Airborne Mid-Infrared Cavity enhanced Absorption spectrometer (AMICA)

• Main Authors: C. Kloss, V. Tan, J. B. Leen, G. L. Madsen, A. Gardner, X. Du, T. Kulessa, J. Schillings, H. Schneider, S. Schrade, C. Qiu, M. von Hobe
• Format: Article
• Language: English
• Published: Copernicus Publications 2021-08-01
• Series: Atmospheric Measurement Techniques
• Online Access: https://amt.copernicus.org/articles/14/5271/2021/amt-14-5271-2021.pdf
• ISSN: 1867-1381; 1867-8548

<p>We describe the Airborne Mid-Infrared Cavity enhanced Absorption spectrometer (AMICA) designed to measure trace gases in situ on research aircraft using Off-Axis Integrated Cavity Output Spectroscopy (OA-ICOS). AMICA contains two largely independent and exchangeable OA-ICOS arrangements, allowing for the simultaneous measurement of multiple substances in different infrared wavelength windows tailored to scientific questions related to a particular flight mission. Three OA-ICOS setups have been implemented with the aim to measure OCS, <span class="inline-formula">CO₂</span>, CO, and <span class="inline-formula">H₂O</span> at 2050 <span class="inline-formula">cm⁻¹</span>; <span class="inline-formula">O₃</span>, <span class="inline-formula">NH₃</span>, and <span class="inline-formula">CO₂</span> at 1034 <span class="inline-formula">cm⁻¹</span>; and HCN, <span class="inline-formula">C₂H₂</span>, and <span class="inline-formula">N₂O</span> at 3331 <span class="inline-formula">cm⁻¹</span>. The 2050 <span class="inline-formula">cm⁻¹</span> setup has been characterized in the laboratory and successfully used for atmospheric measurements during two campaigns with the research aircraft M55 Geophysica and one with the German HALO (High Altitude and Long Range Research Aircraft). For OCS and CO, data for scientific use have been produced with 5 % accuracy (15 % for CO below 60 <span class="inline-formula">ppb</span>, due to additional uncertainties introduced by dilution of the standard) at typical atmospheric mixing ratios and laboratory-measured 1<span class="inline-formula"><i>σ</i></span> precision of 30 <span class="inline-formula">ppt</span> for OCS and 3 <span class="inline-formula">ppb</span> for CO at 0.5 <span class="inline-formula">Hz</span> time resolution. For <span class="inline-formula">CO₂</span>, high absorption at atmospheric mixing ratios leads to saturation effects that limit sensitivity and complicate the spectral analysis, resulting in too large uncertainties for scientific use. For <span class="inline-formula">H₂O</span>, absorption is too weak to be measured at mixing ratios below 100 <span class="inline-formula">ppm</span>. By further reducing electrical noise and improving the treatment of the baseline in the spectral retrieval, we hope to improve precision for OCS and CO, resolve the issues inhibiting useful <span class="inline-formula">CO₂</span> measurements, and lower the detection limit for <span class="inline-formula">H₂O</span>. The 1035 and 3331 <span class="inline-formula">cm⁻¹</span> arrangements have only partially been characterized and are still in development. Although both setups have been flown and recorded infrared spectra during field campaigns, no data for scientific use have yet been produced due to unresolved deviations of the retrieved mixing ratios to known standards (<span class="inline-formula">O₃</span>) or insufficient sensitivity (<span class="inline-formula">NH₃</span>, HCN, <span class="inline-formula">C₂H₂</span>, <span class="inline-formula">N₂O</span>). The <span class="inline-formula">∼100</span> <span class="inline-formula">kg</span> instrument with a typical in-flight power consumption of about 500 VA is dimensioned to fit into one <span class="inline-formula">19</span> in. rack typically used for deployment inside the aircraft cabin. Its rugged design and a pressurized and temperature-stabilized compartment containing the sensitive optical and electronic hardware also allow for deployment in payload bays outside the pressurized cabin even at high altitudes of 20 <span class="inline-formula">km</span>. A sample flow system with two parallel proportional solenoid valves of different size orifices allows for precise regulation of cavity pressure over the wide range of inlet port pressures encountered between the ground and maximum flight altitudes. Sample flow of the order of 1 SLM (standard litre per minute) maintained by an exhaust-side pump limits the useful time resolution to about 2.5 <span class="inline-formula">s</span> (corresponding to the average cavity flush time), equivalent to 500 <span class="inline-formula">m</span> distance at a typical aircraft speed of 200 <span class="inline-formula">m s⁻¹</span>.</p>