JPL MkIV Balloon Interferometer

Instrument:	JPL MkIV Balloon Interferometer
Principal Investigator:	Geoffrey C. Toon (toon@mark4sun.jpl.nasa.gov)
Co-Investigators:	Bhaswar Sen, Jean-Francios Blavier
Organization:	Mail Stop 183-601 Jet Propulsion Laboratory California Institute of Technology 4800 Oak Grove Drive Pasadena, CA 91109-8099

Deployment History: The MkIV interferometer is a high resolution (0.01 cm-1) Fourier Transform Infra-Red (FTIR) spectrometer, built at JPL in 1984 to remotely sense the atmospheric composition. Since 1989, the MkIV has performed eleven balloon flights, the two most recent being from Fairbanks, Alaska, in 1997. It flew on the NASA DC-8 aircraft during the Antarctic campaign of 1987, and the Arctic campaigns of 1989 and 1992. The MkIV instrument has also made over 500 days of ground-based observation, mainly from the South- West USA, but also including measurements from McMurdo, Antarctica, in 1986, and Fairbanks, Alaska, in 1997.

Measurement Description: The MkIV interferometer operates in solar absorption mode, meaning that direct sunlight is spectrally analyzed and the amount of various gases at different heights in the Earth's atmosphere is derived from the shapes and depths of their absorption lines. The optical design of the MkIV interferometer is based largely on that of the ATMOS instrument, which has flown four times on the Space Shuttle. The first three mirrors in the optical path comprise the suntracker. Two of these mirrors are servo-controlled in order to compensate for any angular motion of the observation platform. The subsequent wedged KBr plates, flats, and cube-corner retro-reflectors comprise a double-passed Michelson interferometer, whose function is to impart a wavelength-dependent modulation to the solar beam. This is achieved by sliding one of the retro-reflectors at a uniform velocity so that the recombining beams interfere with each other. A paraboloid then focusses the solar beam onto infrared detectors, which measure the interferometrically modulated solar signal. Finally, Fourier transformation of the recorded detector outputs yields the solar spectrum. An important advantage of the MkIV Interferometer is that by employing a dichroic to feed two detectors in parallel, a HgCdTe photoconductor for the low frequencies (650-1850 cm-1) and a InSb photodiode for the high frequencies (1850-5650 cm-1), the entire mid-infrared region can be observed simultaneously with good linearity and signal-to-noise ratio. In this region over 30 different gases have identifiable spectral signatures including H₂O, O₃, N₂O, CO, CH₄, NO, NO₂, HNO₃, HNO₄, N₂O₅, H₂O₂, ClNO₃, HOCl, HCl, HF, COF₂, CF₄, SF₆, CF₂ClCFCl₂, CHF₂Cl, CF₂Cl₂, CFCl₃, CCl₄, CH₃Cl, C₂H₂, C₂H₆, OCS, HCN, N₂, O₂, CO₂ and many isotopic variants. The last three named gases, having well known atmospheric abundances, are important in establishing the observation geometry of each spectrum, which otherwise can be a major source of uncertainty. Similarly, from analysis of T-sensitive CO₂ lines, the temperature profile can be accurately determined. The simultaneity of the observations of all these gases greatly simplifies the interpretation of the results, which are used for testing computer models of atmospheric transport and chemistry, validation of satellite data, and trend determination.

Although the MkIV can measure gas column abundances at any time during the day, the highest sensitivity to atmospheric trace gases is obtained by observing sunrise or sunset from a balloon. The very long (~ 400 km) atmospheric paths traversed by incoming rays in this observation geometry also make this so-called solar occultation technique insensitive to local contamination.

Platforms:	Balloon, NASA DC-8 Aircraft
Mass:	350 kg
Size:	1.4 x 0.7 x 0.6 m
Power:	250 W
Accuracy:	Depends on the altitude and the gas, but typically 5-10%
Vertical Resolution:	2-3 km in limb occultation mode
Response Time:	90 s per spectrum

References:

Osterman, G. B., et al., The budget and partitioning of reactive nitrogen species in the Arctic stratosphere, Geophys. Res. Lett., 26, 1157-1160, 1999.
Sen, B., et al., Balloon-borne observations of mid-latitude fluorine abundance, J. Geophys. Res., 101, 9045-9054, 1996.
Sen, B. et al., Measurements of reactive nitrogen in the stratosphere, J. Geophys. Res., 103, 3571-3585, 1998.
Toon, G. C., The JPL MkIV Interferometer, Optics and Photonics News, 2, 19-21, 1991.
Toon, G. C, C. B. Farmer, L. L. Lowes, P. W. Schaper, J. F. Blavier, and R. H. Norton, Infrared aircraft measurements of stratospheric composition over Antarctica during September 1987, J. Geophys. Res., 94, 16571-16596, 1989.
Toon, G. C., C. B. Farmer, P. W. Schaper, L. L. Lowes, and R. H. Norton, Composition measurements of the 1989 Arctic winter stratosphere by airborne infrared solar absorption spectroscopy, J. Geophys. Res., 97, 7939-7961, 1992.
Toon, G. C., et al., Comparison of remote and in-situ profiles of atmospheric trace gases, J. Geophys. Res., in press, 1999.