LTEC2007

Frequently Asked Question 4:
What is acid rain?

There are a number of commonly asked questions. Some answers are provided. We welcome your feedback and further questions.

In recent years acid rain has become less of a news story than global warming, the greenhouse effect. The fact is that while it is less publicized it is still occurring. Acid rain is the precipitation of acidic liquids in the form of rain, snow or sleet. This causes several environmental effects and is damaging to human life also.

There are two main acids that fall as rain. These are sulphuric acid (H2SO4) and nitric acid (HNO3). In their raw forms (as in the lab) these acids are very potent. But in this context they are diluted and modified by contact with water in clouds. This makes them less potent in the classic acid burn or metal eating manner, but no less damaging to the natural world.

Sulphuric acid rain is produced from the sulphur oxides created in combustion of fuels containing sulphur such as most diesel fuels, marine fuels and the heavy oil fuel used in industry and power generation. These are namely sulphur dioxide (SO2) and sulphur trioxide (SO3). These combine with oxygen (O2) and hydroxyl (OH) to form sulphuric acid. This is absorbed in rain cloud droplets and falls as rain.

Nitric acid rain is formed more simply from nitrogen dioxide (NO2) and hydroxyl (OH). These combine and are diluted into rain droplets within clouds, these then fall to the ground.

Carbonic acid (H2CO3) has been listed by some as an acid rain contributor but the low potency of the acid (its acidity is rated at pH 5.6, which is close to the natural acidity of rain) has led to some ruling it out. However this acid can be created in the atmosphere when carbon dioxide (CO2) dissolves in water (H2O). This acid primarily attacks carbon based rock and is known to contribute to the formation of limestone caves. While it is a naturally occurring acid within rainfall some suggest that increasing levels of atmospheric carbon dioxide may be contributing to the increasing levels of rain acidity.

The precipitation of acidic rain has many effects. These can be both direct and indirect. The acidity can attack various parts of the bodies of soil and water dwelling animals either killing them outright or weakening them, leaving them more open to infection or less able to forage. The higher acidity of the water also can dissolve elements into the water (such as aluminium and other metals) that are toxic in elevated levels. These processes occur both in bodies of water and in ground water, even that in the top soil.

Vegetation suffers when acid rain falls as nutrients are washed from the soil. In agricultural situations this necessitates the addition of further fertilizers. With trees this weakens the plant making it vulnerable to attack and reducing regenerative growth. It is worth noting that higher altitude forests suffer more as the cloud/fog cover is known to be more acidic.

The cumulative effect of all this is a reduction in ecological vitality for both the regions rained upon and the bodies of water catching the run off. These can include entire countries such as those in Scandinavia and the North Sea and Baltic Sea into which their rivers flow. Global oceanic currents, cycles and conveyors circulate the more acidic waters across the globe. While this dilutes the acidic water, the average acidity accrued increases with time as these pollutants become part of the Earth's oceanic body.

Major sources of the two main acid forming emissions include transport, industry and power generation. Diesel transport emissions include both nitrogen oxides and sulphur oxides (the sulphur levels in fuels have reduced considerably in recent years to virtually zero). Marine transport fuels contain higher levels of sulphur than road transport and their use also produces nitrogen oxides. The heavy fuel oils consumed in industry and power generation contribute the highest amounts of sulphur oxides, a concern for many in government and the industries.