Air pollution is caused by gases and particles, both liquid and solid, which contaminate the environment. Scientists link this kind of contamination of the air to adverse health effects, such as respiratory diseases and even cancer. Some of the other harmful effects are: damage to heritage buildings and artifacts (for example, due to air pollution in the city of Athens, there is evidence of corrosion on the marble statues of the Parthenon), damage to agricultural products causing reduction in the growth of trees and crop yields; reduction in visibility in the atmosphere, climate change, absorption of particulate pollutants by the gases in the atmosphere (smog), global warming, etc.
Various Sources of Air Pollution
Some of the anthropogenic sources of this type of pollution are:
- Power plants that are combustion fired
- Controlled practices of burning carried out in forestry and agricultural management
- Emissions generated by motorized vehicles, marine vessels like cruise and container ships, and airplanes
- Incinerators, furnaces, stoves, fireplaces, and burning wood
- All types of industrial activities, power plants, and oil refineries
- Burning of crop waste in farming and excessive use of chemicals
- Fumes from aerosol sprays, varnish, hair spray, paint, and other solvents
- Waste deposits in landfills, which produce methane
Some of the natural sources are:
- Dust blowing in large areas that have very little vegetation
- Methane given off by cattle due to digestion of food
- Radioactive gaseous elements like radon, formed by radioactive decay that occurs in the Earth's crust
- Carbon monoxide and smoke from wildfires
- Volcanic eruptions, which produce ash particles, chlorine, and sulfur
Air Pollution as an Global Problem
This type of pollution often does not remain confined to any geographical region. For instance, the radioactive cloud that resulted from the nuclear accident in Chernobyl in 1986, spread as far as Ireland. According to a report by the United Nations, Southeast Asia is covered by a two-mile thick haze, called the Asian brown cloud, which has been created by the burning of fossil fuels and wood. This haze could be responsible for thousands of death in the region due to respiratory problems.
Efforts to Reduce Air Pollution
These days, there are many available technologies that can be used to control air pollution, which go hand-in-hand with strategies in urban planning that are designed for the same purpose. For example, there is an debate ongoing worldwide about how to reduce the dependence we have on fossil fuels for our energy requirements, and instead shift to using environmentally friendly renewable sources of energy. Regulations are already in place to use controlling systems at industrial plants, power plants, oil refineries, etc.
Air Pollution Control Equipment
Described below are few control systems, which are being used by vehicles and industries. They help to either remove pollutants from a stream of exhaust before they are emitted into the air, or destroy them.
Systems to Reduce Particulate Matter
Wet Scrubbers: These include a number of devices that remove pollutants from furnace flue gas, as well as other gas streams. The pollutants are removed by the polluted gas stream being forced through a scrubbing liquid, or by using some other method of bringing it into contact with the liquid. Wet scrubbers are used in a number of industries like large power plants, asphalt plants, steel plants, fertilizer plants, and acid plants.
Electrostatic Precipitator (ESP): Also known as Electrostatic Air Cleaners, this control system is a particulate collecting device, which uses the force created by an induced electrostatic charge to remove particulate matter from any flowing gas. These filtration devices are highly efficient, and are very effective in removing fine particles like smoke and dust from the air stream. ESPs are used for controlling particulate emissions in various industries like oil refineries, pulp mills, oil and coal fired utilities that generate electricity and produce smoke, etc.
Dust Cyclones: These are used to remove particulate matter from a gas or air stream without using filters, and instead using vortex separation. Mixtures of fluids and solids are separated by using gravity and rotational effects. There is a large-scale use of cyclones in oil refineries, as well as in the cement industry, wherein they form a part of the kiln pre-heaters.
Systems to Reduce NOx (Nitrogen Dioxide and Nitrogen Oxide)
Exhaust Gas Recirculation (EGR): This is a technique used for reducing Nox, which is utilized in most diesel and gasoline engines. A part of the exhaust of an engine is recirculated back into its cylinders. When the incoming air is intermixed with the recirculated exhaust gas, it results in diluting the mixture with inert gas, reducing the adiabatic flame temperature, and also lowering the excessive oxygen in diesel engines. The peak combustion temperature is also lowered, as the specific heat capacity of the mix is increased by the exhaust gas. Since high temperatures cause Nox to form much faster, EGR helps in limiting it from being generated. Nox is produced when a mixture of oxygen and nitrogen is subjected to high temperatures.
Catalytic Converter: This device is used to diminish the toxicity of emissions, which are produced by internal combustion engines. First introduced in 1975 in the US, in order to comply with the tightening regulations by the Environmental Protection Agency, catalytic converters are still used most commonly in the exhaust systems of motor vehicles. They are also used in trains, mining equipment, forklifts, generator sets, and other machines equipped with engines.
Systems to Decrease Volatile Organic Compounds (VOC):
Gas Flare: Also called a flare stack, this is a chimney that is erected on oil rigs or oil wells, as well as on landfills, chemical plants, and refineries. When flammable gas or unusable waste gas plus liquids are discharged by pressure relief valves, this device is used to burn them off. This technology is also used in landfills to burn and/or vent the waste gas, which is produced by the decomposing materials.
Biofilters: This is a technique for pollution control, which uses living matter to trap and biologically degrade pollutants. The air pollutants are subjected to micro biotic oxidation. In other words, when it is applied for the filtration and purification of air, microorganisms like fungi and bacteria that are embedded in a biofilm, are used to degrade the pollutant.