Measurements of halogenated trace gases in ambient air frequently rely on canister sampling followed by offline laboratory analysis. This allows for a large number of compounds to be analysed under stable conditions, maximizing measurement precision. However, individual compounds might be affected during the sampling and storage of canister samples. In order to assess halocarbon stability in whole-air samples from the upper troposphere and lowermost stratosphere, we performed stability tests using the high-resolution sampler (HIRES) air sampling unit, which is part of the Civil Aircraft for the Regular Investigation of the atmosphere Based on an Instrument Container (CARIBIC) instrument package. The HIRES unit holds 88 lightweight stainless-steel cylinders that are pressurized in flight to 4.5 bar using metal bellows pumps. The HIRES unit was first deployed in 2010 but has up to now not been used for regular halocarbon analysis with the exception of chloromethane analysis. The sample collection unit was tested for the sampling and storage effects of 28 halogenated compounds. The focus was on compound stability in the stainless-steel canisters during storage of up to 5 weeks and on the influence of ozone, since flights take place in the upper troposphere and lowermost stratosphere with ozone mixing ratios of up to several hundred parts per billion by volume (ppbv). Most of the investigated (hydro)chlorofluorocarbons and long-lived hydrofluorocarbons were found to be stable over a storage time of up to 5 weeks and were unaltered by ozone being present during pressurization. Some compounds such as dichloromethane, trichloromethane, and tetrachloroethene started to decrease in the canisters after a storage time of more than 2 weeks or exhibited lowered mixing ratios in samples pressurized with ozone present. A few compounds such as tetrachloromethane and tribromomethane were found to be unstable in the HIRES stainless-steel canisters independent of ozone levels. Furthermore, growth was observed during storage for some species, namely for HFC-152a, HFC-23, and Halon 1301.
Despite their low atmospheric mixing ratios of up to only a few hundred parts per trillion (ppt), halogenated trace gases have a significant impact on the earth's atmosphere. In particular, anthropogenic chlorinated and brominated halocarbons are responsible for stratospheric ozone depletion
The trace gas composition in the upper troposphere and lowermost stratosphere (UTLS) can be analysed from aboard aircraft using in situ instrumentation or air sample collection followed by postflight analysis on the ground. Whereas fast in-flight measurements based on gas chromatography–mass spectrometry provide data of halocarbon and non-methane hydrocarbon mixing ratios in the UTLS at a higher spatial resolution (1–4 min;
The literature on compound stability in sampling canisters mainly deals with volatile organics. For example,
The Civil Aircraft for the Regular Investigation of the atmosphere Based on an Instrument Container (CARIBIC) project (
CARIBIC air sampling was initially limited to 28 glass flasks per series of flights.
Taking advantage of the large spatial coverage of commercial long-distance flights, measurements from these CARIBIC glass flask samples have provided valuable information on the distribution of halogenated trace gases in the upper troposphere and lowermost stratosphere, for example regarding HFC-227ea
Measurements of halocarbons in the tropopause region are sparse. CARIBIC air samples are collected on a regular basis and therefore complement measurements from research aircraft campaigns such as the ATTREX project
The HIRES unit holds 88 lightweight cylinders made of stainless steel (wall thickness 0.25 mm), each with a volume of 1 L, and its total weight is 43 kg.
Samples are pressurized to 4.5 bar in flight at preset time intervals
using two metal bellows pumps (Senior Aerospace Metal Bellows, 28823-7).
Coarse particles are filtered by a 2
Schematic view of the 88 sample cylinders inside the HIRES unit and the six multiposition valves. For simplicity all tubing is omitted to illustrate only the positioning of these main components. Drawing: Laurin Merkel.
Before a flight, the HIRES unit undergoes leak testing with ambient air passed through a 10X molecular sieve (8 Å pore diameter) to prevent contamination of tubing with hydrocarbons.
Cylinders are not preconditioned. On take-off, cylinders will usually hold remnant air from the last research flight or from the leak test. The reason is that
due to the mechanical stability of the thin-walled flasks they should not be evacuated to absolute pressures below 600 mbar.
Tests during the construction phase and monitoring based on NMHC measurements during the sampler's first years of operation have shown that eight iterations of flushing reliably dilute remnants of previous fillings of tropospheric air.
