The infrared method has been used for many years in the measurement of atmospheric trace gases. There have are three main methods of measuring the infrared absorption by the atmospheric gases, as illustrated in the figure here. Drawing a shows the case of looking at the sun from an observing point on the earth's surface. For measurements not concerned with the lower troposphere, an observation point at high altitude is chosen. The higher the observatory, the less is the interference from water vapor. High flying aircraft have also served as the observing platform, for example at the National Center for Atmospheric Research in Boulder, Colorado. (Mankin, et al., 1978).

The second method of recording atmospheric spectra, shown in drawing b, is to observe the setting or rising sun from a balloon or satellite that is above most of the atmosphere. This is called atmospheric limb spectroscopy. In these cases the equivalent optical paths can be hundreds of km-atm. ( One km-atm is 1 km path at sea level). The lowest point in the optical path, called the tangent altitude, depends on how far down the system is pointed. For a viewing direction 94 degrees off the vertical, for example, from an instrument at 30 km altitude, the tangent altitude is about 15 km and the equivalent optical path is about 100 km-atm. The major advantage of sighting along a high altitude path to the sun is that very little water vapor exists in the stratosphere and the water lines do not dominate the spectrum as they do at lower altitudes. A disadvantage of this method is that observations can be made only at sunrise and sunset.

The observation method depicted in drawing c is that which has been used in the urban smog. In this case the path is horizontal, between an infrared radiation source and the detection system. The source may be at one end of the optical path and the detector at the other end, as shown in the figure, or the source and detector may be together at one end, with a reflector at the other end. A single reflection or many reflections may be used. The three-mirror multiple-pass system developed by White (1942), has been used more than any other, and White's system has been of fundamental importance in developing an understanding of the atmospheric chemistry. See the detailed discussions in the topic LONG PATH CELLS. When using a long horizontal path in the urban atmosphere, water vapor absorption is extremely strong, eliminating some spectral regions completely and showing at least some lines in all other regions. One must learn to recognize the ultra-trace gases by their small perturbations on the ever-present water vapor spectrum.

Most of the molecules that were detected and measured in the atmosphere by infrared absorption up to 1984 are listed in the table below. The approximate mixing ratios (ratio of the number of molecules of the trace gas to the number of molecules of all gases), and comments on the distribution of the gases and references to literature reports are also given. These are molecules that were seen in-situ in ambient air. Many additional molecules, mainly industrial pollutants, have been identified by infrared after being captured and taken into the laboratory.

Atmospheric measurements have, of course, continued and there are many additional reports in the literature. An especially valuable group of high resolution spectra was obtained in measurements from the U. S. space shuttle. These results are described, for example, in papers by Zander, et al. (1986), Rinsland, et al. (1986A), Rinsland, et al. (1986B), and Park, et al. (1986).

