6.3.3 Tunable Diode Laser Absorption Spectroscopy  6.3 Measurements of Chemical Composition  6.3.1 Remote Sensing Techniques Using Satellites

6.3.2 Remote Sensing from the Ground and Aircraft

Absorption measurements of many species have been made using long path UV, visible and infrared spectroscopy from the troposphere. These tend to be ground-based or aboard aircraft, in contrast to the infrared and microwave satellite measurements just described. The light source can be either a lamp or light from the Sun. In the case of solar radiation, one can either look directly at the Sun, or use radiation scattered downward by the atmosphere. Sunlight scattered off the Moon has also been used for nighttime measurements. All these measurements apply the Beer-Lambert Law, as described in the earlier section, but with one difference. In an experiment using an evacuable cell, it is possible to measure the intensity of the radiation in the absence of any absorbing gas (I_o). However, in an open path experiment this is not possible. Furthermore, due to the spectral complexity of the atmosphere the spectra do not usually go to the baseline between features. In this case it is necessary to use differential absorption cross sections - that is, the absorption measured between consecutive peaks and valleys in the spectrum (it can be shown very easily that the usual form of the Beer-Lambert Law applies for this case). The efficiency with which one can collect UV-visible spectra has become much higher since the widespread availability of silicon diode array detectors. These allow an entire spectral region to be covered in one measurement, thereby overcoming problems with the speed and reproducibility of scanning monochromators. Analogously, the use of Fourier transform spectroscopy allows the rapid acquisition of infrared spectra.

Using high powered lamps as the light source, the method has been applied to the detection of NO₃, NO₂, HCHO, and SO₂ in the UV-visible. All of these were detected in the troposphere in the early work by Platt, Perner and co-workers in Jülich, Germany. The pathlengths used can be up to several kilometers. The measurements of appreciable amounts of NO₃, a free radical, stimulated a large body of work (which is still growing) on the chemistry of NO₃, and its ability to initiate the oxidation of natural hydrocarbons at nighttime.

The use of the Sun or Moon as a source allows much longer path lengths, and consequently the detection of low concentrations of species. In these experiments, the collection optics for the spectrometer are often pointed away from the Sun, and the measurements are made using scattered radiation. The technique gives the total column amount of a given species above the detection point. By varying the viewing angle, or making measurements as a function of solar zenith angle, information can be derived about the vertical distribution of the molecules.

The use of long path UV absorption to characterize the depletion of ozone in the Antarctic and Arctic stratospheres has proven to be very successful. The detection of chlorine dioxide, OClO, in the Antarctic by Solomon and co-workers during the first National Ozone Expedition in 1986 gave great weight to the proposition that halogen-containing compounds released at the Earth's surface were causing the Ozone Hole. In later campaigns, this was strengthened by the simultaneous detection of bromine monoxide radical, BrO. Solomon and co-workers have also made extensive measurements of the overhead column of the nitrate radical, NO₃, both in Colorado and in the Antarctic. For these measurements the Moon is used, since daytime photolysis reduces the mixing ratio of NO₃ below the detection limit. The paper by Smith et al. (1993) gives a good overview.

For many years Mankin and Coffey at NCAR have utilized high-flying aircraft as a base to measure column amounts of a number of important stratospheric gases (Coffey et al., 1981; Mankin and Coffey, 1983; Mankin et al., 1992). Using a high resolution infrared Fourier transform spectrometer with the Sun as a radiation source, they have recorded the absorption spectrum of the stratosphere from 700 to 5000 cm^-1 (2 to 14 $\mu$m). One of the principal advantages of a Fourier transform spectrometer is that wide ranges of the mid-infrared, where nearly all gases of interest have absorption features, can be quickly and accurately recorded. Analysis of the 0.06 cm^-1 resolution spectra has produced stratospheric column amounts of N₂O, NO, NO₂, HNO₃, ClONO₂, O₃, HCl, HF, OCS, SO₂, HCN, and C₂H₆. The airborne observations can be used to measure gases unattainable by satellite techniques and have been used to describe the latitudinal, seasonal and long term trends of a number of gases important to stratospheric ozone chemistry. 

  
 6.3.3 Tunable Diode Laser Absorption Spectroscopy  6.3 Measurements of Chemical Composition  6.3.1 Remote Sensing Techniques Using Satellites 
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