1.5 Airborne platforms and instrument requirements
1.5.3 Instrument requirements
Due to the relatively low cruising altitude of around 10. . . 12 km of commercial airliners, the air sampled during CARIBIC flights is mostly of tropospheric origin, with occasional strato-spheric intrusions [Zahn, 2008]. In the troposphere, the water vapor mixing ratio ranges be-tween > 5 000 ppmv near the ground to approximately 10 ppmv in the upper troposphere. In the stratosphere in contrast, the water mixing ratio is generally much lower (≈ 3 . . . 5 ppmv).
On the other hand, the HALO research aircraft, having a certified cruising altitude of 15 km, provides the capability of extensive flights into the dry stratosphere. This means that the spec-trometer must be capable to measure isotopic ratios within a large range of water mixing ratios, and must provide the necessary sensitivity (precision).
The expected changes in isotopic ratio (deviation from ocean water) are thereby around δ18O = −160 to −100h for H218O and approximately half of that for H217O. The envisioned measurement precision for δ17O and δ18O is about 20h for water mixing ratios between 25 ppmv and 100 ppmv. Changes in HDO, which is not further discussed in the present work, are much larger (δ D = −800 to −600h), and therefore the required measurement precision is about 50h. Measurements of HDO and consequently δ D will be possible with an upcoming version if this instrument, as discussed in Chapter 2.5.
In addition to the high sensitivity requirements, measurement accuracy is crucial. The envisioned strategy to achieve high accuracy is to perform in-flight calibration of the instru-ment by providing a well characterized measureinstru-ment standard. To this end, a strong effort
led to the development of a suitable calibration gas generator, which is described in detail in Section 2.2.3.
Regardless of the aircraft used, these platforms add mechanical requirements to the mea-surement instrument that very much differ from laboratory or ground-based applications. On airborne platforms one generally has to deal with mechanical vibration and shock, that may induce alignment changes in optical equipment, and thus may potentially lead to a degradation in instrument performance. In addition, aircraft cabin temperature and pressure changes by as much as 30◦C and 200 hPa, respectively, thus inducing mechanical deformation of components of the instrument [Dyroff , 2003; Dyroff et al., 2004]. Therefore the instrument must provide temperature stabilization and a rugged design in order to minimize performance degradation.
On the other hand, and in particular for the CARIBIC and HALO platforms, instrument space and weight are extremely limited. Any of the instruments employed on these platforms need to fit in 19-inch racks, where the height of the present instrument is limited to < 40 cm.
The instrument weight is limited to about 40 kg. In contrast to other high-sensitivity laser spectrometers employed on different airborne platforms [Hanisco et al., 2007; Kormann et al., 2005; Webster et al., 1994; Wert et al., 2003a], these constraints are rather tight, especially because the instrument performance needs to be comparably good, or perhaps better, than standard equipment.
Furthermore, the present instrument is required to operate in a fully autonomous mode, including warmup phase, frequent calibration, and must be able to deal with changing envi-ronmental conditions such as cabin temperature and sample gas pressure. This is in contrast to other, larger airborne platforms, such as the National Center for Atmospheric Research (NCAR) C-130. On these platforms, a human operator attends the flights, handles the instru-ment and can react to perturbations of the measureinstru-ment conditions.
Highly sensitive measurements of water vapor isotopic ratios
In the present Chapter, the design and performance analysis of a highly sensitive laser spec-trometer for water vapor isotopic ratio measurements is presented. Measurements of the iso-topic ratio of water are a valuable tool to study atmospheric transport processes. Despite its usefulness, such measurements have only been performed by a few research groups, and Sec-tion 2.1 reviews the techniques applied and the results that have been achieved.
Section 2.2 then describes the design of the laser spectrometer. This includes the descrip-tion of the optical part of the spectrometer in Secdescrip-tion 2.2.1. Secdescrip-tion 2.2.2 then discusses the selection of suitable absorption lines, which goes hand in hand with the selection of the laser source and the photodetector. A suitable set of strong absorption lines as well as low-noise laser and detector is a prerequisite for high-sensitivity isotopic-ratio measurements. In addi-tion to a high sensitivity, regular calibraaddi-tion is important in order to achieve high accuracy. To this end, a calibration-gas source has been built, and its design is described in Section 2.2.3.
Finally, Section 2.2.4 describes the data acquisition and post-processing procedure, which is used to determine the isotopic ratio of water vapor within a (generally) unknown gas mixture.
The present spectrometer has been extensively tested in order to determine the system sta-bility time and the detection limit for isotopic ratio measurements (Section 2.3). As a first step, the signal-to-noise ratio of the detection system, comprising the laser diode and the detec-tor/preamplifier, has been studied. The analysis is described in Section 2.3.1. This is followed by a description of isotopic-ratio measurements in Section 2.3.2. These measurements have been performed in order to determine the detection limit as well as the stability time of the spectrometer. Several aspects are discussed that may determine the stability time. Finally, the spectrometer response to different isotopic water standards has been tested. The results, which are discussed in Section 2.3.3, are used to determine the accuracy of the spectrometer.
A conclusion (Section 2.3.4) finalizes the this Section.
The performance specifications of the final instrument are summarized in Section 2.4. This Section presents in a compact form the physical properties of the present spectrometer as well as its detection limit and accuracy.
A discussion of techniques that may potentially lead to an even more improved sensitivity is given in Section 2.5.
