Dry deposition refers to pollutants which are deposited on the environment before they have a chance to be absorbed by the moisture in the atmosphere. The pH scale is used as a measure of a substance's acidity or alkalinity. 7 is neutral, less than 7 is acidic and more than 7, alkaline. A log scale is used, so pH6 is ten times more acid than pH7 and pH5 is one hundred times more acid than pH7. The major causes of acid rain are the sulphur dioxide and nitrogen oxides produced when fossil fuels such as coal, oil and gas are burned Sulphur dioxide and nitrogen oxides are released into the atmosphere where they can be absorbed by the moisture and become weak sulphuric and nitric acids, sometimes with a pH of around 3.
Most natural gas contains little or no sulphur and causes less pollution.
Every year about 45 million tonnes of sulphur are put into the atmosphere in Europe. After a drop in the 1980s, amounts are now increasing again. Vehicles, especially cars, are responsible for most of the nitrogen oxides in the atmosphere. Some come from the vehicle exhaust itself, but others form when the exhaust gases react with the air. Exhaust gases also react with strong sunlight to produce poisonous ozone gas which damages plant growth and in some cases, human health.