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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

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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

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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

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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

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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

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(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 from combustion – Nitrogen can be removed by using the combustion process by introducing oxygen in place of air. This gives a very intense flame which needs to be carefully controlled.

In practice, instead of using one of the above methods or any other method alone, best results are obtained by using a suitable combination of several methods.

3.3.2 Vehicular Exhaust

The control methods are exhaust gas recirculation, selective catalytic reduction and a three way catalytic converter.

CHEMISTRY PAPER No. : 4, Environmental chemistry MODULE No. : 20, Oxides of nitrogen, sulphur and carbon

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(a) Exhaust Gas Recirculation – The amount of emitted nitrogen oxide can be reduced by cooling some of the exhaust gas and then redirect it back to the charge air. This reduces the combustion temperature and therefore less amount of nitrogen oxide is produced. This is called exhaust gas recirculation (EGR). (b) Selective Catalytic Reduction – Another method to reduce the concentration of nitrogen oxide is to pass the exhaust gas over a catalytic converter for SCR. While exhaust gas re- circulation can reduce nitrogen oxide emissions by around 40%, SCR removes up to 90% of the nitrogen oxide from exhaust gases. EGR and SCR systems can be combined to ensure better removal. SCR uses oxides of vanadium, molybdenum or tungsten on a ceramic support as catalysts. Platinum or palladium is expensive but more durable as catalyst. (c) A three-way catalytic converter removes hydrocarbon, carbon monoxide and nitrogen oxides from auto-exhaust. Hydrocarbons and carbon monoxide are oxidised while nitrogen oxides are reduced. These oxidation and reduction reactions do not occur in isolation. NO reacts with CO in the exhaust system as shown:

!"#"$%&# 2NO + 2CO N! + 2CO!

Partial reduction to nitrous oxide would also suffice since nitrous oxide is easily decomposed to give nitrogen and oxygen. Nitric oxide and nitrogen dioxide interact with hydrocarbons:

!"#"$%&# NO! + Hydrocarbons N! + CO! + H!O

A three-way catalyst consists of a mixture of platinum, palladium and rhodium deposited on a high surface area honeycomb made of ceramic and metal, or alumina pellets.

4. Oxides of Sulphur in the Atmosphere

SOx is a general term used to refer all oxides of sulphur, out of these, sulphur dioxide (SO2) and sulphur trioxide (SO3) are major ones. Sulphur dioxide is a colourless gas having a pungent, irritating odour and taste. It is water soluble, forming sulphurous acid, which is a weak acid. Sulphur dioxide slowly combines with oxygen in the air to form sulphur trioxide. This then rapidly reacts with water to give sulphuric acid.

4.1 Sources of sulphur oxides

Sulphur dioxide is produced naturally from volcanoes and hot springs. Sulphur dioxide is also obtained by oxidation in air of hydrogen sulphide released from marshes and swamps on land and from biological decay in oceans. Combustion of fossil fuel (especially coal, petroleum and petroleum products) at power plants, other industries and vehicles is the largest anthropogenic source of SO2. Other sources of SO2 emissions include smelting of sulphide ores, refining of petroleum, manufacture of sulphuric acid, paper making and burning of domestic refuse. Coal combustion is the largest source of sulphur dioxide, contributing approximately 60% of it globally. Combustion of petroleum products comes next, contributing around 21%. Sulphide ore CHEMISTRY PAPER No. : 4, Environmental chemistry MODULE No. : 20, Oxides of nitrogen, sulphur and carbon

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smelting and petroleum refining contribute around 7.4% and 6.8% respectively. While the activities mentioned directly release sulphur dioxide in the atmosphere, putrefaction of organic matter including sewage and garbage releases hydrogen sulphide in the atmosphere which is oxidised to sulphur dioxide by oxygen or ozone in air:

2H!S + 3O! → 2SO! + 2H!O SO! + O! → SO! + H!O

Sulphur dioxide, whether from oxidation of H2S or direct emission, persists in the atmosphere for 2-4 days, during which period it is dispersed to distant places and is also oxidised to sulphur trioxide, both chemically and photochemically. Sulphur dioxide molecules in their excited state after absorption of UV light react with oxygen as well as ozone: ʋ SO! + O! SO! + O!

