Rains that harm the Earth

Environmental pollution has been a key concern of the world for a long time. When we look at the variants of environmental pollution, air pollution has been one serious outcome of a range of human activities.

However, the harmful effects on the atmosphere are not confined to the contamination of the air that we breathe. There is an extremely horrifying outcome called 'acid rain' which can wipe out entire forests and literally burn whatever gets caught in its path.

As the name implies, this is simply a rain in which the water drops contain a considerable amount of acidity rather than the neutral water drops that fall on to the Earth as normal rain. The first recorded acid rain took place in England in 1852.

Robert Angus Smith, a Scottish chemist, found the relationship between acid rain and atmospheric pollution. Although acid rain was discovered in 1852, scientists began widely observing and studying the phenomenon towards the 1960s.

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How do acid rains happen?

If you children are familiar with the water cycle, you would know that rains take place when water on the Earth's surface evaporates (turns into vapour) and forms clouds.

Thereafter, the little condensed water droplets in clouds collide and combine to make larger droplets and this is what we see as rain. So, you may wonder where the addition of acid is taking place in this cycle. This involves some chemistry. Let us try to understand this, keeping the complexities to a minimum.

The acids that are commonly found in acid rains are carbonic, sulfuric and nitric acids. These acids are produced up in the atmosphere by oxides of carbon, sulfur and nitrogen respectively. So, how do these compounds exist in the atmosphere? Due to various natural and human-related factors, these are emitted to the atmosphere, and when these oxides of carbon, sulfur and nitrogen come into contact with water, chemical reactions take place and acids are created.

It should be understood that this can happen naturally as well as due to human activities; the issue has become more challenging during the recent past. The main natural phenomena (occurrences) that emit acid-producing gases to the atmosphere are emissions from volcanoes and those from biological processes that take place on the land, in wetlands, and in the oceans.

The main causes of acid rain from human sources are electricity generation, factories and motor vehicles. Coal power plants are one of the most polluting. The gases can be carried hundreds of kilometres in the atmosphere before they are converted to acids and deposited. Since the Industrial Revolution, emissions of sulfur dioxide and nitrogen oxides to the atmosphere have increased.

In a few instances, pH readings of well below 2.4, which is equal to the acidity of vinegar have been reported in industrialised areas. Industrial acid rain is a considerable problem in China, Eastern Europe, Russia and areas down-wind from them. All these areas burn sulfur-containing coal to generate heat and electricity.

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What is an acid?

Over the years, you may have learnt that lime and vinegar are acidic. Some of you may have even done the 'Litmus Test' in school and observed that acidic solutions change the colour of blue litmus to red. The real meaning of an acid lies in its chemical composition.

According to the definition, an acid is a compound which produces a higher concentration of hydrogen ions (charged electrons) when mixed with water. The acidity is determined by what is known as the pH value, which generally ranges from zero to fourteen.

However, when certain super acids are concerned, the range spans from minus five to fourteen. Water, being neutral, has a pH value of 7. Acids have pH values of less than 7; the lower the value, the higher the acidity.

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Harmful effects

The harmful effects of acid rains span across many segments of nature such as surface water, soil, forests, animals and human health. When waterways become acidic due to acid rains, the bio-diversity in them would reduce, damaging fish and other aquatic creatures.

Where the soil is concerned, high acidity can cause certain microbes which aid the fertility of the soil to be killed due to their inability to tolerate such levels. Acid rain can slow the growth of forests and cause leaves and needles to turn brown and fall off. Acid rain can also cause damage to certain building materials and historical monuments.

This is because the sulfuric acid in the rain chemically reacts with the calcium compounds in the stones such as limestone, sandstone, marble and granite to create gypsum, which then peels off. One important monument which has faced this challenge is the famous Taj Mahal.

Compiled by Aravinda Dassanayake