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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.