Atmospheric composition and thermodynamic retrievals from the ARIES airborne TIR-FTS system – Part 2: Validation and results from aircraft campaigns

Abstract

This study validates trace gas and thermodynamic retrievals from nadir infrared spectroscopic measurements recorded by the UK Met Office Airborne Research Interferometer Evaluation System (ARIES) – a Thermal InfraRed Fourier Transform Spectrometer (TIR-FTS) on the UK Facility for Airborne Atmospheric Measurements (FAAM) BAe-146 aircraft. <br><br> Trace-gas-concentration and thermodynamic profiles have been retrieved and validated for this study throughout the troposphere and planetary boundary layer over a range of environmental variability using data from aircraft campaigns over and around London, the US Gulf Coast, and the Arctic Circle during the ClearfLo, JAIVEX, and MAMM aircraft campaigns, respectively. Vertically-resolved retrievals of temperature and water vapour (H₂O), and partial-column retrievals of methane (CH₄), carbon monoxide (CO), and ozone (O₃), over both land and sea, were compared to corresponding measurements from high-precision in-situ analysers and dropsondes operated on the FAAM aircraft. Average Degrees of Freedom for Signal (DOFS) over a 0–9 km column range were found to be 4.97, 3.11, 0.91, 1.10, and 1.62 for temperature, H₂O, CH₄, CO, and O₃, respectively, when retrieved on 10 vertical levels. Partial column mean biases (and 1&sigma; bias) averaged across all flight campaigns were &minus;0.4 (±1.9)%, &minus;6.0 (±13.1)%, &minus;0.6 (±2.1)%, &minus;3.0(±18.4)%, and +4.7 (±24.9)%, respectively, while the typical total a posteriori errors for individually retrieved profiles were 0.4%, 9.5%, 5.0%, 21.2%, and 15.0%, respectively. <br><br> Averaging kernels derived for progressively lower altitudes show improving sensitivity to lower atmospheric layers when flying at lower altitudes. Temperature and H₂O display significant vertically resolved sensitivity throughout the column, whilst trace gases are usefully retrieved only as partial column quantities, with maximal sensitivity for trace gases other than H₂O within a layer 1 km and 2 km below the aircraft. This study demonstrates the valuable atmospheric composition information content that can be obtained by ARIES nadir TIR remote sensing for atmospheric process studies.