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Infrared Spectrum

Atmospheric constituents, primarily carbon dioxide, water vapor, and ozone (COtex2html_wrap_inline2731 , Htex2html_wrap_inline27330, and Otex2html_wrap_inline2735, respectively), provide the medium for radiative interaction due to absorption of upwelling terrestrial radiation. Subsequently, the energy is isotropically re-emitted at the wavelengths where the absorption occurred. Airborne aerosols also contribute to the radiative transfer mechanism and should not be neglected.

Optical depth, in a given spectral band, depends on the absorption line strength and the vertical distribution of atmospheric gases. COtex2html_wrap_inline2737 is well-mixed and is often treated as a permanent species, although its overall concentration changes slowly with time. Htex2html_wrap_inline2739O and Otex2html_wrap_inline2741 concentrations have significant spatial and temporal variation. Atmospheric water vapor content determines the current synoptic situation, where advection of moist or dry air will dictate the local moisture profile. Otex2html_wrap_inline2743 concentration peaks in the stratosphere, between 15 and 30 km, and is produced by photochemical processes. Otex2html_wrap_inline2745 concentration also varies with the synoptic situation, where stratospheric air is often ingested into the troposphere during a cold frontal passage. Otex2html_wrap_inline2747 is often produced from combustion engine exhaust in the lower atmosphere by photochemical reactions.

Figure 1 illustrates downwelling spectral radiance as a function of wavenumber. The dominant species are COtex2html_wrap_inline2749 (600 to 800 cmtex2html_wrap_inline2751, 15 tex2html_wrap_inline2753m), Otex2html_wrap_inline2755 (1000 to 1060 cmtex2html_wrap_inline2757, 9.6 tex2html_wrap_inline2759m), and Htex2html_wrap_inline2761O. The Htex2html_wrap_inline2763O absorption signature is of particular interest due to its spectral distribution. The atmospheric window (800 to 1200 cmtex2html_wrap_inline2765) is the most transparent region in the spectrum. Measurements between water vapor lines within the atmospheric window (referred to as `microwindows' and tabulated in Appendix A) yield observations with the least atmospheric contamination. The Htex2html_wrap_inline2767O absorption lines present an additional problem because the far `wings' of individual Htex2html_wrap_inline2769O lines combine to form the water vapor continuum. Thus, even the microwindows are not completely transparent.

Figure 1: Illustration of atmospheric downwelling radiance relative to values derived from the Planck function for various temperatures. Also noted are the absorption regions for various atmospheric constituents. The spikes in the measured radiance, between 1400 and 1800 cmtex2html_wrap_inline2771, are a result of water vapor absorption lines which become opaque within the instrument.

An overlay of Planck radiance, for several temperature values (200 through 280 K, in 20 K increments), is shown in Figure 1 for comparison to the downwelling radiance. Note the contrast between the atmospheric window and regions of strong absorption, where the atmosphere becomes opaque over a short distance. These values correlate well with a surface measured temperature of 277 K. The spikes in the observed spectrum between 1400 and 1800 cmtex2html_wrap_inline2773 are artifacts of water vapor absorption lines which become opaque within the instrument, causing the system responsivity to approach zero. This results in an increase in calibrated radiance noise due to the small instrumental noise in these opaque spectral regions.

next up previous
Next: IR Radiative Transfer Up: Theory Previous: Theory

Daniel DeSlover
Sun Aug 11 10:02:40 CDT 1996