Tropospheric Chemistry A Complex Interaction Of Biogeochemical Cycles

Somewhat analogous to the chemical cycles just described for the stratosphere, atmospheric chemistry in the troposphere can also be viewed as a complex interaction of chemical cycles. These cycles, however, involve direct interaction with the biosphere and are also greatly complicated by the presence of the more complicated meteorology found only in the lower atmosphere. The biosphere serves as the exclusive source of carbon, and the carbon cycle, in turn, has important linkages that transcend land, ocean, and air.

The dominant form of carbon in the atmosphere is its completely oxidized state, carbon dioxide, which is present in the atmosphere at concentrations of ~ 360 ppmv (parts per million, by volume). Between the time carbon is stored in the biosphere and eventually bccomes C02, many interesting chemical transformations and interactions take place. The second most abundant carbon-containing trace gas is methane, with an atmospheric concentration of ~ 1.8 ppmv, the only other atmospheric trace gas that exists in the concentrations of more than 1 ppmv, even in regions far removed from its sources. From an atmospheric chemistry point of view, methane plays an important role because its oxidation is closely linked to other carbon-containing trace compounds such as carbon monoxide (CO) and formaldehyde (CH20). The role of carbon dioxide, on the other hand, is not directly tied to chemical reactions taking place in the atmosphere, but rather to assessing how natural and anthropogenic processes contribute to increasing the global carbon burden.

The carbon cycle that couples the atmosphere with the biosphere is one of several important biogeochemical cycles in the Earth system. These cycles describe how specific elements and compounds are transferred between the principal global reservoirs—the atmosphere, land, oceans, and biosphere. Chemical and physical processes and transformations determine the partitioning of a material among the reservoirs. For example, for a fixed amount of carbon in these reservoirs, a certain fraction is found in the atmosphere, another fraction in the oceans, and so on; these fractions depend on the way the carbon is transferred between the reservoirs, the sizes of the reservoirs, and other factors. The amount to be found in the atmosphere, where it may have the most substantial effect on climate, is thus determined by the carbon's overall biogeochemical cycle.

The size of these cycles is enormous. For example, the carbon cycle transfers more than 1015 g of carbon per year from the atmosphere to other reservoirs. The terrestrial biosphere is dominated by trees and other vegetation; the amount of land biomass in animal form is much smaller, by approximately a factor of 100. Living biomass on land is also ~ 100 times as massive as the total living biomass in the oceans. Dead biomass is even more plentiful than living biomass. Inanimate organic matter accumulates as plant litter in forests, building up as a layer of humic soil or peat. Eventually, the organic debris is consumed by microorganisms as it decays, or it may be burned; in either case, carbon dioxide is put into the atmosphere. The details of these cycles are complex, and many of the chapters in this section focus on specific key trace gases that are parts of these cycles. Other chapters focus on specific processes that are responsible for the conversion of trace gases to other species that may or may not remain in gaseous form. These processes, however, may provide the dominant vehicle through which various elements are transferred between the various reservoirs within the biogeochemical cycle (e.g., sulfur being removed from the atmosphere and transferred to the land through the formation of acid rain).

In addition to carbon dioxide and methane, the important members of the carbon cycle are carbon monoxide (CO), and a host of nonmethane hydrocarbons [also called volatile organic compounds (VOCs)] consisting of more than one carbon atom and a number of hydrogen atoms (CnHm, where n and m are integers). Other important cycles in Earth's atmosphere include nitrogen, oxygen, and sulfur. In the nitrogen (N) cycle, the key compounds are nitrogen (N2), nitrous oxide (N20), nitrogen oxides (NOx), consisting primarily of nitric oxide (NO) and nitrogen dioxide (N02), nitric acid (HN03), and the nitrate ion (N03)~. Both molecular nitrogen and nitrous oxide are key elements of the biogeochemical nitrogen cycle, although neither is involved in any chemical reactions in the troposphere.

The oxygen cycle in the troposphere is comprised nearly exclusively of molecular oxygen (02), and ozone (03); note that atomic oxygen is not an important player in the troposphere despite its importance on the stratosphere. The primary players in the sulfur cycle are sulfur dioxide (S02), carbonyl sulfide (COS), hydrogen sulfide (H2S), dimethyl sulfide (CH3)2S, sulfuric acid (H2S04), and the sulfate ion (S04)"2.

This section on atmospheric chemistry will include separate articles on reactive nitrogen species, tropospheric ozone, and atmospheric sulfur, as well as the carbon compounds carbon monoxide and methane and nonmethane hydrocarbons. A broad discussion will now be presented on the carbon, nitrogen, and oxygen cycles so that the reader will be better able to envision how the individual trace gases and processes fit into the "big picture."

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