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1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 Fig. 57

Temporal variation of total ozone in the atmosphere over different stations according to Komhyr el al. (1971). Values are expressed in units of difference from the mean. (By courtesy oiNalure and the authors)

1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 Fig. 57

Temporal variation of total ozone in the atmosphere over different stations according to Komhyr el al. (1971). Values are expressed in units of difference from the mean. (By courtesy oiNalure and the authors)

subsection is devoted to the discussion of this problem. We have to emphasize that solar energy absorbed by stratospheric particles is not so important quantitatively. However, the radiation equilibrium of these atmospheric layers is so delicate that the appearance of some new absorbers results in significant alterations. Since we believe that volcanic gases and particles play an important ro'e ¡n the control of the stratospheric aerosol burden (Subsection 4.4.3) it is suggested that variations of volcanic activity are related to changes of stratospheric heat balance. Furthermore, some authors speculate (e.g. Budyko, 1969) that climatic variations in the past and at present can be explained by the effect of volcanic materials on the radiation and heat balance of the Earth-atmosphere system. This idea seems to be confirmed by the fact that in the first part of this century, when the average global temperature increased, the volcanic activity index, which is proportional to the mass of volcanic-gases and particles injected into the stratosphere, was relatively low (SMIC, 1971). Volcanic eruptions with higher intensity were observed, however, after the beginning of the forties when the temperature started to drop. The mass of material emitted by Agung volcano in the equatorial zone in 1963 was considerably significant. However, it must be stressed that this situation may be due to a simple coincidence, because we have no real proof to explain climatic variations by volcanic activity, although the possibility of such an explanation cannot be entirely ruled out.

We have seen in Subsection 3.6.2 that a significant amount of anthropogenic sulfur dioxide is emitted into the troposphere by energy production from fossil fuels. In spite of the fact that the removal of this species in the lower layers of the atmosphere is rather fast, we cannot exclude the possibility that a certain fraction of this sulfur gas reaches the stratosphere. However, it is estimated (SMIC, 1971) that this S02 quantity is negligible compared to that due to the volcanic activity.

Finally, it should be mentioned that supersonic aircraft also emits soot particles and carbon dioxide gas. Although these soot particles can increase the stratospheric temperature in some cases, the effect of carbon dioxide emitted in this way is considered to be negligible (Schneider and Kellogg, 1973).

6.S2 Effects of tropospheric aerosol particles on the radiation balance

It was mentioned in Chapter 4 that aerosol particles scatter and absorb solar radiation. These processes depend upon the concentration, size distribution, form, refractive index and absorption coefficient of the particles, as well as upon the wavelength of the radiation. In the case of water-soluble particles the extinction is also controlled by relative humidity (see Section 4.5). The energy absorbed by particles leads to an increase of temperature, while backscattering produces an energy loss for the system. Sine* this energy loss may be characterized by the albedo, it is proposed to examine first the relation between albedo and temperature in surface air.

Budyko (1969) determined the following empirical relation between the temperature (7) and the radiation intensity emitted (/):

where n is the cloud cover expressed in tenths of the sky, while b{,b2, c, and c2 are empirical constants. Furthermore, for the whole of the Earth we can write that

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