2.1 Carbon Monoxide (CO):

As the first pollutant we will discuss about carbon monoxide (CO). It is a colourless, odourless, tasteless but poisonous gas (b.p. of liquid CO is -192°C). It is as heavier than air (96.5%) and is water insoluble. It plays key role in Green House Effects (GHEs).¹⁰ The basic chemical reactions for yielding and conversion of CO are:

1. Incomplete combustion of fuel or carbon-containing compounds yields CO as follows:

2C + O₂ → 2CO

2. Reaction between CO₂ and carbon-containing materials at elevated temperature
processes, e.g., in blast furnaces follows as below:

CO₂+ C → 2CO

3. Dissociation of CO₂ at high temperatures occurs as follows:

2CO₂ → 2CO + O₂

4. Conversion of CO into CO₂occurs as follows:

2CO+ O₂ → 2CO₂

2.1.1 Sources and Sinks of CO Pollution:

Natural processes, such as volcanic eruption followed by emission of gases, hurricanes’ electrical discharge, seed germination, and marsh gas production during rice cultivation and other agricultural activities, etc. contribute in a small but impactfully with respec to the presence of CO in the atmosphere. However, the significant contribution is from human activities such as out of 350 million tones of annual emission human source contributes 275 and natural source contributes 75 million tones on a global scale. Out of which the USA alone releases more than 100 million tones of CO into the atmosphere as follows: (a) Transportation contributes about 64% of CO-motor vehicles 59.2%, aircrafts 2.4%; and railroads 0.1 %. (b) Next in magnitude is miscellaneous sources, 16.9% the main components are forest fires 7.2% and agricultural burning 8.3%. Agricultural burning implies controlled burning of forest debris, crop residues, brush, weeds and other vegetation. (c) Industrial processes, mainly iron and steel industries and petroleum and paper industries, constitute the third largest contributor of CO (9.6%) to the air. The atmospheric background of CO is 0.1 ppm.^11,12

Sinks-It was found that in every half decade, the annual input concentration of CO in the ambient (surrounding) atmosphere by human activities is expected to be doubled. But the actual increase ambient global CO concentration is much less. The major CO sink is some soil micro-organisms. These include some fungi-Penicillium and Aspergillus. A potting soil sample weighing 2.8kg completely removed, in three hours, 120 ppm CO from ambient air. The same soil sample, when sterilized, failed to remove CO from air. Of 200micro-organisms isolated from the soil and cultured, 16 fungi were found to be active in the removal of CO.^13,14

2.1.2 Concentration Profile:

The soil sinks can take care of atmospheric CO, but it still exists in significant concentrations in the atmosphere. This is because neither CO nor the soil sink is distributed uniformly. As a matter of fact, the largest CO producing areas often have the least amount of soil sink available. The automobile is found to be the biggest contributor of CO pollutant (59.2%), especially in cities.¹⁵

2.1.3 Control of CO Pollution:

The transportation is found responsible for 74% of all CO eruptions and gasoline-fed internal combustions and is primarily accountable for it. Hence, the considerable efforts for control of CO pollution are mainly directed at automobiles. The internal combustion engine emissions have been found to be consisting of CO, NO_x, hydrocarbons and particulate matter. When one of these pollutants is subjected to control measures, the amounts of other pollutants are affected at the same time.^16,17

It follows that a low air-fuel ratio reduces NO_x emission remarkably, but increases CO and HC emissions. The stoichiometric ratio, as indicated by the dashed line, is the ratio at which there is just the right proportion of oxygen in the fuel for complete oxidation of C and H₂ into CO₂ and Hp. Automobile carburettors are adjusted to the fuel-rich phase of the stoichiometric ratio since the engine lends to stall on the rear phase (Lean fuel).^16,17