Chemical oxidation of sulphur dioxide by oxygen takes place in the presence of catalysts such as powdered metal oxides or dust and soot particles:

1 !"#$%&'("$)* SO + O SO + O ! 2 ! ! !

The sulphur trioxide formed immediately combines with water to give sulphuric acid:

SO! + H!O → H!SO!

4.2 Effects of Sulphur Oxides

4.2.1 Primary Effects

(a) Effects on Human Health

Sulphur dioxide is dangerous if breathed in. It causes irritation to nose, throat and airways giving rise to coughing, wheezing, breathing problems and tightness in the chest. Sulphur dioxide is quick acting and symptoms can be felt in 10 or 15 minutes after breathing it in. Asthma patients are most affected by exposure to sulphur dioxide. Sulphur dioxide also causes eye irritation. The effect of sulphur oxides is much worsened when they are adsorbed on soot particles. Sulphur oxides adsorbed on particulates, when inhaled, pass deep within the respiratory tract, dissolve in body fluids and enter the bloodstream, the lymphatic system and the connective tissue in the lungs. This makes the airways contract spasmodically, making breathing difficult and, in turn, putting a strain on the affected person’s heart. Higher levels of exposure to sulphur oxides and particulates lead to bronchitis and lung cancer.

CHEMISTRY PAPER No. : 4, Environmental chemistry MODULE No. : 20, Oxides of nitrogen, sulphur and carbon

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(b) Effects on Vegetation

Sulphur oxides destroy plant cells and interfere with chlorophyll synthesis. High concentration of sulphur dioxide results in the opening of leaf stomata in plants leading to excessive loss of water. The sulphurous pollution reduces the number of plants and the quality of plant yield. Sulphur oxides are more dangerous in combination with other pollutants such as oxides of nitrogen, fluorides, and ozone. At the ecosystem level, it eliminates sensitive species thereby affecting species distribution. Crop yields are reduced in the presence of sulphur dioxide pollution.

(c) Effects on Materials

Stones used in making buildings, statues etc. especially carbonate based stones, undergo severe erosion in the presence of sulphur dioxide, causing pitting and scarring as well as discolouration. This is called ‘stone leprosy’. In case of metal structures, SO2 accelerates corrosion reactions that occur naturally when sufficient moisture is present on a metal surface. Steel panels have been found to corrode 50% more rapidly in the presence of 0.1 ppm sulphur dioxide in conjunction with particulates. Painted surfaces take longer to dry and continuous exposure to sulphur oxides causes discolouration and loss of gloss, requiring frequent repainting. Paper, fabric and leather become discoloured. Paper exposed to sulphur dioxide also becomes brittle.

4.2.2 Secondary Effects

(a) Acid Rain

The SO2 and SO3 in the atmosphere are washed down with rain in the form of sulphurous and sulphuric acid (H2SO3 and H2SO4). The pH of such rain is much below the normal pH of 5.6 associated with normal rainwater and this kind of rain is called acid rain. If the oxides of sulphur and nitrogen get converted into the corresponding acids in the atmosphere and come down with rain, it is wet deposition. Alternatively, the oxides may be directly absorbed by a wet surface – land, vegetation or a structure – and can be transformed into acid upon absorption. This is called dry deposition. Another indirect consequence of the acid rain is that it leaches out nutrients from plant canopy and soil. This results in change of pH of water bodies and disturbs the aquatic environment.

(b) Smog

Industrial smog or London Smog contains two primary components: sulphur dioxide and particulates, which include dust and soot from burning coal. The sulphur dioxide, produced from combustion of coal or other sulphur containing fossil fuels, dissolves in water droplets in the atmosphere and becomes a toxic particle. The worst incident of smog in London in 1952 claimed 4000 lives between 5 December and 9 December and caused major disruption in transport due to reduced visibility. This kind of incident related to smog formation has now fortunately become a thing of the past, partly due to pollution legislation and also due to modern developments, such as the widespread use of central heating.

