• CO(g), H2S(g), SO2(g), SO3(g), NO(g), NO2(g) and CO2(g) are the inorganic compounds which change the atmospheric composition. Organic compounds including hydrocarbons, halo hydrocarbons and particles such as dust and carbon also contribute to change the composition of the atmosphere.
• Some CO(g) in atmosphere is formed by the oxidation of methane, which is formed naturally by the anaerobic degradation of organic matter. CO (g) is emitted from all the incomplete combustion processes including the internal combustion engines of motor vehicles.
• SO2(g) enters the atmosphere from the combustion of sulphur containing fossil fuels,volcanic eruption , biological decay of S containing organic matter, reduction of sulphates and recovery of metals from their sulphides. SO2(g) reacts with oxygen and forms SO3(g). NO(g) increases the rate of oxidation of atmospheric SO2 (g) in to SO3(g).
• NOx(g) [NO(g) and NO2(g)] enters the atmosphere from natural processes such as lightening discharges, and from pollutant sources. The combustion of fossil fuels gives most of the NOx(g). Much of the NOx(g) entering the atmosphere is from the internal combustion engines.
• Microbial decay of S containing organic matter and reduction of sulphate ion are the most common natural sources of H2S.
• Hydrocarbons are widely used as fuels and enter the atmosphere directly or as byproducts in the partial combustion of other hydrocarbons. Uncontrolled vehicle exhausts contain alkanes , alkenes and aromatic hydrocarbons. Methane is produced in large quantities from the anaerobic decomposition of organic matter submerged in water.
Greenhouse effect
• The temperature of the earth is fixed by a steady – state balance between the energy received from the sun and the energy radiated back by the earth. One mechanism for regulating the earth’s temperature is the greenhouse effect.
• The loss of energy from the earth is achieved by means of conduction, convection and radiation. A fraction of the earth’s heat is transmitted to clouds by conduction and convection before being lost by radiation.
• Convection carries heat in the form of the enthalpy of vaporization of water. The water vapour releases heat as it condenses.
• The radiation that carries energy away from the earth is of longer wavelength, in the infrared region.
• If all the outgoing radiation were able to escape, the surface of the earth would be at -16 °C (Same temperature as in the moon).
• Most heteroatomic molecules and some homoatomic molecules (O3) are known as greenhouse gases and these are the ones which contribute to the greenhouse effect.
• Accordingly CO2(g), water vapour, methane, dinitrogen oxide, ozone, SO2 and CFCs absorb radiation given out from the earth and some of it re-radiate back to the earth’s surface. This re-radiation helps to warm the earth andmaintain a climate that will support life. This is called greenhouse effect and these gases are called greenhouse gases.
1. Global warming
• When the greenhouse gases exceed their permissible level. Hence, the temperature of the atmosphere increases. This is called “Global Warming”.
• CO2(g) plays the key role in this global warming. Gases such as NOx and CFCs are the other examples. Though the CFC levels are low they have a longer residence time and they have a high efficiency of absorbing IR radiations. Therefore, their contribution is high.
• The results of global warming include melting of polar ice caps, sinking of low lying countries due to the thermal expansion of sea water, desertification due to loss of soil moisture, drying of fresh water reserviors, changes in biodiversity and weather patterns.
• A significant amount of atmospheric carbon dioxide dissolves in water. Thus, it reduces the contribution of CO2 to global warming. However, with the increase of temperature dissolution of carbon dioxide is reduced and dissolved carbon dioxide returns to the atmosphere.
• Increasing CO2 level in the atmosphere increases the photosynthesis. This is a positive effect of global warming.
• As far as Sri Lanka is concerned global warming can have a higher effect because we are an island situated closer to the equator.
2. Αcid rain
Acidic gases in the atmosphere dissolves in water to contribute to the acidity. Two important factors are
(i) Dissolution of acidic gases in water
(ii) Strength of the resulting acid
In this context even though CO2 levels are high, their contribution to the acidity is very low (pH 5.1 – 5.8) and it is not considered as acid rain. But the SOx and NOx even though they are present in smaller quantities have a higher contribution resulting in a pH of 4 – 5.
