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Air Pollution and Air Pollutants


Air Pollution and Air Pollutants

Clean air is vital to sustain the delicate balance of life on Earth. However the quality of air can be affected by air pollution. Air pollution occurs when certain gases and particles build up in the atmosphere to such levels that they can cause harm to human health, causing breathing and respiratory problems, and even resulting in premature death, as well as damaging the environment around the world. These gases and solid particles (known as pollutants) tend to come from man-made sources, including the burning of fossil fuels such as coal, oil, petrol or diesel, but can also come from natural sources such as volcanic eruptions and forest fires.

Air pollutants are those substances which causes pollution of the atmospheric air. They are those chemical, biological and physical agents which modify the natural characteristics of the atmospheric air.
Air pollutants arise both from natural processes (volcanic activities, oceans, and forests etc.) and human activities (fossil fuel combustion, transportation, power plant emissions, or emissions from other industrial processes).

Air pollution is the contamination of the indoor or outdoor air by a range of gasses and solid particles which modify its natural characteristics. It occurs when gases, dust particles, fumes (or smoke) or odour are introduced into the atmosphere in a way which makes it harmful to humans, animals and plants. Air pollution is often not visible to the naked eye as the size of the pollutants is smaller than the human eye can detect. They can become visible in some situations for example in the form of sooty smoke, and smog etc. The fact that air pollution cannot be seen does not mean that it does not exist. Air pollution threatens the health of humans and other living beings and creates smog and acid rain, causes cancer and respiratory diseases, reduces the ozone layer atmosphere and contributes to global warming. An air pollution control strategy for a region is a specification of the allowable levels of pollutant emissions from sources.

The study of air pollution is based on measuring, tracking, and predicting concentrations of key chemicals in the atmosphere. Four types of processes affect air pollution levels. These are (i) emissions which are chemicals emitted to the atmosphere by a range of sources, (ii) chemistry since many types of chemical reactions in the atmosphere create, modify, and destroy chemical pollutants, (iii) transport of pollutants by winds, and (iv) disposition since materials in the atmosphere return to the earth, either because they are directly absorbed or taken up in a chemical reaction (such as photosynthesis) or since they are scavenged from the atmosphere and carried to Earth by rain, snow, or fog. Air pollution trends are strongly affected by atmospheric conditions such as temperature, pressure, and humidity, and by global circulation patterns.

Study and control of air pollution is carried out in three obviously overlapping but some what distinct areas. These are (i) generation and control of air pollutants at their source, (ii) transport, dispersion, chemical transformation in, and removal of species from the atmosphere which includes all the chemical and physical processes which take place between the point of emission and ultimate removal from the atmosphere, and (iii) effects of air pollutants on human beings, animals, materials, vegetation, crops, and forest and aquatic ecosystems, including the measurement of gaseous and particulate species.

Major health harmful air pollutants include particulate matter (PM), carbon monoxide (CO), ozone (O3), sulphur oxides (SOx), nitrogen oxides (NOx), volatile organic compounds (VOCs), ammonia (NH3), and several toxic metals, such as lead (Pb), arsenic (As), cadmium (Cd), nickel (Ni), or mercury (Hg) etc.

Air pollutants (Fig 1) can be classified as either (i) primary air pollutants, or (ii) secondary air pollutants. Primary pollutants are those substances which are directly produced by a process, such as ash from a volcanic eruption or CO gas from the exhaust of an automobile. Primary pollutants can be transformed in the lower atmosphere by solar radiation and heat or by chemical action in the atmosphere into secondary pollutants, such as O3 and other photochemical pollutants or acid rain. Air pollutants have potential effects on health and the environment.

Fig 1 Air pollutants

Primary air pollutants are emitted directly into the air from sources. They can have effects both directly and as precursors of secondary air pollutants. The major primary and secondary air pollutants are described below.

Primary air pollutants

The major primary air pollutants are described below.

