Airborne Mid-Infrared Cavity enhanced Absorption spectrometer (AMICA)
Authors: Kloss Corinna , Tan Vicheith , Leen J. Brian , Madsen Garrett L. , Gardner Aaron , Du Xu , Kulessa Thomas , Schillings Johannes , Schneider Herbert , Schrade Stefanie , Qiu Chenxi , von Hobe Marc 
Published: August 2021

Abstract. 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, CO2, CO, and H2O at 2050 cm−1;
O3, NH3, and CO2 at 1034 cm−1; and
HCN, C2H2, and N2O at 3331 cm−1. The
2050 cm−1 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 ppb, due to additional uncertainties
introduced by dilution of the standard) at typical atmospheric mixing ratios
and laboratory-measured 1σ precision of 30 ppt for OCS and
3 ppb for CO at 0.5 Hz time resolution. For CO2,
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 H2O, absorption is too weak to
be measured at mixing ratios below 100 ppm. 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 CO2 measurements, and lower the detection limit
for H2O. The 1035 and 3331 cm−1 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 (O3) or
insufficient sensitivity (NH3, HCN, C2H2,
N2O). The ∼100 kg instrument with a typical in-flight
power consumption of about 500 VA is dimensioned to fit into one 19 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 km. 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 s
(corresponding to the average cavity flush time), equivalent to 500 m
distance at a typical aircraft speed of 200 m s−1.