In flight, canisters are therefore flushed with ambient air
eight times; this is achieved by filling a flask to 4 bar followed by venting for 20 s. After this time, ambient pressure is reached, which aboard the aircraft at flight altitude is
Upon the return of the instrument container, the sample collector is retrieved
and postflight gas chromatography analyses for greenhouse gases
The halocarbon measurements are based on adsorption–desorption gas chromatography–mass spectrometry (GC–MS)
Gas chromatography (GC) is performed with an Agilent 7890A instrument equipped with a 7.5 m
precolumn and a 22.5 m main column (both GasPro PLOT, inner diameter 0.32 mm).
The temperature programme of the GC
starts at 50
HIRES samples are measured relative to a laboratory standard which was collected cryogenically at Jungfraujoch (Switzerland) in December 2007. This standard is compared monthly to a tertiary standard of the Advanced Global Atmospheric Gases Experiment (AGAGE) network and was recalibrated versus several AGAGE standards in December 2018. Drift of the working standard can thus be excluded. Mixing ratios are reported in parts per trillion on Scripps Institution of Oceanography (SIO) scales except for trichloroethene, dibromochloromethane, tetrachloroethene, and tribromomethane (reported on scales defined by the University of Bristol (UK), the University of East Anglia (UK), and the National Oceanic and Atmospheric Administration (US)). Mixing ratios of CO and ozone from CARIBIC in-flight measurements are reported in parts per billion by volume (ppbv). Details of the respective calibration of both instruments have been published by
To test a possible influence of ozone on reactive halocarbon species, HIRES sample canisters were pressurized in the laboratory with a mixture of a well-characterized laboratory standard and synthetic air. The standard was filled with an oil-free compressor at Taunus Observatory (50.22
Schematic view of the gas flows for filling HIRES canisters for the storage experiments. Samples can be pressurized with either pure air from a standard gas bottle (flow path III), pure synthetic air (path II), or with a mixture of both. Alternatively, the flow of synthetic air can be directed to pass an ozone-generating UV lamp (path I).
Figure
For a short-term storage test, six individual canisters were pressurized consecutively to an absolute pressure of 4 bar, using 1.7 bar of synthetic air and 2.3 bar of the standard gas. For three of the canisters, the synthetic air was enriched in ozone by directing its flow via the UV lamp. All samples were analysed 1 d after filling and again 1 week later.
Because of the large number of cylinders, measurement of all the samples of one CARIBIC flight series takes several days, depending on the type of analysis performed. The halocarbon measurements described here add up to approximately 53 h total measurement time, not including blank measurements and preparation work. In addition, the HIRES unit needs to circulate between different laboratories in different institutions; therefore, the time between in-flight sampling and postflight sample analyses can be much longer than 1 week.
Canisters have a volume of 1 L and were pressurized up to 4 bar. Due to the mechanical stability of the thin-walled flasks, they may not be evacuated during the measurement.
For a corresponding long-term storage test with six measurements of one gas mixture, the sample volume of one cylinder would not be sufficient.
Therefore, six cylinders were simultaneously filled, and on each measurement day the next canister of such a series was measured.
Canisters were pressurized to an absolute pressure of 4 bar with one of the following gas mixtures:
synthetic air (4 bar); synthetic air (1 bar) and standard (3 bar); synthetic air (1.7 bar) and standard (2.3 bar); synthetic air (1.7 bar), ozone, and standard (2.3 bar); and synthetic air (4 bar) and ozone.
Two subsets of six canisters each were pressurized simultaneously with each of the mixtures, thus giving two subsets with identical composition. One canister from each subset was analysed after a storage time of
1, 8, 15, 29, 51, and 57 d. The full measurement series for the long-term storage test comprises analyses of 60 individual canisters. Between measurements, the sampling unit was stored in an air-conditioned laboratory at temperatures of around 22
Assuming the synthetic air to be free of any of the compounds of interest, mixing ratios in the canisters should be identical to the original mixing ratios weighted by the relative contribution of each gas; i.e., trace gas mixing ratios in the HIRES canisters should be about 75 % of the original mixing ratios of the standard in the case of 1 bar of synthetic air mixed with 3 bar of the standard gas and about 57.5 % in the case of 1.7 bar of synthetic air mixed with 2.3 bar of the standard gas.
