Acid rain results when sulfur oxides (SOx) and nitrogen oxides (NOX) are emitted into the atmosphere and transported by wind and air currents.  The SOx and NOX can react with water, oxygen and other chemicals to form sulfuric and nitric acids. These then mix with water and other materials before falling to the ground. Although carbon dioxide is present in much higher concentrations than nitrogen oxides and sulfur oxides, it does not form a strong acid, as compared to nitric acid and sulfuric acid. Rain has a natural pH between 5-6, which is slightly acidic.

Carbon dioxide source (anthropogenic and natural)
Rain is naturally acidic because of the carbon dioxide present in the atmosphere is able to dissolve in water to form carbonic acid (H2CO3). Dissociation of carbonic acid ultimately leads to slightly acidic rain with a pH around 5.3. Carbon dioxide is also emitted heavily through human activities such as deforestation and cement manufacturing, this surge in the level of carbon dioxide is due to the industrial Revolution.

CO2 (g) + H2O (l) → H2CO3 (aq)

H2CO3 (aq) → H(aq) + HCO3– (aq)

Any rain that is below the pH value of 5 is considered an acid rain.

Main contributors to acid rain:

Nitrogen oxides source (anthropogenic and natural)
Oxides of nitrogen (NOx) can be formed from human activities or natural sources like an intense lightning strike.
The high temperature in a lightning strike causes the oxygen and nitrogen in close vicinity to react and form nitric oxide (NO), which is highly reactive. It reacts with more oxygen molecules in the air to form nitrogen dioxide (NO2). This can be referred as thermal NOx.

N2 (g) + O2 (g) → 2NO (g)
2NO (g) + O2 (g) → 2NO(g)

Human sources such as agriculture activities, industrial power plant combustions, automobiles combustion contributes to increasing nitrogen oxides level in the atmosphere. Fuel NOx involves combustion of fuels that have significant nitrogen content.

These NOx emissions have significant health and environmental effects. Nitric oxide (NO) can combine with haemoglobin to reduce oxygen-carrying capacity of blood. However, it is nitrogen dioxide (NO2) in the atmosphere that is more of a concern these days. It can irritate the lungs ,aggravates asthmatic conditions and causes respiratory symptoms. Prolonged and repeated exposure will cause respiratory diseases. At high concentrations, it is fatal to humans.

NOx also acts as a precursor for formation of ozone at ground level.
Nitrogen dioxide (NO2) is heavier than air and readily soluble in water, forming nitric acid (HNO3) and either nitrous acid (HNO2) or nitric oxide (NO). Dissociation of nitric acid in atmosphere results in acid rain.

2NO2 (g) + H2O (l) → HNO3 (aq) + HNO2 (g)
3NO2 (g) + H2O (l) → 2HNO3 (aq) + NO (g)

HNO3 (aq) → H(aq)  +  NO3 (aq)

Fig.1 Depiction of NOx emissions from automobiles

Sulfur dioxide source (anthropogenic and natural)
Natural sources of sulfur dioxide (SO2) usually come from volcanoes. While human activities such as coal burning power plants produces huge amounts of sulfur dioxide in the atmosphere. Once sulfur dioxide is emitted in the air, it can react with water to form sulfurous acid (H2SO3) which is also a constituent of acid rain, or oxygen molecules to form sulfur trioxide (SO3). Sulfur trioxide can react with water too, to form sulfuric acid (H2SO4). Sulfuric acid readily dissociates in water to give H+ ions , making the rain acidic.

SO2 (g) + H2O (l) → H2SO3 (aq)
SO3 (g) + H2O (l) → H2SO4 (aq)

H2SO4 (aq) → 2H+ (aq) + SO42- (aq)

Fig.2 Industrial pollutants
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Fig.3 Formation and deposition of acid rain

In summary, nitrogen oxides and sulfur dioxide are the major contributors that lead to formation of acid rain. While the emission of these pollutants can come naturally as discussed. Anthropogenic emissions outpace natural emissions greatly. These emissions are usually found greatest in cities that have high coal-burning power plant activities and heavy traffic. These pollutants undergo a series of reactions involving water and oxygen molecules, to form components of acid rain, which can have pH lower than 4.3. Acid deposition can be divided into wet or dry. Wet deposition refers to acidic rain,fog or snow that flows over and through the ground, it involves precipitation. While dry deposition refers to acidic gases and particles that is blown by wind onto buildings and trees. Effects of acid rain on the environment are discussed in the topic ”IMPLICATIONS OF ACID RAIN”.



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