Aircraft Measurements and Analysis of CO, CH4, N2O, CO2, and H2O(v) in Support of TRACE P
Glen W. Sachse(1), Karen B. Bartlett(2,) James Podolski(3) and Nikita S. Pougatchev(4)
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]; 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.
|
Instrument |
Species |
Time Response |
Precision (1s) |
|---|---|---|---|
|
Diode Laser In-Situ |
CO |
1 sec |
1% or 1ppbv |
|
Diode Laser In-Situ |
CH4 |
1 sec |
0.1% |
Diode Laser In-Situ |
N2O |
1 sec |
0.1% |
|
Non-Dispersive IR Analyzer |
CO2 |
1 sec |
50 ppbv |
|
Diode Laser Hygrometer |
H2O(v) |
50 msec |
2% or 0.2 ppmv |
|
Instrument |
Species |
Time Response |
Precision (1 s) |
|---|---|---|---|
|
Diode Laser In-Situ |
CO |
1 sec |
1% or 1 ppbv |
Diode Laser In-Situ |
CH4 |
1 sec |
0.1% |
|
Non-Dispersive IR Analyzer |
CO2 |
1 sec |
50 ppbv |
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.
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.
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.
_____________________
1NASA Langley Research Center, Aerospace Electronic Systems Division, MS 472, Hampton, Virginia, 23681-2199 (g.w.sachse@larc.nasa.gov)
2CSRC EDS Morse Hall, University of New Hampshire, Durham NH 03824, 603-862-2928, karen.bartlett@unh.edu
3NASA Ames Research Center, MS 245-5, Moffett Field, CA 94035-1000, 650-604-4853, jpodolske@mail.arc.nasa.gov
4Christopher Newport University, 1 University Place, Newport News, VA, 23606,757-864-7599, n.s.pougatchev@larc.nasa.gov