Re: Air Pollution And Control Technologies By Anjaneyulu Pdf 29
Stephanie Dejoode
Air pollution is a gas (or a liquid or solid dispersed through ordinary air) released into atmosphere in a large quantity to harm the health of people or other animals, kill plants or stop them growing properly, damage or disrupt some other aspect of the environment (such as making buildings crumble), or cause some other kind of nuisance (reduced visibility, perhaps, or an unpleasant odor).
As with water pollution and land contamination, it's the quantity (or concentration) of a chemical in the air that makes the difference between harmless and pollution. Carbon dioxide (CO2), for example, is present in the air around you at a typical concentration of less than 0.05 percent and breathing it in usually does no harm (you breathe it out all day long); but air with an extremely high concentration of carbon dioxide (say, 5-10 percent) is toxic and could kill anybody in a matter of minutes. Forest fires, erupting volcanoes, and gases released from radioactive decay of rocks inside Earth are just three examples of natural air pollution that can have hugely disruptive effects on people and the planet.
The World Health Organization (WHO) defines air pollution as limited to situations in which the outer ambient atmosphere contains materials in concentrations which are harmful to man and his environment.
Air pollution is broadly defined as the presence in the atmosphere, of one or more contaminants such as fumes, dust, gases, mist. grit, odour, smoke, smog or vapours in considerable quantities and of duration which is injurious to human, animal or plant life or which unreasonably interferes with the comfortable enjoyment of life and property.
Thus, air pollution is generally a disequilibrium condition of air caused by the introduction of foreign elements from natural and man-made sources into the air such that the air becomes injurious to biological communities.
Forest fires (which often start naturally) can produce huge swathes of smoke that drift for miles over neighboring cities, countries, or continents. Giant volcanic eruptions can spew so much dust into the atmosphere that they block out significant amounts of sunlight and cause the entire planet to cool down for a year or more. Radioactive rocks can release a gas called radon when they decay, which can build up in the basements of buildings with serious effects on people's health.
All these things are examples of serious air pollution that happen without any human activity; although we can adapt to natural air pollution, and try to reduce the disruption it causes, we can never stop it happening completely.
Today most of the natural resources are polluted due to increased pace of anthropogenic activities. The increase in population and transportation and industrial development has resulted in the release of all kinds of air pollutants into the environment. The natural capacity of the environment to tolerate and sustain development has dwindled in the face of the ever-increasing discharge of air pollutants.
The assessment of air quality and control of air pollution are difficult tasks facing an environmental manager. Both soil and water can be confined and gathered at one place while air cannot be gathered and confined in one place. Hence air quality assessment, air pollution prevention strategies and control technologies are of great importance in air quality management
Any gas could qualify as pollutant if it reaches a high enough concentration to do harm. Theoretically, that means there are dozens of different pollution gases. In practice, about ten different substances cause most concern:
Sulfur dioxide: Coal, petroleum, and other fuels are often impure and contain sulfur as well as organic (carbon-based) compounds. When sulfur (spelled sulphur in some countries) burns with oxygen from the air, sulfur dioxide (SO2) is produced. Coal-fired power plants are the world's biggest source of sulfur-dioxide air pollution, which contributes to smog, acid rain, and health problems that include lung disease.
Carbon monoxide: This highly dangerous gas forms when fuels have too little oxygen to bum completely. It spews out in car exhausts and it can also build up to dangerous levels inside a home if you have a poorly maintained gas boiler, stove, or fuel-burning appliance.
Carbon dioxide: This gas is central to everyday life and isn't normally considered a pollutant: we all produce it when we breathe out and plants such as crops and trees need to breathe it in to grow. However, carbon dioxide is also a greenhouse gas released by engines and power plants. Since the beginning of the Industrial Revolution, it's been building up in Earth's atmosphere and contributing to the problem of global warming and climate change.
Nitrogen oxides: Nitrogen dioxide (NO2) and nitrogen oxide (NO) are pollutants produced as an indirect result of combustion, when nitrogen and oxygen from the air react together. Nitrogen oxide pollution comes from vehicle engines and power plants, and plays an important role in the formation of acid rain, ozone and smog. Nitrogen oxides are also indirect greenhouse gases (they contribute to global warming by producing ozone, which is a greenhouse gas).
