Everytime you turn on your home oil furnace or drive around in your car, you contribute towards making the writing on some gravestones less legible. What accounts for this effect?

Acid rain! Many gravestones are made of marble, which dissolves in acid. This means that if rain water is acidic, every time there is some precipitation, a bit of the marble is worn away. Acid rain can mostly, but not exclusively, be traced to human activity. The fossil fuels that we rely on extensively for heating and transport are the end product of the long term decomposition of plants and animals in the soil. The carbohydrates, fats and proteins that once made up these living creatures are converted into the hydrocarbons that make up the bulk of petroleum. But many protein molecules also contain sulfur atoms and these remain as a contaminant in fossil fuels. When the fuels burn, the sulfur reacts with oxygen to form sulfur dioxide, which then goes on the react with moisture in the air to form sulfuric acid, and presto, we have acid rain! Cars make a secondary contribution to acid rain as well. The high temperatures inside the engine allow nitrogen and oxygen, the major components of air, to react and form oxides of nitrogen which then react with water to produce nitric acid. Nature also makes a contribution here, as lightening also produces the high temperatures that allow nitrogen and oxygen to react. Now back to our gravestones. Both marble and limestone are made of calcium carbonate, a base that can neutralize acids. But in the process, the calcium carbonate is converted to calcium sulfate or calcium nitrate, which is then washed away. Basically, every time it rains, a bit of marble gravestones, or indeed limestone buildings like our Parliament, are worn down. This kind of destruction, though, is not the only problem with acid rain. It increases the solubility of toxic metals such as copper and lead, so that leaching from water pipes becomes more extensive. It can also damage the leaves of trees, as the maple syrup industry has discovered. Sap production has been reduced as industrialization and traffic have increased. Fish populations in acidified lakes decline, which in turn means that animals that depend on aquatic ecosystems are also affected. Birds that feed on fish start to go hungry. Not all lakes are equally affected by acid rain; the damage depends on the kind of bedrock present. Limestone neutralizes acids and has a buffering effect, but granite, composed of silicates does not undergo any acid-base reaction. So a lake surrounded by granite rock is likely to suffer more damage from acid rain. But granite’s lack of reaction with acid also means that gravestones made of granite are going to last longer than those made of limestone.

Joe Schwarcz

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