Modification , Characterization and Evaluation of a Quantum / Interband Cascade Laser Spectrometer for simultaneous airborne in situ observation of CH 4 , C 2 H 6 , CO 2 , CO and N 2 O

Achieving an improved understanding of the anthropogenic influence on climate due to man made greenhouse gas emissions is of major interest for the global civilization. Sources, sinks and transport of climatologically-relevant gases in the Earth’s atmosphere are still insufficiently understood, implying a fundamental need for accurate, spatially and temporally dense observations. Tunable diode laser absorption spectroscopy is a widely used technique for in situ sensing of atmospheric composition. Mid-infrared spectrometers have become commercially available, since continuous wave quantum 5 cascade (QCL) and interband cascade lasers (ICL) today achieve excellent performance, stability and high output power at typical ambient conditions. Aircraft deployment poses a challenging environment for these newly-developed instruments. Here, we demonstrate the successful adaption of a commercially available QCL/ICL based spectrometer for airborne in-situ trace gas measurements. The instrument measures methane, ethane, carbon dioxide, carbon monoxide, nitrous oxide and water vapor simultaneously, with high 1σ-precision (740ppt, 205ppt, 460ppb, 2.2ppb, 137ppt, 16ppm, respectively) and high frequency 10 (2Hz). We estimate a total measurement uncertainty of 2.3ppb, 1.6ppb, 1.0ppm, 7.4ppb and 0.8ppb in CH4, C2H6, CO2, CO and N2O, respectively. The instrument enables truly simultaneous and continuous (zero dead-time) observations for all targeted species. Frequent calibration allows for a measurement duty cycle ≥ 90% while retaining high accuracy. A custom retrieval software has been implemented and instrument performance is reported for a first field deployment during NASA’s Atmospheric Carbon and Transport America (ACT-America) campaign in fall 2017 over the eastern and central U.S.. This 15 includes an inter-instrumental comparison with a calibrated cavity ring-down greenhouse gas analyzer (operated by NASA Langley Research Center, Hampton, USA) and periodic flask samples analyzed at the National Oceanic and Atmospheric Administration (NOAA). We demonstrate excellent agreement of the QCL/ICL based instrument to these concurrent observations within the combined measurement uncertainty. 1 Atmos. Meas. Tech. Discuss., https://doi.org/10.5194/amt-2018-312 Manuscript under review for journal Atmos. Meas. Tech. Discussion started: 5 October 2018 c © Author(s) 2018. CC BY 4.0 License.


