Synonyms: 
CRYSTAL-FACE
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Closed-path Laser Hygrometer

The University of Colorado closed-path tunable diode laser hygrometer (CLH) is based on the water vapor hygrometers designed by R. D. May (Maycomm, Inc.). CLH is coupled to a heated, forward-facing inlet that enhances particulate water by anisokinetic sampling. Ice water content (IWC) is derived from the measurement of enhanced total water, with knowledge of the instrument sampling characteristics, particle size distributions and ambient water vapor.

In contrast to the open-path systems of similar heritage, the CLH, which was designed for operation in the troposphere on commercial aircraft, has a single-pass absorption cell (27.62 cm long). The light source is a room-temperature solid-state laser that puts out 3-5 mW of radiation at 1.37 mm (7306.752 cm-1).

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Gulfstream V - NSF, WB-57 - JSC
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Chemical Ionization Mass Spectrometer

The NOAA chemical ionization mass spectrometer (CIMS) instrument was developed for high-precision measurements of gaseous nitric acid (HNO3) specifically under high- and variable-humidity conditions in the boundary layer. The instrument’s background signals (i.e., signals detected when HNO3-free air is measured), which depend on the humidity and HNO3 concentration of the sample air, are the most important factor affecting the limit of detection (LOD). A new system to provide HNO3-free air without changing both the humidity and the pressure of the sampled air was developed to measure the background level accurately. The detection limit was about 23 parts per trillion by volume (pptv) for 50-s averages. Field tests, including an intercomparison with the diffusion scrubber technique, were carried out at a surface site in Tokyo, Japan, in October 2003 and June 2004. A comparison between the measured concentrations of HNO3 and particulate nitrate indicated that the interference from particulate nitrate was not detectable (i.e., less than about 1%). The intercomparison indicated that the two independent measurements of HNO3 agreed to within the combined uncertainties of these measurements.

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Argus Tunable Diode Laser Instrument

Argus is a two channel, tunable diode laser instrument set up for the simultaneous, in situ measurement of CO (carbon monoxide), N2O (nitrous oxide) and CH4 (methane) in the troposphere and lower stratosphere. The instrument measures 40 x 30 x 30 cm and weighs 21 kg. An auxiliary, in-flight calibration system has dimensions 42 x 26 x 34 cm and weighs 17 kg.

The instrument is an absorption spectrometer operating in rapid scan, secondharmonic 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 CH4 and CO detection. The electronic processing of the second harmonic spectra is done by standard phase sensitive amplifier techniques with demodulation occurring at twice the laser modulation frequency of 40 kHz. To optimize the secondharmonic 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.

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Aircraft Laser Infrared Absorption Spectrometer

ALIAS (Aircraft Laser Infrared Absorption Spectrometer) measures total water, total water isotopes, carbon monoxide, and carbon dioxide isotope ratios. No other instrument provides real-time measurements of carbon dioxide isotope ratios which are clear identifiers of atmospheric transport (18O/17O/16O for stratospheric intrusion, 13C/12C for anthropogenic signals). ALIAS easily adapts to changing mission priorities and can be configured to measure HCl, CH4, SO2, and N2O by simply replacing a semiconductor laser. These measurements contribute to Atmospheric Composition Focus Area research by providing key data on how convective processes affect stratospheric composition, the development of cirrus particles and their affect on Earth's radiative balance, and health of the ozone layer through measurement of chlorine partitioning.

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