The fourth possible solution to automotive emission problem is alternate power sources, e.g. steam, electric and gas turbine engines. But none of these are economically viable as compared to the third possible approach to CO-pollution problem is a substitute fuel for gasoline. Autogas in both compressed (CNG) and liquefied forms (LNG) has been used as fuel. Although it is an attractive pollution-free fuel, there are problems of steady supply and economic storage. Alcohols are other substitutes, but their combustion products, aldehydes, are eye irritants. The fourth possible solution to automotive emission problem is alternate power sources, for instance, steam, electric and gas turbine engines. But none of these are economically viable as compared to gasoline.^16,17

2.2 Nitrogen Oxides (NO_x):

Nitorgen oxides (NO_x) contain atmospheric gases such as nitric oxide (NO) and nitrogen dioxide (NO₂) which primarily contribute to the air pollution. Out of these two, NO is a colourless, odourless gas, but NO₂has a reddish-brown colour and pungent smell.¹⁸

The formation of NO and NO₂(temperature 1210-1763 °C) follows the reactions as:

N₂+ O₂ → 2NO

2NO+ O₂ → 2NO₂

The NO is formed favorably at high temperatures, in general, yielded during many air-based-combustion processes (1210 to 1763°C). Rapid cooling of these combusted products prevents dissociation of NO-this is characteristic of combustion processes.

At temperature about 1100°C the second reaction is also favoured, but the amount ofNO₂ formed is usually less than 0.5% of the total NO_x at 1100°C. Photolytic
reaction is also being followed in yielding NO₂. The hydrocarbons disrupts this photolytic cycle and gives rise to photochemical smog, which is discussed in a subsequent section.¹⁹

2.2.1 Sources and Sinks of NO_x:

Bacterial processes occur in nature yield about 5 × 10³tonnes of NO_x per year in the world, exclusively in the form of NO, while man-made sources annually yield 5 × 10⁷tonnes of NO_x. Uniform global distribution of NO_xwas found for natural yields but for man-made sources the distribution varies with type of areas i.e. urban/rural areas. Specifically, we can say that the urban areas yield around 10 to 100 times bigger NO_xthan in rural areas. Thus, in USA and Canada, the concentrations of NO and NO₂are 2 ppb (parts per billion) and 4 ppb, respectively, from natural sources, but the same in urban areas it may reach 500ppb (0.5ppm).^20,21 In the USA, it is estimated that out of an annual input of NO_xof 10 million tonnes from stationary combustion sources, power plants contribute 4 million, industries 4.8million and domestic heating plants 1.2 million tonnes per year. On an average NO and NO₂resides about 4 days and 3 days in the atmosphere. Within this duration NO_xare being converted into HNO₃ which is precipitated as nitrate salts in either rainfall or as dust naturally by following photochemical reactions.^20,21 The formation of HNO₃follows the mechanism as shown below:

1. Before daylight, NO and NO₂ levels remain fairly stable at concentrations slightly higher the daily minimum.

2. As the traffic rush begins and increases (6 to 8 A.M.), the level of NO increases and become maximum.

3. At mid-morning, with increased ultraviolet light, the NO₂ level increases (9 to 10 A.M.) due to conversion of NO into NO₂.

4. O₃ builds up as NO levels drop below 0.1 ppm.

5. In the evening (5 to 8 P.M.), the NO level again goes up during the evening traffic rush.

6. O₃accumulated during daytime reacts with NO during night with the result that NO concentration goes slightly up and O₃ level drops.

Continuous monitoring in busy cities with heavy automobile traffic has revealed that the maximum values of NO and NO₂ levels are 1-2 and 0.5 ppm, respectively.

2.2.2 Control of NO_x Pollution:

In order to control automotive emissions catalytic converters can be utilized^22,23 for removing NO_x in the first stage, as described in preceding sections:

Plants for power generation emit about 50 to 1000ppm of NO_x. Such emission can be reduced by 90% by two-stage combustion process:

1. The fuel (coal/oil/gas) is fired at a relatively high-temperature with a sub stoichiometric amount of air, say 90-95% of the stoichiometric requirement. However, limited yield of NO is observed in the absence of excess O₂.

