Acid rain can be defined as ‘any form of precipitation which is more acidic than usual due to dissolved substances’[sc:1]. It is an extremely harmful process to both the environment and to human infrastructure, with effects ranging from the erosion of buildings and monuments to mass freshwater fish kills[sc:2]. This has terrible and long-lasting consequences with the potential to change entire ecosystems as acid sensitive species disappear.
Pure water has a pH of 7, which is neither acidic nor alkaline. Rain is naturally a little acidic (pH of about 5.6) due to the presence of carbon dioxide in the atmosphere, which dissolves in water to form carbonic acid. However, rainwater in some areas of the world can have a pH as low as 3 – over a thousand times more acidic than normal rainwater – due to pollutants released during human activities[sc:2].
Acid rain is formed when one or both of sulphur dioxide (SO2) and nitrogen oxides (NOx) react with water, oxygen, and other chemicals in the atmosphere. This leads to the formation of sulphuric acid (H2SO4) and nitric acid (HNO3).
Sulphur dioxide is naturally present in the atmosphere due to volcanic eruptions, but only in very low concentrations of around 0.01 parts per million (ppm). Human activity has led to much higher levels of SO2, which is formed during the combustion of fossil fuels. The current atmospheric concentrations of SO2 range from 0.1 to 2ppm. About 75% of acid rain is caused by SO2 in the form of sulfuric acid[sc:2].
Nitrogen oxides are also naturally present in the atmosphere in similar concentrations as SO2. They are also formed by high temperature fossil fuel combustion, especially in vehicle engines. Atmospheric NOx levels have risen from around 0.01 ppm to 0.2 ppm, and nitrogen oxides are the cause of around 25% of acid rain[sc:2].
Acid rain comes in two main forms: wet and dry. Wet rain is any form of precipitation involving water, and could include snow, fog, hail, rain, or dew. Dry deposition occurs when acid particles are formed in the atmosphere in the absence of water. These can then attach themselves to particles in the air, and are deposited on the ground during events such as dust-storms. Dry acid rain is especially common in areas which receive a low rainfall, such as deserts[sc:3].
When acid rain reaches the earth, it has many ecological effects. Not only does it affect the area where it falls, but it flows across the land, polluting waterways, lakes, and even groundwater. Most ecosystems are affected in some way when touched by acid rain, but the greatest impact occurs in freshwater aquatic environments[sc:4]. It also can have a noticeable effect on woodlands, alpine ecosystems, and in places where the soil is naturally acidic. A range of human infrastructure is damaged every year as acid rain erodes and dissolves[sc:5].
Since acid rain is caused by human pollution, the only way to stop it from happening and reduce its effects is to stop polluting. Everyone can play a role: conserving electricity will mean that less fossil fuels need to be burnt, meaning that less pollutants will be emitted. If you regularly drive places, you could consider walking or cycling, since motor vehicles are the second major source of the pollutants causing acid rain[sc:6].