Climate in cities

Basics

Unit 3:
Acid rain

Water is essential for life on Earth and its quality is, therefore, of great importance.  Clean rain is naturally slightly acidic but when the pH of rain falls below 5.6, we call it acid rain.  Emissions of the two air pollutants, nitrogen oxides (NOx) and sulphur dioxide (SO₂) are the main reasons for acid rain formation.  Emissions of these pollutants are decreasing in Europe and North America but are still increasing in Asia.

Acid rain affects the whole environment.  Polluted rain enters surface waters and permeates into the groundwater.  It makes the aluminium in the soil reactive and leaches out the nutrients.  Trees and plants are damaged both through the decrease in soil nutrients and also directly by the acidic rain falling on them.  Biodiversity in areas affected by acid rain is likely to be poor. 

Areas affected by acid rain are found mainly in Europe, the United States of America and in China.  Affected regions are generally close to highly urbanised and industrialised areas but long range transport of air pollutants can result in acid rain damage far away from the pollution sources.