In-flight stability of quantum cascade laser-based infrared absorption spectroscopy measurements of atmospheric carbon monoxide
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Airborne carbon monoxide (CO) measurements based on Quantum cascade Laser infrared Absorption Spectroscopy (QLAS) were performed on the German High-Altitude Long-range Observatory (HALO) aircraft during test flights in January 2015. Here we investigate the in-flight stability of TRISTAR (TRacer In-Situ Tdlas for Atmospheric Research), a multilaser QLAS instrument for the detection of tropospheric CO, methane and formaldehyde (HCHO). During one test flight the instrument was probed with tank air to measure a constant mixing ratio of CO and zero air for HCHO. Here we investigate the instrument stability for the CO channel of TRISTAR and identify potential noise sources as well as environmental processes that limit the stability of the instrument. The 1σ reproducibility of the constant CO measurement yields a value of 1.2% (2.9 ppbv) corresponding to an optical density limit of 0.001 for a 5-s average. The CO precision is ultimately limited by an etalon fringe originating from the double corner-cube White cell, whose phase and amplitude changes with the aircraft heading.
KeywordsHCHO Quantum Cascade Laser Test Flight Infrared Absorption Spectroscopy Tunable Diode Laser Absorption Spectroscopy
Mid infra-red absorption spectroscopy is a versatile tool to quantify mixing ratios of atmospheric trace gases at ppbv and sub-ppbv levels. In particular, the use of tunable infra-red lasers has found widespread application during the past four decades. While instruments in the 1980s and 1990s deployed lead-chalcogenide tunable diode lasers [1, 2, 3, 4], since about the year 2000 tunable quantum cascade lasers have mostly been used due to their superior behavior with respect to laser power, single mode operation and stability [5, 6, 7]. In particular, the deployment of Tunable Diode Laser Absorption Spectroscopy (TDLAS) or Quantum cascade Laser Absorption Spectroscopy (QLAS) instruments on research aircraft require compact, low weight and rigid designs to face the challenging demands due to vibrations, as well as cabin pressure and temperature fluctuations on the platform [5, 6, 8, 9, 10, 11, 12]. Since 1997, we have deployed the multi-laser TRacer In-Situ Tdlas for Atmospheric Research (TRISTAR) on a number of airborne platforms during a total of 15 measurement campaigns, on 170 research flights with more than 800 flight hours for tropospheric and stratospheric measurements of carbon monoxide (CO), carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and formaldehyde (HCHO) [13, 14]. In the present configuration TRISTAR deploys three liquid nitrogen cooled continuous wave quantum cascade lasers for CO, CH4 and HCHO. As mentioned above, the measurement precision during airborne applications suffers from a non-ideal environment. In particular, changes of the cabin temperature and the cabin pressure can affect the optical alignment and the operation conditions of lasers and infra-red detectors. For species with mixing ratios in the ppbv to ppmv range like CO (typical tropospheric mixing ratios of 80 ppbv) and CH4 (~2 ppmv) the absorptions correspond to optical densities in the 10−2 range and the reported in-flight precisions, based on the reproducibility of in-flight calibrations, are in the sub per cent range , similar to results obtained with other airborne QCL spectrometers [15, 16]. For HCHO, whose tropospheric mixing ratio is in the sub-ppbv range, corresponding to optical densities of the order of 10−5, the detection limit is more important than instrument precision. Reported values based on the reproducibility of zero air measurements are in the low tens of pptv range and are limited by optical fringes . In order to improve both the precision of CO and CH4 measurements and the detection limit for HCHO, the instrument has to be stabilized with respect to the changing environmental processes, such as pressure, temperature and aircraft movements and vibrations. This requires a detailed knowledge of the parameters affecting the instrument stability during airborne operations. In January 2015 TRISTAR together with other trace gas instruments was installed onboard the German High-Altitude Long-range Observatory (HALO) aircraft for the Oxidation Mechanism Observations (OMO) project. Since the payload was new, a number of test flights were performed during the first phase of OMO-Europe (OMO-EU). During one of these flights we had a chance to test the in-flight instrument stability under controlled conditions. For this purpose TRISTAR was operated in a dual channel mode (CO and HCHO), sampling either a CO standard from a tank or zero air from scrubbing HCHO from the same tank. Since the CO and HCHO measurements were made with a constant signal input, the in-flight data provide insight into the instrument stability under changing environment conditions such as temperature, pressure and position changes due to flight level changes and aircraft heading. Here we will discuss results for the CO measurements. Results for HCHO will be addressed in a separate study. Please note that TRISTAR was autonomous in operation. Although 4 operators were supervising the instruments operated on HALO (15 instruments in total), no trained operator was dedicated to the operation of TRISTAR.
