Acid Deposition

By Aaron Lee

Acid deposition is when chemical compounds and other substances in the atmosphere are transformed through chemical processes, and then deposited back on to the earth. More specifically, it is defined as a complex chemical and atmospheric phenomenon which involves the emissions of sulfur and nitrogen compounds and other substances. These compounds and substances are transformed in the atmosphere through chemical processes and then deposited back to the earth either in a wet or dry form. The wet form of acid deposition is commonly referred to as acid rain, but in reality it can also be in the form of snow, fog, or mist. The dry form of acid deposition is commonly found as acidic gases or acidic particulates.

Oxides of Nitrogen

One reason for acid deposition is the formation of oxides of nitrogen, NOx. Nitrogen oxides form for a variety of reasons, one being lightning strokes. The high temperature caused by the lightning stroke causes the nitrogen and oxygen in air in it's vicinity to react and form nitric oxide, NO. The nitric oxide then reacts very quickly to the surrounding oxygen in the air, thus forming nitrogen dioxide.

Fig. 1 - Formation of nitrogen oxides
[Fig. 1] Formation of nitrogen oxides

In the case of acid rain, the nitrogen dioxides would react with the rain water to form nitrous acid, HNO2, and nitric acid, HNO3. Due to these acids forming from the reaction between nitrogen dioxide and rainwater, it causes the rainwater to have a lowered pH, thus resulting in acid rain.

[Fig. 2] Formation of nitric acid

Besides lightning strokes, nitrogen oxides can also form from a variety of other reasons, such as the combustion of coal and oil in power plants and the combustion of gasoline in automobiles. Nitrogen and oxygen do not react naturally, because the energy provided by the temperature is not enough; the presence of combustion in power plants and automobile provide the nitrogen and oxygen with enough energy to react, thus forming nitrogen oxides. Nitrogen oxides only form if enough heat is provided for the nitrogen and oxygen in the atmosphere.

Oxides of Sulfur

The main sulfur oxide that contributes to acid deposition is sulfur dioxide, SO2. Sulfur dioxide can come from many sources,such as industrial sources, gasoline combustion, and coal and oil combustion from power plants.

[Fig. 3] Anthropogenic sources of sulfur dioxide
[Fig. 3] Anthropogenic sources of sulfur dioxide

Sulfur dioxide would then react with the surrounding oxygen in the air to form sulfur trioxide, SO3.

2SO2 (g) + O2 (g) --> 2SO3 (g)

And then when it starts to rain, sulfur trioxide will react with the rainwater to form sulfuric acid, H2SO4.

SO3 (g) + H2O (l) --> H2SO4 (aq)

As sulfuric acid is an acid, it results in a lower pH of rainwater, thus it becomes acid rain.


Ammonia, NH3, is found in the earth, from agriculture (livestock waste & fertilizers), as well as in the atmosphere, from the decaying of vegetable matter. Ammonia is capable of neutralizing sulfuric acid, as shown in the following reaction:

NH3 + H2SO4 --> (NH4)2SO4

as well as neutralize nitric acids, as seen below:

NH3 + HNO3 ---> NH4NO3

So we can observe that the main role of ammonia in acid deposition is to help counteract the effects of sulfuric acid and nitric acid on the acidity of rainwater, thereby helping to make the rain water less acidic.


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