Although human pollution, through the burning of fossil fuels, has contributed to acid deposition, rainwater is naturally acidic as a result of carbon dioxide in the air dissolving in the water. In addition, natural sources of sulphur and nitrogen emissions can contribute further to the acidity of rainwater.
Natural sources of sulphur dioxide include release from volcanoes, biological decay and forest fires. Actual amounts released from natural sources in the world are difficult to quantify; in 1983 the United Nations Environment Programme estimated a figure of between 80 million and 288 million tonnes of sulphur oxides per year (compared to around 69 million tonnes from human sources world-wide).
Natural sources of nitrogen oxides include volcanoes, oceans, biological decay and lightning strikes. Estimates range between 20 million and 90 million tonnes per year nitrogen oxides released from natural sources (compared to around 24 million tonnes from human sources worldwide).