![](media/image1.wmf){width="5.997222222222222in" height="0.75625in"}

Principal Investigator: Max Loewenstein

Co-Investigators: Jimena Lopez, Jeffrey Grose

Organization: Mail Stop 245-5

Atmospheric Chemistry and Dynamics Branch

Ames Research Center

National Aeronautics and Space Administration

Moffett Field, CA 94035-1000

**Instrument Description**

Argus is a two channel, tunable diode laser instrument set up for the
simultaneous, ![](media/image2.jpeg){width="2.7in" height="2.7in"}*in
situ* measurement of CO (carbon monoxide), N~2~O (nitrous oxide) and
CH~4~ (methane) in the troposphere and lower stratosphere. The
instrument measures 40 x 30 x 30 cm and weighs 21 kg.

![](media/image3.png){width="3.3in" height="2.901388888888889in"}The
instrument is an absorption spectrometer operating in rapid scan,
second-harmonic mode using frequency-modulated tunable lead-salt diode
lasers emitting in the mid-infrared. Spectra are co-added for two
seconds and are stored on a solid state disk for later analysis. The
diode laser infrared beam is shaped by two anti-refection coated lenses
into an f/40 beam focused at the entrance aperture of a multi-pass
Herriott cell. The Herriott cell is common to both optical channels and
is a modified astigmatic cell (New Focus Inc., Santa Clara, California).

The aspherical mirrors are coated with protected silver for optimal
infrared reflectivity. The cell is set up for a 182-pass state for a
total path of 36m. The pass number can be confirmed by visual spot
pattern verification on the mirrors observed through the glass cell body
when the cell is illuminated with a visible laser beam. However,
instrument calibration is always carried out using calibrated gas
standards with the Argus instrument operating at its infrared design
wavelengths, 3.3 and 4.7 micrometers respectively for CH~4~ and CO/N~2~O
detection. The electronic processing of the second harmonic spectra is
done by standard phase sensitive amplifier techniques with demodulation
occrring at twice the laser modulation frequency of 40 kHz. To optimize
the second-harmonic signal amplitude in a changing ambient pressure
environment the laser modulation amplitude is updated every 2 seconds to
its optimal theoretical value based upon the measured pressure in the
Herriott cell.

**Calibration**

Argus is regularly calibrated in the laboratory against a CMDL (Climate
Monitoring and Diagnostics Lab) whole air standard for CH~4~, N~2~O and
CO. Flow-mixing of this standard with a zero gas provides us with
absolute calibration and linearity data as inputs to the in situ flight
recorded analysis. During field operations Argus is calibrated in the
lab both before and after every flight operation with the instrument
maintained in its flight setup configuration throughout the
calibrations.

**Performance Quality at 18 km**

+-----------------+------------+----------+-------------+-------------+
| **Tracer**      | >          | **Data   | **Precision | **Accuracy  |
|                 |  **Typical | Rate     | (ppb)**     | (%)**       |
|                 | > Mixing   | (Hz)**   |             |             |
|                 | > ratio    |          |             | **1-sigma** |
|                 | > (ppb)**  |          |             |             |
+-----------------+------------+----------+-------------+-------------+
| **CO**          | 35         | 0.5      | 1.3         | 3           |
+-----------------+------------+----------+-------------+-------------+
| **CH~4~**       |            | 0.5      | 9.8         | 1.5         |
+-----------------+------------+----------+-------------+-------------+
| **N~2~O         |            | 0.5      | 3.9         |             |
| (estimated)**   |            |          |             |             |
+-----------------+------------+----------+-------------+-------------+

**Data Analysis**

The post-flight data reduction uses a non-linear, least squares
Marquardt-Levenberg fitting procedure where theoretical fitting function
is the second Fourier component of the modulated Voigt absorption
line-shape. One parameter returned by the fitting procedure is the
molecule number density (CO, N~2~O and CH~4~) in the Herriott cell. This
is converted to mixing ratio using the measured gas density in the cell.
Argus then reports molecule mixing ratio at 0.5 Hz data rate to the data
archive.