This easterly jet is consistent with convective outflow from the large convective complexes to the east of Manus (Fig. 9) reaching up to the tropopause during this period but not extending into the stratosphere.

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The corresponding contour plot of ozone concentration is shown in Fig. 11. This clearly shows the "S" shape expected of tropical ozone soundings, with low values near the surface and in the TTL, and a maximum in the mid-troposphere. Minimum values of < 20 ppbv are frequently shown in the TTL, around 14 km during the first Introduction Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | meteorological period and then up to 16.5 km during the second period. The periods of precipitation in Manus (Fig. 7) both correspond to ozone concentrations < 20 ppbv reaching the tropopause, and indeed in the very wet period between 20 and 22 February, when the TTL easterly jet was at its most intense, ozone minimum concentrations fell to < 15 ppbv. The lowest measured value was 8.2 ppbv on 21 February -a similar 5 minimum to that measured in Darwin during ACTIVE. This may have been an outlier (its background current was 60 nA), but five sondes reached between 12 and 13 ppbv (e.g. sonde 34 on 22 February, Fig. 5) and a further four between 13 and 15 ppbv.
To confirm that the very low ozone measured in the TTL is consistent with uplift from the deep convection to the east of Manus, back-trajectory calculations were performed 10 using the HYSPLIT on-line model. As an example, Figs. 12 and 13 show four-day HYSPLIT back-trajectories initiated over Manus at 02:00 UTC on 22 February (corresponding to sonde 34), between 13 km (180 hPa) and 15 km (130 hPa). The trajectories clearly indicate extensive uplift from the lower troposphere in the 48 h before the measurement, indicating that the source of the low ozone in the TTL is indeed the lower 15 troposphere north of the Solomon Islands. Of course, the HYSPLIT trajectories cannot represent ascent in individual cloud systems, and so cannot determine whether the air is really of boundary-layer origin, but they do confirm that the meteorological conditions at this time were consistent with widespread deep uplifting of air. Figure 11 shows that the low-level ozone over Manus also showed two distinct peri-20 ods, consistent with the meteorology. Ozone concentrations < 15 ppbv extended up to 2 km in the dry period and persisted below 1 km up to 14 February, but in the very wet period the lowest values were in the range 15-20 ppbv, more than the minima measured in the TTL. However, the ground-level measurements from the TECO-49 ozone monitor (Fig. 14)  conditions set in by 13 February, with the diurnal ozone variation largely disappearing in the steady north-westerly breeze. Ozone concentrations in the range 9-13 ppbv predominated up to 19 February, with 12-14 ppbv thereafter. These values are in fact consistent with the minimum values measured in the TTL (save for the very low value on 21 February) -and with the sondes, which generally measured a steep increase 5 in ozone in the bottom 200 m of the profile (the altitude scale in Fig. 11 obscures this point). If the lower tropospheric ozone in the uplift region to the east of Manus was similar to that over the island, this would suggest that the air reaching the very top of the TTL in the wet period originated very near to the surface and was lifted to the tropopause without significant mixing with surrounding air, consistent with the sugges-10 tion of Kley et al. (1996). Of course, if the uplifted ozone more closely resembled the Manus profiles during the dry period, such stringent restrictions on the convective uplift would not be required, but the evidence suggests a strong land-based local influence on the ozone profiles during the dry period which would not apply over the ocean.

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One of the aims of this paper was to determine the best way to correct ozonesonde profiles from a tropical station for the effect of the background current. We were very fortunate that the Gulfstream V flight RF14 was able to fly by Manus during the period when very low ozone concentrations were observed in the TTL by the sondes. Ozonesondes 34 and 35 were free of contamination, and when using a constant background cur-20 rent measured just before launch their measurements agreed with the Gulfstream V to within 3 ppbv (the realistic limit on the accuracy of the ozonesonde at 100 mb due to background current uncertainty). We conclude that for a well-prepared sonde -i.e. (for the batch used here) one where I bg ∼ 50 nA -a constant background current correction is the best choice.

