In-flight stability of quantum cascade laser-based infrared absorption spectroscopy measurements of atmospheric carbon monoxide

Abstract

and sub-ppb v levels. In particular, the use of tunable infrared 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 (CO 2 ), methane (CH 4 ), nitrous oxide (N 2 O) and formaldehyde (HCHO) [13,14]. In the present configuration TRISTAR deploys three liquid nitrogen cooled continuous wave quantum cascade lasers for CO, CH 4 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 ppb v to ppm v range like CO (typical tropospheric mixing ratios of 80 ppb v ) and CH 4 (~2 ppm v ) 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 [14], similar to results obtained with other airborne QCL spectrometers [15,16]. For HCHO, whose tropospheric mixing Abstract 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 ppb v ) 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 doub...

Upper tropospheric CH<sub>4</sub> and CO affected by the South Asian summer monsoon during the Oxidation Mechanism Observations mission

“…We employed the TRISTAR instrument (Tracer In Situ TD-LAS for Atmospheric Research -where TDLAS stands for Tunable Diode Laser Absorption Spectrometry), which is an infrared (IR)-quantum cascade laser absorption spectrometer for airborne measurements of trace gases (CO, CH 4 , and HCHO) on board HALO; the instrument has a compact and robust design, consisting of an optical set-up and the instrument electronics. TRISTAR has been described in more detail in previous publications (Schiller et al, 2008 and references therein;Tadic et al, 2017). The electronic portion of the instrument, including the laser controller, data acquisition, etc., is integrated into the upper section of half a 19 rack.…”

“…CH 4 is calibrated against a working standard based on the NOAA 2004 scale by Dlugokencky et al (2005) and has an uncertainty of ±0.3 ppbv relating to the CMDL83 standard (Dlugokencky et al, 2005). Furthermore the in situ CO and CH 4 data are drift corrected by interpolation between regular in-flight calibrations (Tadic et al, 2017).…”

An aircraft gas chromatograph–mass spectrometer System for Organic Fast Identification Analysis (SOFIA): design, performance and a case study of Asian monsoon pollution outflow

“…The base of operation in the eastern Mediterranean was Paphos airport, in Cyprus, and flights generally headed east over the Arabian Peninsula to intercept monsoon flows heading west. Methane, measured by IR absorption spectroscopy (Schiller et al, 2008;Tadic et al, 2017), was used to identify monsoon outflowinfluenced air masses. A threshold methane value was derived to identify such plumes based on the average of profiles (4-10 km height; 12 flights) over Cyprus, Italy and Germany, which represented the European background without monsoon influence.…”

TDLAS Monitoring of Carbon Dioxide with Temperature Compensation in Power Plant Exhausts

“…Compared with the current spectroscopic techniques such as Fourier transform infrared (FTIR) [1] and non-dispersive infrared (NDIR) spectroscopy [2], tunable diode laser absorption spectroscopy (TDLAS) with high sensitivity and short response time [3] offers significant advantages for in situ CO 2 measurements in the hot, humid, and dusty environments of power plants. With the rapid advancements in semiconductor laser industry, TDLAS has been widely applied in different fields such as combustion diagnostics [4][5][6][7][8], isotopic analysis [9][10][11][12][13], and atmospheric gas detection [14][15][16][17][18]. Because of their exquisite constituents and compact packaging, as well as low cost, tunable semiconductor diode lasers are nearly the ideal source for highly-sensitive and highly-spectral-resolved measurements in industrial fields [19].…”