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Atmospheric constituents, primarily carbon dioxide, water vapor, and ozone (CO , H0, and O, 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. CO is well-mixed and is often treated as a permanent species, although its overall concentration changes slowly with time. HO and O 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. O concentration peaks in the stratosphere, between 15 and 30 km, and is produced by photochemical processes. O concentration also varies with the synoptic situation, where stratospheric air is often ingested into the troposphere during a cold frontal passage. O 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 CO (600 to 800 cm, 15 m), O (1000 to 1060 cm, 9.6 m), and HO. The HO absorption signature is of particular interest due to its spectral distribution. The atmospheric window (800 to 1200 cm) 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 HO absorption lines present an additional problem because the far `wings' of individual HO 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 cm, 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 cm 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.