For the case of looking at the sun from an observing point on the earth's surface, an early and noteworthy example was the work carried out at Jungfraujoch, Switzerland, in the early 1950's, and reported as an Atlas of the Solar Spectrum (Migeotte et al. 1956) In that work a large grating spectrometer was used from a laboratory at an altitude of 3,600 meters. When the viewing direction was 45 degrees off the vertical, the equivalent path was 7.3 kilometer atmospheres.

            The atlas demonstrates an important aspect of spectral observations: they are a permanent record of the condition of the atmosphere, and this record may continue to be useful in later years. For example, Rinsland, et al., (1985a and 1985b) have recently used the 1951 spectra to determine


column amounts of methane and carbon monoxide that are significantly smaller than those known to exist today. (Rinsland, et al. 1985a and 1985b).             Another early example of atmospheric measurements by infrared absorption were published by Scott, et al. in 1956. They measured ozone and other pollutants in the Los Angeles smog. Figure 1 is taken from their publication.  


            As an example of what has been done from aircraft, Fig. 2 shows the carbonyl sulfide lines seen among the carbon dioxide lines in the spectral region 2050 to 2060 cm-1 . This spectrum was obtained by Mankin et al. (1979) using an airborne Fourier transform spectrometer and solar tracking apparatus. In the figure, asterisks mark the COS lines used for analysis. The upper curve was from solar elevation 9.2°; the lower curve from solar elevation 6.5°. These lines indicate a COS mixing ratio of about 5 x 10-10.

            Many recordings of the solar spectrum from balloons have been presented by Prof. D. Murcray and his colleagues at the University of Denver. These observations have been carried out over a span of 20 years and have had an especially important impact on the understanding of atmospheric composition and chemistry. The balloon-borne instruments usually observed the sun from an altitude of about 30 km. Figure 3 shows a spectrum recorded in 1968 (Murcray et al., 1969). The main feature of this spectrum is the nitric acid band, with peaks at 879 cm-1 and 896 cm-1. However,the spectrum also offers an example of


how one can derive valuable information from a spectrum years after it was recorded. In 1968 people were not aware of the accumulation of chloroflurocarbons in the atmosphere, but in 1975 when concern arose over the danger of chlorofluorocarbons leading to stratospheric ozone depletion, the 1968 spectrum was re-examined and the absorption feature of CF2C2 at 921 cm-1 was detected. This is indicated by the arrow in the figure.

Further spectra of the halogenated pollutants were obtained in 1975 by the University of Denver group (Williams et al., 1976). Although the chlorofluorocarbon threat to stratospheric ozone had been outlined by Molina and Rowland (1974) on the basis of knowledge of the atmospheric chemistry of the compounds involved, it was still necessary to confirm their predictions by physical measurements. The spectra of Fig. 4 made that confirmation in a most convincing way. Comparison of the heights of the HNO3 peaks to the CF2CL2 peak indicated that between 1968 and 1975 the ratio of CF2CL2 to HNO3 in the stratosphere increased by five-fold or more.


An example of the value of "upper limit" calculations from atmospheric spectra is also drawn from the work of the University of Denver group. In the discussions of the late 1970's on on chlorine-ozone interactions the suggestion arose that much of the chlorine in the stratosphere might be tied up in the compound chlorine nitrate. No one had measured chlorine nitrate in the air, but laboratory chemistry showed that the compound should exist. There were not enough kinetic data available to allow accurate predictions, but some kineticists were saying that the ClNO3 could be the major sink for Cl in the stratosphere and that predictions of ozone depletion by chlorofluorocarbons should be scaled down. However, when one examined spectra obtained by balloon-borne instruments and compared them to a laboratory spectrum of ClNO3, no evidence of ClNO3 was seen.

                                                                                It was concluded that the mixing ratio of ClNO3 in the stratosphere had to be below about 1 x 10-9 (Murcray et al. 1977). Subsequently, solar spectra recorded from balloons showed a weak absorption feature near 1,292 cm-1 that is probably due to ClNO3 in the 20 to 30 km altitude region. In this case the mixing ratio was estimated to be in the range of 6 x 10-10 to 1 x 10-9 (Murcray et al. 1979).


                                                                                 The horizontal optical path at ground level has been used mainly in the study of urban and industrial pollution. An example of a long single path is seen in the work of W. F. Herget (1979). He used a quartz-iodine lamp as light source and projected the radiation along a path on the order of 1 to 2 km to a receiving telescope and Fourier transform spectrometer. When observing in the vicinity of an oil refinery, species detected included ethylene, propylene and sulfur dioxide (Fig. 5).


            In the the vicinity of a fertilizer plant he detected hydrogen fluoride (Fig. 6).


Each of these spectra shows the column amount of trace gas distributed along the whole path. Such a single long path can be used to monitor the total pollution crossing the boundary of an industrial site.

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