Context:

Iran faces the severe challenge of air pollution and acid rain. 

About Acid Rain

• Acid rain is a broad term that includes any form of precipitation with acidic components, such as sulfuric or nitric acid that fall to the ground from the atmosphere in wet or dry forms. 
• Formation: It results when sulphur dioxide (SO2) and nitrogen oxides (NOX) are emitted into the atmosphere and transported by wind and air currents. 
• • When these molecules dissolve in water droplets and the droplets precipitate, the precipitation that is formed is known as acid rain. The typical pH of acid rain is around 4.2-4.4. 
• Major sources of SO2 and NOX:
  • Burning of fossil fuels to generate electricity.
  • Vehicular emission.
  • Manufacturing, oil refineries and other industries.
• Challenges: 
  • Impact on Biodiversity: It renders the water inhospitable and destroys some bacteria which in turn adversely affects forests and other large ecosystems in complex ways. 
  • A Global Concern: Winds can blow SO2 and NOX over long distances and across borders, making acid rain a global problem.
  • Corrosive in Nature: It is highly corrosive, hence impacts the infrastructure too.
  • Acid rain also leads to lower visibility.
• Mitigation: 
  • Flue-gas Desulphurisation: Flue-gas desulfurization is a set of technologies that removes sulphur dioxide (SO2) from flue gases produced from industrial combustion.
  • Liming Process: Liming adds acid-neutralising substances (usually powdered lime or limestone) into the affected lake or river. 
    • The lime neutralises some of the acid and raises the pH of the water. 
  • Reducing emissions from vehicles, industries, etc. 
  • Collaborative Action: Many governments worldwide have also been working together to minimise acid rain. Example- The Acid Deposition Monitoring  Network in East Asia (EANET) in Asia.