Introduction
With steadily increasing greenhouse gas concentrations in the Earths atmosphere an improved understanding of the anthropogenic influence on climate is of major interest for the global civilization.Globally averaged carbon dioxide (CO 2 ) mixing ratios have increased by 40% since 1750.Methane (CH 4 ) mixing ratios have more than doubled since the pre-industrial era, where over 60 % of this increase is estimated to be of anthropogenic nature (IPCC, 2014).Nitrous oxide (N 2 O) is a strong greenhouse gas and is expected to remain the most important ozone-depleting anthropogenic impact throughout the 21st century (Ravishankara et al., 2009).Ethane (C 2 H 6 ) is a powerful tracer commonly used to discriminate between different types of methane sources (Smith et al. (2015); Barkley et al. (2017); Peischl et al. (2015)) and carbon monoxide (CO) is a marker for incomplete combustion processes and relates to the formation of tropospheric ozone (Klemm et al., 1996).
Aircraft provide a flexible basis for satisfying the fundamental need for accurate, temporally and spatially dense observations of these climatologically-relevant gases from local to regional scales.On-board meteorological data acquisition systems allow for concurrent observations of important atmospheric state variables like the local wind field, which is particularly useful to estimate emission fluxes.Spectroscopic instruments making use of molecular ro-vibrational absorption allow for high temporal coverage through fast instrument response times (Chen et al., 2010).Some have already been used for airborne research, e.g.well-established cavity ring-down instruments (O'Shea et al. (2013); Santoni et al. (2014) ;Cambaliza MOL (2015); Filges et al. (2015)).With the commercial availability of continuous-wave lasers emitting in the mid infrared (IR) region near ambient temperature (Capasso (2010); Vurgaftman et al. (2015); Kim et al. (2015), Beck et al. (2002)) several new instrument designs have emerged (McManus et al. (2015); Zellweger et al. (2016)).QCL/ICL based systems exploit several orders of magnitude stronger molecular absorption features in the mid infrared compared to near infrared based cavity ring-down instruments.Richter et al. (2015) reported on a custom-built QCL spectrometer for simultaneous in-situ detection of formaldehyde (CH 2 O) and C 2 H 6 providing best-in-class detection sensitivities of 40 ppt and 15 ppt, respectively.The custom-built airborne QCL spectrometer described by Catoire et al. (2017) allows for simultaneous observation of CO, CH 4 and nitrogen dioxide (N O 2 ) with in-flight precisions of 0.3 ppb, 5 ppb and 0.3 ppb for a sampling time of 1.6 s.McManus et al. (2011) reported on the development of a high-sensitivity trace gas instrument based on quantum cascade lasers and astigmatic Herriott cells with up to 240 m path length.This design is commercially available from AERODYNE RESEARCH INC., Billerica, USA, and allows for simultaneous observation of a multitude of gases, depending on the wavelength of the installed lasers.Unlike many established cavity ring-down instruments measuring different species sequentially, the described spectrometer allows for truly concurrent sensing of the selected observables and faster response times.These instruments have already been operated on different research aircraft.Santoni et al. (2014) reported on overall instrument performance for over 500 flight hours.Pitt et al. (2016) found a strong cabin pressure dependence on retrieved methane mixing ratios.Recently, Gvakharia et al. (2018) described a fast calibration strategy to overcome this cabin pressure dependence.
Here, we describe the setup and performance of our flight-proven (over 100 flight hours) airborne QCL/ICL system developed for simultaneous airborne measurements of CH 4 , C 2 H 6 , CO 2 , CO, N 2 O and H 2 O. Section 2 summarizes the refinements over the commercial system for use on aircraft.We show that frequent two-point calibration can mitigate cabin pressure de-pendencies.Section 3 describes our custom-built retrieval software developed for tuning the retrieval process to yield optimum output.Sections 4 and 5 report on instrument performance in the laboratory and in the field during NASA's ACT-America fall 2017 campaign, respectively, including an inter-instrumental comparison with a calibrated cavity ring-down instrument and periodically taken flask samples.Section 6 summarizes our findings and concludes the study.

The airborne DLR QCL/ICL spectrometer
The spectrometer system used here builds upon the Dual Laser Trace Gas Monitor, a commercial tunable IR laser diode absorption spectrometer (TILDAS) available from AERODYNE RESEARCH INC., Billerica, USA.The basic instrument has already been extensively described in McManus et al. (2011).We will therefore only briefly introduce the basic instrument setup followed by a description of the refinements required to operate the instrument on research aircraft.

Basic instrument setup
The spectrometer is split into an electronics compartment and an optics compartment.The electronics compartment includes an embedded computing system, thermoelectric cooling (TEC) controllers, power supplies, etc..The optics compartment includes the lasers, the sample cell, the pressure controller and all optical elements.output spectrum every half of a second.Before reaching the sample cell, the laser beam travels approximately 1.6m inside the instrument under ambient conditions.This will be referred to as the open-path of the instrument, which is heavily influenced by variations in cabin pressure, temperature and humidity during airborne operation.After passing through the sample cell, the combined output from both lasers hits a single TEC-cooled detector.A second, identical detector collects radiation from two auxiliary paths.The first auxiliary path contains a small, sealed reference cell filled with CH 4 and N 2 O.This allows for spectral referencing during system startup.The second path introduces an etalon into the beam, allowing for experimental determination of the laser tuning rate, which relates laser supply current and emitted wavelength.

