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

Glen W. Sachse(1), Stephanie A. Vay(2,) and Bruce E. Anderson(3)

Tracer gas measurements will be provided using three separate techniques: a folded-path, differential absorption, tunable diode laser spectrometer for CO, CH4, and N2O [Sachse et al., 1987, 1991]; a non-dispersive infrared analyzer for CO2 [Anderson et al., 1993, 1996a]; and an external path, tunable diode laser hygrometer for H2O(v) [Collins et al., 1995; Vay et al., 1998a]. Instrumentation types slated for the DC-8 and P3-B aircraft as well as their performance characteristics are listed in Tables 1 and 2 respectively followed by brief instrument descriptions.

Diode Laser In-Situ (DC-8)

The spectrometer system, referred to as "DACOM" (Differential Absorption CO Measurement), includes three tunable diode lasers providing 4.7, 4.5, and 3.3mm 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 detectors - 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.

DACOM is currently being upgraded to reduce its requirements on the DC-8 aircraft while maintaining the performance outlined in Table 1. Projected changes in PEM-Tropics A requirements are a substantial weight savings of ~ 400 lbs, a several ampere reduction in consumption of 60 Hz power by tapping into aircraft 400 Hz power, and the freeing of an entire instrument bay for other investigators. Further reductions in aircraft requirements may be realized particularly if suitable 400 Hz air sampling pumps are available.

DACOM II: Diode Laser In-Situ (P3-B)

The mid-IR diode laser instrument (DACOM II) on the P-3B is functionally very similar to DACOM, the major difference being that only CO and CH4 are measured. The CO and CH4 performance (time response and precision) are the same as the corresponding DACOM channels (see Table 2). DACOM II is also being upgraded resulting in a substantial reduction in aircraft requirements.

Non-Dispersive IR Analyzer (DC-8 and P-3B)

Carbon dioxide measurements will be provided by modified Li-Cor model 6252 non-dispersive infrared (NDIR) spectrometers. These instruments were adapted by the investigators for airborne sampling and have been used successfully in numerous missions including AASE-II [Anderson et al., 1993], PEM West A [Anderson et al., 1996a], TRACE-A [Anderson et al., 1996b], PEM West B, and PEM-Tropics A [Vay et al., 1998b] expeditions. 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 mm 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 <0.05 ppmv (1s) for 1 Hz sampling rates are typical for our present airborne CO2 systems.

Each Li-Cor will be physically located on the optical table of the respective diode laser in-situ instrument to reduce vibrationally-induced noise. The CO2 analyzers and diode laser instruments share: gas sampling systems that precondition incoming air flow by removing H2O(v) and bringing 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 (DC-8) A diode laser-based hygrometer, which has flown in several field missions including PEM-Tropics A, TOTE, VOTE, SUCCESS, and SONEX, will be flown on the DC-8. This novel sensor includes 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 this 28.5m external path. This instrument approach has a number of important advantages including its compactness, simple installation, fast response time (~50 msec), no wall or inlet effects, and wide dynamic measurement range (several orders of magnitude). 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., in press, 1998a.

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., accepted for publication, 1998b.

_____________________

1NASA Langley Research Center, Aerospace Electronic Systems Division, MS 472, Hampton, Virginia, 23681-2199 (g.w.sachse@larc.nasa.gov)

²NASA Langley Research Center, Atmospheric Sciences Division, MS 483, Hampton, Virginia, 23681-2199 (s.a.vay@larc.nasa.gov)

3NASA Langley Research Center, Atmospheric Sciences Division, MS 483, Hampton, Virginia, 23681-2199 (b.e.anderson@larc.nasa.gov)