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20, Oxides of Nitrogen, Sulphur and Carbon ____________________________________________________________________________________________________ Subject Chemistry Paper No and Title Paper 4: Environmental Chemistry Module No and Title 20: Oxides of Nitrogen, Sulphur, Carbon and Their Effects Module Tag CHE_P4_M20 CHEMISTRY PAPER No. : 4, Environmental chemistry MODULE No. : 20, Oxides of nitrogen, sulphur and carbon ____________________________________________________________________________________________________ TABLE OF CONTENTS 1. Learning Outcomes 2. Introduction 3.Oxides of Nitrogen in the Atmosphere 3.1 Sources of Nitrogen Oxides 3.2 Effects of Nitrogen Oxides 3.3 Control Measures for Nitrogen Oxides 4. Oxides of Sulphur in the Atmosphere 4.1 Sources of Sulphur Oxides 4.2 Effects of Sulphur Oxides 4.3 Control Measures for Sulphur Oxides 5. Oxides of Carbon in the Atmosphere 5.1 Sources of carbon Monoxide 5.2 Effects of Carbon Monoxide 5.3 Control Measures for Carbon Monoxide 6. Summary CHEMISTRY PAPER No. : 4, Environmental chemistry MODULE No. : 20, Oxides of nitrogen, sulphur and carbon ____________________________________________________________________________________________________ 1. Learning Outcomes After studying this module, you will • Know about the sources of the oxides of nitrogen, sulphur and carbon in the atmosphere. • Learn about the effects of these oxides on humans, animals, plants and materials. • Learn about the control measures for atmospheric pollution due to these oxides. 2. Introduction By this time, you are quite familiar with the composition of the atmosphere. As you know, the oxides of nitrogen, sulphur and carbon are introduced into the atmosphere through several natural processes. We will discuss the effects of these oxides in this module. Carbon dioxide is a natural constituent of the earth’s atmosphere. However, excessive amounts of atmospheric carbon dioxide upsets the natural heat balance of the atmosphere due to what is known as ‘Greenhouse Effect’ that results in global warming. Carbon monoxide is formed in the atmosphere due to oxidation of methane produced in swamps and bogs and oxidation of chlorophyll in mature leaves. Elemental nitrogen is the major constituent of the earth’s atmosphere. There are several natural processes by which nitrogen oxides enter the atmosphere e.g. chemical combination of atmospheric nitrogen with oxygen in the presence of lightning during thunderstorms to form nitric oxide, which further reacts with oxygen to give nitrogen dioxide. Nitrous oxide (N2O) is released due to the action of soil microorganisms. Sulphur dioxide enters the atmosphere during volcanic eruptions. It is also produced by the oxidation of hydrogen sulphide formed by decomposition of organic matter in the absence of air. Over millennia, a certain global balance of these gases from natural sources has been achieved in the earth’s atmosphere. However, when present in excessive amounts due to human activity, most of these gases exert detrimental effects and become air pollutants. 3. Oxides of Nitrogen in the Atmosphere Nitrogen forms a series of oxides on combining with oxygen: nitrous oxide N2O, nitric oxide NO, nitrogen dioxide NO2, dinitrogen trioxide N2O3, dinitrogen tetroxide N2O4 and dinitrogen pentoxide N2O5.However, only the first three, i.e. N2O, NO and NO2 are important from the point of view of environmental pollution. They are collectively referred to as NOx. These oxides act as primary pollutants by producing toxic reactions themselves and they also act as secondary pollutants by combining with other constituents of the atmosphere to give rise to photochemical smog, acid rain and aerosols. 3.1 Sources of Nitrogen Oxides CHEMISTRY PAPER No. : 4, Environmental chemistry MODULE No. : 20, Oxides of nitrogen, sulphur and carbon ____________________________________________________________________________________________________ As mentioned above, nitrogen oxides can be obtained from both natural and anthropogenic sources with the latter accounting for only 8% of the global total. However, local concentrations, especially in urban areas, are elevated several hundredfold. The burning of fossil fuels in power plants and automobiles are known to be anthropogenic sources of nitrogen oxides. In all the cases, nitrogen and oxygen in air react to form nitrogen oxides due to the heat produced by combustion with a lesser contribution from combustion of nitrogen present in the fuel. Nitric oxide is first formed by combination of nitrogen and oxygen: �� + �� → ��� Then nitrogen dioxide is formed by reaction of nitric oxide and oxygen. 