H2SO4aq CaCO3S H2Ol CO2g CaSO4S2H2Ow eqn 328

Sulphuric acid converts limestone (CaCO3) into gypsum (CaSO4 .2H2O). The deterioration is severe because gypsum is soluble and dissolves in rain. Perhaps more importantly, gypsum occupies a larger volume than limestone, which adds mechanical stress so that the stone almost explodes from within.

The diesel engine is no longer confined to large vehicles in Europe, as passenger cars have taken advantage of potentially lower fuel costs. The fuel injection process of the diesel engine leads to the fuel dispersing as droplets within the engine. These may not always burn completely, so diesel engines can produce large quantities of smoke if not properly maintained. Diesel smoke now makes a significant contribution to the soiling quality of urban air.

In the modern urban atmosphere, O3 may be the pollutant of particular concern for health. However, it is a reactive gas that will also attack the double bonds of organic molecules (see Section 2.7) very readily. Rubber is a polymeric material with many double bonds, so it is degraded and cracked by O3. Tyres and windscreen wiper blades are especially vulnerable to oxidants, although newer synthetic rubbers have double bonds protected by other chemical groups, which can make them more resistant to damage by O3.

Many pigments and dyes are also attacked by O3. The usual result of this is that the dye fades. This means that it is important for art galleries in polluted cities to filter their air, especially where they house collections of paintings using traditional colouring materials, which are especially sensitive. Nitrogen oxides, associated with photochemical smogs, can also damage pigments. It is possible that nitrogen oxides may also increase the rate of damage to building stone, but it is not really clear how this takes place. Some have argued that NO2 increases the efficiency of production of H2SO4 on stone surfaces in those cities that have moderate SO2 concentrations.

SO2(g) + NO2(g) + H2O(l) ^ NO(g) + H2SO4(aq) eqn. 3.29

Others have suggested that the nitrogen compounds in polluted atmospheres enable microorganisms to grow more effectively on stone surfaces and enhance the biologically mediated damage. There is also the possibility that gas-phase reactions produce HNO3 (eqn. 3.14) and that this deposits directly on to calcareous stone. Diesel soot that increasingly disfigures buildings may also carry organic nutrients to the surface that could enhance biological damage.

Finally, we should remember that it is not just materials that are damaged by photochemical smog, since plants are especially sensitive to the modern atmospheric pollutants. Recollect that it was this sensitivity that led Haagen-Smit to recognize the novelty of the Los Angeles smog. Ozone damages plants by changing the 'leakiness' of cells to important ions such as potassium. Early symptoms of such injury appear as water-soaked areas on the leaves.

Urban air pollution remains an issue of much public concern. While it is true that in many cities the traditional problems of smoke and SO2 from stationary sources are a thing of the past, new problems have emerged. In particular, the automobile and heavy use of volatile fuels have made photochemical smog a widespread occurrence. This has meant that there has been a parallel rise in legislation to lower the emission of these organic compounds to the atmosphere.

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