Oxides of nitrogen – Nitrogen oxides (NOx) are a group of gases made up of varying amounts of oxygen (O2) and nitrogen (N2) molecules. Two most important oxides of N2, which are air pollutants, are nitric oxide (NO) and nitrogen dioxide (NO2). These two oxides are frequently lumped together under the designation NOx, although analytical techniques can distinguish clearly between them. Of the two, NO2 is the more toxic and irritating compound.

NO is a principal by-product of combustion process, arising from the high temperature reaction between N2 and O2 in the combustion air and from the oxidation of organically bound N2 in certain fuels such as coal and oil. The oxidation of N2 by the O2 in combustion air occurs primarily through the following two reactions known as the Zeldovich mechanism.

N2 + O = NO + N
N + O2 = NO + O

The first reaction above has relatively high activation energy, due to the need to break the strong N2 bond. Because of the high activation energy, the first reaction is the rate-limiting step for NO production, proceeds at a somewhat slower rate than the combustion of the fuel, and is highly temperature sensitive. NO formed via this route is referred to as ‘thermal NOx’. The second major mechanism for NO formation in combustion is by the oxidation of organically bound N2 in the fuel. A portion of this organically bound N2 is converted to NOx during combustion. (The remainder is generally converted to N2).

While NO is the dominant NOx compound emitted by most sources, NO2 fraction from a source varies somewhat with the type of source. Once emitted, NO can be oxidized quite effectively to NO2 in the atmosphere through atmospheric reactions.

NO is colourless, odourless gas. It is nonflammable and is soluble in water. It is a toxic gas. NO2 is a reddish-orange-brown gas with sharp and pungent odour. It is toxic and highly corrosive. It absorbs light over much of the visible spectrum.

Oxides of nitrogen can be seen as the brown haze dome above or plume downwind. NO2 is one of the most prominent air pollutants. This reddish-brown toxic gas has a characteristic sharp, biting odour. It can increase the likelihood of respiratory problems, as it inflames the lining of the lungs, and can reduce immunity to lung infections. This can cause problems such as wheezing, coughing, colds, flu and bronchitis.

Sulphur oxides (SOx) – Sulphur oxides (SOx) are a group of compounds made up of sulphur (S) and O2 molecules. The most common sulphur oxide is sulphur dioxide (SO2). SO2 is a chemical compound with the formula SO2. It is a major component of SOx. SO2 is a gas formed from the oxidation of S contained in fuel as well as from certain industrial processes which utilize S-containing compounds. Anthropogenic emissions of SO2 result almost exclusively from stationary point sources. A small fraction of SOx is emitted as primary sulphates, gaseous sulphur trioxide (SO3), and sulphuric acid (H2S04).

SO2 is toxic at high concentrations, but its principal air pollution effects are associated with the formation of acid rain and PM. SO2 in the presence of a catalyst such as NO2 forms H2SO4. SO2 dissolves in cloud droplets and oxidizes to form H2SO4, which can fall to Earth as acid rain or snow or form sulphate particles (PM) in the atmosphere.

SO2 is a colourless gas with irritating, and pungent, burnt match type odour. It is detectable by taste at levels of 0.3 ppm (parts per million) to 1 ppm. It is highly soluble in water (10.5 grams per 100 cubic centimetres at 20 deg C.

SO2 can cause breathing difficulties if inhaled into the body. It is also toxic to plants and can cause acid rain when it reacts with moisture in the air.

Carbon monoxide – CO is a colourless, odourless, flammable, non-irritating but very poisonous gas. It is a product by incomplete combustion of carbon (C) in fossil fuel such as natural gas, coal, oil, or wood. Vehicular exhaust is a major source of CO. CO gas is slightly soluble in water.

CO is dangerous to humans, once inhaled it competes with O2 by attaching on to haemoglobin in red blood cells and starving vital organs such as the brain, nervous system tissues and the heart of O2, reducing their ability to work properly. It interferes with the blood’s ability to carry O2, slowing reflexes and causing drowsiness. Headaches and stress on the heart can result from exposure to CO. Low exposures can aggravate cardiac ailments, while high exposures cause central nervous system impairment or death. It also plays a role in the generation of ground-level O3.

