Argus Tunable Diode Laser Instrument

Mesurement Description: Argus is a two channel, tunable diode laser instrument which measures atmospheric methane and nitrous oxide in the upper troposphere and stratosphere on the OMS balloon platform up to an altitude of 32 km using second harmonic detection. Argus is an evolution from the very successful ATLAS (Airborne Tunable Laser Absorption Spectrometer) instrument, which is a single-channel diode laser spectrometer operating on the NASA ER-2 aircraft. ATLAS performs a ratioing of the peak of an atmospheric second harmonic absorption line with a CMDL traceable reference gas, whereas Argus records full spectra. Argus was designed for use on remotely piloted aircraft or light-payload balloons and weighs less than 20 kg. We have recently flown Argus and ATLAS together on the ER-2 platform for comparison. The two Argus channels each have their own laser, optics, detector and signal processing chains. We sample methane in the 3-band at 3.3 µm and nitrous oxide in the 3-band at 4.5 µm. The gas is sampled in a multipass cell which has a 26 cm base path length and a total path of 18.8 m. Each laser is current- and temperature-controlled by a dedicated microprocessor which also manages the acquisition and integration of the spectra. A third microprocessor, the host, controls the overall system and records spectra and housekeeping data to a solid state disk. We sweep the laser at 10 Hz over the absorption feature and modulate with a 40 kHz sine wave; a phase sensitive amplifier and integrator detects the second harmonic data. 100 spectra are co-added resulting in a sample rate of 0.1 Hz. The analysis is performed offline by applying direct fits to the measured spectra using the non-linear Marquardt-Levenberg algorithm.

Proposed Modifications for SOLVE: We plan to permanently adapt Argus for the ER-2, improve key electronic components, increase the path length in the sample cell by using a New Focus 36 m astigmatic Herriott cell, essentially doubling the path length. We will improve the data analysis for the aircraft environnment, and increase the data rate to 0.5 Hz. An extensive intercomparison of N₂O instruments, including ATLAS, in September 1999 will allow determination of the accuracy and precision of the modified Argus and assure the continuation of the ATLAS data series.