Several nitrogen oxides are known, including N20 (nitrous oxide), NO (nitric oxide), N02 (nitrogen oxide), N03 (nitrogen trioxide), N203 (dinitrogen trioxide), N204 (dinitrogen tetroxide) and N2Os (dinitrogen pentoxide). At normal temperatures and small partial pressures N203 and N204 decompose according to the following equations (Junge, 1963):
For example, owing to this last decomposition, the N204 concentration is 10"6 /ig/m\ if the nitrogen dioxide level is 20 ¿tg/m3.
Furthermore under normal atmospheric conditions N03 decomposes by photolysis or combines with NO. Thus Crutzen (1974) showed:
For these reasons, only nitrous oxide and NOv (x = 1 or 2) can be identified in practice in the air, if we disregard HNO, formed by the interaction of nitrogen dioxide and water vapour (see later).
In addition to the above gases, ammonia (NH3) is also an important atmospheric trace substance. An essential characteristic of ammonia and NOx is that these trace gases transform in the air into ammonium and nitrate-containing aerosol particles. These particles are of importance for the control of many atmospheric processes (see Chapter 4).
In the following, the atmospheric cycle of gaseous nitrous oxide will first be discussed. As we shall see, the pathway of this compound is related to the NOx cycle. After the discussion of N20, the abundance of ammonia gas (ammonium particles) will briefly be presented; these can also be converted in the air to nitrogen oxides. Finally, the atmospheric cycle of NO, will be outlined, including particulate nitrate.
The presence of nitrous oxide in the atmosphere was first identified by optical methods (Adel, 1938). These early spectroscopic measurements gave a concentration interval of 0.25-0.60 ppm with a mean value of 0.40 ppm (Junge, 1963). Results of more recent gas chromatographic analyses show that the N20 mixing ratio is near the lower limit of the range mentioned (Schiitz et al, 1970). These recent studies also demonstrate that the concentration of N20 decreases only very slowly with increasing height in the troposphere. However, the decrease becomes significant in the stratosphere. This pattern is supported by the observations of Ehhalt et al (1977), according to which the mixing ratio at an altitude of 35 km is one-tenth its value at the tropopause. The analyses of air samples collected in the vicinity of Mainz (Federal Republic of Germany) indicate no effects of industrial pollution on the level of this compound. This finding is confirmed by Soviet data showing no differences in N20 content in air columns over Moscow and over Siberia (Achmedov et al, 1978).
In the U.S.A., N20 was measured in Massachusetts by Goody (1969). His results suggested that NzO concentration increased during 1967 and 1968. More recently Rasmussen and Pierotti (1978) have studied the N 20 level in the air over Antarctica, the Pacific Ocean and eastern Washington. Their data do not show hemispheric differences. The mean concentration has been found to be 0.33 ± 0.003 ppm, which is larger than the average tropospheric NzO level (0.26 ppm) reported by Schiitz et al. (1970) However, Rasmussen and Pierotti's results also show a uniform mixing ratio in the troposphere. It is believed that this discrepancy is at least partly due to analytical problems. However, as Rasmussen and Pierotti (1978) point out, data gained in 1976 and 1977 at a rural station in eastern Washington do not exclude a small temporal increase of the concentration. The rate of this increase is estimated to be less than 2 %. Considering that the measurement precision is at best 1 %, it is obvious that further observations are needed in this field.
Schiitz et al. (1970) estimated, on the basis of their measurements, a N20 column concentration of 3.30xlO"4 g cm-2 in the troposphere. The corresponding stratospheric figure is 0.625 x 10" 4 gem'2. Taking into account the surface of the
Earth (5.1 x 1018 cm2), a rounded-off value of 1700 x 1061 can be calculated for the whole of the troposphere, while the global atmospheric N20 burden is equal to 2000 x 1061. The data of Rasmussen and Pierotti (1978) lead to a somewhat higher total quantity in the troposphere. However, on the basis of several recent vertical profile measurements, Ehhalt et al. (1977) give a global tropospheric nitrous oxide mass of 1660 x 106 t which is in excellent agreement with Schutz's estimate.
It was shown by experiments carried out under laboratory conditions that nitrous oxide can be liberated from different soils since under some conditions bacterial denitrification processes lead to the formation of N20 gas (Delwiche,
Schutz et al. (1970) estimate that the global strength of this source is 50 x 1061 yr ~1, while Robinson and Robbins (1970) propose a value of 592 x 1061 yr"1. In a recent and careful study Sdderlund and Svenssofl (1976) speculate that this source yields between 25 and 110x10® t yr-1 which essentially confirms Schutz's estimate. Moreover, Hahn (1974) demonstrated that ocean water was supersaturated with N20 as compared to atmospheric concentration. In another paper he hypothesizes that oceanic denitrification deliberates yearly (30-130) x 106 t of N20 (see: Sdderlund and Svensson, 1976). On the basis of the above data it can be proposed with caution that the global strength of the N20 biological source is around 100 x 1061 yr"1. Taking into account the total global burden of 2000 x 1061, we can calculate a .value of 20 years for the atmospheric residence time.
It has been mentioned in Section 1.1 that for atmospheric constituents the residence time linearly increases with the decrease of the variability of the concentration. Junge (1974) speculates that the product of the residence time and the relative standard deviation of the concentration is constant and equal to 0.14. By using this relation Rasmussen and Pierotti (1978) estimate, on the basis of their measurements, residence times of between 25 and 30 years, which is in acceptable agreement with the above figure.
It should be stressed that the activity of man also produce atmospheric N20. Several authors argue (e.g. Bremner and Blackmer, 1978) that N20 can be released to the atmosphere through denitrification of fertilizer-derived nitrate and ammonia This concept seems to be supported by the possibility that the N20 mixing ratio is increasing in the troposphere. It is obvious, however, that much work remains to be done to clarify this problem.
Unfortunately, the sinks -of the atmospheric nitrous oxide are very poorly understood. In any case we can accept the view that a part of N20 is chemically removed from the air by the following reactions (Crutzen, 1973):
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