The study of atmospheric chemistry focuses on how chemical constituents cycle through the atmosphere. Excluding water vapor (which can account for as much as 2 to 3% of the volume of the atmosphere under extremely moist conditions), more than 99.9% of the remaining dry atmosphere is comprised of nitrogen (78.1%), oxygen, (20.9%), and argon (0.93%). Unlike the study of conventional meteorology, where the atmosphere is generally treated as a bulk medium, atmospheric chemistry focuses on each individual constituent (commonly referred to as trace gases) and the chemical reactions that take place among them.
When discussing atmospheric chemistry, it is perhaps most convenient to separate the discussion into two distinct chemical regimes: the stratosphere and the troposphere. In the stratosphere, the most important trace gas is ozone, 03, whereas in the troposphere, it can be argued that one of the most important trace gases is carbon dioxide, C02. Both of these trace gases are intimately tied to the issue of global change as measurements over the past several decades confirm that stratospheric ozone is decreasing and that carbon dioxide is increasing. Ozone in the stratosphere is vital for shielding the biosphere from harmful ultraviolet radiation; a decrease in the amount of ozone in the stratosphere will result in damage to biota at the ground. On the other hand, carbon dioxide is an important trace gas (second in importance to water vapor) that keeps infrared radiation within the lower atmosphere, and it is generally agreed that an increase in C02 may have important climatic implications and lead to global warming.
The source of energy that drives the chemical processes in the atmosphere is the same source that drives Earth's weather engine, namely the sun. Furthermore, the high-energy ultraviolet radiation emitted by the sun initiates a series of reactions in
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the upper atmosphere as these high-energy photons break the stable molecules, N2 and 02, apart into their atomic components. This high energy not only is capable of breaking these very strong molecular bonds apart, but it is also capable of stripping away electrons creating a source of ions in the atmosphere above ~ 50 km. This region of the atmosphere is called the ionosphere, and its chemistry will not be discussed in this section. For more information about the chemistry of the ionosphere, mesosphere, and thermosphere, see Brasseur and Solomon's (1986) Aero-nomy of the Middle Atmosphere, Chapter 6 and various sections in Chapter 5. These ions and atoms can feed some of the chemical cycles that take place in the stratosphere, such as supplying reactive nitrogen species (e.g., see Fig. 1).
From an atmospheric chemistry point of view, important cycles take place in both the stratosphere and the troposphere; this section will concentrate on the chemistry taking place in these regions of the atmosphere. To a certain extent, the chemistry of the stratosphere is somewhat less complex than the chemistry in the troposphere because only large-scale meteorological processes are present at these high altitudes; smaller scale processes such as precipitation can be generally neglected. Also important is the fact that the sources of trace species in the stratosphere are not determined from small-scale sources and can thus can be quantified using a simplified methodology.
In the stratosphere, observing and gaining an understanding of how the distribution of ozone evolved was the primary research emphasis from the 1930s through the 1960s. Understanding how its abundance and distribution has been perturbed by anthropogenic inputs has been the focus of intense research efforts since the 1970s.
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