Airborne Measurements of CO, CH4, N2O, CO2, and H2O(v)


 

Instrument:Airborne Measurements of CO, CH4, N2O, CO2, and H2O(v)
Principal Investigators:Glen W. Sachse
Organization:Mail Stop 472
Langley Research Center
National Aeronautics and Space Administration
Hampton, VA 23681-2199
Co-Investigators:Stephanie A. Vay, and Bruce E. Anderson
Organization:Mail Stop 483
Langley Research Center
National Aeronautics and Space Administration
Hampton, VA 23681-2199
Measurement Description: Tracer gas measurements will be provided using three separate techniques: a folded-path, differential absorption mid-IR diode laser spectrometer for CO, CH4, and N2O (DACOM) [Sachse et al., 1987, 1991]; a non-dispersive infrared analyzer for CO2 [Anderson et al., 1993, 1996a, 1996b; Vay et al., 1999]; and an external path, near-IR diode laser hygrometer for H2O(v) (DLH) [Collins et al., 1995; Vay et al., 1998]. Instrumentation types slated for the DC-8 aircraft as well as their performance characteristics are listed in Table 1 followed by brief instrument descriptions.

Table 1.
InstrumentSpeciesTime ResponsePrecision (%)
DACOMCO1 sec1% or 1ppbv
DACOMCH41 sec0.1%
DACOMN2O1 sec0.1%
Non-Dispersive IR AnalyzerCO2 1 sec50 ppbv
DLHH2O(v)50 msec2% or 0.2 ppmv


Diode Laser In-Situ (DACOM): The spectrometer system, referred to as DACOM (Differential Absorption CO Measurement), includes three tunable diode lasers providing 4.7, 4.5, and 3.3µm radiation for accessing CO, N2O, and CH4 absorption lines respectively. The three laser beams are combined by the use of bandpass 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 optical bandpass filters and are then directed to three InSb detectorsm, one for each laser wavelength. A wavelength reference cell containing several torr each of CO, CH4, and N2O is used to wavelength lock the operation of the three lasers to the appropriate absorption lines. Ambient air is continuously drawn through a Rosemont inlet probe and a permeable membrane dryer which removes H2O(v) 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 100 Torr. At 4 SLPM mass flow rate, the absorption cell volume is exchanged nearly twice every second assuming piston flow. 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 (~ every 10 minutes) flowing calibration gas through this instrument. By interpolating between these calibrations, slow drifts in instrument response are effectively suppressed yielding the high precision values shown in Table 1. Measurement accuracy is closely tied to the accuracy of the reference gases obtained from NOAA/CMDL, Boulder, CO.

Non-Dispersive IR Analyzer: Carbon dioxide measurements will be provided by a modified Li-Cor model 6252 non-dispersive infrared (NDIR) spectrometer. This instrument was adapted by the investigators for airborne sampling and has been successfully deployed during numerous missions including AASE-II, TOTE/VOTE, SUCCESS, SONEX and GTE's PEM series. The basic instrument is small (13 x 24 x 34 cm) and composed of dual 11.9 cm3 volume sample/reference cells, a feedback stabilized infrared source, 500 Hz chopper, thermoelectrically-cooled solid state PbSe detector, and a narrow band (150 nm) interference filter centered on the 4.26 µm CO2 absorption band. Measurements are based on the difference in absorption of infrared radiation between reference and sample gases that flow continuously through identical optical absorption cells. Thus, by selecting a reference gas of approximately the same concentration as background air, very minute fluctuations in atmospheric concentration can be quantified with high precision. When operated at 250 Torr sample pressure, precisions of <0.07 ppmv (1) for 1 Hz sampling rates are typical for our present airborne CO2 system. The Li-Cor will be physically located on the DACOM optical table to reduce vibrationally-induced noise and both instruments have in common: a gas sampling system that preconditions the incoming air by removing H2O(v) and brings the air flow to thermal equilibrium with the cabin temperature; the calibration system which uses NOAA/CMDL standards having a CO2 concentration established relative to primary standards traceable to the World Meteorological Organization Central CO2 Laboratory at the Scripps Institution of Oceanography; and the data acquisition system.

Diode Laser Hygrometer (DLH): The DLH has been successfully flown during several previous field campaigns including VOTE, SUCCESS, SONEX, and the PEM-Tropics Series. This novel sensor will measure water vapor (H2O(v)) during SOLVE via absorption of a strong, isolated line at 7139.1 cm-1 (1.4007 m) and is comprised of a compact laser transceiver mounted to a DC-8 window plate and a sheet of high grade retroflecting road sign material applied to an outboard DC-8 engine housing to complete the optical path. Using differential absorption detection techniques, H2O(v) is sensed along the 28.5m external path negating any potential wall or inlet effects inherent in extractive sampling techniques. A laser power normalization scheme enables the sensor to accurately measure water vapor even when flying through clouds. An algorithm calculates H2O(v) concentration based on the differential absorption signal magnitude, ambient pressure, and temperature, and spectroscopic parameters that are measured in the laboratory.

References:

Anderson, B. E., J. E. Collins, G. W. Sachse, G. W. Whiting, D. R. Blake, and F. S. Rowland,AASE-II Observations of trace carbon species distributions in the mid to upper troposphere, Geophys. Res. Lett., 20, 2539-2542, 1993.
Anderson, B. E., G. L. Gregory, J. E. Collins, Jr., G. W. Sachse, T. J. Conway, and G. P. Whiting,Airborne observations of the spatial and temporal variability of tropospheric carbon dioxide, J. Geophys. Res., 101(D1), 1985-1997, 1996a.
Anderson, B. E., et al.,Aerosols from biomass burning over the south tropical Atlantic Region: Distributions and impacts, J. Geophys Res., 101(D19), 24,117-24,138, 1996b.
Collins, J. E., Jr. G. W. Sachse, L. G. Burney, and L. O. Wade, A novel external path water vapor sensor, presented at Atmospheric Effects of Aviation Project 5th Annual Meeting, April 23-28, 1995.
Sachse, G. W., G. F. Hill, L. O. Wade, and M. G. Perry, Fast-response, high-precision carbon monoxide sensor using a tunable diode laser absorption technique, J. Geophys. Res., 92, 2071-2081, 1987.
Sachse, G. W., J. E. Collins, Jr., G. F. Hill, L. O. Wade, L. G. Burney, and J. A. Ritter,Airborne tunable diode laser sensor for high precision concentration and flux measurements of carbon monoxide and methane, SPIE Proceedings, 1991.
Vay, S. A., B. E. Anderson, G. W. Sachse, J. E. Collins, Jr., J. R. Podolske, C. H. Twohy, B. Gandrud, K. R. Chan, S. L. Baughcum, and H. A. Wallio,DC-8-based observations of aircraft CO, CH4, N2O, and H2O(g) emission indices during SUCCESS, Geophys. Res. Lett., 25, (10), 1717-1720, 1998.
Vay, S. A., B. E. Anderson, T. J. Conway, G. W. Sachse, J. E. Collins, Jr., D. R. Blake, and D. J. Westberg, Airborne observations of the tropospheric CO2 distribution and its controlling factors over the South Pacific Basin, J. Geophys. Res., 104 (D5), 5663-5676, 1999.