High-Resolution Infrared Laboratory Spectroscopy
to Support
Atmospheric Measurements
Research Objectives
Detailed knowledge of the molecular spectra of ozone and other infrared-active atmospheric species is needed for accurate calculation of atmospheric heating and cooling rates in climate models. Remote sensing experiments on numerous satellite and Space Shuttle missions have shown that space-based measurements of infrared absorption or emission can be used to accurately determine the concentrations and distributions of stratospheric species on a global scale. The objective of this research is to improve knowledge of the spectroscopic line parameters (positions, intensities, assignments, halfwidths, and pressure-induced shifts) of key atmospheric constituents through laboratory measurements.
Progress and Results
In 1992 new high-quality spectra of methane-nitrogen and methane-air mixtures
at temperatures down to -63oC was recorded. These measurements utilized the
McMath-Pierce high-resolution Fourier Transform Spectrometer (FTS) at the
National Solar Observatory on Kitt Peak, Arizona, and a coolable absorption cell
designed and built at NASA Langley Research Center. The spectra cover several
methane absorption bands in the 2- to 5-µm region and are being analyzed
together with previously recorded room-temperature spectra to determine the
behavior of pressure broadening and shift coefficients over the temperature
range found in the terrestrial upper atmosphere. Analysis of air-broadening and
shifts in the 2.3-µm bands has been completed (Malathy Devi et al., 1994), and work is in progress on the
3-µm region. This is a continuation of a comprehensive study of methane line
broadening involving over 175 spectra recorded at room temperature from 1984
through 1988 and at low temperatures in 1989 and 1992. In addition,
"contaminant" carbon dioxide and water vapor features appearing in the 1989 low-
temperature methane spectra due to residual gas in the optical path have been
analyzed. These studies helped to determine nitrogen-broadened line widths and shifts in two water vapor bands in the 3-µm region and in the strong 4.3-µm band
of carbon dioxide. Some additional broadening and shift results for deuterated
water lines in the 3-µm region were obtained by analysis of room-temperature
spectra recorded in 1990.
Continuing analysis of room-temperature ozone
absorption spectra recorded in previous years has resulted in improved line positions, intensities, and assignments in a
number of bands. Data analysis is performed primarily at Langley Research
Center, and the theoretical interpretation of the results is being done in
collaboration with investigators at several other institutions in the United
States and France. Recent results include analyses of hot-band transitions in
the 3.3-µm region and the fundamental absorption bands of the 17O-enhanced
isotopic ozone species in the 10-µm region; none of these bands had been
previously measured. Analysis of N2-, O2-, and air-broadening coefficients and
pressure-induced shifts of ozone lines in the 3-µm region has been completed and
published (Smith et al., 1994).
There is also a continuing effort to analyze low-temperature self-broadened and
air-broadened ozone spectra in the 4- to 15-µm region recorded with the
McMath-Pierce FTS and the NASA Langley coolable cell in 1990 and 1991. The
spectra are being analyzed using a recently-developed multi-spectrum nonlinear
least- squares technique (Benner et al., 1995) to determine the pressure-broadening and shift
coefficients and their temperature- dependences for numerous ozone lines.
Analysis of the air-broadened spectra in the 9-µm and 13-µm regions is complete (Smith et al., 1996; Malathy Devi et al., 1996), and work on other regions is in progress.