Refinements for airborne operation
The key challenges for a successful deployment on research aircraft are limited space and power, the occurrence of linear and angular accelerations and large pressure, temperature and humidity fluctuations in both cabin and sampled air.Airborne instrumentation further requires a fast system response time, owing to the rapid movement of aircraft in the atmosphere.The response time is controlled by the time it takes to completely exchange the air in the sample cell which is driven by the highest achievable volumetric flow rate given a specific pump and sample cell volume.
Here, a scroll pump has been chosen to enable a constant sample flow through the sample cell.The lubricant-free scroll pump runs very smoothly, avoiding injecting large vibrations into the measurement system, yet providing good pumping performance with a nominal value of 500 liters per minute at standard conditions.This translates to a net flow rate of 25 SLP M when operating with a cell pressure of 50 hP a.Earlier experience showed that large electrical inrush currents have jeopardized nominal system startup (priv.comm.Stefan Müller, MPI Mainz).The original motor has therefore been exchanged with a synchronous three-phase motor (BAUMUELLER NUERNBERG GMBH, Velbert, Germany).This DC motor provides a rated power of 627 W at 28 V DC.By using a digital motor controller the maximum startup current can be limited amongst various other tuning options.From previous studies the motor is known to emit a considerable amount of heat; a forced airflow provided by a standard axial fan ensures motor temperatures stay in the rated range.
Aircraft deployment requires the entire system to operate with a maximum of 50A at 28V DC.Power consumption of the instrument is mainly dominated by the pump and the thermoelectric cooling making up more than 3/4 of the total power requirement.Both components have been electrically converted without the need for power inverters from 230V AC to 28V DC to increase overall efficiency.The spectrometer and its internal computer are driven by a power inverter.to withstand linear accelerations of up to 9g on the aircraft forward axis, 8g on the downward axis, 6g on the upward and 2.25g sidewards.Due to aircraft certification issues, pure water is used as process fluid for the liquid cooling/heating circuit instead of the intended propylene glycol / water mixture.
A 3/8" inner diameter hose made out of Polytetrafluoroethylene (PTFE) has been chosen for the sample air intake as a compromise between pressure drop across the inlet and to minimize lag time between the inlet and the sample cell.Inside the instrument and upstream of the sample cell, an aerosol filter holds back particles bigger than 2 µm.The inlet is rear facing, preventing large particle entrainment and protecting the instrument from liquid water and ice.Owing to the small diameter, the intake flow is inside the turbulent regime at all times (Re ∼ 4000).
Finally, the sample cell pressure is regulated by means of a fully-configurable pressure controller (BRONKHORST High-Tech B.V., Ruurlo, Netherlands).A chip-scale temperature-compensated pressure transducer (Measurement Specialties (Europe), Ltd.) and a humidity sensor (Sensirion AG, Staefa ZH, Switzerland) have been built into the optics compartment, to allow for monitoring the open path state variables (see section 2.1).

In-flight calibration strategy
A custom-built calibration system has been implemented as illustrated in Fig. 2. Using mass flow controllers (MFCs, BRONKHORST High-Tech B.V., Ruurlo, Netherlands), two gases can be mixed at arbitrary ratios.The calibration gas mixture has been chosen to resemble "target" gas mixing ratios close to atmospheric ambient conditions.The cylinders have been cross-calibrated against NOAA standards and are thus traceable to World Meteorological Organization (WMO) standards for the species CH 4 and CO 2 .C 2 H 6 , CO and N 2 O are compared to NOAA flask samples taken during the ACT-America field campaigns, which are also traceable to WMO standards.We use ultra-pure synthetic air as "zero" gas instead of pure nitrogen (N 2 ) to be in accordance with aircraft safety regulations and because the mixing ratio of synthetic air (79.5% N 2 and 20.5% O 2 ) is chemically closer to sampled atmospheric air.Our calibration setup allows the net flow rate from the calibration cylinders to be slightly higher than the sample flow rate, minimizing pressure variations in the sample cell during switchover from normal to calibration sampling.To avoid contamination with cabin air, leak tests have been carried out on a regular basis during the ACT-America field campaign.
Owing to the high sensitivity of the retrieved mixing ratios to changes in ambient conditions during flights (Gvakharia et al., 2018), calibration cycles are carried out automatically every 5 to 10 minutes.Each cycle consists of a pre-programmed sequence of flushing the sample cell with zero gas for 10 seconds followed by another 10 seconds of calibration gas.These time intervals have been found to be a good compromise between calibration gas cylinder endurance and measurement duty cycle.settle to an approximately constant value within the first two seconds after switchover from calibration gas to sample air and vice versa.The only exception is water vapor, which is observed to settle after approx.30 seconds because of its stickiness and because the inlet tubing is made out of PTFE.The observed decay in H 2 O is different from the decay in other species in that a slow, almost linear decay follows the initial exponential decay, due to remaining water vapor in the inlet tubing and the sample cell.

