Atmospheric Spectroscopy


Figure: Ozone Production and Destruction.
Simplified diagram which shows the ozone prod-
uction and destruction cycles in the upper atmo-
sphere.


One of the principal applications of the THz spectral region has been the remote sensing of the atmosphere of the Earth. This has been an extremely successful application because the characteristics of THz spectroscopic interactions are especially well suited to this task. As a result, a number of sophisticated and powerful instruments have been developed [1]. Because of the complexity of the models designed to predict future trends and the effects of policy decisions on these trends, remote sensing data is vital not only to monitor the concentrations of the ozone itself, but also of the many contributors to the ozone cycle.

Figure: Ozone Production and Destruction shows that major contributors to ozone destruction are catalytic cycles of the type: X + O3 → XO + O2 and XO + O → X + O2, with the net result O + O3 → 2O2. The catalyst X is typically NO, OH, or Cl, which is often produced by either photochemistry or reactions with atomic oxygen from long-lived precursors.

Figure: Mixing Ratios illustrates mixing ratios of a number of these species as a function of altitude. Because many of the processes are catalytic, even small concentrations of key species have major impact on the overall cycle. As a result it has been necessary to develop instruments capable of measurement of concentrations below the ppb (10-12)level.

Figure: Mixing Ratios. Mixing ratios for a number of
important contributors to the ozone cycle.

THz instruments have been able to meet this challenge because of the fundamental characteristics of molecular interactions discussed in Section II. Of equal importance, it has been possible to develop instruments that in many cases approach fundamental theoretical limits. This has been in many ways a remarkable achievement in a spectral region which is often considered to be technology limited and speaks to the progress that can be achieved by requirement driven and tested technology development.

The most obvious foundation for these successes is the strong interactions between THz radiation and molecules. However, an important additional factor is that the THz region lies energetically below kT at atmospheric temperatures. As a result, it is possible to observe the thermal emissions from these molecular species in a limb sounding geometry against the colder background of space. In these experiments, the beam of the instrument is scanned vertically through the atmosphere and sophisticated deconvolution techniques make possible the recovery of vertical profiles similar to those shown in Figure: Mixing Ratios at near diffraction limited angular resolution. In contrast, most infrared instruments work in absorption, using the sun as a source, limiting their observations to sunrise and sunset.

Another contributor to this success is that much of the ozone cycle occurs at relatively high altitude. This reduces the pressure broadening (~ 5 GHz at sea level) and gives rise to a spectroscopic richness which can approach that obtained in the laboratory studies discussed above. Not only does this provide unique spectroscopic signatures, but it also makes it possible to choose spectral regions for the study of trace species that are not contaminated by spectra from the much more abundant species such as O3 and H2O. In order to take advantage of this complexity, considerable effort has been devoted to the characterization and ultimately cataloging [9].of the spectra of atmospheric species. This effort has to be comprehensive because it is necessary not only to know the spectroscopy of the target species, but also that of any possible interlopers.

In this section two complementary approaches to atmospheric spectroscopy in the THz will be discussed: systems based on the extension of "microwave" technology to higher frequencies and systems based on the extension of "infrared" technology to longer wavelength. There are a number of examples of both approaches, but we will focus on Microwave Limb Sounder (MLS) [6].as an example of the latter. Both are important and interesting in their own right, but they more generally serve as examples of the different approaches to spectroscopy in the THz.

The similarities and contrasts of systems based on each approach are noteworthy. Because sensitivity, resolution, and measurement speed are major issues for both, optimal use of each photon emmitted by the atmosphere is required. As a result both are multiplex instruments, but with very different implementations. Similarly, both represent technological challenges as their core technologies are extended into the THz, but again these challenges are very different.

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