Reactions of atmospheric SO2 (g)
(i) SO2(g) + H2O(l) 
H2SO3(aq)
H2SO3(aq) + H2O(l) H3O+(aq) + HSO3–(aq)
HSO3–(aq) + H2O(l) H3O+(aq) + SO32-(aq)
(ii) Oxidants in the atmosphere can oxidize SO2 to SO3.
SO2(g) → SO3(g)
O2(g), O(g), OH(g) and peroxides can serve as oxidants. Some salts can catalyse the oxidation. Then SO3(g) dissolves in H2O to form H2SO4.
(iii) Both SO2 and the oxidant (normally O2) can dissolve in a rain drop. Rain drop facilitates the oxidation process by bringing two chemicals together.
2SO2(aq) + 2H2O(l) + O2(aq) → 2H2SO4(aq)
H2SO4(aq) + 2H2O(l) → 2H3O+(aq) + SO42-(aq)
• Similarly NOx reacts in the atmosphere.
2NO(g) + O2(g) → 2NO2(g) 4NO2(aq) + 2H2O(l) + O2(aq) → 4HNO3(aq)
HNO3(aq) → H+(aq) + NO3–(aq)
• Acid rain damages plants and causes the death of fish in the lakes. Acids such as sulphuric acid and nitric acid dissolve aluminium from aluminosilicate materials of soil giving free Al3+ to water. It interferes with the operation of fish gills.
• Acid rain water, draining through soils washes out nutrients and liberates aluminum ions.The roots of the trees may take up the aluminum ions instead of essential nutrients.
eg. Ca2+ and Mg2+.
• Limestone, metallic structures, bridges, ships, motor vehicles, etc. are also affected.
Change of the composition of earth’s surface due to acid rain
• Dolomite, limestone or marble are soluble in acidic water.
• Under mild acidic conditions;
CaCO3(s) + H+(aq) → Ca2+ (aq) + HCO3–(aq)
MgCO3(s) + H+(aq) → Mg2+(aq) + HCO3–(aq)
CaCO3.MgCO3(s) + 2H+(aq) → Ca2+(aq) + Mg2+ (aq) + 2HCO3–(aq)
Here, insoluble substances become soluble.
• Under strong acidic conditions;
CaCO3(s) + 2H+(aq) → Ca2+(aq) + H2O(l) + CO2(g)
MgCO3(s) + 2H+(aq) → Mg2+(aq) + H2O(l) + CO2(g)
CaCO3.MgCO3(s) + 4H+(aq) → Ca2+(aq) + Mg2+(aq) + 2H2O(l) + 2CO2(g)
• Many other salts in the rocks and sand also dissolve in the acid rain. Soil becomes gradually more acidic in the natural course of events. Cations are removed from the soil solution by plants and replaced by H+ ions. Minerals such as sulphides are oxidized to form acids. At low pH, hydrogen ions displace other cations from soil. Not only Al3+, Mg2+, Ca2+ but heavy metal ions are also displaced by H+ ions. The leaving of these ions deprives plants of the nutrients required for healthy growth. The acidic water passing through the soil, causes to leaching Al3+ and other minerals and also weathering of rocks. The Ca2+ and Mg2+ concentration increase in water, and hardness of water also increases. The acidity, salinity and nitrogen concentration also increase insurface water. Concentration of the heavy metal ions also increases in the surface water.
3. Photochemical smog
• Motor vehicle emissions contain NOx and unburnt hydrocarbons (CxHy). They are converted to ozone, aldehydes, peroxyacetyl nitrate (PAN), peroxy benzoyl nitrate (PBN), etc. in the presence of sunlight and temperatures above 15 °C.
• This is known as photochemical smog as those chemicals are formed in the presence of sunlight
• Smog is a yellowish haze which reduces visibility and causes eye irritation.
• The word smog is used to describe the combination of smoke and fog.
• The starting reaction of photochemical smog is dissociation of NO2 to NO and ‘O’.
• The steps in the formation of a photochemical smog are given below.
(i) NO2 absorbs sunlight and undergoes photolysis.
NO2 hυ→ NO + O
(ii) The resulting atomic oxygen combines with O2 molecules
a) to form ozone.