The paper is structured as follows: Sect. 2 provides a short description of TRISTAR. Since the main features of the instrument have been discussed in a number of previous publications [13, 14, 17], we will mainly address recent changes and specific conditions for the operation on HALO. The results from the in-flight stability tests will be presented in Sect. 3, while in Sect. 4 applications of ambient CO data obtained during two measurement flights will be discussed to illustrate the data quality. Finally (Sect. 5), we will summarize the obtained results and give an outlook for future instrument improvements.
2 Instrument set-up
2.1 Optical and mechanical set-up
The optical and mechanical set-up consists of an optical board with a 66 cm × 52 cm footprint, a 2-l liquid nitrogen cryostat (ILK, Dresden) housing three quantum cascade lasers (Alpes Lasers, Lausanne, Switzerland) for CO (2158.30 cm−1), HCHO (1759.72 cm−1) and CH4 (1298.98 cm−1) operating at constant temperatures between 95 and 135 K and two cryogenic (~78 K) photovoltaic mercury-cadmium-telluride detectors (Kolmar Technologies Inc., Newburyport, USA), a double-corner cube multi-pass cell following the design of White  and a set of reflecting optics for beam collimation and stirring. Time multiplexing of the three laser beams is achieved via pneumatically driven pop-up mirrors. Details of the optical set-up are documented in [13, 14, 17].
Both the optical alignment and the laser operation conditions are sensitive to changes in the surrounding temperature and to a lesser extent to changes in the cabin pressure. Therefore, the optical board and the laser cryostat are enclosed in an aluminum frame with 3 cm of insolation foam (melanin foam) mounted on the inside. Additionally heating foils are glued to the underside of the optical board. The combination of the controlled heating of the optical board and cooling due to boiling-off from the liquid nitrogen cryostat allows the temperature inside the enclosement to be regulated to (35 ± 0.1) °C, even at cabin temperatures up to 40 °C. These measures guarantee a constant temperature of the optics independent of cabin temperature. The time required from start-up of the instrument until stabilization of the instrument temperature is of the order of 2 h. Unfortunately, the aircraft safety instructions require a power shut-off during roll-out and refueling of the plane, resulting in an approximately 30-min interruption of the active temperature stabilization during the final phase before take-off. Although the enclosement is still able to keep the temperature of the optics close to its set-point, the first part of a measurement flight is still affected by slight changes in the optical temperature.
The main effect of pressure variations inside the cabin (at higher altitudes the cabin pressure is reduced to approximately 840 hPa) is the change in boiling temperature of the liquid nitrogen. This can in principle lead to changes in the temperature settings of the lasers and detectors with subsequent changes of the operation conditions. Although the optical board is mounted onto the rack with shock-mounts, accelerations of the aircraft due to ascent, decent or changing of the heading (curves) can potentially induce alignment changes. Additional changes of the cabin pressure can lead to changes in the optical alignment in particular that of the White cell.
2.2 Signal processing and data acquisition
The electronic set-up has been described in detail by Schiller et al. . Briefly, saw-tooth current ramps of 66 ms duration are applied to each laser, sweeping the emission frequency across the target absorption lines. Additionally a 20-kHz sinusoidal component is added to the laser current for wavelength modulation. The amplified detector output from a signal and a reference channel (for active line-locking) undergoes phase-sensitive detection at 40 kHz (2f) with analog lock-in amplifiers (Femto Messtechnik, model LIA-BV-150-S, Berlin, Germany). The data acquisition is very flexible and can be adjusted to the measurement tasks. Since HCHO mixing ratios in the troposphere are 3 orders of magnitude smaller than for CO, the majority of the sampling time is dedicated to this species. Three different measurement modes are distinguished: in-flight calibration (CO, CH4), background (HCHO) and ambient (CO, CH4, HCHO) measurements. During calibrations a standard is supplied to the instrument from a tank (6 l composite tank, Auer GmbH, Germany) to acquire a reference spectrum of known concentration. Ambient spectra are fitted to these calibration spectra. In-flight calibrations for CO and CH4 are performed every 30 min for 1 min. In-flight calibrations for HCHO are presently not performed. Instead, pre- and post-flight calibrations are performed using a permeation source. In general HCHO measurements are more affected by optical fringes that induce background structures to the spectrum, which are larger than the gas absorption itself. Therefore, regular in-flight background measurements are performed with cabin air after passing a scrubber (platinum pellets heated to 80 °C) removing HCHO completely every 144 s for a duration of 35 s. These background spectra are subtracted from ambient HCHO measurements. In-between background measurements ambient measurements are performed in cycles, each consisting of HCHO (5 s), CO (1 s) and CH4 (1 s). A complete cycle lasts 12 s (including delays due to mirror adjustments) and an ambient measurement period consists of 7 cycles (105 s).
