Volume mixing ratio (ppm) of carbone monoxide in the troposphere (Seiler, 1974). (By courtesy of Tellus)

Volume mixing ratio (ppm) of carbone monoxide in the troposphere (Seiler, 1974). (By courtesy of Tellus)

Investigation of the cycle of CO in the air was stimulated by the development of a continuous chemical monitoring technique (Seiler and Junge, 1970). This method, based on the destruction of mercuric oxide by CO, was widely used by Seiler (1974) to determine the carbon mqnoxide concentration in the air as a function of geographical location and altitude. His results are summarized in Fig. 7. It can be seen that in surface air maximum concentrations are found in the Northern Hemisphere (>0.2 ppm). Data also show that at the latitudes where the CO level has a maximum the variations of concentration are also rather significant. In the vicinity of the equator, in the intertropical convergence zone4, the volume mixing ratios are between 0.10 and 0.16 ppm. The concentration begins to increase in the Northern Hemisphere just north of the convergence zone.

The concentration of CO in the upper troposphere also varies in agreement with the latitudinal changes observed in surface air. However, the magnitude of upper air

* Zone where the southeast trade winds of the Southern Hemisphere meet the northeast trade winds of the Northern Hemisphere. This "convergence" at the surface is the result of continual rising of air at the point of meeting.

variations is small compared with fluctuations in the lower troposphere. In the Northern Hemisphere the concentration decreases with increasing tropospheric altitudes. These vertical variations become insignificant in the Southern Hemisphere. Finally, Seiler's measurements show that above the tropopause the concentration decreases rapidly in the first 1 km layer, then it remains practically constant in the stratosphere (0.05 ppm).

According to Seiler (1974) and Seiler et al (1978) the atmospheric carbon dioxide is mostly produced by the following sources (see Table 7):

(a) anthropogenic sources;

(b) emission of CO from the ocean;

(c) burning of bushes and forests;

(d) oxidation of non-methane hydrocarbons;

(e) direct production by plants;

(f) oxidation of methane.

Robinson and Moser (1971) speculate that the majority of anthropogenic CO (64 %) is emitted into the air by automobile exhausts mainly over the Northern Hemisphere. Seiler (1974) reports a slightly higher figure (70-80 %). The strength of global anthropogenic source was estimated by Robinson and Moser (1971) on the basis of data from 1966. They concluded that the total emission is 285 x 106 t yr~'. According to JafTe (see Seiler, 1974), who used data relative to 1971, the rate of anthropogenic CO formation is equal to 460 x 106 t yr~l. Seiler (1974) believes that even this latter value can be considered as a lower limit. He argues that the authors underestimated the effect of domestic heating. Furthermore, they did not take into consideration emissions from some industrial processes. Finally, the above emission rates were estimated mainly on the basis of data from USA, where the emissions are rather severely controlled. For these reasons Seiler gives a global value of 640 x 106 t yr"1.

According to the results of measurements carried out in the ocean (Seiler, 1974) the carbon monoxide concentration in near surface marine layers is 5 x 10"5 mil"1 on an average. This water concentration would be in equilibrium with a surface air CO level of 2.5 ppm. However, the carbon monoxide concentration in air over the ocean surface is between 0.04 ppm and 0.20 ppm, which means that the ocean water is supersaturated with CO. It follows from these data that the ocean is a CO source, the global strength of which is about six times less than the total anthropogenic emission (Table 7).

It is believed that CO formation caused by natural burning is also small as compared to the production rate of man-made atmospheric carbon monoxide. The same is probably true in case of the oxidation of terpenoid hydrocarbons emitted by vegetation (see Subsection 3.3.3). However, as Wilks(1959) pointed out green plants can directly emit carbon monoxide due to the photodecomposition of pigments and

Table 7

Estimated annual production rates and sinks of atmospheric CO according to Seiler (1974) and Seiier el al. (1978)

N. Hemisphere S. Hemisphere Toul

Sources [106 t yr1] Anthropogenic Oceans

Bush- and forest fires Oxidation of hydrocarbons CH4 oxidation Direct production by plants Sinks [106 t yr"'] Soil uptake Oxidation

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