Acid rain is a rain or any other form of precipitation that is unusually acidic, meaning that it has elevated levels of hydrogen ions (low pH). It can have harmful effects on plants, aquatic animals and infrastructure. Acid rain is caused by emissions of sulfur dioxide and nitrogen oxide, which react with the water molecules in the atmosphere to produce acids. Some governments have made efforts since the 1970s to reduce the release of sulfur dioxide and nitrogen oxide into the atmosphere with positive results. Nitrogen oxides can also be produced naturally by lightning strikes, and sulfur dioxide is produced by volcanic eruptions. Acid rain has been shown to have adverse impacts on forests, freshwaters and soils, killing insect and aquatic life-forms, causing paint to peel, corrosion of steel structures such as bridges, and weathering of stone buildings and statues as well as having impacts on human health.

This shows the process of the air pollution being released into the atmosphere and the areas that will be affected:

The chemical process:

Combustion of fuels produces sulfur dioxide and nitric oxides. They are converted into sulfuric acid and nitric acid:

The combustion of gasoline from cars and coal in thermoelectric plants generates oxides of nitrogen (NO and NO₂) that cause respiratory problems. Between 2005 and 2011 the population of this areafrom the USA it has grown and yet the pollution has been reduced. What do you think has happened?
a) People have become aware, so they have decided to consume less electricity and use less car.
b) New laws and technological improvements have been applied in means of transport and in industries.