ACID RAIN

Acid rain

Acid rain

Introduction

Acid rain is rain or any other form of precipitation that is unusually acidic, i.e. elevated levels of hydrogen ions (low pH). It has harmful effects on plants, aquatic animals, and infrastructure. Acid rain is mostly caused by emissions of compounds of sulfur, nitrogen, and carbon which react with the water molecules in the atmosphere to produce acids. However, it can also be caused naturally by the splitting of nitrogen compounds by the energy produced by lightning strikes, or the release of sulfur dioxide into the atmosphere by phenomena of volcano eruptions. (Charlson, 2002)

Acid rain is a widespread term used to describe all forms of acid precipitation (rain, snow, hail, fog, etc.). Atmospheric pollutants, particularly oxides of sulphur and nitrogen, can cause precipitation to become more acidic when converted to sulphuric and nitric acids, hence the term acid rain. Acid deposition, acid rain and acid precipitation all relate to the chemistry of air pollution and moisture in the atmosphere. Scientists generally use the term acid deposition but all three terms relate to the same issue.

The term acid rain was first used by Robert Angus Smith, a scientist working in Manchester in the 1870s. The problem of acid rain is hence not a new one but the nature of the problem has changed from being a local problem for towns and cities to being an international problem. In Smith's time, acid rain fell both in towns and cities whilst today pollutants can be transported thousands of kilometres due to the introduction of tall chimneys dispersing pollutants high into the atmosphere.

Precipitation is naturally acidic because of carbon dioxide in the atmosphere. The burning of fossil fuels (coal, oil and gas) produces sulphur dioxide and nitrogen oxides which can increase the acidity of rain or other precipitation. Sources of sulphur dioxide and oxides of nitrogen may be natural such as volcanoes, oceans, biological decay and forest fires, or may arise from combustion sources. The increasing demand for electricity and the rise in the number of motor vehicles in recent decades has meant that emissions of acidifying pollutants have increased dramatically from human sources, particularly since the 1950s. Emissions of such pollutants are heavily concentrated in the northern hemisphere, especially in Europe and North America. As a result, precipitation is generally acidic in these countries. (Herman, 2007)

In the 1970s and 1980s, Scandinavian countries began to notice the effects of acid deposition on trees and freshwaters. Much of the pollution causing this damage was identified as being transported from other more polluting countries. Acid rain became an international concern.

The pH scale is used to measure the acidity of acid rain which is determined by the hydrogen ion content (H+). This scale was invented by a Danish scientist called Sorenson in 1909. The pH scale ranges from 0, which is strongly acid, to 14 which is strongly alkaline, the scale point 7 being neutral. The pH scale is logarithmic rather than linear, so there is a ten-fold increase in acidity with each pH unit, ...