Measurements of the pure synthetic air, however, revealed contamination of the synthetic air with
carbonyl sulfide (12.8 ppt), chloromethane (24.3 ppt), HFC-152a (2.8 ppt), and tetrachloroethene (1.1 ppt). This needs to be taken into account for the calculation of the trace gas mixing ratios expected in the HIRES canisters after mixing synthetic air and standard gas.
To reduce uncertainties related to the mixing of synthetic air and standard gas, which are mainly caused by the uncertainty of the pressure readings of approximately 0.1 bar, all compounds were evaluated relative to CFC-12 (CCl
The short-term storage test was performed to investigate the influence of ozone being present during pressurization. The HIRES unit is usually operated in the UTLS region and ozone mixing ratios are commonly up to 800 ppbv, depending on flight route and season. Figure
Results of the ozone and short-term storage test for HFC-134a
For HFC-134a, the ratio of its mixing ratio to that of CFC-12 agrees with the expected value within the experimental uncertainty range on both the first and eighth day. For dichloromethane, samples influenced by ozone (dashed lines) show a significantly lower ratio to CFC-12 than expected. No systematic change from day one to day eight was measured for either of the two compounds. While dichloromethane was stable over a storage time of 1 week, it was influenced by ozone and already exhibited depleted mixing ratios in the canisters 1 d after pressurization. It can thus not be reliably measured using HIRES canisters from the lowermost stratosphere.
The substances that were found to be depleted in HIRES canisters when pressurized in the presence of ozone were dichloromethane (CH
It should, however, be noted that the experiment does not adequately mimic stratospheric conditions. In the laboratory tests presented here, the reference gas is mixed with the ozone-enriched synthetic air during the filling procedure. In flight, stratospheric air masses with high ozone levels will be in some state of mixing and continuously chemically processing. In addition, contact with hot surfaces, such as inside the metal bellows pumps, will destroy ozone.
Among the substances influenced by ozone, dibromochloromethane (CHBr
The long-term storage test comprised measurements of pressurized canisters after storage times of 1, 8, 15, 29, 51, and 57 d. While for the short-term test individual canisters were measured on day one and day eight, this was not possible for the long-term test. For the long-term test, six cylinders were simultaneously filled, and on each measurement day the next canister in the series was measured. Thus, it cannot be fully excluded that stability might not only depend on the substance investigated, but it might be a feature of an individual canister, for example related to the quality of welding seams.
Figure
Some substances that were found to be stable during the 1-week short-term test decreased after longer storage times, for example dichloromethane, shown in Fig.
Results of the ozone and long-term storage test for HFC-134a
Figure
Results of short-term (8 d) storage test, long-term (57 d) storage test, and ozone interference. “X” indicates that substances were found to be stable in the respective storage test, and arrows indicate whether mixing ratios were increasing (
As an example of air collected in the atmosphere under real conditions,
samples from CARIBIC flight 544, which took place on 22 March 2018 travelling from Munich (Germany) to Denver (US), were analysed.
These measurements were performed approximately 5 weeks after the flight; thus only substances that were shown to be stable in the long-term stability experiment are expected to yield reliable results. Figure
Time series of ozone (red), CFC-12 (yellow), and dichloromethane (blue) during a flight from Munich to Denver on 22 March 2018. Ozone high-resolution data represented by the red line are integrated over the sampling period of each whole-air sample (red diamonds).
CFC-12 anticorrelates with mixing ratios of ozone, and this is also found for the other long-lived compounds which were stable in the storage experiments. Such a behaviour is expected, because ozone-rich stratospheric air masses are aged and should contain lower mixing ratios depending on a substance's stratospheric lifetime and transport pathway.
Three of the canisters analysed from this flight were collected in tropospheric air masses characterized by lower mixing ratios of ozone levels.
Mixing ratios of CFC-12 measured in these samples are around 510 ppt, which is consistent with current tropospheric mixing ratios observed at ground sites
Dichloromethane mixing ratios are below 1 ppt in most samples, which is close to the 0.4 ppt limit of detection as derived from the 3-fold noise level. Only three samples have mixing ratios above 5 ppt; they are at the same time characterized by higher mixing ratios of CFC-12 and low mixing ratios of ozone, which is indicative of tropospheric air.
