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Sulphur dioxide (SO2)
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page last modified:
11 January 2005
   
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Sulphur dioxide

Sulphur dioxide, SO2, enters the atmosphere as a result of both natural phenomena and anthropogenic activities, e.g.:
  • combustion of fossil fuels
  • oxidation of organic material in soils
  • volcanic eruptions
  • biomass burning
Coal burning is the single largest man-made source of sulphur dioxide, accounting for about 50% of annual global emissions, with oil burning accounting for a further 25 to 30%. Sulphur dioxide reacts on the surface of a variety of airborne solid particles (aerosols), is soluble in water and can be oxidised within airborne water droplets, producing sulphuric acid. This acidic pollution can be transported by wind over many hundreds of kilometres, and is deposited as acid rain.

Changes in the abundance of sulphur dioxide have an impact on atmospheric chemistry and on the radiation field, and hence on the climate. Consequently, global observations of sulphur dioxide are important for atmospheric and climate research.

Effects of volcanic eruptions may have an impact on air traffic, as such eruptions are important sources of ash (aerosols) and sulphur dioxide in the atmosphere. A near-real time retrieval of sulphur dioxide concentrations would enable monitoring of such events and can thus assist in aviation control. Off-line retrieval, on the othe hand, is more suitable for monitoring anthropogenic pollution aspects.
 

Volcano eruptions

Volcano eruptions, for example, are important sources of gases in the atmosphere, but:
  >   emissions are sporadic and intermittent;
  >   emissions often occur in uninhabited regions.
In-situ assessment of emissions is therefore difficult or even impossible. Observations of sulphur dioxide from space provides important information to earth scientists.

Eisinger & Burrows have shown that the retrieval of sulphur dioxide from GOME spectra is possible with a Differential Optical Absorption Spectroscopy (DOAS) technique. Within the TEMIS project an optimal estimation inversion method will be used for a combined retrieval of sulphur dioxide and ozone concentrations. The following image shows an example result of such a retrieval.

 

SO2 over Nyamuragira volcano, 4-6 Dec. 1996
Sulphur dioxide plume travelling westward from the Nyamuragira volcano (Democratic Republic of Congo) after its eruption on 1 December 1996, as derived from observations by GOME using an optimal estimation inversion method. The three bands of ground pixels on the right are from 4 Dec., when GOME passed directly over the volcano (marked by an asterisk), while the three bands on the left are from 6 Dec.

 

Sulphur dioxide in troposphere and stratosphere

The lifetime of sulphur dioxide molecules in the troposphere is a few days. The amount is highly variable, above a low background concentration.

It is removed from the troposphere

  • in gas phase by formation of suphuric acid, which forms condensation nuclei for aerosols and clouds and acidifies the rain;
  • directly, by way of an uptake on aerosols and clouds, which leads to dry and wet acid depositions.
Clean continental air contains less than 1 ppb of sulphur dioxide, which corresponds to a total column density < 0.2 Dobson Units (DU) in a boundary layer of 2 km.

The lifetime of sulphur dioxide molecules in the stratosphere, on the other hand, is several weeks, during which is produces sulphate aerosols. This makes sulphur dioxide from volcanos one of the two most important sources of stratospheric aerosols.