Environmental Chemistry

Environmental chemistry is defined as the study of the sources, reactions, transport, effects, and fates of chemicals in the environment, and the effect of human and biological activities on these.

Environmental Pollution is the introduction of undesirable substances into the environment that causes adverse effects on living organisms. It can occur from the chemical substances or energy, like noise, heat or light. One of the major types of environmental pollution is atmospheric or air pollution.

Atmospheric or Air Pollution

Air pollution is the unwanted introduction of air pollutants such as chemicals, particulates, biological matter which harms the living organisms and the environment sustaining them. Air pollutants can be classified as primary and secondary. Primary pollutants are mostly produced from a process, like ash produced from a volcanic eruption. Secondary pollutants are not produced directly but they form in the air to the reaction or interaction of primary pollutants.

Major Primary Pollutants

• Sulfur dioxide (SO₂) is produced by volcanoes and in many industrial processes. Sulfur dioxide is also produced on combustion of coal and petroleum. In the presence of nitrogen dioxide, sulfur dioxide forms sulfuric acid (H₂SO₄) which causes acid rain.
• Nitrogen dioxide (NO₂) is produced due to high temperature combustion, and also during thunderstorms by electric discharge. This is visible as a brown haze above the cities.
• Carbon monoxide (CO) is a colourless, odourless, toxic and non-irritating gas. It is produced due to incomplete combustion of fuels like natural gas, coal or wood. CO is also produced majorly from vehicular exhaust.
• Methane (CH₄) is a very efficient greenhouse gas that contributes to global warming.
• Particulate Matter (PM) is minute particles of solid or liquid suspended in a gas, whereas aerosol refers to combined particles and gas. Some particulates originate from volcanoes, dust storms, forest fires, and sea spray. Aerosols are produced due to burning of fossil fuels in vehicles, power plants and many industrial processes. The elevated levels of fine particles in the air are health hazards to human beings.
• Chlorofluorocarbons (CFCs) are the gases that are released from air conditioners, refrigerators, aerosol sprays, etc. On being released into the air, CFCs go to stratosphere where these interact with other gases and damage the ozone layer. This permits the ultraviolet rays to reach the Earth’s surface leading to various diseases like skin cancer and damage to the plants.
• Ammonia (NH₃) is released from agricultural processes. Ammonia reacts with oxides of nitrogen and sulfur to produce secondary particles in the atmosphere.

Major Secondary Pollutants

• Particulates are produced from gaseous primary pollutants and compounds in photochemical smog which is a type of air pollution. Classic smog is produced from huge quantities of coal burning in an area due to mixing of smoke and sulfur dioxide. Modern smog is produced due to vehicular and industrial emissions which are acted on in the atmosphere by UV rays from the sun to form secondary pollutants that combines with the primary emissions to produce photochemical smog.
• Peroxyacetyl nitrate (PAN) is present in photochemical smog. It is thermally unstable and disintegrates into peroxyethanoyl radicals and nitrogen dioxide gas. Its formation becomes harmful when ethanol is used as a vehicular fuel. This results into increased emission of acetaldehyde which then reacts in the atmosphere to form smog.

Acid Rain: Acid rain is a resultant of air pollution. When any kind of fuel is burnt, various other chemicals are produced. The smoke released from a fire or the fumes that come out of a car exhaust don't just consist of the sooty grey particles but they also contain many invisible gases that can be even more harmful to the atmosphere. Some of these gases (such as nitrogen oxides and sulphur dioxide) react with the minute droplets of water in clouds to form sulphuric and nitric acids. This rain falls as a very weak acid giving it the name “acid rain”. Acid rain can also take the form of snow, mist and dry dust. Acid rain can have a serious effect on soil, trees, buildings and water.

Image Courtesy: www.image.slidesharecdn.com