In the tropospheric air samples, dichloromethane varied between 14 and 49 ppt. These mixing ratios agree with mixing ratios in tropospheric samples in the dataset presented by
Figures
Correlation of CFC-12
Correlation of CFC-12
Dichloromethane does not correlate with ozone or with CO in the stratosphere (Figs.
In order to assess the potential of halocarbon analysis from samples collected with the HIRES unit from the CARIBIC instrument package, the sample collection unit was intensively tested, focusing on compound stability in the stainless-steel canisters and the influence of ozone. Sampling during CARIBIC flights takes place in the upper troposphere and the lowermost stratosphere, with ozone mixing ratios of up to several hundred parts per billion by volume. Therefore samples were pressurized with a mixture of a dry standard gas, containing typical tropospheric mixing ratios of a wide range of halogenated hydrocarbons, and synthetic air. The synthetic air could be enriched in ozone by passing an ozone-generating UV lamp. Final ozone mixing ratios were estimated to range from 400 to 600 ppbv. This is representative of the mixing ratios typically encountered at flight levels in the lowermost stratosphere. Several short-lived halocarbons were found to be depleted in canisters pressurized in the presence of ozone. COS was found to exhibit higher mixing ratios in this case.
In one experiment, samples were analysed 1 d after pressurization and again after a storage time of 1 week. While bromomethane and chloromethane were found to have already grown in this short period, tribromomethane and trichloroethene had decreased; tetrachloromethane was found to be stable, but its mixing ratio was significantly below the value expected. Of the 28 compounds investigated, 23 were found to be stable over storage of up to 1 week.
This changed in the long-term stability test, which was conducted in several time steps with periods of up to 57 d. All compounds influenced by high levels of ozone were found to show the same behaviour (decreasing or increasing) during the long-term test. In addition, dichloromethane, trichloromethane, tetrachloroethene, tetrachloromethane, and bromochloromethane also showed a tendency to decrease after storage for more than 2 weeks. HFC-152a, HFC-23, and Halon 1301 started to increase significantly. The tests showed that reliable measurement results for a number of halogenated tracers can only be achieved if measurements are performed within a few days of a flight. If this is not possible (which for CARIBIC samples is often the case, as they circulate between several laboratories at different institutions), results must be interpreted with care.
Measurements of samples from a CARIBIC flight in March 2018 that took place about 5 weeks after the flight confirmed the results from the stability tests. Mixing ratios of compounds found to decrease in the stability tests were in general very low, often below their respective detection limits. Additionally the results of the ozone test were confirmed, as mixing ratios of compounds found to be sensitive to ozone were low in canisters sampled in the stratosphere at high ozone mixing ratios. With the current HIRES unit it is not possible to study aspects such as the vertical gradients of mixing ratios above the tropopause for short-lived species. Compounds that had grown during the storage test were not evaluated.
Currently, we are in the process of constructing a second high-resolution air sampler for use inside the CARIBIC container. Based on the measurements presented here, close attention will be given to the manufacturing of the stainless-steel cylinders, which will be made of electropolished stainless-steel foil, and welding will be performed under vacuum. In contrast to the canisters tested here, which were microplasma welded from similar stainless-steel foil that was not electropolished, we expect to obtain cleaner inside surfaces and cleaner welding seams. During the construction of the new sampling unit, more stability tests like those presented here will be performed.
Data are available from the corresponding author upon individual request.
TS, AKB, and AE designed and performed the storage experiments. TS, AKB, and ER performed the data analysis. CAMB designed and constructed the HIRES unit. JW coordinated the HIRES handling and scientific deployment. AZ coordinated the CARIBIC project. TS wrote the first draft of the manuscript. All co-authors took part in the discussion of the results and contributed to the paper.
The authors declare that they have no conflict of interest.
The authors acknowledge the contribution of technical staff who perform regular maintenance of the CARIBIC container and handle the air sampling unit. We would also like to thank Martin Vollmer (EMPA) for calibration of the laboratory standard and Laurin Merkel for drawing the HIRES unit.
This paper was edited by Tim Arnold and reviewed by two anonymous referees.