Molecule	Location of Bands or Lines (cm^-1)	Approximate Mixing Ratio	Comments	Some References
H₂O	Everywhere in spectrum	10^-6 in stratosphere; 10^-2 in troposphere	Interferes with detection of nearly everything else.	All Spectra
CO₂	2,380, 670	3 x 10^-4	Being a linear, symmetrical molecule, CO₂ has a relatively simple spectrum, which does not interfere seriously in the detection of other compounds.	All Spectra
CH₄	3,020, 1,305	1.5 x 10^-6	Quite uniformly distributed, but in urban areas, slightly higher than in others.	Most Spectra
N₂O	2,220, 1,280	3 x 10^-7	Uniformly distributed; not significantly higher in urban areas than in non-urban areas. May be used as an internal standard, allowing ratios of other compounds do be calculated in reference to N₂O.	Most Spectra
CO	2,146	2 x 10^-6	Higher in northern hemisphere than in southern; in urban areas CO can be 10 to 100 times higher than in non-urban areas.	Migeotte et al. 1956. Zimmerman et al. 1978
O₃	1,050	10^-6 in stratosphere 10^-7 in lower troposphere	Shows in all solar spectra. O₃ is beneficial in stratosphere, but poisonous in urban areas.	Most Spectra
NO	1900	10^-6 in stratosphere 10^-10 in clean troposphere 10^-7 in urban smog	Difficult to see because of H₂O interference, but seen in urban smog and stratosphere.	Farmer 1985 Hanst et al. 1982 Fontanella et al. 1975 Batherwick et al. 1980 Coffey et al. 1981
NO₂	2,920, 1,620	10^-6 in stratosphere; 10^-9 in clean troposphere 10^-7 in urban air	Lines of strong band near 1,620 cm^-1 can be seen in some solar spectra. In urban smog these lines are obscured by water vapor, but one can see the lines of the weaker band near 2,920 cm^-1.	Goldman et al. 1978 Coffey et al. 1981 Fontanella et al. 1975 Blatherwick et al. 1980 Hanst et al. 1982
N₂O₅	1,240	10^-9 in stratosphere ?	N₂O₅ is undoubtedly formed in the urban smog, but it hydrolyzes and photolyzes. It has not yet been detected in smog, but has been seen in sunrise measurements from the space shuttle.	Farmer 1985
HNO₂	791	10^-9 in urban smog	Nitrous acid has a well-known infrared spectrum, but it can be more easily measured by its UV absorption. 5 x 10^-9 atm. of HNO₂ have been seen in urban smog early in the morning. The compound is photolyzed by sunlight.	Perner and Platt 1979 Hanst et al. 1982
HNO₃	879, 967	10^-9 in clean troposphere 5 x 10^-9 in stratosphere 10^-8in urban areas	End product of nitrogen oxide oxidation. Washed out of troposphere. Prominent in stratospheric spectra.	Coffey et al. 1981 Hanst et al. 1982 Murcray et al. 1969
NH₃	932, 967	10^-8 - 10^-9	NH₃ is seen near farms and feed lots. Spectra show it to be absent from urban smog. It has not been seen in the stratosphere.	Tuazon et al. 1978 Hanst et al. 1982
ClNO₃	780, 809, 1,293	10^-9	Partial reservoir for atmospheric chlorine.	Murcray et al. 1977
HCl	2,924, 2,926	10^-9 in stratosphere not seen in troposphere	Sink for Cl in stratosphere. Washes out of troposphere	Farmer et al. 1980 Williams et al. 1976 Farmer et al. 1976 Ackerman et al. 1976
HF	4,039, 4,174	10^-10 in stratosphere 10^-8 near fertilizer plant	HF is also of concern in aluminum production facilities	Herget 1979 Farmer et al. 1980
HCOOH	1,105	10^-8 in urban smog 10^-9 in upper troposphere	Product of oxidation of organic compounds.	Hanst et al. 1971 Hanst et al. 1982 Goldman et al. 1984
H₂CO	2,780, 2,870	10^-8 in urban smog 10^-10 in upper atmosphere	Product in oxidation of methane and other organic compounds.	Jouve 1980 Hanst et al. 1982
HCN	3,270-3,290	10^-10	Detected in stratosphere	Coffey et al. 1981 Smith and Rinsland 1985
Chloro-fluorocar-bons	various	10^-9	see text
Non-Methane hydro- carbons	2,850-3,000	10^-7 in smog 10^-9 in stratosphere	see text

Infrared methods also are used to confirm and calibrate chemical methods of atmospheric analysis. Chemical methods can be extremely sensitive, but they also are notoriously subject to interference. In many instances, reports of chemical measurements of atmospheric pollution have been disproved or corrected by the application of infrared methods.

To account for the strong differences in the spectra seen along the various optical paths, the figure at the left shows the amounts of trace gases in terms of darkened areas. Starting with the lower atmosphere case, at the bottom of the figure, one sees that a 2 km sea level path may show an amount of water vapor equivalent to a 20 meter path through one atmosphere of pure water vapor (steam). The amount of CO₂ is much smaller, being equivalent to 0.66 m-atm. The ultra-trace gases in which one is interested are so much smaller in concentration that they cannot fairly be represented by areas on the graph. In most cases, the area of a dot is too large. The middle of the figure shows that on the path from a balloon to the sun, CO₂ is dominant, with the amount of water vapor being even smaller than the amount of ozone. The upper part of the figure shows that when looking at the sun from a high altitude location, (mountain top), the amounts of water vapor and carbon dioxide in the path are comparable.