Nitrous Oxide

Another significant greenhouse trace gas is nitrous oxide, N20, also known as "laughing gas"; its molecular structure is NNO rather than the more symmetrical NON. Its bending vibration absorbs IR light in a band at 8.6 /xm, i.e., within the window region, and in addition one of its bond-stretching vibrations is centered at 7.8 jxm, on the shoulder of the window and at the same wavelength as one of the absorptions for methane. Per molecule, N20 is 296 times as effective as C02 in causing an immediate increase in global warming. Like that of methane, the atmospheric concentration of nitrous oxide was constant until about 300 years ago, at which time it began to increase, although the level has increased from 275 ppb (preindustrial) by only 16% to 320 ppb. The yearly growth rate in the 1980s was about 0.25% but fell significantly in the early 1990s for reasons that are uncertain. The increased amounts of nitrous oxide that have accumulated in the air since preindustrial times have produced about one-third of the amount of the additional warming that methane has induced.

Less than 40% of nitrous oxide emissions currently arise from anthropogenic sources. In 1990 it was discovered that the traditional procedure, using nitric acid, HN03, of synthesizing adipic acid (a raw material in the preparation of nylon) resulted in the formation and release of large amounts of nitrous oxide. Since that time, nylon producers have instituted a plan to phase out N20 emissions.

The greater part of the natural supply of nitrous oxide gas comes from release from the oceans, and most of the remainder is contributed by processes occurring in the soils of tropical regions. The gas is a by-product of the biological denitrification process in aerobic (oxygen-rich) environments and in the biological nitrification process in anaerobic (oxygen-poor) environments; the chemistry of both processes is illustrated in Figure 6-17. In denitrification, fully oxidized nitrogen in the form of the nitrate ion, NOj", is reduced mostly to molecular nitrogen, N2. In nitrification, reduced nitrogen in the form of ammonia or the ammonium ion is oxidized mostly to nitrite, N02-, and nitrate ions. Chemically, the existence of the nitrous oxide by-product in both processes is simple to rationalize: Nitrification (oxidation) under oxygen-limited conditions yields some N20, which has less oxygen than the intended nitrite ion; denitrification (reduction) under oxygen-rich conditions yields some NzO, which has more oxygen than the intended nitrogen molecule. Nitrification is more important than denitrification as a global source of N20. Normally, about 0.001 mole of nitrous oxide is emitted per mole of nitrogen oxidized, but this value increases substantially when the ammonia or ammonium concentration is high and relatively little oxygen is present. Overall, the increased use of fertilizers for agricultural purposes probably accounts for the majority of anthropogenic emissions of nitrous oxide. The decomposition of livestock-produced manure under aerobic conditions, including its use

Oxidation state of nitrogen

Nitrification nh3 _

Ammonia and ammonium ion

S Nitrite

\ ion

v By-product nor


Nitrous oxide f NjO )

Nitrous oxide

Diatomic nitrogen

Nitrate ion


FIGURE 6-17 Nitrous oxide production as a by-product during the biological cycling of nitrogen.

as a fertilizer, contributes significantly to nitrous oxide emissions; manure produces very little N20 if decomposed anaerobically.

Apparently, nitrous oxide released from new grasslands is particularly significant in the years following the burning of a forest. Some portion of the nitrate and ammonium fertilizers used agriculturally, particularly in tropical areas, is similarly converted (an unintended effect, to be sure) to nitrous oxide and released into the air. Tropical forests in wet areas are probably a huge natural source of the gas.

At one time it was believed that fossil-fuel combustion released nitrous oxide as a by-product of the chemical combination of the N2 and 02 in air, but this belief was based on faulty experiments. Only when the fuel itself contains nitrogen, as do coal and biomass (but not gasoline or natural gas), does N20 form; apparently, N2 from air does not enter into this process at all. However, some of the NO produced from atmospheric N2 during fuel combustion in automobiles is unavoidably converted to N20 rather than to N2 in the three-way catalytic converters currently in use and is subsequently released into air. Some of the newer catalysts developed for use in automobiles do not suffer from this flaw of producing and releasing nitrous oxide during their operation.

As mentioned in Chapter 1, there are no sinks for nitrous oxide in the troposphere. Instead, all of it rises eventually to the stratosphere, where each molecule absorbs UV light and decomposes, usually to N2 and atomic oxygen, or reacts with atomic oxygen.

Continue reading here: CFCs and Their Replacements

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