Evaluation of the Airborne Quantum Cascade Laser Spectrometer (QCLS) measurements of the carbon and greenhouse gas suite – CO<sub>2</sub>, CH<sub>4</sub>, N<sub>2</sub>O, and CO – during the CalNex and HIPPO campaigns

Abstract

Abstract. We present an evaluation of aircraft observations of the carbon and greenhouse gases (CO2, CH4, N2O, and CO) using a direct-absorption pulsed quantum cascade laser spectrometer (QCLS) operated during the HIPPO and CalNex airborne experiments. The QCLS made continuous 1 Hz measurements with 1-sigma Allan precisions of 20, 0.5, 0.09, and 0.15 ppb for CO2, CH4, N2O, and CO, respectively, over > 500 flight hours on 79 research flights. The QCLS measurements are compared to two vacuum ultraviolet (VUV) CO instruments (CalNex and HIPPO), a cavity ring-down spectrometer (CRDS) measuring CO2 and CH4 (CalNex), two broadband non-dispersive infrared spectrometers (NDIR) measuring CO2 (HIPPO), two onboard gas chromatographs measuring a variety of chemical species including CH4, N2O, and CO (HIPPO), and various flask-based measurements of all four species. QCLS measurements are tied to NOAA and WMO standards using an in-flight calibration system and mean differences when compared to NOAA CCG flask data over the 59 HIPPO research flights were 100, 1, 1, and 2 ppb for CO2, CH4, N2O, and CO, respectively. The details of the end-to-end calibration procedures and the data quality-assurance and quality-control (QA/QC) are presented. Specifically, we discuss our practices for the traceability of standards given uncertainties in calibration cylinders, isotopic and surface effects for the long-lived greenhouse gas tracers, interpolation techniques for in-flight calibrations, and the effects of instrument linearity on retrieved mole fractions.

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

“…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…”

The development and evaluation of airborne in situ N<sub>2</sub>O and CH<sub>4</sub> sampling using a quantum cascade laser absorption spectrometer (QCLAS)

“…However, in-flight calibration is commonly used to account for instrumental drift when using optical-based measurements (e.g. O'Shea et al, 2013b;Santoni et al, 2014), as external variables such as temperature and pressure can undergo significant variation during a flight. Our system employs three calibration standards to scale the data and assess instrument performance, as described in Sect.…”

“…Daube et al, 2002;Peischl et al, 2010;Santoni et al, 2014). The advantage of this approach is obvious, as any empirically derived correction for the influence of water vapour will contribute, often significantly, to the overall uncertainty of the measurements.…”

“…A leak (ingress) into the system also appears implausible, as one would expect this to have the opposite effect on the high and low cylinder measurements, pulling both towards the mole fractions of CH 4 and N 2 O present in the aircraft cabin. Santoni et al (2014) warn of issues associated with fluctuations in sample cell pressure. However, the offsets in retrieved QCLAS mole fractions observed here were found to exhibit no dependence on either sample cell pressure, sample cell temperature or the rate of change for these variables (as recorded by the pressure and temperature sensors within the sample flow).…”

“…Infrared (IR) spectroscopy is frequently employed for airborne measurement of greenhouse gas mole fractions (Chen et al, 2010;O'Shea et al, 2013b;Santoni et al, 2014), enabling high frequency measurement (usually ≥ 1 Hz) and fast instrument response times (of the order seconds). Many instruments make use of the superior lasers, optics and detectors available in the near-IR region around ∼ 6000 cm −1 (Baer et al, 2002;Crosson, 2008).…”