Volatile Organic Compounds (VOCs): These carbon-based (organic) chemicals evaporate easily at ordinary temperatures and pressures, so they readily become gases. That's precisely why they're used as solvents in many different household chemicals such as paints, waxes, and varnishes. Unfortunately, they're also a form of air pollution: they're believed to have long-term (chronic) effects on people's health and they also play a role in the formation of ozone and smog.
Particulates: These are the sooty deposits in air pollution that blacken buildings and cause breathing difficulties. Particulates of different sizes are often referred to by the letters PM followed by a number, so PM 10 means soot particles of less than 10 microns (10 millionths of a meter or 10μm in diameter). In cities, most particulates come from traffic fumes.
Ozone: Also called trioxygen, this is a type of oxygen gas whose molecules are made from three oxygen atoms joined together (so it has the chemical formula O3), instead of just the two atoms in conventional oxygen (O2). In the stratosphere (upper atmosphere), a band of ozone (the ozone layer) protects us by screening out harmful ultraviolet radiation (high-energy blue light) beaming down from the Sun. At ground level, it's a toxic pollutant that can damage health. It forms when sunlight strikes a cocktail of other pollution and is a key ingredient of smog (see box below).
Chlorofluorocarbons (CFCs): Once thought to be harmless, these gases were widely used in refrigerators and aerosol cans until it was discovered that they damaged Earth's ozone layer. We discuss this in more detail down below.
Unburned hydrocarbons: Petroleum and other fuels are made of organic compounds based on chains of carbon and hydrogen atoms. When they bum properly, they're completely converted into harmless carbon dioxide and water; when they bum incompletely, they can release carbon monoxide or float into the air in their unbumed form, contributing to smog.
Lead and heavy metals: Lead and other toxic heavy metals can be spread into the air either as toxic compounds or as aerosols (when solids or liquids are dispersed through gases and carried through the air by them) in such things as exhaust fumes and the fly ash (contaminated waste dust) from incinerator smokestacks.
The concentration of any pollutant can be expressed in a number of ways involving units of mass, or volume per unit mass or volume per volume of air. There are four ways in which the concentrations of pollutants can be expressed.
where P is the total pressure in bars, Mp is the molecular w eight of the pollutant. T is the absolute temperature in K, and 8.314 x 10 -2 is the value of the universal gas constant expressed in units of m bar/kg mol -K. At the standard temperature (25 C) and pressure (1.0133 bars) the above equation reduces to
Any mass per unit volume measurement must take into account changing temperature as well as pressure while the mass per unit volume of an ideal gas is independent of temperature and pressure. 1 g mole of any gas at STP conditions occupies 22.4 litres (64 g of SO2. or 30 g of NO or 46 g of NO2 or 48 g of O3, all of them occupy 22.4 litres at 0C and 760 mm pressure while the volume occupied is 24.45 litres at 25C and 760 mm pressure.).
The important point is that mass per unit volume (i.e., pg/m) expressions do not allow an immediate comparison of one pollutant with another in terms of numbers of molecules. In contrast, an atmosphere containing 1 μ l-1 SO2 contains the same number of pollutant molecules as one containing 1 μl l-1 O3 or NO2 because the molecular weight and the volume of gas containing 1 gram molecule have already been taken into account. Interconversions from ppm to μg/m and vice versa at different temperatures for common air pollutants are given in Table 1.2 and Table 1.3.
The Central Pollution Control Board standards for NO2, SO2, and CO for sensitive, industrial and residential areas (exposure limits at atmospheric pressure and 25 C) are given in Table 1.4. Express their concentrations in ppm.
Pollutants are also classified into the following categories: gaseous pollutants, particulate matter pollutants, aerosol pollutants, pesticides, metallic contaminants, carcinogenic pollutants, radioactive pollutants and biological contaminants. Gaseous pollutants include oxides of nitrogen, oxides of sulphur, CO and CO2, hydrocarbons, etc.
Natural sources produce considerable pollutants but most of them are essential components of a balanced ecosystem. None of these natural pollutants normally accumulate to a level that is dangerous for life. However, many industrial activities (Table 1.6(b)) produce air pollutants in levels that exceed the normal natural assimilation processes. Air pollution problems are concentrated in and around urban areas because industries are concentrated in or around urban areas spewing poisonous gases, and particulates and increasing ambient temperature. Vehicular traffic is also dense in urban areas causing pockets of intense air pollution. All recorded air pollution episodes are around developed cities only. There is a qualitative difference between air pollution due to industries and air pollution due to vehicles, as can be seen by an analysis of the air polluted by them.
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