Air Pollutants PDF

AIR POLLUTANTS TYPES, SOURCES, PARTICLE SIZE AND CHEMICAL NATURE "Air pollution" is defined by (WHO) World Health Organisation as follows : "Air pollution occurs when one or several air pollutants are present in such amounts for a long period in the outside air that they are harmful to humans, anim...

AIR POLLUTANTS TYPES, SOURCES, PARTICLE SIZE AND CHEMICAL NATURE "Air pollution" is defined by (WHO) World Health Organisation as follows : "Air pollution occurs when one or several air pollutants are present in such amounts for a long period in the outside air that they are harmful to humans, animals, plants or properties, contribute to damage or may impair the well being to a measurable degree". The atmosphere is composed primarily of nitrogen, oxygen and several noble gases. However, there are a number of gases and that occur in relatively small and sometimes highly variable amounts. Water vapour, carbon dioxide and ozone fall in the latter category, and so do the gases considered to be the common urban air pollutants. The air pollution problem can be simply depicted as a system consisting of three basic components : 1.Pollutants (Emission sources) 2. mixing & chemical transformation (Atmosphere) 3. Receptors The major emission sources are (1) transportation (3) refuse burning (5) industrial processes. (2) electric power generation (4) industrial and domestic fuel burning The waste products produced by the above sources mainly comprise of sulphur dioxide, suspended particulate. matter, oxides of nitrogen, carbon monoxide, hydrocarbons and lead, which add greatly to the pollution of urban areas. The development of new industries introduce the problem of toxic chemicals, while nuclear power production and atomic-weapons testing highlighted ionizing radiation as a pollutant. Air Pollutants: Any agent or combination of such agents, including any physical, chemical, biological, radioactive substance or matter which is emitted into or otherwise enters the ambient air and can, in high enough concentrations, harm humans, animals, vegetation or material. Classification of air pollutants : The air pollutants are classified according to chemical composition. They are further c~assified according to physical state, that is, gaseous, liquids or solids. Air pollutants are classed according to the manner in which they reach the atmosph ere. 1. Primary po11utants: Those emitted directly from the source 2. Secondary pollutants : Those formed in the atmosophere by chemical interactions among primary pollutants and normal atmospheric constituents The first type of air pollutant was recognized to be high concentrations of sulfur compunds (SO2 & Sulfates) and particles resulting from combustion. The second type of air pollutant has been called smog (Photochemica1smog). This photochemi ca1 smog occurs with high temperatures, bright sun light and low humidity. The main primary pollutants in photochemical smog are nitric oxide and hydrocarbons, which are rapidly converted to secondary pollutants, Ozone, Organic nitrates, oxidized hydrocarbons and so-called photochemical aerosols. Air pollution is judged by the presence of five important components : Oxides of Sulphur, Suspended Particulate Matter, Oxides of Nitrogen, Oxides of Carbon and Hydrocarbons. Oxides of sulphur: Sulfur occurs primarily as metal sulfides and pyrites in the earth surface, as sulfate salts in the oceans and as sulphur dioxide, hydrogen sulfide and sulfate aerosol in the atmosphere. Oxides of sulfur are emitted during petroleum and metal refining, mobile sources, sulphuric acid production, coking and forest fires. On a world wide basis, in volcanoes and geothermal activity, sulphur dioxide and sulphur trioxide are produced during combustion. Hydrogen sulfide is emitted in large quantities in nature from biological decay processes. Sulfur dioxide can react catalytically or photochemically with other pollutants to form sulfur trioxide, rapidly hydrating to sulphuric acid and sulphates. Suspended particulate matter : The term 'suspended particulate matter' refers to the wide range of finely divided solids or liquids dispersed into the air from combustion processes (heating and power generation) 1ndustrial activities; and natural sources. These particul2te matter range in size from 0.1 upto about 25 micro meter in diameter. Particles less than 2.5 micrometer in diameter are generally referred to as fine and those greater than 2. 5 micrometer diameter as coarse. The particle size ranges from a few tens of Angstroms to several micrometers. Oxides of Nitrogen : Diatomic nitrogen constitutes approximately 78% of our atmosphere. Nitrogen plays a dominant role in the chemistry of all living organisms. In the atmosphere, nitrogen exists at trace levels as ammonia, nitrous oxides, nitric oxide and nitrogen dioxide. Only nitric oxide and nitrogen dioxide exist as important air pollutants. The oxides of nitrogen are produced by natural processes, including bacterial action in the soil, lightning and volcanic eruptions. The principal emissions from human activity are from the combustion of fossil -fuels in stationary sources like heating, power generation and in motor vehicles. Oxides of Carbon : Carbon compounds that are important in local or regional air pollution problems are carbon monoxide and the number of hydrocarbons emitted as a result of fossil fuel combustion. Carbon cycle revolves around the photosynthetic conversion of atmospheric carbon dioxide to oxygen and carbohydrates with subsequent decomposition of the organic material back to carbon dioxide. On global basis, motor vehicles contribute 55% of carbon monoxide. Hydrocarbons : In the atmosphere, volatile hydrocarbons typically range from c1 to c10 compounds. The hydrocarbon composition includes the unburned hydrocarbons from fuels, which are formed during combustion and natural hydrocarbons emitted by vegetation. A particular group of hydrocarbons causing increasing environmental concern is the polynuclear aromatic hydrocarbons, PAH.. These are unsaturated fused benzene rings varying from napthalene (2 rings) to corene (8 rings). The PAH compounds are associated with fine particulate matter present in air. These compounds originate from incomplete combustion. The major causes of PAH pollution in cities are emission from motor cars, burning of coal for domestic cooking, a number of small factories, very frequent traffic jams are major sources of polycyclic aromatic hydrocarbons. The atmospheric pollution occurs primarily in the lower layers of the atmosphere and the long term changes due to pollution affect the entire atmosphere of the earth.  