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Future energy demand and supply
Future energy demand and supply

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3.2 Sulphur dioxide, acid rain and air quality

Emissions of sulphur dioxide (SO2) and nitrogen oxides by burning fossil fuels cause globally significant environmental problems (Figure 14) due to acid rain. Sulphur dioxide reacts with water vapour to become first sulphurous acid (H2SO3) and then sulphuric acid (H2SO4) as a result of photochemical oxidation involving sunlight and other gases such as ozone (O3), hydrogen peroxide vapour (H2O2) and ammonia (NH3) which act as catalysts. Depending on the amount of moisture in the air, up to 80% of emitted sulphur dioxide may become acidic. Nitrogen oxide (NOx) emissions also react with water vapour, to form dilute nitrous and nitric acid. If sulphur dioxide reaches the very dry stratosphere, rather than being 'rained-out', sulphuric acid forms minute droplets (aerosols) that have a very different effect (Section 3.3).

Figure 14 The chain of pollution caused by acid rain.

Acid rain increases the acidity of groundwater, which helps to dissolve metal ions from soil and rocks. Some of these, such as aluminium ions, are poisonous to plants and animals (Figure 14). Acid rain also attacks carbonate-rich soils and limestones exposed at the surface to release CO2 into the atmosphere. Even if sulphur dioxide levels from power stations are strictly controlled, as they are in the USA and several European countries, NOx emissions from increasing numbers of cars and power stations are capable of continuing the production of acid rain on about the same scale.

Current research suggests that even if SO2 and NOx pollution was halted today, the effects of acid rain outlined in Figure 14 would remain for decades. Acidity and aluminium would continue to poison lakes and streams. One UK investigation suggests that halving acid precipitation over the Scottish hills immediately would merely maintain the acidity of the local lakes at their present-day levels.

Activity 3

Figure 15 shows the pattern of global atmospheric fallout of sulphur (mainly as sulphates) during the 1990s and anticipated fallout during 2030 (based on atmospheric models). Analyse Figure 15, using the following tasks as a guide:

  • a.Identify those regions that are likely to receive decreased sulphur pollution by 2030.
  • b.Which regions are likely to expect an increased problem with acid rain?
  • c.Suggest reasons for the main regional changes in acid rain between 1990 and 2030.
  • d.Australia has one of the highest per capita energy consumption rates in the world. Why is acid rain so low there?
Figure 15 Global fallout of sulphur from acid rain (colours show ranges in g m-2 yr-1), during (a) the 1990s and (b) estimated for 2030.


  • a.Europe and North America are likely to receive less sulphur-rich acid rain pollution by 2030.
  • b.Asia is the major region likely to experience an increase in acid rain pollution by 2030, although other regions such as Africa, the Middle East, South and Central America are also anticipated to experience increases in acid deposition.
  • c.Europe and North America will probably experience less pollution, due mainly to the introduction of pollution abatement legislation. Between 1990 and 2030 technological developments that allow reduced emissions of sulphurous compounds will lead to less pollution, even though the economies of these regions are predicted to continue to grow. In Asia and, to a lesser extent, in Africa, South and Central America and the Middle East, economies are predicted to grow by 2030. The accompanying rapid demand for energy that will mostly be met by the consumption of fossil fuels would result in an overall increase in sulphurous acid rain pollution, even if emission abatement legislation were introduced there in the intervening time.
  • d.Although the per capita energy consumption is high, the population density of Australia is low, so energy use per unit land area is likely to be lower than in more densely populated developing regions such as Asia.