2NO + O! → 2NO! Nitrogen dioxide can also be formed when nitric oxide reacts with ozone: �O + O! → NO! + O! Nitrous oxide, which is produced by the action of soil bacteria on nitrate fertilisers, undergoes photolysis in the stratosphere to nitric oxide or free oxygen atoms: N!O + ℎʋ → NO + N N!O + ℎʋ → N! + O The nitric oxide so formed is then oxidised and nitrogen dioxide is formed. The free oxygen atoms interact with nitrous oxide to form nitric oxide. N!O + O → 2 NO This is further oxidised to yield nitrogen dioxide. Nitrogen dioxide reacts with water vapours, producing nitric acid, which is washed down as acid rain or reacts with atmospheric ammonia to give ammonium nitrate aerosols: 3NO! + H!O → 2HNO! + NO NH! + HNO! → NH!NO! 3.2 Effects of Nitrogen Oxides CHEMISTRY PAPER No. : 4, Environmental chemistry MODULE No. : 20, Oxides of nitrogen, sulphur and carbon ____________________________________________________________________________________________________ Nitrous oxide and nitric oxide ultimately form nitrogen dioxide in the atmosphere. It is nitrogen dioxide which is responsible for the detrimental effects of nitrogen oxides. Nitrogen dioxide is a primary pollutant itself and produces secondary-polluting effects as well. 3.2.1 Primary Effects Let us first discuss about the effects of nitrogen dioxide as a primary pollutant. (a) Effects on human health Nitrogen dioxide affects the inner lining of lungs and increases chances of lung infections. This can give rise to wheezing, coughing, colds, flu and bronchitis. Continuous or frequent exposure to higher concentration of nitrogen dioxide than the normal level in ambient air may increase frequency of acute respiratory illness in children. Exposure to nitrogen dioxide near roadways is harmful for the elderly, children and people having asthma. Pulmonary oedema (accumulation of fluid in the lungs) may occur in susceptible people several days after exposure, resulting in death. The maximum allowable limit for nitrogen dioxide exposure is 8 ppm for an eight-hour period. However, even at much lower levels of exposure, nitrogen dioxide can cause increased airway resistance in adults and acute bronchitis in children. Nitrogen dioxide is also a powerful eye irritant (b) Effects on vegetation High levels of NO2 can have a damaging effect on vegetation, including leaf damage and reduced growth. It can make vegetation more susceptible to disease and frost damage. Even at the low concentration of 0.3 ppm, NO2 suppresses plant growth. At higher concentrations, it causes leaf injury through chlorosis (insufficient chlorophyll production) which is clearly visible on the leaf. Food crops exposed to NO2 display chlorosis and reduced yield. 3.2.2 Secondary Effects Nitrogen dioxide is also involved in the following secondary environmental pollution phenomena: (a) Acid Rain Nitrogen dioxide reacts with water vapour in the atmosphere to give nitric acid as shown here, which is washed down as acid rain: ���� + ��� → ����� + �� (b) Photochemical Smog In the presence of hydrocarbons and sunlight, oxides of nitrogen form photochemical smog. This contains a variety of chemical species such as ozone and organic compounds including peroxo compounds, aldehydes, ketones, acetylnitrates etc. CHEMISTRY PAPER No. : 4, Environmental chemistry MODULE No. : 20, Oxides of nitrogen, sulphur and carbon ____________________________________________________________________________________________________ (c) Aerosols Some of the nitric acid obtained from the reaction of nitrogen dioxide and water combines with ammonia to form ammonium nitrate aerosols: NH! + HNO! → NH!NO! 3.3 Control Measures for NOx Pollution To remove NOx pollution due to its two major sources, i.e. thermal power plants and automobile exhaust, control measures have to be put in place in both cases. 3.3.1 Power Plants Reducing Combustion Temperature – In power plants combustion temperature can be reduced to control the NOx pollution. This technique keeps away from the ideal stoichiometric ratio because with this ratio higher temperatures are produced, generating higher concentrations of NOx. There are several techniques which can be applied to reduce the combustion temperature: (1) use of fuel rich mixtures to control the amount of oxygen available; (2) use of fuel to control temperature by dilution energy; (3) injecting cooled flue gas to dilute energy; (4) injecting cooled flue gas with added fuel. Chemical Reduction of NOx– This technique provides a reducing agent, such as urea, to react with nitrogen oxides and reduce them to nitrogen, carbon dioxide and water: NH!CONH! + NO + NO! → 4N! + CO! + 2H!O Oxidation of NOx – This can be done by making use of catalyst or by introducing hydrogen peroxide or ozone to the air. The N2O5 produced is scrubbed with water to give nitric acid which can be either collected as such or neutralized by an alkaline scrubber and then sold as calcium or ammonium nitrate. Removal of nitrogen
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