2. However, in excess air fuel burnout is completed even at a relatively low-temperature. Under this condition O₂is not formed.

A possible approach to NO_x removal from stack gas is the chemical aborption process using H₂SO₄ solution or alkaline scrubbing solution containing Ca(OH)₂ and Mg(OH)₂ NO is converted into N₂O₃ which is easily absorbed. Then NO₂ is recycled.

NO₂ + NO → N₂O₃

Four steps are involved in this scrubbing process: (i) Flue gas and NO₂ are introduced into an oxidizer. Gases NO and NO₂ react to form N₂O₃ which is scrubbed by H₂SO₄ in a scrubber. The cleaned flue gas is released into the atmosphere

N₂O₃ + 2H₂SO₄ → 2NOHSO₄ + H₂O

The reaction product from the scrubber is next decomposed in a decomposer and the resulting H₂SO₄ recycled to the scrubber:

2NOHSO₄ + H₂O → 2H₂SO₄ + NO₂ + NO

(ii) NO₂ produces HNO₃ in the HNO₃ reactor:

3NO₂ + H₂O → 2HNO₃ + NO

and excess NO₂ and NO are recirculated through the oxidiser in step (i).

About 85 million tonnes of CH₄has been generated each year just from domesticated animals and resides about 3 to 7 years in the atmosphere. Anthropogenic sources (human activities), majorly automobile sectors, are found responsible for contribution of almost 15% of the hydrocarbons per year. The breakage of about 570 × 10⁶tonnes per year of global hydrocarbon emissions is as follows: petroleum 55%; coal 3.3%; wood 2.2%; burning 28.3%; solvent evaporation 11.3%. High populated area is responsible for high human activities and hence high emission of hydrocarbons is being recorded and damages human beings and plants at a maximum level. For instance, an area of heavy vehicular traffic such as Los Angeles city of the USA was researched for hydrocarbon pollutants and resulted that about twenty different hydrocarbons were present contributed with ethane and ethylene (102 ppb total), acetylene and propane (76 ppb total), n-butane (46 ppb), isopentane (35 ppb), toluene (30 ppb), n-pentane (21 ppb), m-xylene (12 ppb) and isobutane (12 ppb).

In order to remove hydrocarbons from the atmosphere several chemical and photochemical processes have been followed. Mostly, oxidation processes can be used in a series of steps to convert these hydrocarbons into CO₂and solid organic particulate matter as end products that settle from the atmosphere or water-soluble products, e.g., acids and aldehydes, which are washed by rain.

2.3 Photochemical Smog:

Several photochemical reactions yield those products which are found accountable for most of the harmful impacts of hydrocarbon pollution. Generally, hydrocarbons react photochemically produced substances but not to the sunlight. We can understand this by the fact that photochemical oxidants in the atmosphere are formed via exposure of large quantities of automobile exhausts loaded in the atmosphere and trapped in an inversion layer (stagnant air masses) to intense sunlight. This phenomenon is called as photochemical smog which is an oxidizing smog having a high concentration of oxidants and hence it is different from normal smog, a combination of smoke and fog, which is chemically a reducing smog. Photochemical smog appears as brown and hazy fumes, irritates the eyes and lungs, and cracks rubber and damages plant life, extensively. Maximum hydrocarbon level was noticed during early morning traffic rush hours followed by decrement in it during the remaining daylight hours as it is consumed in the smog-formation reactions. For instance, nitric oxide concentration peaked at the same time and then falls as NO₂ concentration increases. Subsequently, concentrations of oxidants (aldehyde, PAN) increaseand contribute to the irritating ingredients of smog.^24,25

2.3.1 Control of Hydrocarbons and Photochemical Pollutants:

Carbonyl compounds, O₃ and PAN are secondary pollutants, so their control ultimately depends on the control of their primary precursors, hydrocarbons and nitrogen oxides. The control of NO_x has been described earlier and that of hydrocarbons has been discussed in connection with the control of auto-exhaust emissions.²⁶