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In preparing these sondes we found it necessary to change solutions in the cells up to three times during a day-of-flight preparation in order to ensure a sufficiently low background current. Other than for sondes 3 and 4, we also did not expose the sondes to ozone during the day-of-flight preparation, which removes the problem of the slow decay in I bg after such exposure (Vömel and Diaz, 2010). Both these changes in standard procedures are recommendations from this work. For the sondes exposed to contamination during first preparation a hybrid background current correction was 5 adopted after the laboratory investigation. Using this, the profile for sonde 6 was found to agree remarkably well with the aircraft profile from RF09 (Fig. 4), lending confidence to this somewhat arbitrary correction. Care must be taken not to generalise this result too far, but we can conclude (both from the CAST sondes from Manus and the ACTIVE sondes from Darwin) that a background current in excess of 70 nA is too high for a constant I bg correction -as shown by Vömel and Diaz (2010) this leads to a substantial underestimate of the TTL ozone and even to negative ozone in some cases (e.g. Rex et al., 2014).
The minimum reproducible ozone concentration measured in the TTL during CAST was 12 ppbv -very similar to the minimum measured in Darwin with well-prepared son-15 des (12 and 11 ppbv on 23 January and 14 February 2006 respectively) in air whose origin, according to back-trajectory calculations, lay in deep convective uplift east and north-east of New Guinea. In both campaigns an isolated example of a lower concentration, around 8-9 ppbv, was also measured. The CAST measurements confirm Vömel and Diaz (2010)'s conclusions that ozonesonde measurements < 5 ppbv in the 20 TTL are artifacts of the background current correction, and suggest that the Rex et al. (2014) measurements made in the same region in November 2009 need verifying.
The lowest ozone concentrations measured in the TTL above Manus occurred around 16 km during a period when widespread deep convection was occurring near and to the east of the island. This is consistent with the "hot spot" idea proposed by low as 12 ppbv be found. This suggests that the widespread deep convection was able to lift air from the lower boundary layer into the upper TTL without significant mixing -a hypothesis we cannot pursue further here but which will be the subject of future investigations. 5 When the pattern found in Fig. 2 was discovered, the records of the CAST field campaign were examined (Sect. A1) and a series of laboratory experiments devised to ascertain the reasons why the background current generally decreased between sondes 5 and 14 yet the minimum measured current in the TTL remained reasonably constant. It was observed that when an ozonesonde drew air from the TSC01 ozonizer unit, a high current was registered. This was identified in the laboratory experiments as being contamination, rather than high concentrations of ozone, as explained in Sect. A2.

Appendix A: Laboratory experiments
Neither the source nor the identity of the contamination was known, and so an experiment was devised to determine the response of an ozonesonde to pressure with various degrees of contamination, by placing it into a bell jar and varying the pressure.

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The results of this experiment are described in Sect. A3. The contamination gradually disappeared over time, so the bell jar experiments were neither reproducible nor did they replicate exactly the conditions that were experienced on Manus, but they serve as a check on the validity of the hybrid background current correction. 20 The first five ozonesondes were normal on first preparation. Between the fifth ozonesonde being prepared for the first time and the following day when the first ozonesonde was being prepared for flight, the ozonizer was found to be causing the cell current to increase dramatically even when it was supplying "no-ozone" air. This affected the first two ozonesondes' day-of-flight preparations, and their background Introduction Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | current remained well above that of a normal working ozonesonde. Thenceforth, an external ozone destruction filter was used instead of the ozonizer to produce no-ozone air and the sonde was not exposed to the ozoniser during the day-of-flight preparation. However, ozonesondes 6 to 14 were briefly exposed to the ozonizer during their first preparation to check the response of the sonde to ozone. Since exposure to the 5 ozoniser was resulting in elevated background currents, the ozonesonde sample tube was only connected to it for a few seconds before being removed. However, this turned out to be long enough to allow the contaminant to get into the ozonesonde where it remained throughout the preparations.

A1 Examination of records
Ozonesondes 15 onwards were not exposed to the ozoniser at all, and were there-10 fore the most reliable ozonesondes launched during CAST.

A2 Identification of contamination
In order to investigate the cause of the high background currents in the first fourteen ozonesondes, laboratory investigations were conducted after the equipment was returned from Manus to Manchester, some two months after the campaign ended.

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First, the response of an ozonesonde was compared with that of the TECO-49 ultraviolet photometric ozone monitor. When sampling laboratory air, both TECO-49 and the ozonesonde measured comparable concentrations (∼ 22 ppbv), and when drawing air through the external charcoal filter the sonde measured 2 ppbv while the TECO-49 measured 12 ppbv. However, when sampling supposedly ozone-free air from the 20 ozoniser (air drawn through an internal charcoal filter) the sonde measured 189 ppbv while the TECO-49 again measured 12 ppbv. Clearly, therefore, the ozoniser was acting as a source of some contaminant which produced a positive signal in the ozonesonde but not in the photometric ozone monitor -i.e. this substance was not ozone. (The 12 ppbv signal measured by the TECO through the filters is understandable as the flow 25 rate of the TECO-49 is much higher than the ozonesonde and exceeds the capacity of the filters). Further investigation, dismantling the ozoniser and examining different parts, identified the source of the contamination as the tube which is illuminated by 16673 Introduction

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Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | a mercury lamp to generate ozone. However, contamination was found even on the PTFE manifold at the outlet of the ozoniser.

