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.
Table 1. Instrumentation for DC-8
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
|
Table 2. Instrumentation for P-3B
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.
_____________________
1 NASA Langley Research Center, Aerospace Electronic
Systems Division, MS 472, Hampton, Virginia, 23681-2199 (g.w.sachse@larc.nasa.gov)
2 CSRC EDS Morse Hall, University of New Hampshire,
Durham NH 03824, 603-862-2928, karen.bartlett@unh.edu
3 NASA Ames Research Center, MS
245-5, Moffett Field, CA 94035-1000, 650-604-4853, jpodolske@mail.arc.nasa.gov
4 Christopher Newport University, 1 University
Place, Newport News, VA, 23606,757-864-7599, n.s.pougatchev@larc.nasa.gov