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Air pollution effects: Acid Rain

What is acid rain?

Acid rain, or acid deposition, can be defined as any form of precipitation with high levels of nitric and sulfuric acids. It can occur in the form of snow, fog and even dry materials that settle to earth.

Types of acid deposition

  • Wet deposition: It is “the image” that comes to your mind when you imagine acid rain. More in detail, is when any kind of precipitation (rain, snow, fog, hail, sleet or dew) removes acids from the atmosphere and delivers it to the Earth’s surface.
  • Dry deposition: It occurs when particles and gases stick to the ground, vegetation, buildings or other surfaces. Between 20 and 60% of total acid rain is dry deposition, but it depends on the amount of rainfall received (the less water deposited over the year, the higher ratio of dry deposition).

PH of acid rain

PH scale is a logarithmic scale used to specify the acidity or basicity of an aqueous solution. It goes from 0 (acidic) to 14 (alkaline), with 7 as neutral point. Some common examples of pH are: battery acid (pH = 0), distilled water (pH = 7), blood (pH = 7,4) or liquid drain cleaner (pH = 14).

Normal or clean rain is between 5 and 6 pH, especifically 5,6, while acid rain has a pH closer to 4, generally between 4,2 and 4,4.

These levels are not enough acidic to harm our skin like people commonly imagine. If lemon juice or vinegar don’t harm us and have a pH of 2, acid rain is not going to cause any health problems in our skin if we are not usually exposed to it.

What causes acid rain?

Most acid rain is caused by human activities, especially when people burn fossil fuels, the main components of acid rain, sulfur dioxide (SO2) and nitrogen oxides (NOx, the combination of NO and NO2), are released into the atmosphere.

Winds and air currents can carry and spread these gases over thousands of kilometers. Then, some of their components are deposited and react with water, oxygen, and other substances to form sulfuric and nitric acid.

Major sources of sulfur dioxide and nitrogen oxides are anthropological:

  • Electric power generators using coal release into the atmosphere between ⅔ and ¼ of SO2 and NOx total emissions, respectively.
  • Vehicles and heavy equipment because of the burning of fossil fuels.
  • Manufacturing, oil refineries and other industries.

Just a little part of the total SO2 and NOx emissions are released naturally by volcano eruptions, electrical activities in the atmosphere (like lightning or acidic deposits in glacial ice), among others.

How is the chemical process of acid rain formation?

For those who are interested in knowing the stoichiometric equations or formula for the formation of sulfuric acid, here it is:

  • Burning of fossil fuels release sulfur dioxide: S (in coal) + O2 → SO2
  • Sulfate ion is made from the oxidation of sulfur dioxide: 2 SO2 + O2 → 2 SO3
  • The reaction between hydrogen atoms and sulfate ion creates sulfuric acid: SO3 + H2O → H2SO4

Effects of acid rain on humans and the environment

Acid rain is one of the air pollution phenomena that practically only affects the environment, as it does not affect humans directly. Wind can spread easily sulfur dioxide and nitrogen oxides to long distances, so effects can show up far away from its original sources.

After falling, acid rain enters water systems of runoff and sinks into the ground. This can make water toxic to crayfish, clam, fish, and other aquatic animals. The rest of the food chain, including non-aquatic species such as birds, are often affected as well. Furthermore, it not only pollutes lakes or little water concentrations, acid rain also contributes to ocean acidification, making it difficult for coastal species to survive.

Acid rain also harms forest and vegetation damaging trees’ leaves, releasing harmful substances into the soil (such as aluminium), robbing the soil of essential nutrients and making it hard for trees to take up water. Affected trees have growth difficulties due to the lack of nutrients and minerals, it makes it harder to photosynthesise properly or makes them more vulnerable to insects, diseases and bad weather.

The erosion process by wind, water, snow and ice is stronger when it is complemented with acid rain. It will help this natural process to become quicker, and can affect statues, building, vehicles, pipes and cables.

Despite acid rain has no direct effects on humans’ health, its components (SO2 and NOx) have it, causing heart and lung problems, such as asthma and bronchitis. As mentioned in other posts, sensitive groups such as kids, elderly or asthmatics will suffer harder symptoms.

How to prevent acid rain?

Designing cleaner power plants and using fewer fossil fuels, the number of pollutants that create acid rain could be reduced. Since a huge amount of emissions are human-made, governments should invest on finding new sources of power for industries and vehicles or at least control those vehicle emissions.

Furthermore, the damage done to lakes and rivers can be restored by adding powdered limestone to neutralise the water. This process is called liming, and despite being a continuous expensive treatment, it can restore the environment and its wildlife.

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