Air Pollution System

Emissions

There are two types of sources of emissions,natural and anthropogenic.

A.Natural Emissions

There are many natural biogenic and geogenic sources of primary pollutants known.A wide variety of parameters such as temperature,sunlight,etc. can alter the emission rates.These emissions are significant in compression to anthropogenic emissions on global scale.They generally do not play role in the atmospheric chemistry of polluted urban areas.Important primary pollutants and the sources of the natural emission are summarized below:

i)Sulphur compounds

Sulphur compounds(e.g.SO₂,H₂S,dimethyl sulphide,CS₂,etc.)are emitted to air from volcanic eruptions,sea spray and biogenic processes.

ii)Particles

Natural sources of particles are mainly terrestrial dust caused by the winds,sea spray,biogenic emissions,volcanic eruptions and windfires.

iii)Nitrogen oxides(NO_x)

The major natural sources of emission for NO_x are lightning,stratoscopic injection,ammonia oxidation,burning of biomass including wild fires and soil emission.

iv)Non-methane hydrocarbons(NHMC)

The major sources for non-methane hydrocarbons such as isoprene and α-pinene is the biogenic process.The magnitude and chemical composition of these emission varies largely.

v)Carbonmonoxide(CO)

The major natural sources of CO emission are:CH₄ oxidation,oxidation of natural hydrocarboons,microbial activity in ocean and emission from plants.

B.Anthropogenic Emissions

i)Sulphur compounds

The most important pollutant in this category is SO₂.The major sources of SO₂ is the combustion of sulphur containing fuels.On global scale about 60% of the SO₂ produced is from coal combustion and 30% from petroleum refining.

ii)Particles

Most of the anthropogenis emission of particles occur from:

a)stationary fuel combustion

b)industrial process

c)solid waste disposal

d)transportation

e)other emissions(forest fire,agricultural burning,coal burning,etc.)

iii)Nitrogen oxides(NO_x)

High temperature combustion processes produces NO from the reaction of N₂ and O₂.

$$N_2+O_2 \rightarrow 2NO$$

This may also be produced from the oxidation of N₂ in the fuel.Small amounts of NO₂ are produced by further oxidation of NO.The two major sources of NO_x emissions are fossil fuel combustion and biomass burning.

iv)Non methane hydrocarbons(NHMC)

Transportation,petroleum refining and gas production accounts for more than 50% of NHMC.Use of organic solvents in the industries accounts for another 25% of total emission.

v)Carbob monoxide(CO)

CO is produced due to incomplete combustion of fossil fuel in the urban area.The automobile exhaust is another major sources of CO emission.

vi)Lead

Gasoline combustion in the motor vehicles was the greatest source of lead emission.Lead which was used as ant knocking agent as TEL(Tetraethyl lead) got emitted in the form of particles and organ lead compounds.Nowadays,lead free fuel is available which uses BTX(Benzene-Toulene-Xylene) mixture as anti-knocking agent.

Hence,there are five primary pollutants which together contribute more than 90% of global air pollution.They are as follows:

i)Carbon oxides,CO & CO₂

ii)Nitrogen oxides(NO_x)

iii)Sulphur oxides(SO_x)

iv)Hydrocarbons(HC)

v)Particulates

Some primary pollutants may react with one another or with the basic components of air to form new pollutants called the Secondary pollutants.Eg,HNO₃,H₂SO₄,H₂O₂,O₃,NO₃^-,SO₄^-- salts.Long lived pollutants travel for before and they return to the earth as particles,droplets or chemical dissolved in precipitation.The primary and secondary pollutant composes ambient air.

Carbonmonoxide as Pollutant in Air

CO is a colourless,odourless and tasteless gas.It is heavier than air and has low solubility in water.It has been found in the samples from trace amounts to 0.5 ppm.The global concentration average around 0.1 ppm.

Origin and sources of CO

The CO is formed due to the following three proceses.

i)Incomplete combustion of fuel or carbon containing compound

$$2C+O_2 \rightarrow 2CO$$

ii)Reaction between CO₂ and carbon containing material at elevated temperature in the industrial process.e.g.in the blast furnace.

$$CO_2+C \rightarrow 2CO$$

iii)Dissociation of CO₂ at elevated temperature

$$CO_2 \rightleftharpoons CO+O$$

Many sources of CO production are known today.They have been divided into the two classes.

A.Natural sources

Many geophysical and biological processes are natural sources of CO.The processes include volcanic eruptions,natural gas emission,electrical discharge during stroms,seed germination and seeding growth and marsh gas production.The contribution of CO is small from these natural sources.Forest fire comtributes significantly.

B.Artificial sources

major sources of CO emission is incomplete combustion of fossil fuels.All combustion process gives off CO:Stoves,furnaces,open fires,brush fires,burning of agricultural and other solid waste,awste disposal,factories,power plants,automobile exhaust,etc.The industrial processes are also contributing significantly to CO concentration in the atmosphere.

The CO Sink Animaly

An interesting explaination has been put forward for CI sink anomaly according to which oxidation of CO to CO₂ near the surface of the soil is occurring with the help of microorganisms present in the soil.It has been found that air containing nearly 125 ppm of CO was completely freed of CO in only 3 hour duration if brought in contact with 3 kg of soil.There are 16 different Fungi known present in the soil those are capable of removing CO from atmosphere.Hence,the mechanism by which CO is removed from the lower atmosphere involved a natural sink that is dependent upon particular soil micro-organism for its operation.

Effect of CO on humans

Carbon monoxide is a non-irritating gas.Mostly it is referred silent killer,as it is highly toxic.it is hazardous to human beings.The physiological mode of action of CO toxic effect have been demonstrated in terms of ability of this gas to bind to haemoglobin.The product formed is a carboxyhaemoglobin(Hb.CO).This reaction completes to the formation of oxyhaemoglobin necessary for oxygen transport by blood to the site where it is required inside the cell.

$$Hb+OC \rightarrow Hb.O_2$$

$$Hb+CO \rightarrow Hb.CO$$

$$Hb.O_2+CO \rightarrow Hb.CO+O_2$$

Haemioglobin has 204 times the affinity for CO as for O₂,with a result,the capacity of haemoglobin to carry oxygen decreases.The presence of CO thus reduce the concentration of haemoglobin available in the blood to carry oxygen to body cells and with less O₂ normal metabolism is impaired and the ability to perform mental task is hindered when the level of Hb.CO in blood is about 2%.Blood concentration of Hb.CO greater than 10% cause muscular weakness,respiratory ailments,dizziness and cause damage to heart.

Prevention of CO pollution(Control of CO emission)

There are four main ways to control CO emission.They are as follows:

i)Modification of internal combustion engines for reducing the amount of pollutants formed during fuel combustion.

ii)Development of exhaust system reactors which will complete the combustion process and emit CO₂,a more acceptable material into atmosphere.

iii)Development of substitute fuels for gasoline which produce low concentration of pollutants upon combustion.

iv)Development of new types of engines to replace combustion engine.

References

Poudel, Bhoj Raj Poudel and Dr.Megh Raj. A Text Book Of Inorganic Chemistry. Kathmandu: National Book Centre, 2012.

Sons, John Wiley and. J.D Lee Concise Inorganic Chemistry. 2007.