Hydrogen and helium 321 Hydrogen

The first acceptable hydrogen analyses in the atmosphere were carried out by Schuftan in Germany (see Junge, 1963). According to his data, taken in 1923, the concentration of hydrogen in the air is 0.5 ±0.10 ppm. Subsequently, many hydrogen measurements were made in polluted atmospheres. In really clean environments, the first samplings for hydrogen analysis were done by Bainbridge (see Schmidt, 1974) in remote areas of the Pacific Ocean. Later, air samples were taken over territories of the U.S.A. by means of an aircraft (Ehhalt and Heidt, 1973). In these programs individual air samples were analyzed. Thus, the number of data is rather limited. Hydrogen measurement in the atmosphere was facilitated by a refinement of the mercuric oxide method for carbon monoxide, which makes continuous monitoring possible.

This method was widely used in recent years by Schmidt (1974). He carried out measurements in the air near the surface as well as in the upper troposphere and lower stratosphere. His results obtained near ground level can be divided into three categories:

(1) polluted atmosphere (Mainz, FRG);

(2) Atlantic Ocean, Northern Hemisphere;

(3) Atlantic Ocean, Southern Hemisphere.

One of most interesting features of his results is the fact that the hydrogen concentration in polluted air is higher than in clean air. The atmospheric hydrogen level is lowest in the clean air of the Southern Hemisphere. The mean concentration values for the categories mentioned are 0.800 ppm, 0.575 ppm and 0.550 ppm, respectively. The more recent data of Schmidt (1978) confirm essentially these

2 This means that the concentrations given are generally so-called background concentrations.

concentrations. He reports that the average mixing ratio is 0.576 ppm in the Northern and 0.552 ppm in the Southern Hemisphere. The troposhere was found to be rather well mixed vertically and horizontally in both hemispheres. According to Schmidt's aircraft measurements carried out over France the H2 volume mixing ratio in the air does not change in passing from the upper troposphere to the lower stratosphere. Figure 5 gives his results obtained during two flights around the tropopause. In Fig. 5 the temperature profile and the vertical variation of the ozone concentration are also plotted. This last was determined simultaneously with the H2 measurement. It can be seen that the ozone concentration increases while the H2 level remains constant with increasing height.

Finally, it should be noted that Schmidt (1974) also measured the hydrogen concentration of ocean waters. He found that the H2 content of ocean waters is generally three times higher than the equilibrium value calculated on the basis of atmospheric measurements.

O3 mixing ratio [pg/g]

O3 mixing ratio [pg/g]

H2 mixing ratio [ppm] Temperature [*c]

The vertical profile of hydrogen and ozone in the vicinity of the tropopause according to Schmidt (1974).

(By courtesy of Tellus)

H2 mixing ratio [ppm] Temperature [*c]

The vertical profile of hydrogen and ozone in the vicinity of the tropopause according to Schmidt (1974).

(By courtesy of Tellus)

Taking into account the above findings Schmidt (1974) hypothesized four H2 sources:

(a) human activity;

(b) surface of oceans;

(c) soil surface;

(d) photochemical formation in the troposphere.

The anthropogenic H2 is emitted into the air in automotive exhaust gases, which contain H2 in the range of 1-5 % by volume. The nature of the oceanic source is not entirely clear but it is probably due to microbiological activity. However, the supersaturation of ocean waters unambiguously indicates hydrogen gas formation there. The emission from soils is caused by the fermentation of bacteria.

From the point of view of air chemistry the photochemical H2 formation is the most interesting source. Although this formation process is possible by the direct photolysis of water molecules (see reaction [2.2]), it is generally accepted that methane plays an important part in the reaction chain. The atmospheric methane (see Section 3.4) first reacts with OH radicals. These very reactive electrically uncharged atomic groups, called free radicals, can be formed partly by the photolysis of water (see later) and partly by the following process3 (Warneck, 1974):

where the asterisk denotes that the oxygen atom is in excited state (see Subsection 3.4.2). The excited atomic oxygen comes from the destruction of ozone (see later). By the interaction of OH radicals and CH4, formaldehyde is formed, which dissociates under the influence of solar radiation with wavelengths of 0.30-0.36 fan (Calvert et al, 1972):

According to Schmidt (1974) there are two important H2 sinks:

(a) chemical destruction in the troposphere;

The chemical destruction in the troposphere is also promoted by the presence of OH radicals which react with hydrogen in the following way:

The product of this reaction is partly water vapour and partly another chemically active free radical (H). The interaction of processes [3.2] and [3.3] results in an equilibrium H2 concentration.

The numerical values of source and sink terms are given in Table 5 (Schmidt, 1974). It should be mentioned that the table does not contain the global soil source strength. It is speculated that its lower limit is around 0.1 x 1061 yr "Considering that the sum of known global source terms is about 66 % of the H2 sinks, soil sources

3 In polluted atmosphere free radicals are formed by other reaction steps, too (Haagen-Smit and Wayne, 1976).

3 M&zärot must be about 12.5 x 106 t yr"1 in order to balance sources and sinks. Further research is needed, however, to validate the numerical values given in Table 5.

On the basis of the H2 measurements discussed above it can be calculated that the atmospheric reservoir contains 204 x 106 t of hydrogen. More than the half of this quantity can be found in the atmosphere over the Northern Hemisphere. Dividing this figure by the global source or sink strengths, we obtain for values of between about 6 and 8 years for the atmospheric residence time.

Table 5

Sources and sinks of atmospheric H2 expressed in 1061. The data are combined from two tables of Schmidt (1974)

Table 5

Sources and sinks of atmospheric H2 expressed in 1061. The data are combined from two tables of Schmidt (1974)

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