Acid Deposition

The topic of acid deposition can be found in the SL section of the acids and bases unit, section 8.5.

As a teacher, when I first read this title I had no idea what it meant, but, as with most things in the IB, the title is actually quite descriptive, once you know what the title means.

The key word is ‘deposition’. It means (according to the definition in the online dictionary by Merriam-Webster), simply, ‘the act of being deposited’.¹

So acid deposition means depositing acid—or, as you may have been taught previously, or read, ‘acid rain’. But acid deposition does not just cover acid rain—it also covers acid snow, acid fog, and acid dew. In fact, it covers any method in which acid in the atmosphere is deposited onto the surface of the earth.

The two main sources of acid come from sulfur oxides and nitrogen oxides. Both are formed naturally as well as by man-made means.

Sulfur oxides are formed when sulfur in fossil fuels (an impurity) is combusted along with the fuel (man-made) or through volcanic eruptions (natural).

Nitrogen oxides are formed in internal combustion engines where the very high temperatures allow nitrogen and oxygen in the air to react (man-made). Nitrogen oxides are also formed naturally during lightning strikes where, again, temperatures are high enough to let oxygen and nitrogen in the air react.

Sulfur oxides will react with water to produce sufurous acid or sulfuric acid, whereas the nitrogen oxides will react with water to produce nitrous acid or nitric acid.

It should be noted that acid deposition only occurs when the pH of the acid being deposited is less than pH 5.6. This is because water in the atmosphere is naturally acidic, partly due to the two processes outlined about that produce natural sources of acid, but mainly due to carbon dioxide in the atmosphere that will naturally dissolve in water to produce a weak acid, carbonic acid.

The effects of acid deposition are widely reported. Acid deposition will affect vegetation (killing plan life), water sources (killing aquatic animals), buildings (reacting with limestone / marble) and human health (causing respiratory illnesses).

Source: Lovecz, via Wikimedia Commons

Naturally, these are things that we would not like to happen and, for this reason, sulfur is now, in some parts of the world, removed from fossil fuels before combustion or sulfur dioxide is removed after combustion. Catalytic converters in internal combustion engines can remove nitrogen oxides before they enter the atmosphere.

Is acid rain a problem in your part of the world? If so, what are the effects of it to your environment and does it affect your living conditions?

¹ https://www.merriam-webster.com/dictionary/deposition
[Accessed 8 January 2018]