Organic compounds – Organic air pollutants are sometimes divided according to particulate organic compounds (POCs) and VOCs, although there are some species which are actually distributed between the particulate and gaseous phases. The emission of unburned or partially burned fuel from combustion processes and escape of organic vapours from industrial operations are the major anthropogenic sources of organic air pollutants.

There are a large number of VOCs. They include hydrocarbons (CxHy) and also other organic chemicals which are emitted from a very wide range of sources, including fossil fuel combustion, industrial activities, and natural emissions from vegetation and fires.

VOCs are an important outdoor air pollutant. In this field they are often divided into the separate categories of methane (CH4) and non-methane VOCs (NMVOCs). Major anthropogenic sources of CH4 include natural gas production and use, coal mining, livestock, and rice paddies. CH4, the simplest and most long-lived VOC, is an extremely efficient greenhouse gas which contributes towards increased global warming. Other hydrocarbon VOCs are also significant greenhouse gases via their role in creating O3 and in prolonging the life of CH4 in the atmosphere, although the effect varies depending on local air quality. Within the NMVOCs, the aromatic compounds benzene (C6H6), toluene (C7H8), and xylene (C8H10) are suspected carcinogens. The compound 1, 3-butadiene (C4H6) is another dangerous compound which is often associated with industrial uses.

VOCs are also of interest as chemical precursors of ground-level O3 and aerosols. The importance of VOCs as precursors depends on their chemical structure and atmospheric lifetime, which can vary considerably from compound to compound.

Large VOCs oxidize in the atmosphere to produce nonvolatile chemicals which condense to form aerosols. Short-lived VOCs interact with NOx to produce high ground-level O3 in polluted environments.

VOCs contribute to smog formation and can cause serious health problems such as cancer. They may also harm plants. The aromatic compounds C6H6, C7H8, and C8H10 can lead to leukemia through prolonged exposure.

Particulate matter – PM is also known as aerosols and is a term used to describe very small solids. Smoke, ash, soot, dust, metals, and other particles from burning fuels are examples of some of the materials which make up particulate matter. It can consist of hundreds of different chemicals, including C, S, N2, and metal compounds. In fact, it refers to everything emitted in the form of a condensed (liquid or solid) phase.

PM originates from many different sources, including construction sites, vehicle exhausts, industrial plants, unpaved roads, and come in many shapes and sizes. Some are large enough to be seen with the naked eye, whereas others can only be seen through powerful microscopes.

PM in the atmosphere typically measures between 0.01 micrometers to 10 micrometers in diameter. Most of the PM is found in the lower troposphere, where it has a residence time of a few days. The PM is removed when rain or snow carries it out of the atmosphere or when larger particles settle out of suspension due to gravity. Large PM particles (usually 1 micrometer to 10 micrometers in diameter) are generated when winds blow sea salt, dust, and other debris into the atmosphere. Fine PM particles with diameters less than 1 micrometer are mainly produced when precursor gases condense in the atmosphere. Major components of fine PM are sulphate, nitrate, organic C, and elemental C. Sulphate, nitrate, and organic C particles are produced by atmospheric oxidation of SO2, NOx, and VOCs.

Coal and, to a lesser extent, oil combustion contributes most of the particulate emissions. Coal is a slow-burning fuel with relatively high ash (incombustible inorganic) content. Coal combustion particles consist primarily of C, silica (SiO2), alumina (AI2O3), and iron oxides (FeO and Fe2O3). In contrast to coal, oil is a fast-burning, low-ash fuel. The low ash content results in formation of less PM, but the sizes of particles formed in oil combustion are generally smaller than those of particles from coal combustion. Oil combustion PM contains Cd, Ni, Co (cobalt), Cu (copper), and V (vanadium).

Elemental C particles are emitted by combustion, which is also a major source of organic C particles. Light-absorbing C particles emitted by combustion are called black C or soot. They are important agents for climate change and are also suspected to be particularly hazardous for human health.

