Atmospheric Composition
Document ID: 221
Anderson, G. P.1
Muench, H. S.1
Good, R. E.2
Philbrick, C. Russell3
Swinder, W.4
1 Section 21.1: ATMOSPHERIC OZONE, BELOW 50 KM
2 Section 21.2: MINOR CONSTITUENTS IN THE STRATOSPHERE
3 Section 21.3: MESOSPHERE
4 Section 21.4: IONIZED CONSTITUENTS
Abstract
The thin envelope of gas that surrounds the earth has evolved into the present state routinely accepted as our atmosphere. While the evolutionary history of the atmosphere has been studied by several scientists, this chapter deals only with our current understanding of the gas species present. The density and temperature structure and the nomenclature adopted to describe it have been discussed in Chapters 14 through 16.
The atmosphere can be viewed as a giant photochemical and dynamical experiment where aeronomiers try, with limited snapshot glimpses, to understand and model its complexities. Nitrogen and oxygen in molecular and atomic forms contribute 99% of the total atmospheric composition to altitudes near 500 km. However, it is the minor species which comprise less than 1 % of the atmosphere that are most important in establishing the chemical and dynamical behavior of the atmosphere. The minor species of interest include all forms of the molecules that result following dissociation by sunlight of not only the major species but also the less abundant molecules containing hydrogen and carbon. The noble gases also contribute to the sum of the minor species and provide inert tracers for examining the dynamical processes in the upper atmosphere. Sources such as meteorites, interplanetary dust, and the solar wind contribute additional minor species to the top of the atmosphere. Tropospheric clouds, aerosols, ocean spray, anthropogenic sources, and industrial pollution contribute sources of minor species from below. Within the past 10 to 15 years, the potential impact of manmade pollution in modifying the bulk properties of the atmosphere has been realized. The excited states of several significant minor species must be examined separately because of the special importance they have on particular chemical processes.
The role of the minor species in the atmosphere is most important to the following topics:
- Absorbing and shielding the earth's surface from solar ultraviolet radiation
- Establishing the radiation balance by absorption and emission of infrared radiation which establishes the thermal structure of the atmosphere
- Absorption of the extreme ultraviolet radiation which leads, through ion chemistry, to the formation of the D, E, and F regions of the ionosphere
During the past 10 years significant advances in mass spectrometry, optical spectrometry, radiometry, and interferometry have rapidly improved our knowledge of the atmospheric composition. The region between 50 and 140 km can be directly sampled only by rocket borne instruments, but recent advances in remote sounding techniques are beginning to provide additional data of a more synoptic nature in this region. The lower altitudes can be probed by aircraft and balloons while the higher altitudes have been examined during the past 20 years by satellite instruments.
| Citation: | G. P. Anderson, H. S. Muench, R. E. Good, C. R. Philbrick, W. Swinder, Atmospheric Composition, U.S. Air Force, 1985, 67 pages |