A3 Ozonesonde behaviour at different pressures
The behaviour of the contamination was then investigated. The pressure dependency of an ozonesonde was investigated by putting it into a bell jar and lowering the pressure.

5
Three ozonesondes of varying levels of exposure to the contaminant were used in the experiment: the first was heavily contaminated, the second slightly contaminated, and the third not contaminated at all. The ozonesonde was placed in the bell jar and left to settle to a constant background current for about five minutes. The bell jar was then pumped down to a target pressure using a rotary pump, and then the rotary pump was 10 switched off. The ozonesonde was left for five minutes to settle and reach a constant background current, and then a new target pressure was chosen. The first, heavily contaminated ozonesonde emulated the first two ozonesondes launched in CAST, which were prepared just after the contamination episode, but before the contamination was recognised. The slightly contaminated ozonesonde em-15 ulated ozonesondes 3 to 14, which were only contaminated on first preparation. Ozonesondes 15 onwards were not contaminated, like the third test ozonesonde in this experiment.
The heavily contaminated ozonesonde was contaminated on both first preparation and the day-of-flight preparation and had a background current of 132 nA, which is 20 comparable to the early ozonesondes in CAST. Figure A1 shows the result of the bell jar experiment. The current was erratic, which was observed with the contaminated ozonesondes during CAST: the current occasionally spiked by ∼ 20 nA, possibly due to the cell picking up more contamination. The most likely behaviour of the ozonesonde was a decay of the background current from 135 nA at surface pressure to 115 nA at 25 20 hPa, still well above the expected value for a well-functioning sonde. This confirms that a reliable background current estimate could not be made for the first two CAST sondes. The ozonesonde used in this experiment was subjected to a further prepara-16674 Introduction tion cycle (without exposure to contaminant) to investigate whether it could be cleaned. Its background current reached 40 nA after 15 min of no-ozone-air treatment, indicating that the contamination was changing its character over time: changing solutions in the Manus sondes did not remove the contamination. The second ozonesonde was initially contaminated in first preparation, and then pre-5 pared cleanly in the day-of-flight preparation, similar to ozonesondes 3-14 in Manus. However, as with the first test sonde, the contamination was found to disappear so that the "day-of-flight" background current was 55 nA -consistent with a clean ozonesonde. It appears than that the contaminant changed its nature and became less adhesive over the three-month period since the contamination event. More contaminant was therefore added at the end of the second preparation, bringing the background current to 80 nA. The bell jar experiment showed little consistency in the background current as a function of pressure, but a clear decay over time (Fig. A2). Since in a normal ozonesonde launch pressure decreases as a function of time, this gives weight to the idea that a decaying background current correction with pressure is appropriate for the slightly 15 contaminated ozonesondes. The uncontaminated ozonesonde was prepared cleanly both times, and had a background current of 45 nA. Figure A3 shows the result of the bell jar experiment. The experiment was split into two sections, one in which the ozonesonde remained above 200 hPa at all times, followed by another in which the pressure was pumped down to 20 70 hPa. The current decreased slightly between 1000 and 100 hPa (45-40 nA), before decreasing to 27 nA at 70 hPa. This is similar to the result found by Thornton and Niazy (1983), which was attributed to a change in the mass transfer inside the ozonesonde. Within experimental accuracy of ±10 nA, therefore, a constant background current is appropriate to the uncontaminated ozonesondes up to 100 hPa, with a possible 25 decrease above this level. As the tropopause pressure encountered in Manus was > 90 hPa, with the ozone concentration increasing rapidly into the stratosphere, we have used a constant background current throughout the profile for uncontaminated 16675 hesive over time. This is consistent with the general decrease of background current between sondes 5 and 14 in Manus, despite their identical preparation procedure. Nevertheless, the behaviour is sufficiently similar to the CAST sondes as to provide support for the method used in Eq.
(2) to calculate the background current.
The bell jar experiments show that the background current in this batch of ozoneson-10 des was largely constant in the absence of contamination, while that in a slightly contaminated ozonesonde reduced with time to a "clean" value over a period of ∼ 30 minutes. The heavily contaminated ozonesonde did not reduce to an acceptable background current, confirming that data from the heavily contaminated ozonesondes launched in CAST should be discarded.