PM can reduce visibility and causes a variety of respiratory problems. High concentration of PM is a major cause of cardiovascular disease. PM has also been linked to cancer. It can also corrode metal, and can erode building and sculptures, as well as soil fabrics. Larger particles (greater than 10 micrometers) are generally filtered out of the body via the nose and throat. Particles having size of 10 micrometers or smaller, can be inhaled into the deepest parts of the lungs. Fine particles are smaller than 2.5 micrometers and are small enough to pass from the lungs. They are serious threats because they are small enough to be absorbed deeply into the lungs, and sometimes even into the blood stream.

The health impacts of PM depend on the level of exposure (frequently expressed in micro grams per cubic meters) and the duration of exposure (which can be either short term e.g. 8 hours or 24 hours or long term e.g. annual). Individual sensitivity to the health impacts of the PM can vary. Short term exposure to PM is likely to cause acute health reactions such as irritation to the eyes, nose, and throat, coughing, wheezing and increased frequency of acute lower respiratory infections, deep in the lungs. More prolonged and continued exposure to either high or lower levels of air pollution can also lead to an increased risk of respiratory infections, exacerbation of asthma, bronchitis or serious chronic effects including reduced lung function, ischaemic heart disease, stroke, lung cancer and premature death.

PM also has important radiative effects in the atmosphere. Particles are said to scatter light when they alter the direction of radiation beams without absorbing radiation. This is the principal mechanism limiting visibility in the atmosphere, as it prevents the people from distinguishing an object from the background. Air molecules are inefficient scatterers because their sizes are orders of magnitude smaller than the wavelengths of visible radiation (0.4 micrometers to 0.7 micrometers). PM, by contrast, is efficient scatterer. When relative humidity is high, PM absorbs water, which causes them to swell and increases their cross-sectional area for scattering, creating haze. Without PM pollution the normal visual range is typically around 300 kilometers, but haze can reduce the visibility significantly.

PM has a cooling effect on Earth’s climate when they scatter solar radiation since some of the scattered light is reflected back into space. In contrast, some PM particles such as soot absorb radiation and have a warming effect. This means that estimating the net direct contribution to global climate change from PM particles needs detailed inventories of the types of PM particles in the atmosphere and their distribution.

PM particles also influence Earth’s climate indirectly. They serve as condensation nuclei for cloud droplets, increasing the amount of radiation reflected back into space by clouds and modifying the ability of clouds to precipitate. The latter is the idea behind ‘cloud seeding’ in desert areas, where specific kinds of mineral PM particles which promote ice formation are injected into a cloud to make it precipitate.

Lead – Pb is a metal found naturally in the environment as well as in manufactured products. Small solid particles of Pb can become suspended in the air. Pb can then be deposited on soil and in water. The major source of Pb is metal processing. Other sources include waste incinerators, utilities, and Pb acid battery manufacturers.

Pb accumulates in the body. It causes brain and nervous system damage, especially in children (lead poisoning) and renal system damage. Exposure to Pb can cause blood, organ and neurological damage in humans and animals. Pb can also slow down the growth rate in plants.

Mercury – Hg is a toxic pollutant. It is ubiquitous in the environment and is unique among metals in that it is highly volatile. When materials containing mercury are burned, as in coal combustion or waste incineration, Hg is released to the atmosphere as a gas either in elemental form, Hg (O) or oxidized divalent form Hg2+. The oxidized form is present as water-soluble compounds such as HgCl2 which are readily deposited in the region of their emission. By contrast, Hg (O) is not water-soluble and is required to be oxidized to Hg2+ in order to be deposited. This oxidation takes place in the atmosphere on a time scale of one year, sufficiently long that Hg can be readily transported around the world by atmospheric circulation. Hg thus is a global pollution issue.

Deposition of anthropogenically emitted Hg to land and ocean has considerably raised Hg levels in the biosphere. This accumulation is visible from sediment cores which provide historical records of Hg deposition for the past several centuries.