2.3 Inlet configuration
3 Results from the test flight
On this particular day, the aircraft was moved out of the hanger around 9:00 in the morning (local time). Before roll-out the instruments were under power for about 2 h in the hangar. During roll-out the power supply for all experiments was interrupted for about 30 min, with repowering around 9:30. As shown in Fig. 3c the plate temperature decreased from the set value of 31.5to 8 °C during the power interruption. After restart of the instrument the optical plate is slowly heated up and finally reached the setting point after approx. 2 h. A faster heating would result in a strong swing of the temperature above the setting point, further delaying the temperature stabilization. The CO data for calibrations (green in Fig. 3a) and “ambient” (red) are calculated by fitting to an average master spectrum obtained on the ground around 9:40 (black. The measured “ambient” CO data varied between 241 and 266 ppbv relative to the master spectrum (mean: 257.1 ppbv; 1σ standard deviation: 4.91 ppbv), exhibiting a long-term drift towards higher values and sometimes a sinusoidal drift within individual “ambient” data packages. The long-term drift closely follows the changes in cabin and plate temperature. Although the change in cabin pressure that varies between 950 hPa at low flight levels and 840 hPa at the highest flight level affects the DC signal of the MCT detector, it has no significant influence on the “ambient” data. Obviously, changes in pressure will mainly affect the boiling temperature of the liquid nitrogen used for cooling and thus the operating temperature of the MCT detectors which is not actively stabilized, while the temperature setting of the laser is not affected due to its active temperature control.
This analysis demonstrates that the quality of the CO data is ultimately limited by spectral background structures (fringes) at an optical density level of 0.001. While long-term drifts can be addressed by in-flight calibrations or a de-trending based on a relationship with the temperature of the optical table, the variability of the short-term fluctuations cannot be removed by these measures. To reduce the etalon effect one could use background subtraction, similar to those performed for HCHO. Unfortunately, the scrubber used for HCHO does not remove CO completely. The scrubbing efficiency for CO was determined to be 0.84 ± 0.01% and showed a slight dependency on the temperature of the scrubber and the aircraft’s angle of attack. So in order to use CO background measurements, the scrubbing efficiency for CO has to be increased to 100%. It is not clear if this can be achieved with the present design of the scrubber. An alternative method to reduce the influence of the optical fringes on the precision of the CO measurements is the application of wavelet transformation to the CO spectra. As demonstrated by Li et al.  for ground-based CO data this can reduce the noise level due to fringes by roughly a factor of 2. In the next chapter we will demonstrate that the actual precision of the CO data in its present form is sufficient for atmospheric studies during OMO-EU.
4 Results from two measurements flights
Here we used a test flight on the HALO aircraft during the OMO-EU mission to identify noise sources and stability limitations for CO measurements obtained from mid-IR laser absorption spectroscopy. During the flight, the instrument was probed with a constant mixing ratio from an on-board gas tank, normally used for in-flight CO and CH4 calibrations. Ideally this should produce a constant signal during all phases of the flight. Deviations, in particular a long-term drift, are due to slow changes in the temperature of the optical plate during the first 2 h of a measurement flight. Although the temperature of the optics is actively stabilized, an unavoidable power break during the roll-out of the aircraft out of the hangar is responsible for the temperature drift of the optics and subsequently the drift in measured CO data. Regular in-flight calibrations or a temperature-CO mixing ratio polynomial relation can be used to successfully de-trend the data. Ultimately, the precision is limited to values around 3 ppbv by changes in the amplitude or phase of optical fringes similar to those limiting the detection limit for HCHO . To overcome this problem, regular high-frequency background measurements for CO would be required. Such background measurements are already performed for HCHO, but the scrubber used has an insufficient scrubbing efficiency for CO, which is of the order of 85% (100% for HCHO), requiring a redesign. An alternative solution to suppress the influence of the optical fringes could be the application of a wavelet transformation to individual absorption spectra, as has been demonstrated for ground-based CO measurements . Nevertheless, an analysis of ambient data obtained during two measurement flights demonstrates that the CO data quality is sufficient for atmospheric studies.
Open access funding provided by Max Planck Society. This research was part of the OMO-EU campaign. Thanks to Andreas Zahn and his colleagues from Karlsruhe Institute of Technology for providing the ozone data. Also we would like to thank Torsten Schmitt for his contribution to the post-processing of the data sets. Last but not least, we want to thank the whole OMO community for realizing the campaign.
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