  The remote sensing of gases in complex mixtures at atmospheric pressure is a
challenging problem and much attention has been paid to it. The most
fundamental difference between this application and highly successful
astrophysical and upper atmospheric remote sensing is the line width associated
with atmospheric pressure broadening, ~ 5 GHz in all spectral regions. In this
paper, we discuss quantitatively a new approach that would use a short pulse
infrared laser to modulate the submillimeter/terahertz (SMM/THz) spectral
absorptions on the time scale of atmospheric relaxation. We show that such a
scheme has three important attributes: (1) The time resolved pump makes it
possible and efficient to separate signal from atmospheric and system clutter,
thereby gaining as much as a factor of 10^6 in sensitivity, (2) The 3-D
information matrix (infrared pump laser frequency, SMM/THz probe frequency, and
time resolved SMM/THz relaxation) can provide orders of magnitude greater
specificity than a sensor that uses only one of these three dimensions, and (3)
The congested and relatively weak spectra associated with large molecules can
actually be an asset because the usually deleterious effect of their
overlapping spectra can be used to increase signal strength.
