In Situ Ozone and Carbon Dioxide Measurements From the NASA DC-8 and P3-B  Aircraft

 Melody A. Avery, Stephanie A. Vay

Description of Ozone Instrument Features and Capabilities

Our in situ ozone sensors are capable of fast, sensitive ozone measurements over a large dynamic range and a wide variety of atmospheric conditions.  The measurements are performed by combining pure reagent nitric oxide (NO) with incoming sample air in a small volume reaction chamber, and by measuring light from the resulting NO₂ chemiluminescence.  The reaction chamber is maintained at constant temperature and pressure (25 Torr) by buffering ambient pressure changes with a larger-volume prechamber maintained at 125 Torr.  The technique, as adapted for use on aircraft, is described in detail in the references listed below.  Sampled air enters the aircraft through a forward-facing, J-shaped probe that has been shown to be insensitive to aircraft attitude.  Approximately 2 standard liters/minute of air is pulled from RAM flow through the probe into the instrument prechamber.  Finally, sample flow into the reaction chamber is 500 standard cc/minute.  The instruments are calibrated by reference to the NIST standard ozone photometer.  Corrections for water vapor quenching are applied, post-flight, using in flight pressure, temperature and water vapor data.  These corrections are largest (up to 13% of the measured ozone) in the marine boundary layer, and are negligible at altitudes above 3 km.  Final data from the TRACE-P mission are available from the GTE ftp site.  Some instrument specifications are listed below.

Ozone Instrument Specifications

Technique:                 Chemluminescent reaction of ozone with nitric oxide
Dynamic Range:             0.8 – 1500 ppb
Accuracy:                       5% or 2 ppb
Precision:                        2% or 0.8 ppb
Response:                       2-3 Hz
Spatial Resolution:          50 m vertical, 200 m horizontal
Data Coverage:              96.2% DC-8, 98.9% P3-B

References

Eastman, J.A. and D.H. Stedman, A fast response sensor for ozone eddy-correlation flux measurements, Atmos. Environ., 11, 1209-1211, 1977.

Gregory, G.L., C.H. Hudgins, J. Ritter and M. Lawrence, In situ ozone instrumentation for 10-Hz measurements: Development and evaluation, Proceedings of sixth symposium on Meteorological Observations and Instrumentation, New Orleans, LA, Jan 12-16, 136-139, 1987.

Pearson R.W. and D.H. Stedman, Instrumentation for fast response ozone measurements from aircraft, Atmos Tech, 12, 1980.

Carbon Dioxide

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 in situ sampling and have been used successfully in numerous airborne campaigns since 1991.  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.  Using synchronous signal detection techniques, it operates by sensing the difference in light absorption between the continuously flowing sample and reference gases occupying each side of the dual absorption cell. Thus, by selecting a reference gas of approximately the same concentration as background air (~ 372 ppmv), very minute fluctuations in atmospheric concentration can be quantified with high precision.  Precisions of 0.07 ppmv (1 s) for 1 Hz sampling rates are typical for our present airborne CO₂ systems when operated at a constant pressure of 300 torr.

During ambient sampling, air is continuously drawn through a Rosemount inlet probe, a permeable membrane dryer to remove H₂O_(v), the LI-COR, and finally exchanged through a diaphragm pump that is vented downstream of all investigator inlets.  Frequent but short calibrations with well-documented and stable reference gases, critical to achieving both high precision and accuracy, are accomplished by periodically (~ 15 minutes) flowing reference gas through the instrument’s sample cell.  By interpolating between these calibrations, slow drifts in instrument response are effectively suppressed, yielding high precision values.  Temperature control of the units minimizes instrument drift permitting reduction in the frequency of calibrations thus maximizing our ambient sampling time.  CO₂ measurement accuracy is closely tied to the accuracy of the reference gases obtained from NOAA/CMDL, Boulder, CO that have calibrations traceable to the WMO primary standards.

The DC-8 LI-COR will be incorporated with the ozone instrument into half of a DC-8 high bay rack, have a separate inlet and flow system, but share the same data acquisition system.  The fast-response CO₂ and O₃ instruments on the P-3B also require half of a standard aircraft rack and will be conveniently packaged with the TAMMS system.  Data from this 10 Hz response-time CO₂ unit can be combined with TAMMS wind measurements to generate CO₂ fluxes.  The CO₂ instrument will have a separate inlet and flow system optimized for making fast response measurements and the CO₂ data will be recorded at 66 Hz using the TAMMS data acquisition system.

References:

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, 1996.

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.