Robinson and Robbins (1970) think that these processes are effective mainly in the stratosphere. This concept is reasonable since O* concentrations in the stratosphere are greater than in the troposphere. Furthermore, the intensity of U V radiations is also greater above the tropopause. According to Ehhalt et al (1977) the flux of N20 from the troposphere to the stratosphere is between 6 x 108 and 30 x 108 molecules cm "2 s"1 which are equivalent to (7-35) x 1061 yr "These values are based partly on the N20 concentration gradient measured in the lowest three kilometers of the stratosphere and partly on the net transport coefficient of 1 x 103 cm2 s"1 and
5 x 103 cm2 s"1. Taking into account the average tropospheric column concentration given by these authors (4.5 x 1018 molecules cm-2) calculations of the tropospheric residence time range between 47 and 233 years. Since these values are high compared to the atmospheric lifetimes calculated above (20-30 years) we can conclude that other sink mechanisms must operate in the troposphere. The nature of this tropospheric sink is not yet clear. We cannot exclude the possibility (Schütz et ai., 1970) that chemical removal processes are also effective in the troposphere. Robinson and Robbins (1970) speculated that a largj part of atmospheric N20 returns to the pedosphere. It should be mentioned in this respect that Brice et ai (1977) demonstrated in England that N20 concentration varied during the day inversely with the level of radon of soil origin. On the basis of this finding they postulated that soils provide a nitrous oxide sink with a stength sufficient to balance the sources mentioned.
An important consequence of reaction [3.42] is that N20 plays a certain role in the chemistry of ozone formation. Although a small part of the nitric oxide formed in this way returns into the troposphere by slow diffusion (see later Fig. 15), the majority of NO molecules takes part in stratospheric chemistry as discussed in Subsection 3.4.3. This suggests that N20 arising from the use of nitrogen containing fertilizers may pose a threat to the stratospheric 03 layer.
Ammonia was first identified in atmospheric precipitation (Junge. 1963). NH3 molecules (actually ammonium ions) in precipitation water are due to ammonia gas and ammonium containing aerosol particles in the air (see Chapter 5). Results of atmospheric measurements show that the ammonia concentration in surface air is between 4-20 fig m"3 for unpolluted continental areas (Georgii, 1963). The corresponding range for the oceanic environment is 0.2-1.3. ¡ig m-3 (Junge, 1963; Tsunogai and Ikeuchi, 1968). In tropical continental air NH3 concentrations are relatively high (Lodge et al, 1974). Robinson and Robbins (1970) propose a value of
6 ppb (4.6 ¡ig m "3 STP) for the mean atmospheric concentration, which seems too high on the basis of the above data. In the recent compilation of Söderlund and Svensson (1976) figures between 0.4 and 12 ppb (0.3-9.1 /igm"3 STP)can be found.
The first aircraft flights to measure NH3 concentration in the troposphere over continents were carried out by Georgii and Müller (1974), while Gravenhorst (1975)
S Mcszäros published the first oceanic vertical profiles. The solid lines of Fig. 14 represent the mean results of Georgii and Müller obtained over Germany. These authors divided their data in two categories:
(1) when the surface air temperature was higher than 18 °C (summer);
(2) when the surface air temperature was below 10 °C (late fall, winter, early spring).
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