3 Data Retrieval & Post-processing
The standard approach to retrieve dry-air mixing ratios from the Aerodyne QCLS instruments is by making use of the software supplied by the manufacturer (TDLWintel).Here we utilize a custom retrieval software (JFIT) developed to double check the output of the TDLWintel software and to enhance the ability of tweaking the retrieval process to yield optimum output.database using a conventional Voigt profile approach.Ethane line-by-line data have been taken from high-resolution FTIR spectra due to deficiencies in the HITRAN data for this particular species/wavenumber combination (Harrison et al., 2010).
The computation of the Voigt profile has been adopted from Abrarov and Quine (2015) for improved efficiency.Open-path water is also included in the model.
of the spectrum with no molecular absorption, are considered to represent the spectral baseline.The shape of this baseline is mainly controlled by laser characteristics, the detector response function and optical properties of the installed mirrors and windows inside the instrument.
The spectrum is broken down into 3 micro windows for the retrieval process (see Fig. 5).These were chosen based on best overall performance found in retrieval tests and named after the chemical species included.A synthetic spectrum, including a polynomial representing the spectral baseline, is generated and fitted using an unbounded Levenberg-Marquardt least-squares algorithm (Marquardt, 1963).The degree of the background-fitting polynomial has been adjusted empirically for each micro window.Species independent shift parameters have been included allowing individual absorption features to freely move on the spectral axis.Special care has been taken to group weak and strong absorption features together in a single shift parameter, to provide sufficient certainty on their spectral positions.In other words, not every absorption line has its own shift parameter, but they are grouped as schematically shown in Using this approach, we found a clear improvement in the C 2 H 6 data quality including a higher precision and the absence of discontinuities.The associated spectral baseline is modeled as a second-order polynomial.
The CO 2 -CO-N 2 O micro window covers the entire second laser and is the most complex spectral scene.It includes several overlapping absorption features making the retrieval of mixing ratios of the targeted species challenging.As illustrated in Fig. 5, a single CO 2 absorption line is surrounded by two N 2 O lines.The CO line is directly adjacent to one of the N 2 O lines.This results in comparatively large signal noise and deteriorated accuracy on the retrieved mixing ratios due to crosstalk between the N 2 O, CO and CO 2 absorption lines.However, the spectral range includes another N 2 O line at 2227.843 cm −1 , which is slightly stronger than the other two (see Fig. 5).Our approach is to fix the mixing ratios of the first two N 2 O lines to the stronger third one, in order to reduce the uncertainty on retrieved N 2 O and hence the noise on the CO 2 and CO retrieval.The

