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Acid rain

QUESTION : What is acid rain? K. Dyesh, Chennai

ANSWER 1 : Acid rain is any form of precipitation - rain, snow, sleet, fog - which contains high levels of acid. Precisely, it is precipitation with dilute solutions of two strong acids - sulphuric and nitric. Of these acids, 70 per cent derive from oxidation and hydrolysis of sulphur dioxide and hydrogen sulphide, 30 per cent from various nitric oxides and other compounds.

Both sulphur and nitrogen emissions are attributed to the burning of fossil fuels particularly high-sulphur coal and oil and the latter is tied directly to industrial development.

There has been a rise in carbon-dioxide and sulphur and nitrogen oxides in the last 150 years during which fossil fuel consumption has increased.

Ammonia from live stock sources and agricultural fields, hydrogen from sea spray, volatile organic compounds (alkanes, polychlorinated aldehydes, esters and hydrocarbons) also add up to the ``acid rain cocktail.''

In acid rain, which contains sulphuric acid, the first ingredient is the sulphur dioxide gas emitted by power stations, cars and lorries.

When it meets highly reactive chemical agents called OH radicals in the atmosphere it is oxidised to form sulphur trioxide, which is also a gas.

This then combines with rain water to form sulphuric acid. It is estimated that the raindrops containing even 12 molecules of water, in this situation can bring about sulphuric acid almost as effectively as in a large droplet containing nearly 200 molecules. This water cluster appears to have a double role.

The first two molecules are catalytic and the extra ten molecules cluster round the reactants acting like a solvent. When two water molecules and the single molecule of sulphur trioxide combine a short lived ring compound is formed which breaks up to leave the sulphuric acid and water molecule.

This reaction is faster in a large droplet of rain water which results in the formation of acid rain.If the rain is very pure its pH should be 7, but theoretically it is slightly lower than 7 i.e., it is mildly acidic. In industrially advanced cities like Toronto and Los Angeles of the USA sometimes the average pH of rainfall becomes 3.5 and 2.2 respectively.

The most acidic rainfall recorded in the U.S. to date is 1.4 in West Virginia which is not much above the acidity of battery which has a pH of 1. In our country the lowest natural rain pH of 3.5 was recorded in Chembur in Mumbai and Indraprasth in Delhi.

Acid rain will cause direct damage to architectural structures by corroding famous monuments. Most fish species die when the pH is less than 5. The aerosol of acid rain can cause respiratory disorders in man. If drinking water is drawn from ponds and lakes, then the aluminium leaching is likely to pose dangers.Aluminium toxicity is linked to kidney failure and bone softening. Copper leached from water pipes containing acid rain water can cause diarrhoea in children.

S. Palaniappan , Pudukottai

ANSWER 2 : Clean dry air is composed of nitrogen 78 per cent, oxygen 21 per cent, argon 0.93per cent , carbon monoxide 0.03 per cent and traces of other elements.

The five most important air pollutants are carbon monoxide, sulphur oxides, hydrocarbons, nitrogen oxides and particles, both liquid and solid of industrial dusts, ash and spray.

The major sources from which these are derived are automobile industry, electric power plants, space heating and refuse.

Complex reactions occur between these contaminants and water vapour that lead to the formation of carbonic acid, sulphuric acid, nitric acid etc.

These acids are the primary factor in acid rain which is now recognised as one of our most serious environmental factors.

Acid rains create complex problems and their impacts are far reaching.

M. Vaitheeswaran, Tuticorin

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