Pollutant Average residence time in the atmosphere CHEMICAL NATURE He 10^7 years Time of residence of substance in the Atmosphere N2 10^6 to 2 x 10^7 years 1. Organic Compounds Organic air pollutants are sometimes divided according to O2 5 x 10^3 to 1 o^4 years volatile organic compounds (VOCs) and particulate CO2 5 to 10 years organic compounds (POCs), although there are some species that will actually be distributed between the gaseous and H2 4 to 8 years particulate phases. The emission of unburned or partially burned CH 2 4 to 7 years fuel from combustion processes and escape of organic vapors from industrial operations are the major anthropogenic sources of N2O 2. 5 to 4 years organic air pollutants. O3 o.3 to 2 years Alkyl nitrites (RONO) undergo photolytic dissociations to form alkoxy radicals (RO) and nitric oxide: CO o.2 to 0 5 years RONO + hν → RO + NO NO 2 8 - 11 days These processes are so fast that the alkyl nitrites are not readily H 2O 10 days observed in the atmosphere. SO4^2- 10 days Formaldehyde is known to photolyze in NO 9 days the tropospherically important wavelength region of 290–356 nm via two pathways: NH 3 5 - 6 days HCHO + hν → HCO + H NH4^+ 6 days HCHO + hν → H2 + CO For acetaldehyde (CH3CHO), three pathways for photolytic NO3^- 5 days reaction are possible: SO2 2 to 4 days CH3CHO + hν → CH3 + HCO CH3CHO + hν → CH4 + CO H2S 0. 5 t 0 4 days CH3CHO + hν → CH3CO + H 0 rg-anic- carbon 2 days 2. Oxygen and Ozone in the Troposphere In Earth's troposphere, the sun is the light source for the important photochemical reactions. The radiation reaching the troposphere is strongly filtered by molecular oxygen (O2) and ozone that is produced in the stratosphere by oxygen photolysis, known as the Chapman cycle. O3 + hν → O2 + O Photolysis of ozone is extremely important, because the reaction of O with water vapor (H2O) is extremely fast and produces hydroxyl radical (OH) O + H2O → 2 OH 3.Oxides of Nitrogen Nitric oxide (NO) and nitrogen dioxide (N02) are the two most important nitrogen oxide air pollutants. Nitric oxide is a principal by-product of combustion processes, arising from the high-temperature reaction between N2 and O2 in the combustion air and from the oxidation of organically bound nitrogen in certain fuels such as coal and oil. The oxidation of N2 by the O2 in combustion air occurs primarily through the two reactions N2 + O-- NO + N N + O2 -- NO + O known as the Zeldovich mechanism. The first reaction above has a relatively high activation energy, due to the need to break the strong N2 bond. Nitrogen Dioxide Another important species for the photochemistry of the troposphere is nitrogen dioxide. The removal of ultraviolet radiation of very high energy by stratospheric absorption of oxygen and ozone means that the solar radiation reaching the troposphere has insufficient energy to break the O–O bond. Therefore, oxygen cannot be photolyzed in the troposphere to produce oxygen atoms and ozone. Nitrogen dioxide, however, can be photolyzed in the region of 290–423 nm NO2 + hν (290–423 nm) → NO + O Subsequent reaction of the ground-state oxygen atom with molecular oxygen can produce ozone in the troposphere: O + O2 + M → O3 + M Nitrous Acid Another important photochemical reactant is nitrous acid (HONO). Nitrous acid photolyzes in this region to produce hydroxyl radical: HONO + hν (310–400 nm) → OH + NO Nitrate Radical Ozone and nitrogen dioxide can react with each other to form nitrate radical (NO3-): O3 + NO2 + M → NO3- + M Nitrate radical is a very reactive species in nighttime processes. There are two possible pathways for the photolytic dissociation of nitrate radical: NO3- + hν → NO2 + O (1) NO3- + hν → NO + O2 The photolysis actually proceeds via pathway (1) and produces an oxygen atom that rapidly forms ozone in the troposphere, leading to the regeneration of the starting reactants, ozone and nitrogen dioxide. 4. Volatile Organics (Carbon Monoxide and Methane ) The reaction of hydroxyl radical with carbon monoxide in polluted air would lead to the formation of H atoms and subsequently the hydroperoxyl radical in the presence of molecular oxygen: OH + CO → CO2 + H H + O2 → HO2 This same chemistry could also occur with methane (CH4) if the hydroxyl radical abstracted a hydrogen atom and formed the methyl radical (CH3) and subsequently the methylperoxy radical, which can react with nitric oxide to form nitrogen dioxide: OH + CH4 → CH3 + H2O CH3 + O2 → CH3O2 CH3O2 + NO → CH3O + NO2 5.Hydrogen Peroxide Hydrogen peroxide photolyzes to form two hydroxyl radicals: H2O2 + hν → 2 OH 6. Sulfur Oxides Sulfur dioxide (S02) is formed from the oxidation of sulfur contained in fuel as well as from certain industrial processes that utilize sulfur-containing compounds. Anthropogenic emissions of S02 result almost exclusively from stationary point sources. Sulfur dioxide (S02) is formed from the oxidation of sulfur contained in fuel as well as from certain industrial processes that utilize sulfur-containing compounds. Anthropogenic emissions of S02 result almost exclusively from stationary point sources. (Add more of its chemical behaviour)