CHEMISTRY PAPER No. : 4, Environmental chemistry MODULE No. : 20, Oxides of nitrogen, sulphur and carbon

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4.3 Control Measures for SOx Pollution

High sulphur content of fuel increases the emitted sulphur. SOx emissions can be reduced by making use of low-sulphur fuel like natural gas, low-sulphur oil, or low-sulphur coal. The additional advantage of using natural gas is that it does not emit particulate matter on burning. There are two broad categories of methods for reducing the amount of sulphur oxides released into the environment from burning of coal – fuel desulphurisation techniques and flue gas desulphurisation techniques.

(a) Fuel Desulphurisation

Physico-chemical method – Up to70% of the sulphur in high-sulphur coal is in mineral sulphate form, present as iron pyrites, FeS2 not chemically bonded to the coal. The rest is organically bonded sulphur. Ground coal is subjected to hydraulic washing resulting in the settling down of the pyrites, which is heavier. The sulphur-free coal floats on the surface, from where it is collected. Organic sulphur is removed by passing hydrogen gas over the coal in the presence of a mixed cobalt–molybdenum oxide catalyst. The organic sulphur is converted into hydrogen sulphide, which is absorbed into diethanolamine. The sulphur can be regenerated by oxidation afterwards.

Microbial method – The organic sulphur can also be removed by a microbial method in which a specific bacterial strain, cultured in a sulphur-limited medium where the supply of all other nutrients is abundant, is allowed to draw out the sulphur required for its metabolism from coal. It selectively metabolises the sulphur from coal without degrading the coal. However, the method is too slow to be commercially viable.

Petroleum desulphurisation is achieved during the refining process, in which heat and pressure convert the organic sulphur into hydrogen sulphide. The gas rich in hydrogen sulphide is passed through a sulphur recovery plant where the H2S is oxidised to give SO2 and water and the SO2 is catalytically reduced to give sulphur in elemental form.

(b) Flue Gas Desulphurisation

Wet Lime-Limestone Scrubbing – Slurry is made by mixing crushed lime and water which is then sprayed into the flue gases containing sulphur. Aqueous slurry of calcium sulphite is formed after the reaction of sorbent with SO2. Subsequently, calcium sulphite is oxidised to calcium sulphate by introducing compressed air into the slurry. Calcium sulphate so formed is then subjected to removal of excess water and disposed of in landfills. SO2 removal can be close to 90% with this method. The limestone could also be mixed in powdered form with powdered coal in the boiler. Limestone decomposes at high temperature to give lime, which reacts with SO2 to give calcium sulphite and sulphate: ∆ CaCO! CaO + CO! CaO + SO! → CaSO! 1 CaO + SO + O → CaSO ! 2 ! ! CHEMISTRY PAPER No. : 4, Environmental chemistry MODULE No. : 20, Oxides of nitrogen, sulphur and carbon

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Though effective in reducing SOx emissions, the method generates large amounts of sludge for disposal.

Wellman-Lord Process – This process can be categorised into two stages: 1) Absorption: In this stage the hot flue gases are passed through a pre-scrubber and ash, hydrogen chloride, hydrogen fluoride and SO3 are removed out of the flue gas. The gases are then cooled down and delivered to the absorption tower. A saturated solution of sodium sulphite is sprayed onto the flue gases at the top of the tower; the sodium sulphite reacts with the SO2 and forms sodium bisulphite. The concentrated bisulphite solution is then collected and subsequently passed to an evaporation system for regeneration.

2) Regeneration: Sodium bisulphite can be broken down using steam and the resulting sodium sulphite recycled to the flue gases. The released SO2 is then transformed into elemental sulphur, sulphuric acid or liquid SO2. The advantage of the system is that the sorbent is regenerated during the combustion process which is continuously recycled.

Fluidised Bed Combustion – In this process, sand is used to make the bed on which combustion of coal takes place. Air is blown up from beneath the bed at high velocities. As velocity increases, particles are forced upwards and reach a level at which they remain suspended in the air stream. The bed in this state is called fluidised, as it behaves like a fluid. In order to absorb the heat generated, water-containing tubes are immersed in the bed (this water is converted to steam which is used to drive a steam turbine thus producing electricity). The movement of this fluidised material in the combustion chamber results in a transfer of heat to the water filled tubes and therefore operating temperatures are lower than in a conventional system. Addition of sorbent such as lime or limestone can control the SO2 emissions because it absorbs the SO2 present. The SO2 released from the coal is absorbed by the limestone and is retained within the ash, which can be regularly removed. Effective combustion takes place without causing the ash to soften, making it easy to remove the ash containing the absorbed SO2.

5. Oxides of Carbon in the Atmosphere

Carbon dioxide and carbon monoxide are the main oxides of carbon. Both of them are colourless, odourless and non-irritating. Carbon dioxide is non-toxic, non-combustible and a non-supporter of combustion while carbon monoxide is highly toxic and burns with a blue flame.

5.1 Sources of Carbon Oxides

Carbon dioxide is generally present in the atmosphere at a concentration of 0.04 per cent (400 ppm) by volume. It is utilised by plants, algae, and cyanobacteria in the process of photosynthesis. During the day, plants overall absorb carbon dioxide because the carbon dioxide required for photosynthesis is more than that produced during respiration.

During respiration, carbon dioxide is produced and exhaled by humans and animals as well as plants. At night, when there is no photosynthesis, some carbon dioxide is produced by plants during respiration. Volcanoes, hot springs and geysers are natural sources of carbon dioxide. CO2 CHEMISTRY PAPER No. : 4, Environmental chemistry MODULE No. : 20, Oxides of nitrogen, sulphur and carbon

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is found in lakes, deep in the sea and mixed with oil and gas deposits. It is a major greenhouse gas. The burning of carbon-based fuels increases the concentration of carbon dioxide in the atmosphere, leading to global warming.

Carbon monoxide is generated by both natural and anthropogenic sources. In nature, it is produced by the oxidation of methane, a gas released from swamps, bogs and rice paddies. Mature leaves produce copious amounts of carbon monoxide due to degradation of chlorophyll. The surface waters of all the earth’s oceans release carbon monoxide to the atmosphere. Small amounts of carbon monoxide are produced in normal animal metabolism. Combustion processes in nature such as forest fires and volcanoes also produce carbon monoxide.

The background concentration of carbon monoxide is increasing every year due to human activity. When carbon containing substances are partially burnt, carbon monoxide is formed.

!"#$ 2C + O! 2CO

Carbon monoxide is produced when there is not enough oxygen to form carbon dioxide, such as when operating a stove or an internal combustion engine in a surrounded space. The reaction of CO to give CO2 is slow owing to high activation energy, hence it occurs only when the temperature is high or there is excess of oxygen. When there is heavy traffic, a high concentration of CO is released in the atmosphere. In cities, a high percentage of all CO emissions may be released from automobile exhaust. Industrial processes and non-transportation fuel combustion are other sources of carbon monoxide. Fuel-burning appliances are source of indoor pollution by CO and contribute to serious health risks to building occupants.

Cigarette smoke contains a high concentration of carbon monoxide (up to 2%) due to incomplete combustion of tobacco. Some of the inhaled gas (0.04%) passes into the lungs of the smoker. Thus exhaled breath of smokers contains higher amounts of carbon monoxide than that of non- smokers.

5.2 Effects of Carbon Monoxide

Carbon monoxide present in air may cause headaches, nausea, fatigue and impairment of judgment. If it is breathed in excessively, it can prove fatal. This happens because when inhaled, carbon monoxide combines with the haemoglobin in the blood, which has approximately 210 times more affinity for CO than for O2. Thus instead of oxyhaemoglobin, the complex formed in blood on inhalation of oxygen.

Hb + O! ⇋ HbO!

Carboxyhaemoglobin is formed:

Hb + CO ⇋ COHb

CHEMISTRY PAPER No. : 4, Environmental chemistry MODULE No. : 20, Oxides of nitrogen, sulphur and carbon

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The inhaled oxygen is carried to different parts of the body as oxyhaemoglobin. The oxygen- carrying ability of blood is impaired in the presence of carbon monoxide because many haemoglobin molecules remain complexes to carbon monoxide and are unavailable for carrying oxygen. In addition, in the presence of CO, it becomes difficult for oxyhaemoglobin to release oxygen after reaching different parts of the body. The combined effect results in severe impairment of oxygen transporting ability of blood. High levels of carbon monoxide in the body thus lead to severe oxygen shortage for various bodily functions. The danger from CO exposure is greatest in infants, the elderly, and those suffering from chronic heart and lung illnesses. Distance vision, night vision, hearing ability and mental performance are adversely affected by CO poisoning, increasing the incidence of accidents. Carbon monoxide can cross the placental barrier, thus reducing oxygen availability for the foetus. This results in low birth weight of infants whose mothers are exposed to carbon monoxide during pregnancy e.g. pregnant women who are smokers.

Higher blood levels of carbon monoxide lead to an increase in haematocrit, i.e. percent volume of red blood cells in blood, possibly because the body needs to make up for the RBC in which the haemoglobin is tied up with CO. Smokers have been found to have increased haematocrit. This increases the possibility of clot formation in blood vessels. Smokers thus have high cardiovascular morbidity and are at risk for fatal artery blockage at an early age. Where oxygen deficiency already exists, e.g. in individuals with anaemia, chronic lung disease or impaired circulatory system, the effects associated with carbon monoxide poisoning increase. This also happens in high altitude areas where the atmospheric oxygen concentration is already low. Normally the threshold limit value for carbon monoxide is 50 ppm for 8-hour exposure but for those living at an altitude of 5000-8000 feet, the limit is 25 ppm.

5.3 Control Measures for Carbon Monoxide Pollution

Let us now discuss about the control measures for pollution caused by carbon monoxide. These include engine tuning in vehicles, catalytic converter and good ventilation to prevent indoor air pollution. Engine tuning in vehicles – Carbon monoxide emissions from vehicles can be reduced by maintaining properly tuned engines and regularly checking for defects in the exhaust system. Vehicles must not be left with running engines in attached garages and warming up of vehicles should also be done only with the garage door open. Catalytic converter – The typical catalytic converter is used to combine oxygen and carbon monoxide and form non-poisonous carbon dioxide. It is installed in most of the new models of cars and trucks. The concentration of carbon monoxide in gasoline engines in the absence of catalytic converters are in the range of 30,000 to over 100,000 ppm. The three-way catalytic converter for taking care of NOx, hydrocarbons and carbon monoxide simultaneously has been described earlier. Good ventilation system – Indoor carbon monoxide pollution due to use of charcoal grills, stoves, heating systems etc. can be controlled by keeping the home or workplace well-ventilated.

6. Summary

Let us recapitulate what we have learnt in this module. We have discussed about various sources of the oxides of nitrogen, sulphur and carbon. The oxides of nitrogen, sulphur and carbon are CHEMISTRY PAPER No. : 4, Environmental chemistry MODULE No. : 20, Oxides of nitrogen, sulphur and carbon

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introduced into the atmosphere through several natural processes. However, addition due to human activity raises their proportion in air to harmful levels. We have examined the damaging effects of high levels of NO2 on humans, animals and also on vegetation, including leaf damage and reduced plant growth. The burning of fossil fuels and automobile emissions are known to be anthropogenic sources of nitrogen oxides. Secondary pollution due to nitrogen oxides has also been discussed. Continuous exposure to high concentration of oxides of nitrogen results in acute respiratory illness in children and respiratory discomfort in adults. Sulphur oxides are released from combustion of sulphur-containing fossil fuels. We have discussed about the two broad categories of methods for reducing the amount of sulphur oxides released into the environment which include fuel desulphurisation techniques and flue gas desulphurisation techniques. Incomplete combustion of carbon-containing material results in the formation of carbon monoxide, a poisonous gas, which when inhaled, affects the oxygen-carrying capacity of blood.

So now you are familiar with the oxides of sulphur, nitrogen and carbon present in the atmosphere, their sources, effects on human health and environment and control measures.

CHEMISTRY PAPER No. : 4, Environmental chemistry MODULE No. : 20, Oxides of nitrogen, sulphur and carbon