Once deposited, oxidized Hg can be converted back to the elemental form Hg (O) and re-emitted to the atmosphere. This repeated re-emission is called the “grasshopper effect,” and can extend the environmental legacy of Hg emissions to several decades. The efficacy of re-emission increases with increasing temperature, which makes Hg (O) more volatile. As a result Hg tends to accumulate to particularly high levels in cold regions where re-emission is slow.

Divalent Hg deposited to ecosystems can be converted by bacteria to organic methyl mercury, which is absorbed easily during digestion and accumulates in living tissues. It also enters fishes’ bodies directly through their skin and gills.

Hg interferes with the brain and central nervous system. Since Hg is transported on a global scale, its control requires a global perspective. In addition, the legacy of past emissions through re-emission and Hg accumulation in ecosystems is to be recognized.

Ammonia – NH3 forms secondary pollutants with the acid pollutant of SO2 and NOx to produce ammonium (NH4+). These can then be moved by the air over large distances from the initial source. It is primarily released from animal waste and fertilizer use, vehicle exhaust and other processes.

NH3 is a very soluble colourless gas with a strong pungent smell. When NH3 and the NH4+ pollutants fall to the ground, they add to the N2 enrichment effects, increasing the growth of some plants, including trees.

Secondary air pollutants

The major secondary primary air pollutants are described below.

Ozone – O3 is a gas not usually emitted directly into the air. It has no primary sources. It is formed as a secondary pollutant by chemical reactions in the atmosphere involving hydrocarbons and oxides of N2 in the presence of sunlight and heat. Primary pollutants can be transported over long distances by wind, generating O3, meaning that rural areas can experience high O3 levels.

Ground level O3 is a colourless and toxic gas which is a major component of atmospheric smog. O3 is slightly soluble in water. Some ground level O3 also comes from higher in the atmosphere. One can actually smell O3 after a heavy thunder storm.

O3 can cause irritation to the respiratory tract and eyes, causing chest tightness, coughing and wheezing, especially amongst those with respiratory and heart problems. Repeated exposure can cause permanent lung damage. O3 also is a damaging air pollutant to plants since it damages leaves of trees and other plants. It decreases the ability of plants to produce and store food, and reduces crop yield. O3 also affects buildings and building material.

Acid rain – Acid rain refers to precipitation with pH values below 5, which generally happens only when large amounts of manmade pollution are added to the atmosphere. Acid rain is formed when SO2 and NO2 react in the atmosphere with water, O2 and other chemicals to form various acidic compounds. These compounds are transported in the air by the wind, until they fall to the ground in either wet or dry form.

When the compounds fall to the ground, they can cause damage to plants, including trees. They can also increase the acidity levels of the soils, rivers, lochs and streams, affecting the delicate balance of ecosystems that live in these environments.

The pollutants (SO2 and NO2) which cause acid rain can also be damaging to human health. As these gases interact in the atmosphere, they can form fine sulphate and nitrate water droplets which can irritate the air ways and cause irritation to eyes. Acid rain also accelerates the decay of irreplaceable buildings, statues, and sculptures.

The main components of acid rain are sulphuric acid and nitric acid. These acids form when SO2 and NOx are oxidized in the atmosphere. Sulphuric and nitric acids dissolve in cloud water and dissociate to release H+ as given below.

HNO3 (aq) —> (NO3) – + H+
H2SO4 (aq) —> (SO4)2- + 2H+

Human activity also releases large amounts of NH3 to the atmosphere, mainly from agriculture, and this NH3 can act as a base in the atmosphere to neutralize acid rain by converting H+ to the ammonium ion (NH4+). However, the benefit of this neutralization is illusory because NH4+ releases its H+ once it is deposited and consumed by the biosphere. The relatively high pH of precipitation in many places is due in part to NH3 from agriculture and in part to suspended calcium carbonate (limestone) dust. Acid rain has little effect on the environment since it is quickly neutralized by naturally present bases after it falls.

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