Water vapor correction
In the current instrument setup, water vapor is not removed from sampled air before entering the sample cell.Therefore, the influence of water vapor on the retrieved mixing ratios has to be corrected in order to report dry-air mixing ratios.Here, we correct for both, dilution and water broadening effects.The first describes the fact that concentrations appear smaller when analyzing moist air, although the dry air mole fraction might be constant.This effect can be remedied for if the absolute water concentration is known for each individual sample using Eq. 1 where c d is the dry-air mole fraction, c x is the raw concentration of a particular species of interest diluted in moist air and c H2O is the water vapor concentration (Harazono et al., 2015).Spectroscopic water broadening effects are approximately an order of magnitude smaller than dilution effects, yet they do have to be corrected for to obtain precise measurements.HITRAN's air broadening parameters are listed for a particular chemical composition of air excluding water vapor.H 2 O, however, can be a more potent broadening agent than nitrogen or oxygen (Kooijmans et al., 2016).These coefficients have been determined using the setup depicted in Fig. 6 and are summarized in Tab. 1.Therefore, the pressure broadening has to be modified to include this effect.Under dry air conditions it is common to split the pressure broadening into two parts: self-broadening and airbroadening.The self-broadening coefficient allows computation of the broadening induced by mutual collisions of a particular species of interest.The air-broadening coefficient can be used to approximate the broadening induced through collisions of a  particular species with all the other species in a given air standard excluding the species itself.From the HITRAN definitions, the pressure-broadened half width at half maximum for a gas at pressure p and temperature T is given by where T ref is a fixed reference temperature (T ref = 296 K), p self is the partial pressure of a particular species of interest and n air is the coefficient of the temperature dependence of the air-broadened half width.This model has been extended to include   Fig. 7 shows the Allan variance for common averaging times τ for the individual trace gases monitored.For most species averaging up to 20s will decrease the signals standard deviation, before deteriorating effects (i.e.drift) occur.Figure 7 also addresses retrieved mixing ratio linearity.Linearity checks have been carried out for all species using the calibration system described in Section 2.3.All retrieved species are linear within error margin.CH 4 is used in Fig. 7 for demonstration purposes.
Typical shift parameters (as introduced in Sect.3) for ground-based operation are depicted in Fig. 8 for the CH 4 -H 2 O and CO 2 -CO-N 2 O micro windows.These shift parameters can be considered as a tracer for instrument stability for both lasers.Overall spectral stability is in the range of ±10 −3 cm −1 .Apart from expected low-frequency instability (due to thermal changes) on the lasers spectral output, high-frequency shifts are evident, including discontinuities.The source of these discontinuities remains unclear.They could be introduced by the software based frequency lock mechanism, by instabilities of the laser itself or by timing changes in the sampling.The shape of the individual shifts match and so does their trend over time, 10 which is a good indicator for a stable tuning rate during ground-based operation.The lowermost panel of Fig. 10 provides water vapor mixing ratios obtained from an onboard hygrometer, from the G2301-m PICARRO analyzer and from the QCLS.The QCLS water vapor data is used to correct for water vapor effects during the retrieval of dry-air mixing ratios from the QCLS raw spectra as described in Sect.3.1.By taking a closer look on the upper two panels, the benefit of simultaneously measuring several species can be readily identified.Figure 10 shows enhanced CH 4 without coinciding C 2 H 6 enhancements for the first low-altitude leg.For the second low-altitude leg above the Marcellus area, however, concurrent CH 4 and C 2 H 6 enhancements suggest natural gas being the dominating source.between the QCLS and PICARRO/FLASK datasets.This constant bias has been corrected for.The large CO 2 bias most possibly results from a difference in the isotopic composition of the calibration standard relative to the sampled air.Since we are reporting retrieved mixing ratios relative to the WMO scale, however, only the working standard reproducibility contributes to the total uncertainty.Uncertainty on the other measured species is taken from the ACT-America dataset to allow for WMO traceability.The uncertainty on calibration sequence evaluation (see 2.3) is estimated with the double of the measurement precision and the uncertainty introduced by the H 2 O correction is estimated from Eq. 1 using an assumed relative error on retrieved water vapor of 2%.Errors originating from instrument drift are considered negligible due to our frequent calibration strategy (see Section 2.3).The total uncertainty is given by the quadrature sum of the individual contributors, listed in Tab. 3. A severe cabin pressure dependence in excess of 0.3 ppb hP a −1 in CH 4 mixing ratio has been previously reported for airborne Aerodyne TILDAS instrumentation (Pitt et al., 2016).We were not able to reproduce this large cabin pressure dependence during operation of the QCLS instrument aboard the C130 using the calibration strategy from Sect.2.3.later 1.5 s, the PICARRO is insensitive to CH 4 .Therefore, it is difficult to mimic the PICARRO sampling by averaging the QCLS data as it would be required for a one-to-one comparison.Instead we decided to linearly interpolate QCLS data to the PICARRO timescale.The fast response time of the QCLS instrument allows for better sampling of spatially narrow plumes, as can be seen from the right hand side panel in Fig. 12.This panel zooms in on a relevant portion of the methane data from Fig. 10 and demonstrates that two mutually-separated plumes can be identified from the high frequency QCLS data at 18:47 UTC, where only a single enhancement can be seen from PICARRO data.Furthermore, absolute enhancement and area beneath the peak(s) differ for the two instruments, due to the different sampling patterns.Figure 13 compares the QCLS mixing ratios to the PICARRO instrument and to the flask samples after correcting for a bias constant for the whole measurement series.The upper panels show differences in retrieved mixing ratios between the QCLS and the cavity ring-down instrument for the flight on Oct. 3, 2017, exhibiting a near normal distribution.This hints towards residuals originating from random processes, i.e. noise.
Although interpretation of the differences to flask samples is difficult for high-variability flight segments, the lower panels of  2016)) on the retrieved mixing ratios are effectively minimized using a frequent (5 to 10mins interval) two-point calibration approach obtained by flushing the sample cell with "zero" and "target" gases.This allows for a measurement duty cycle of ≥ 90 % when operating at sample flow rates near 23 SLP M .A custom retrieval software has been developed to allow for independent processing of raw spectra.We minimize retrieval artifacts by introducing a new way to handle spectral shifts.We reduce fitting residuals by implementing open path water vapor absorption using an auxiliary sensor mounted inside the instruments optics compartment.Apart from low frequency laser instability we identify high frequency "jumps" on the spectral axis, possibly due to the instruments frequency lock mechanism.In-flight performance has been assessed using data obtained during the research flight on the 3rd Oct. 2017 above the eastern U.S..We identify two precision regimes whether flying within the planetary boundary layer or above, due to aircraft vibration propagating into the instrument shown to be capable of detecting isotopologue level mixing ratios, which will be picked up in the near future to modify the instrument for airborne isotope ratio analysis.
Monika Scheibe from DLR and Martin Nowicki from NASA WFF for engineering support.Furthermore we would like to thank everyone involved during the ACT-America field campaigns for their relentless dedication and the helpful discussions, especially Alan Fried, Bing

