NASA - National Aeronautics and Space Administration

MOPITT (Measurements Of Pollution In The Troposphere) is a carbon monoxide and methane remote sounder launched in 1999 with the Terra spacecraft. An aircraft replica (MOPITT-A) was developed at the University of Toronto to perform validation of MOPITT radiances as well as small-scale pollution studies. MOPITT-A is based on the engineering model of MOPITT, modified for flight in NASA's ER-2 research aircraft. The instrument was first tested over California from the NASA Dryden Flight Research Center in July 2000.

The in‐situ diode laser spectrometer system, referred to by its historical name DACOM, includes three tunable diode lasers providing 4.7, 4.5, and 3.3 μm radiation for accessing CO, N2O, and CH4 absorption lines, respectively. The three laser beams are combined by the use of dichroic filters and are then directed through a small volume (0.3 liter) Herriott cell enclosing a 36 meter optical path. As the three coincident laser beams exit the absorption cell, they are spectrally isolated using dichroic filters and are then directed to individual detectors, one for each laser wavelength. Wavelength reference cells containing CO, CH4, and N2O are used to wavelength lock the operation of the three lasers to the appropriate absorption lines. Ambient air is continuously drawn through a Rosemount inlet probe and a permeable membrane dryer which removes water vapor before entering the Herriott cell and subsequently being exhausted via a vacuum pump to the aircraft cabin. To minimize potential spectral overlap from other atmospheric species, the Herriott cell is maintained at a reduced pressure of ~90 Torr. At 5 SLPM mass flow rate, the absorption cell volume is exchanged nominally twice per second. Frequent but short calibrations with well documented and stable reference gases are critical to achieving both high precision and accuracy. Calibration for all species is accomplished by periodically (~4 minutes) flowing calibration gas through this instrument. Measurement accuracy is closely tied to the accuracy of the reference gases obtained from NOAA/ESRL, Boulder, CO. Both CO and CH4 mixing ratios are provided in real-time to investigators aboard the DC‐8.

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.

ATLAS uses a tunable laser to detect an infrared-active target gas such as N2O, methane, carbon monoxide, or ozone. The laser source is tuned to an individual roto-vibrational line in an infrared absorption band of the target gas, and is frequency modulated at 2 kHz. The instrument detects the infrared target gas by measuring the fractional absorption of the infrared beam from the tunable diode laser as it traverses a multipass White cell containing an atmospheric sample at ambient pressure.

Synchronous detection of the resultant amplitude modulation at 2kHz and 4kHz yields the first and second harmonics of the generally weak absorption feature with high sensitivity (DI/I < 1E-5). Part of the main beam is split off through a short cell containing a known amount of the target gas to a reference detector. The reference first harmonic signal is used to lock the laser frequency to the absorption line center, while the second harmonic signal is used to derive the calibration factor needed to convert the measurement beam second harmonic amplitude into absolute gas concentration. A zero beam is included to correct for background gas absorption occurring outside the multipass cell. The response time of the instrument is set by the gas flow rate through the White cell, which is normally adjusted to give a new sample every second. Periodic standard additions of the target gas are injected into the sample stream as a second method to calibrate the measurement technique and as an overall instrument diagnostic.

ACATS-IV is a 4-channel gas chromatograph with electron capture detection (ECD) that measures a variety of halocarbons and other long-lived trace gases in the stratosphere. The instrument is currently configured to measure CFC-11 (CCl3F), CFC-12 (CCl2F2), CFC-113 (CCl2FCClF2), methyl chloroform (CH3CCl3), carbon tetrachloride (CCl4), halon-1211 (CBrClF2), chloroform (CHCl3), methane (CH4), and hydrogen (H2) every 125 s, and nitrous oxide (N2O) and sulfur hexafluoride (SF6) every 250 s. Each channel is comprised of a sample loop (2-10 cm3 volume), gas sampling valve (GSV), chromatographic column pair, ECD, electrometer, and several flow, temperature, and pressure controllers. In-flight calibration is carried out every 625 s (1250 s for N2O and SF6) by injecting a dried, whole air standard containing approximately 80% of tropospheric mixing ratios.

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.

The Airborne Laser Infrared Absorption Spectrometer (ALIAS-II) is a very high resolution scanning tunable diode laser spectrometer which makes direct, simultaneous measurements of selectable combinations of HCl, NO2, CO, CO2, CH4, and N2O at sub-part-per-billion levels over a 3-30 second integration time. The measurement technique is based upon using tunable lead-salt and/or quantum cascade lasers operating from 3.4 to 8 microns wavelength scanning over absorption lines at 10 Hz recording second harmonic spectra. The instrument features an open-cradle multipass Herriott absorption cell with 15.24-cm diameter spherical zerodur mirrors coated with gold on chrome. The separation between the mirrors is adjustable allowing for a relatively small cell (0.75-m to 1.5-m) to contain an optical path length up to 120-m, depending on the spacing of the mirrors. Lasers and detectors are contained in a lightweight aluminum liquid nitrogen Dewar which can achieve a 28-hour hold time with only a 2 liter charge of liquid nitrogen. The instrument features custom laser current drives, signal chains, InSb detectors and preamps, 16-bit signal averager, analog signal conditioner, and digital I/O which are controlled by an onboard Pentium processor. Data is written to a ruggedized 2-Gb hard disk every 30 seconds and simultaneously transmitted via telemetry to ground station computers which provide backup storage of the data. The instrument weighs 36 kg and requires <56 watts for operation. Additional power up to 250 watts is available for structural heaters and current draw varies with atmospheric conditions.