O + O2 + M → O3 + M
(M is known as third body which absorbs excess energy. M can be an airborne particle
or a gas.)
b) to form OH radicals.
O + H2O → 2OH•
(iii) The resulting HO• converts other airborne chemicals into radicals and they start a set of reactions to produce aldehydes, PAN, PBN, etc.
Effects of photochemical smog are given below.
• Effects on human health and comfort : Photochemical smog affects the respiratory system.It causes coughing, wheezing, etc.
• Damage to materials : Ozone causes rubber to deteriorate through fission of the double bond and also reduces the quality of fabrics and bleaches dyes.
• Effects on the atmosphere : Aerosol particles scatter light and reduce the visibility.
• Toxicity to plants : Most of the photochemical smog products are toxic to plants. Plant growth is inhibited by the prodcts from photochemical smog. This can effect the food production.
4. Depletion of ozone layer
• There is a layer of ozone in the stratosphere. The ozone layer prevents high energy UV light from reaching the troposphere.
• Some reactions involving O2(g) and O3(g) are;
a) The O2(g) is dissociated by solar UV radiations.
b) Some of the atomic oxygen (O) combines with dioxygen molecules to form trioxygen molecules (O3).
O (g) + O2(g) + M → O3(g) + M
c) O3(g) absorbs UV light with different frequencies and dissociates.
O3(g) → O2(g) + O(g)
d) O3 molecule reacts with O atom and forms O2 molecule.
O3(g) + O(g) → 2O2(g)
There is a natural balance which keeps the ozone layer at a constant thickness.
• Ozone is destroyed by reactions with and other free radicals. These radicals act as catalysts and destroy thousands of O3 molecules. The catalysed destruction of ozone in the stratosphere is known as ozone layer depletion.
• Chlorine radicals from the chlorofluorocarbon have been recognized as a major contribution to ozone layer depletion . This chloroflurocarbons are stable in the atmosphere but produce radicals in stratosphere with UV radiation.
• There is a strong connection between UV radiation and the cataract formation as well as incidence of both non-fatal and fatal skin cancer in humans. The ozone layer protects us.
To minimize the environmental and health effect of the above mentioned global issues, emission of pollutant gases has to be minimized. Such several remedial actions are given below.
• Minimization of fuel combustion
Motor vehicles, industries and routine household activities (such as cooking) releases large amount of carbon dioxide to the environment. Some of the releases can be controlled. For example, we can reduce drastically the number of vehicles on our roads. The development of an efficient public transport system involving electric trains and electric cars is an alternative.Using fuels of lower carbon to hydrogen ratio will minimize the CO2 in combustion. Use of the other energy sources such as nuclear and solar energy instead of fossil fuel is another option. Proper vehicle inspection and burning fuel only (without impurities) when necessary will help in this issue.
• Absorption of CO2 by trees
Carbon dioxide produced by the respiration of living organisms and the normal activities of man is fixed by the green plants during the photosynthesis. Photosynthetic organisms utilize solar radiation to convert carbon dioxide and water to carbohydrates using a chlorophyll catalyst.
6 CO2(g) + 6 H2O(l) → C6H12O6(s) + 6 O2(g)
• Here green plants in a way purify our air since oxygen is produced as a byproduct of
photosynthesis.
• Tropical rain forests are warm and humid. These conditions are ideal for photosynthesis.Their destruction is one of the factors that is causing an increase in atmospheric carbon dioxide level. Therefore, preservation of forest areas and planting are the best ways of controlling CO2 increase.
• Complete combustion
• Carbon monoxide is a major air pollutant which is formed due to incomplete combustion of fuels. The largest amount of CO comes from motor vehicle exhaust.
• The combustion of butane, for example requires 6.5 moles of oxygen per mole of hydrocarbon. If only six moles of oxygen are present one mole of CO will result.
2C4H10(g) + 13 O2(g) → 8 CO2(g) + 10 H2O(g)
C4H10 (g)+ 6 O2(g) → 3 CO2(g) + CO(g) + 5 H2O(g)
• Maintaining the air / fuel ratio(by mass) leads to a complete combustion.The equation for the complete combustion of octane is;
2 C8H18(l) + 25 O2(g) → 16 CO2(g) + 18 H2O(g)
From the stoichiometry of the equation it follows that;
(mass of air)/(mass of octane) = 14 : 7. This is called the air/fuel ratio
.