Fig. 1 Figure 1 .
Fig. 1 shows a top-view photograph of the optics compartment.A combination of a continuous wave (CW) QCL and ICL measures mixing ratios of CH 4 , C 2 H 6 , CO 2 , CO, N 2 O and H 2 O simultaneously by direct absorption spectroscopy.The sample cell is an astigmatic Herriott cell with approximate physical dimensions of 15cm x 15cm x 50cm (WxHxL) made from Atmos.Meas.Tech.Discuss., https://doi.org/10.5194/amt-2018-312Manuscript under review for journal Atmos.Meas.Tech.Discussion started: 5 October 2018 c Author(s) 2018.CC BY 4.0 License.Large parts of the wiring harness have been exchanged from standard PVC cables to aviation-grade fire-resistant wiring.Mandatory electromagnetic compatibility/interference (EMC/EMI) tests have been carried out to comply with Federal Aviation Administration (FAA) regulations.The rack-mounted instrument sums up to a total mass of approx.115 kg and has been tested

Figure 2 .
Figure 2. Schematic showing the main components with emphasis on the calibration system.A mass flow meter allows for measuring the sample flow rate.Two reference gases can be mixed at any arbitrary ratio by means of two calibrated mass flow controllers.A 2µm particle filter upstream of the sample cell avoids cell contamination.

Figure 3 .Figure 4 .
Figure3.A typical raw spectrum as recorded in binary format by the instrument.Arrows have been added to ease identification of the observed chemical species.Channel numbers on the abscissa can be converted to spectral units using the laser tuning rate.The intensity offset can be corrected by shifting the entire spectrum to yield zero intensity when lasers are turned off.

Fig. 4 .Figure 5 .
Figure 5. Typical, normalized spectra for each micro window including fits and associated residuals.The first micro window (top left) includes CH4 and H2O absorption features.The top right micro window depicts C2H6 absorption.The lower left spectrum shows CO, CO2 and N2O absorption.

CO 2
absorption line originates from a molecular transition of the 13 C 16 O 2 carbon dioxide isotopologue, resulting in reduced accuracy if the isotopic composition of the sample is not accurately constrained.The spectral baseline has been split into two parts, the first covering the first two N 2 O, CO 2 and CO lines, and the second covering the individual N 2 O line only.Both are modeled as second-order polynomials.

Figure 6 .
Figure 6.Schematic depicting the water correction lab setup.A reference gas can be humidified to typical atmospheric values between 0% and 2% absolute water using mass flow controllers and an electronically controlled vaporizer.A downstream pump allows for simulation of different flight levels.

5
collisions with H 2 O molecules yielding γ (p, T ) = T ref T nair (γ air (p − p self − p H2O ) + γ self p self + γ H2O p H2O ) (3) with the partial pressure of water vapor p H2O and the water broadening coefficient γ H2O .The former can be computed from the measured water vapor concentration.The latter can be empirically determined.Two MFCs are used to precisely modify mixing ratios of water vapor in a clean and dry calibration gas.Theoretical computation of the water vapor mole fraction follows under 10 the assumption that there is no water deposition on the enclosing flow channel surfaces and therefore the existence of a steady 11 Atmos.Meas.Tech.Discuss., https://doi.org/10.5194/amt-2018-312Manuscript under review for journal Atmos.Meas.Tech.Discussion started: 5 October 2018 c Author(s) 2018.CC BY 4.0 License.state flow condition.The amount of each constituent n in mols can be computed using the mass flow rate ṁ (integrated over a suitable interval of time) and the corresponding molar mass M .An additional downstream pump allows, in combination with a manually-controlled needle-valve, tuning the absolute pressure at the instrument inlet to simulate altitude changes.For these tests, the QCLS instrument has been operated at low flow rates of approx. 1 SLP M due to limitations on the two mass flow controllers.The water broadening coefficient γ H2O has been adjusted iteratively until reported dry-air mixing-ratios of the species of interest remained constant for the set of water vapor mixing ratios.4 Ground-based performance Extensive ground-based instrument checks have been conducted, including tests in a pressure chamber at the Karlsruhe Institute of Technology (KIT) and laboratory tests at DLR Oberpfaffenhofen, Germany.These tests confirmed the presence of an ambient pressure dependence found in earlier studies (i.e.Pitt et al. (2016)).Here, we show in-field, ground-based instrument checks conducted in Hangar N-159 at NASA Wallops Flight Facility, Wallops Island, USA, to ensure proper instrument operation and determine instrument precision.Power drawn from the aircraft remained under 50 A at all times and settled at approximately 40 A. The volumetric flow rate stabilized at 23 SLP M for a sample cell pressure regulated at 50.0 ± 0.2 hP a(measured 1-sigma @5Hz, excluding absolute error).Typical precision (standard deviation for 1s averaging) for ground-based operation is summarized in Tab. 2. These values are in good agreement with the values reported by Aerodyne, Inc..

