Although it has revolutionised our lives, and mainly for the better, the internal combustion engine has always been a source of nuisance. As long ago as 1914, the Oxford classical scholar A.D. Godley pointed out as much in an amusing little verse which may appeal to those who still remember their Latin declension:
What is this that roareth thus?
Can it be a Motor Bus?
Yes, the smell and hideous hum
Indicat Motorem Bum;
How shall wretches live like us
Cincti Bis Motoribus?
Domine, defende nos
Contra hos Motores Bos.
But in more recent times it has become apparent that the motor bus, and others of its ilk, are the source of a nuisance more insidious; the internal combustion engine, through its production of nitrous oxides, is a prime contributor to acid rain.
The motor car, of course, is not the only culprit. Indeed the main source of acid rain is sulphur dioxide, which is a byproduct of the burning of fossil fuels. Chemical reactions in the atmosphere, often brought about by the effect of sunlight, transform these substances into sulphuric and nitric acids, which are then washed from the air by rain.
Acid rain is not directly harmful to humans to any significant extent. But it does damage our environment, and in particular it affects buildings and monuments by causing the stonework to deteriorate and crumble away; some of the architectural wonders of Greece have required major refurbishment for this reason, especially those constructed with a very porous building stone.
Other materials are affected too; iron, zinc, steel, and even paint are susceptible to damage from acid rain. The effect may not be catastrophic, but the impact, both economic and cultural, is by no means insignificant.
The precise mechanism by which acid rain causes damage to buildings is still imperfectly understood. A complex combination of chemical reactions is known to occur, involving both "dry" deposition - where chemicals fall directly on to the fabric as dry solid particles - and "wet" deposition - essentially acid rain.
Until recently it was thought that sulphuric acid in the atmosphere was the main culprit, but during the 1980s, emissions of sulphur dioxide in the London area were dramatically reduced, and yet weathering has so far continued to occur at much the same rate. It may be that chemical reactions from earlier pollution are still going on - or alternatively, it may be that other constituents of acid rain are more important than previously thought.