2.4 Sulphur Dioxide, SO₂:

Combustion of any sulphur-bearing material produces sulphur dioxide, SO₂ (accompanied by a little SO), which is a colourless gas with a pungent odour. This mixture is denoted by SO_x and found responsible for acid rain (droplets of H₂SO₄) through reaction with water vapourunder normal humid conditions of the atmosphere. Under the influence of photolytic and catalytic processes involving O₃, hydrocarbons and NO, in photochemical smog SO₂ is partially oxidised to SO.²⁷

2.4.1 Control of SO_xPollution:

The major anthropogenic sources of SO_x pollution are the power plants and that’s why it constitutes with tall smokestacks which are utilized to disperse the plume over a wide area. It may reduce the local problem but often create problems in areas far from the power plants.²⁸

The four plausible approaches to control the SO_x emission can be summarized as:

1. By controlling emission of SOx from fuel gases.

2. By controlling emission of sulphur from fuel burning.

3. By using low-Sulphur content based fuels.

4. By using alternative energy sources for fuel combustion.

5. The method appears economical, but is not favoured by power plant official since the primary drawback is that CaSO₃ in large amount poses a waste-disposal problem.

6. An alternative process is based on a reaction between HSO₃ ions (from SO₂) and citrate ions.

7. The flue gas is cooled to 50°C or lower and freed from particulates and traces of H₂SO₄, It is then led into an absorption tower followed by reaction with a solution containing citrate ions (H₂Cit).

3. Consequences: Impacts of Polluted Air:

3.1 Health Impacts:

As per the WHO reports several major health conditions such as respiratory infections, heart disease, and lung cancer and minor health conditions such as breathing, wheezing, coughing, asthma and aggravation of existing respiratory and cardiac conditions are caused by air pollution. Eventually, it leads to increment in the usage of medication, visitation of doctors or emergency rooms, hospital admissions and premature deaths. In this vigorous condition, the degree of exposure to these air pollutants, the individual's health status and genetics may add to the fuel. Thus, in other words we may say that it impacts our respiratory system and the cardiovascular system.²⁹

The air pollution is constituted of several gases such as ozone, nitrogen dioxide, and sulfur dioxide as well as particulates. The indoor air pollution also plays part along with outdoor pollution in approximately 3.3 million causalities in the whole world. When we look into the age-wise category we came to know that the newborn to preschooler children of developing countries are the most vulnerable ones because of high exposure to air pollutants which lead to increased risk of attack of asthma, pneumonia and other lower respiratory infections. As per WHO report 2.4 million people die each year due to air pollution out of which 1.5 million deaths are attributed to the indoor air pollution.³⁰

If we talk about worst short term civilian air pollution crisis, we may begin with the example of year1984 Bhopal Disaster (India), Union Carbide factory leaking (the USA), year 1952 Great Smog (London, U. K.), year 1979 leak of anthrax spores from a biological warfare laboratory (Sverdlovsk, USSR) which lead to 4,000 to 25,000 death outright and injury anywhere from 150,000 to 600,000.³⁰

3.2 Environmental Impacts:

The components of polluted air produce acid rain and increase the ground level ozone which eventually destroys plants, trees, crops, farms, animals along with the water bodies.

3.3 Economical Impacts:

The polluted air also affects our economy. In simple language, we can understand that the economy of a nation boots when its citizen are found to be healthy. However, business of a nation also depends on cultivated raw materials and natural resources which directly depend upon the environmental air’s cleanliness. By reducing air pollution we may be able to improve the decrement in the agricultural crop and commercial forest yields and that will directly improve our economy by billions of money each year.

3.4 Polluted Air’s Control:

Polluted air can be controlled by reducing pollution part from polluted air but it is not an easy task to do and that’s why prevention protocols are always a better way to be implemented for the purpose. These controllable can be incorporated either through incorporation of government (laws) or by implying individual actions. Nowadays, almost all the big cities have installed air pollution measuring equipments at many points so that authorities read them regularly to check the air quality index.