• A rich mixture [which is having more hydrocarbons (fuels) or having oxygen less thanthe stoichiometric proportion] gives an exhaust gas which is high in CO and partially combusted organic products. A lean mixture (with an excess of air or less fuel) gives an exhaust gas with less CO but has more oxides of nitrogen (NOx). The best way to control the emissions is “tuning up” (adjusting the air to fuel ratio for the optimum condition) and the use of a catalytic converter to convert the pollutants to harmless products.
• The control of emission from the internal combustion engine is the best hope of reducing CO level.
• Several soil microorganisms have enzymes which catalyze the oxidation and remove CO from the atmosphere.
• Both N and S will form different oxides and they are acidic in nature. Hence the burning of any material containing N or S in air can produce SO2 and NOx. Atmospheric N2 is not reactive due to a strong triple bond between two nitrogen
atoms. But, if the temperature is greater than 900 °C this bond can be cleaved forming NOx’s (NO and NO2). The burning temperature exceeds 900 °C in most of the combustions including internal combustion engines, burning cigarettes and in the cooking stoves. Also it happens naturally, with thundering and lightening. The best way to minimize the emissions of SO2 and NOx is lowering the temperature of the combustion process and reducing the burning of S and N containing material.
Following methods can also be used to reduce the release of acidic gases to the atmosphere.
• Absorption methods
Acidic gases can be neutralized by reacting with a base. We have enough natural bases such as limestone (CaCO3) and magnesium oxide (MgO) that can be used to remove (scrub) the acidic gases. The products that are formed can be converted to the valuable industrial chemical, sulphuric acid.
(i) Slurry of limestone and lime is used to “scrub” the fuel gases.
CaCO3(s) + SO2(g) → CaSO3(s) + CO2(g)
CaO(s) + SO2(g) → CaSO3(s)
2 CaSO3(s) + O2(g) + 2 H 2O(l) → 2 CaSO4.2H 2O(s)
(ii) Slurry of magnesium oxide is used as a scrubber.
MgO(s) + SO2(g) → MgSO3(s) →Δ MgO(s) + SO2(g)
The MgSO3 is heated to give MgO which is recycled and SO2 at a concentration high enough is used in the manufacture of sulphuric acid.
(iii) A solution of sodium sulphite can be used for scrubbing.
Na2SO3(s) + H2O(l) + SO2(g) → 2NaHSO3(aq)
The NaHSO3 produced can be heated to give Na2SO3 for recycling and SO2 can be sold to sulphuric acid manufacturers.
• Minimization of pollutant gases from car exhaust
The most significant pollutant gases in vehicle exhausts are CO, NOx and un-burnt or partially burned hydrocarbons. The partial combustion is due to lack of oxygen. This can be reduced by adjusting the air to fuel ratio as discussed above and this is known as “tuning up” of a vehicle. Toxic gases in automobile exhaust fumes could be controlled by installing catalytic converters along the exhaust pipes of the vehicles. An efficient catalytic converter should oxidize carbon monoxide and unburnt hydrocarbons to carbon dioxide and water and also reduce nitric oxide and nitrogen dioxide to nitrogen and oxygen. These oxidations and reductions are done at two stages on the catalytic surfaces of the catalytic converter which is fixed to the silencer (muffler) of a vehicle. Hot exhaust gases are fed through catalytic converter containing thin film of an inert metal such as platinum and transition metal oxides such as copper and chromium oxides.
2 NO(g) + 2 CO(g) → N2 (g) + 2 CO2(g)
2 CO(g) + O2(g) → 2 CO2(g)
C7H16(g) + 11 O2(g) → 7 CO2(g) + 8 H2O(l)
Three way catalytic converters (oxygen monitor fitted) transform harmful exhausts of CO,NOx and CxHy to relatively harmless N2, CO2 and H2O. The catalytic converters do not start working until the catalyst has reached a temperature about 200 °C. So they are not effective until the engine has warmed up.