Figure 7 .
Figure7.Allan variance for all measured chemical species during ground-based operation (left panel).The right panel demonstrates linearity for methane is within achievable error bounds during ground-based operation using the online calibration gas mixing system from Section 2.3.

Figure 9 .
Figure 9.A typical fair-weather flight during ACT-America.This figure shows the flight pattern for Oct. 3, 2017 with color coded altitude.The flight includes two low-altitude (≈ 1000 f t AGL) legs downwind and upwind of parts of the Marcellus shale area.High-altitude transects between the two low-altitude legs include two en route descents and ascents in West Virginia. 10

Figure 10 .Figure 11 .
Figure 10.A direct comparison between dry-air mixing ratios retrieved from different measurement techniques for a complete flight on Oct. 3, 2017.Depicted are methane (uppermost panel), ethane (center panel) and water vapor (lowermost panel) mixing ratios.QCLS-retrieved methane data is in good accordance with PICARRO and flask data.QCLS-retrieved ethane data is in good agreement with flask data too.Water vapor sensed by an onboard hygrometer does differ from the PICARRO and QCLS data.

Figure 12 .
Figure12.The left panel shows the cabin pressure dependence for a typical flight on Oct. 3, 2017.The large cabin pressure dependence in excess of 0.3 ppb hP a −1 reported byPitt et al. (2016) could not be reproduced.The right hand side panel shows a temporal zoom on the methane data at 18:47 UTC to emphasize the benefit of high-frequency measurements.

Figure 13 .
Figure 13.Comparison of QCLS derived mixing ratios to well-established in-flight PICARRO data and flask samples after correcting for a bias constant for the whole measurement series.Interpretation of the errors against flask samples is difficult for high-variability flight segments, due to the large flask sampling time.The residual plots show color-coded data from 5 typical flights on 10/03/2017, 10/11/2017, 10/14/2017, 10/18/2017 and 10/20/2017.
optics and related slight changes in optical alignment.Typical in-flight precision figures for boundary layer flights (standard deviation for 1s averaging) are 740 ppt, 205 ppt, 460 ppb, 2.2 ppb, 137 ppt, 16ppm for CH 4 , C 2 H 6 , CO 2 , CO, N 2 O and H 2 O respectively.Precision figures improve to approximately the half for flights above the PBL.We estimate a total measurement uncertainty of 2.3 ppb, 1.6 ppb, 1.0 ppm, 7.4 ppb and 0.8 ppb in CH 4 , C 2 H 6 , CO 2 , CO and N 2 O, respectively.We demonstrate an excellent agreement to concurrent flask sample and cavity-ringdown measurements within combined measurement uncertainty for all targeted species.The instrument retrieves carbon dioxide mixing ratios via a 13 C 16 O 2 absorption line and is thus Atmos.Meas.Tech.Discuss., https://doi.org/10.5194/amt-2018-312Manuscript under review for journal Atmos.Meas.Tech.Discussion started: 5 October 2018 c Author(s) 2018.CC BY 4.0 License.

Table 1 .
Empirically determined water vapor foreign broadening coefficients

Table 2 .
Achieved ground-based performance

Table 3 .
Typical in-flight performance including contributions to overall uncertainty.The total measurement uncertainty at 1s temporal resolution is given by the quadrature sum of the individual contributors.We found a bias constant for the whole measurement series of ∼ 2 ppb for CH 4 and ∼ 10 ppm for CO 2