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2.1 Existing techniques for airborne isotopic ratio measure-ments
Water isotopologues have found increasing interest during the last decade as a means to bet-ter understand the transport processes of wabet-ter in the atmosphere. The first measurements have been made by remote sensing from balloon or spaceborne platforms [Dinelli et al., 1991;
Moyer et al., 1996; Rinsland et al., 1984, 1991], and a number of further remote measure-ments with different instrumeasure-ments have followed [Herbin et al., 2007; Johnson et al., 2001;
Nassar et al., 2007; Payne et al., 2007; Steinwagner et al., 2007; Urban et al., 2007; Worden et al., 2006]. While these measurements can improve our understanding of large-scale pro-cesses, in-situ measurements are necessary to provide inside into small scale propro-cesses, such as isolated convective cells in tropical latitudes. However, the number of in-situ instruments and measurements is still very limited until to date.
Zahn[2001] and Franz and R¨ockmann [2005] have measured water isotopic ratios by col-lecting water samples in situ using a cold trap, with subsequent laboratory-based analysis by mass spectrometry. While mass spectrometry is capable to deliver high precision measure-ments (about 0.2h), a relatively large water sample is required. This in turn requires long sampling times (about 20 min) during the flight, especially at low water concentrations, which limits the spatial resolution of the measurement. In addition, the technique requires substan-tial sample preparation due to mass-overlap of the H217O and HDO water isotopologues (both M=19).
Webster and Heymsfield [2003] have measured H2O isotopic ratios in the 1 484 cm−1 (6.74 µm) spectral region employing the ALIAS tunable diode laser spectrometer [Webster et al., 1994], which is based on wavelength-modulation spectroscopy using a 80 m multipass absorption cell. This instrument was employed for a total of eleven flights aboard the NASA WB-57 aircraft during the CRYSTAL-FACE1campaign in 2003 [Jensen et al., 2004; Webster and Heymsfield, 2003, see also supplemental online material]. The instrument calibration is performed pre-flight using certified water samples, and the reported uncertainty is 50h for water vapor mixing ratios> 10 ppmv, and larger for water vapor mixing ratios < 10 ppmv.
Kerstel et al.[2006] measured H2O isotopic ratios in the 7 183 cm−1(1.39 µm) spectral re-gion using their IRIS instrument. The instrument is based on optical feedback cavity-enhanced absorption spectroscopy (OF-CEAS) [Morville et al., 2005], where a laser beam is coupled into the fundamental mode of a high finesse optical cavity. A detector behind the cavity measures the transmission spectrum of the cavity. Due to the high reflectivity of the cavity mirrors, an effective optical path length of > 4 km (∼ 20 µs residence time) has been reached. The instrument calibration is performed in the laboratory using two different isotopic standards.
The instrument was flown on a few engineering flights aboard the NASA WB-57 aircraft in 2006. At very stable conditions, the uncertainties in isotopic ratios are reported to be 1h, 3h, and 9h for δ18O, δ17O, and δ D, respectively, for data averaged over 30 s and at a total water mixing ratio of about 200 ppm. However, the precision achieved is generally worse because the averaging time is limited by systematic drift [Kerstel et al., 2006; Kerstel, 2007].
Hanisco et al. [2007] report on the application of two instruments developed simultane-ously, the integrated cavity output spectrometer (ICOS) [Sayres, 2006] to measure δ18O, δ17O,
1Cirrus Regional Study of Tropical Anvils and Cirrus Layers Florida Area Cirrus Experiment
and δ D, and the laser induced fluorescence spectrometer (HOxotope) to measure δ D [St.Clair et al., 2006]. Both instruments flew on the NASA WB-57 aircraft as part of the AVE-WIIF2 campaign during June and July 2005.
The ICOS device measures in the 1 484 cm−1 (6.74 µm) spectral region, where the laser beam is injected into a high finesse cavity at an off-axis direction. The laser beam therefore produces a spot pattern much like in a Herriott cell, however providing a much greater effective optical path length of up to 4.2 km [Sayres, 2006] as the laser beam does not fully exit the cavity after fulfilling the reentrant condition (compare with Eq. 3.6). The system is calibrated in-flight using a 20 ppmv standard from a gas cylinder [Sayres, 2006]. This gives rise to potential fractionation effects as water sticks to the cylinder walls, thus leading to a slightly different isotopic ratio (remaining water in the cylinder is expected to be heavier than outgoing water). The calibration uncertainty of this instrument is reported to be 50h [Hanisco et al., 2007]. The instrument weights about 250 kg, and requires about 1.5 kW of power [Moyer et al., 2007].
Within the HOxotope instrument, water is first photolyzed with an excimer lamp at 172 nm, producing ground state OH and OD radicals. The radicals are excited with laser radiation at 287 nm from a frequency-doubled dye laser. Fluorescence of neighboring OH and OD emis-sion lines is detected at 309 nm [St.Clair et al., 2006]. The calibration with water standards is performed in the laboratory, and the calibration uncertainty of this instrument is reported to be 50h [Hanisco et al., 2007; St.Clair et al., 2006].
The three groups mentioned above are — to the best of my knowledge — at present the only groups worldwide that work (or have worked) on in-situ measurement of water isotopic ratios. Their instruments have been successfully employed during relatively short measure-ment campaigns to study rather isolated atmospheric phenomena. However, due to the scarce-ness of the conducted measurements, the transport of water in the upper troposphere and lower stratosphere (UT/LS) is presently still relatively poorly quantified. In this context I believe that the unique possibilities for long-term employment of the present isotopic ratio spectrometer aboard the CARIBIC as well as HALO aircraft (see Section 1.5) will ultimately lead to a better understanding of transport processes in Earth’s atmosphere.