An Environmental Case Study - Acid Rain: Causes, Effects and Solutions

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An Environmental Case Study - Acid Rain: Causes, Effects and Solutions

Acid rain is the broad term used to describe rainfall, snow, fog or sleet which has a higher level of acidity than that of natural, unpolluted rain. The acidity of a substance is measured by its pH. The pH of deionised water is pH 7, or neutral. Anything above 7 is classed as being an alkaline or base and anything below pH 7 is acidic. The pH of normal rain is 5.6 due to the reaction of water and carbon dioxide in the atmosphere to form a mildly acidic carbonic acid. The pH of acid rain ranges between pH 5-3 with most acidic deposition ranging between pH 5.0-4.3.

Acid rain is formed when gases such as sulphur dioxide and nitrogen oxides. These gases are by products of human activity and are released during industrial processes, most notably the burning of fossil fuels to produce electricity. Smaller quantities are released by the burning of fuels for cars, but these emissions have been greatly reduced due to the introduction of three-way catalytic convertors which reduce the nitrogen oxides released in exhaust fumes. By far, the most common cause of acid rain is sulphur dioxide which makes up 70% of acid rain formation with the various oxides of nitrogen being responsible for the remaining 30%. Around 70Tg(S) per year SO2 is released through human activity, 7-8Tg(S) per year through volcanic emissions and 2.8Tg(S) per year from wildfires.

Acid rain formation begins in the . Sulphur dioxide is oxidised by reacting with a  radical as SO2 + OH·  HOSO2·

This is followed by HOSO2· + O2  HO2· + SO3 

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In the presence of water,  (SO3) is converted rapidly to  

SO3(g) + H2O(l)  H2SO4(l) 

 is formed by the reaction of OH with  NO2 + OH·  HNO3 

Acid deposition occurs as both wet and dry deposition. Wet deposition of acid occurs when rain, snow, fog etc are formed from acids such as nitric and sulphuric acid when gases in the atmosphere react with water as outlined previously. These acids are delivered to the Earth’s surface either by deposition of acids produced in the raindrops or by precipitation removing the acids in clouds or below clouds.

Dry deposition occurs in the absence of precipitation. ...

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