3.5 Government (or community) law based preventive measures:

· In order to reduce our dependency on fossil fuels which cause heavy air pollution, most of the developing and developed authorities around the world have already introduced utilization of green energy such as wind energy and solar energy, as well as other renewable energy.

· Another step is taken by most of the governments by implementing strict laws for industries for their manufacturing works so that even polluted air can be controlled to a large extent.

3.6 Individual Level Preventive Measures:

· By encouraging people to use public transport such as bus, taxi, metro, local train, etc. for commuting so that there will be fewer vehicles on road and lesser polluted air can be observed in terms of fumes.

· By saving energy with respect to electricity or electric equipments we can reduce the electricity demand which is found responsible for burning of fossil fuels.

· By recycling and re-using the things, we can minimize the production of new things. Remember manufacturing industries create a lot of air pollution, so if we can re-use things such as plastic bags, clothing and bottles, it can help.

· Under corporate-social responsibility companies are taking several steps to support the programs organized for controlling air pollution.

3.7 Control devices:

Several items can be utilized to control polluted air by destroying or removing contaminants using exhaust prior to its emission into the atmosphere and hence called as control devices for air pollution. These apparatuses are being utilized by industries or transportation devices.

· Mechanical collectors: dust collectors or multi-cyclone separator can be used for paper and pulp industry, food processing industry, copper recycling plant, etc.

· Electrostatic cleaners or precipitators: It is used to remove fine particulates such as dust and smoke from a blowing gas (such as airstream particles) with the help of an induced electric charge.

· Bag houses: For removing load of dust particles, bas houses consisting of a blower, dust filter, a filter-cleaning system, and a dust remover can be used which is quite different from other air cleaners which utilize disposable filters.

· Particulate scrubbers: In this type of polluted air control apparatus wet scrubber is used in which gas pollutants are cleaned by passing the polluted gas stream through the scrubbing liquid.

4. CONCLUSION:

In this review we have focused on the elaboration of how polluted air affects our health and how it can be prevented and/or controlled. To large extent, if individuals and businesses stop using toxic substances that cause air pollution in the first place we can get success in it. This would require the cessation of all fossil fuel-burning processes, from industrial manufacturing to home use of air conditioners. This is an unlikely scenario at this time. However, stringent regulations need to be set for manufacturing and energy industries so that harmful emissions into the Earth's atmosphere can be reduced.

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Received on 08.07.2024 Revised on 17.08.2024

Accepted on 22.09.2024 Published on 22.10.2024

Available online from October 31, 2024

Asian J. Research Chem.2024; 17(5):271-277.

DOI: 10.52711/0974-4150.2024.00047

Pollutant	Description	Sources	Health Impacts	Environmental Effects
Carbon Monoxide (CO)	Colorless, odorless gas	Motor vehicle exhaust, indoor sources include kerosene stoves.	Headaches reduced mental alertness, heart attack, cardiovascular diseases, impaired fetal augmentation and casualty.	Contribute to the formation of smog.
Sulfur Dioxide (SO₂)	Colorless gas, after dissolving in water yields acid which interacts with other gases and particles.	Coal-based power plants, petroleum refineries, preparation of sulfuric acid and smelting of sulfur-based ores.	Eye-issues, asthmatic issues, chest tightness, lung damage.	Responsible for the acid rain, visibility impairment, aesthetic issue.
Nitrogen Dioxide (NO₂)	Reddish brown, highly reactive gas.	Vehicles, electric equipments and other sources of fuel burning.	Respiratory-issue (e.g. cough, chest-pain, difficulty breathing).	Responsible for smog, acid rain, water quality deterioration, global warming, and visibility impairment.
Ozone (O₃)	Gaseous pollutant, formed in the troposphere.	Vehicle exhaust and certain other fumes. Other air pollutants in sunlight.	Eye and throat issues, coughing, respiratory tract problems, asthma, lung damage.	Plant and ecosystem damage

1. INTRODUCTION: