Primary air pollutants and their sources

What are primary air pollutants (PAPs)?

Primary air pollutants (PAPs) are those that are emitted directly into the air from sources. Examples – Sulphur dioxide, nitrogen oxides, carbon monoxide, volatile organic matter, particulate matters, lead, ammonia etc.

How PAPs impact environment?

They can have effects both directly and as precursors of secondary air pollutants (chemicals formed through reactions in the atmosphere), which are discussed in the following section.

What are secondary air pollutants?

A secondary pollutant is not directly emitted as such, but forms when other pollutants (primary pollutants) react in the atmosphere.

Examples of a secondary pollutant include ozone, which is formed when hydrocarbons (HC) and nitrogen oxides (NOx) combine in the presence of sunlight; NO2, which is formed as NO combines with oxygen in the air; and acid rain, which is formed when sulfur dioxide or nitrogen oxides react with water.

We will take up Secondary air pollutants is the next post. Now let us read about some common primary air pollutants.


1. Sulfur dioxide (SO2)

SO2 is a gas formed when sulfur is exposed to oxygen at high temperatures during fossil fuel combustion, oil refining, or metal smelting. SO2 is toxic at high concentrations, but its principal air pollution effects are associated with the formation of acid rain and aerosols.

SO2 dissolves in cloud droplets and oxidizes to form sulfuric acid (H2SO4), which can fall to Earth as acid rain or snow or form sulfate aerosol particles in the atmosphere.


2. Nitrogen oxides (NO and NO2, referred together as NOx)

NOx are highly reactive gases formed when oxygen and nitrogen react at high temperatures during combustion or lightning strikes. Nitrogen present in fuel can also be emitted as NOx during combustion. Emissions are dominated by fossil fuel combustion at northern mid-latitudes and by biomass burning in the tropics.

In the atmosphere NOx reacts with volatile organic compounds (VOCs) and carbon monoxide to produce ground-level ozone through a complicated chain reaction mechanism. It is eventually oxidized to nitric acid (HNO3). Like sulfuric acid, nitric acid contributes to acid deposition and to aerosol formation.


3. Carbon monoxide (CO)

CO is an odorless, colorless gas formed by incomplete combustion of carbon in fuel. The main source is motor vehicle exhaust, along with industrial processes and biomass burning.

Carbon monoxide binds to hemoglobin in red blood cells, reducing their ability to transport and release oxygen throughout the body. Low exposures can aggravate cardiac ailments, while high exposures cause central nervous system impairment or death.

It also plays a role in the generation of ground-level ozone.


4. Volatile organic compounds (VOCs)

VOCs including hydrocarbons (CxHy) but also other organic chemicals are emitted from a very wide range of sources, including fossil fuel combustion, industrial activities, and natural emissions from vegetation and fires. Some anthropogenic VOCs such as benzene are known carcinogens.

VOCs are also of interest as chemical precursors of ground-level ozone and aerosols. Large VOCs oxidize in the atmosphere to produce nonvolatile chemicals that condense to form aerosols. Short-lived VOCs interact with NOx to produce high ground-level ozone in polluted environments. Methane (CH4), the simplest and most long-lived VOC, is of importance both as a greenhouse gas and as a source of background tropospheric ozone. Major anthropogenic sources of methane include natural gas production and use, coal mining, livestock, and rice paddies.


5. Particulate Matter

Particle pollution (also called particulate matter or PM) is the term for a mixture of solid particles and liquid droplets found in the air. Some particles, such as dust, dirt, soot, or smoke, are large or dark enough to be seen with the naked eye. Others are so small they can only be detected using an electron microscope.
Particle pollution includes “inhalable coarse particles,” with diameters larger than 2.5 micrometers and smaller than 10 micrometers and “fine particles,” with diameters that are 2.5 micrometers and smaller.

These particles come in many sizes and shapes and can be made up of hundreds of different chemicals. Some particles, known as primary particles are emitted directly from a source, such as construction sites, unpaved roads, fields, smokestacks or fires. Others form in complicated reactions in the atmosphere of chemicals such as sulfur dioxides and nitrogen oxides that are emitted from power plants, industries and automobiles. These particles, known as secondary particles, make up most of the fine particle pollution in the country.


6. Lead

In the past, motor vehicles were the major contributor of lead emissions to the air. As a result of regulatory efforts to reduce lead in on-road motor vehicle gasoline, air emissions of lead from the transportation sector, and particularly the automotive sector, have greatly declined over the past two decades.

Major sources of lead emissions to the air today are ore and metals processing and piston-engine aircraft operating on leaded aviation gasoline. The highest air concentrations of lead are usually found near lead smelters. Other stationary sources are waste incinerators, utilities, paints and lead-acid battery manufacturers.


7. Ammonia

Ammonia is a colorless, pungent, hazardous caustic gas composed of nitrogen and hydrogen. Though ammonia is used for different applications in many sectors, agriculture is its largest consumer and producer. Livestock farming, animal waste and fertilizer application are the biggest sources of atmospheric ammonia emissions within the agricultural sector.

Gaseous ammonia is a dangerous air pollutant. Breathing in large amounts can cause death.


Natural Sources of air pollution


Apart from the anthropogenic (man-made) sources, there are also natural sources of air pollution.The natural sources of air pollution emissions include:

1. Volcanoes: Volcanic activity produces smoke, ash, carbon dioxide, sulfur dioxide and other air pollutants.

2. Geysers: The air pollutants emitted by geysers include hydrogen sulfide, arsenic and other heavy metals.

3. Digestive gases: Methane and other gases generated by the digestion of food and emitted by animals such as cattle.

4. Oceans, Rivers and Estuaries: These are sources of methane emissions thought to be caused by the digestive systems of marine life, methanogenesis in sediments and drainage areas along coastal regions, and possibly seepage from methane hydrates on the ocean floors.

5. Dust: Windblown dust from areas with little or no vegetation such as desert areas.

6. Sea salt: Wind-blown sea water which evaporates in the atmosphere and releases sodium chloride and other particulates into the atmosphere.

7. Radioactive decay: Radon gas is released into the atmosphere by radioactive decay occurring in the Earth’s crust.

8. Forest fires: Forest fires created by lightning, or other natural causes, result in the formation and release of smoke, ash, dust, carbon dioxide, nitrogen oxides and other air pollutants.

9. Plants and trees: Biogenic sources such as pine trees and certain other plants and trees which release volatile organic compounds (VOC). About 80% of the overall emissions of VOC are from biogenic sources.

10. Wetlands: Microbial action in wetlands result in significant amounts of methane being formed and released to the atmosphere. In fact, wetlands are the largest natural source of methane emissions.

11. Termites: Termites are the second largest natural source of methane emissions. The methane is produced by their normal digestive process.

12. Lightning: Lightning converts atmospheric nitrogen to nitrogen oxides.

13. Soil outgassing: Another biogenic source wherein microbial action in soils result in the formation and release of significant amounts of nitrogen oxides.


How UPSC may ask questions on these?


1. Identifying the type of pollutant – primary or secondary; particulate or gaseous


Which of the following is/are example of particulate air pollutants?

1. aldehydes
2. chlorine
3. soot
Select the correct answer using the codes given below.
a) 1 and 3 only
b) 3 only
c) 2 and 3 only
d) none                            (from our GS Prelims Test Series 2015)


Which of the following is/are primary air pollutant?
1. ground level ozone
2. photochemical smog
3. sulphur dioxide
Select the correct answer using the codes given below.
a) 1 and 2 only
b) 1 and 3 only
c) 3 only
d) 2 and 3 only   (from our GS Prelims Test Series 2015)


2. Identifying the sources of a particular pollutant


Consider the following: (asked in UPSC Prelims 2011)

1. Photosynthesis.
2. Respiration.
3. Decay of organic matter.
4. Volcanic action.
Which of the above add carbon dioxide to the carbon cycle on earth?
a) 1 and 4 only
b) 2 and 3 only
c) 2, 3 and 4 only
d) 1, 2,3 and 4


Lead ingested or inhaled, is a health hazard. After the addition of lead to petrol has been banned, what still are the sources of lead poisoning? (asked in UPSC Prelims 2012)

1. Smelting units
2. Pens and pencils
3. Paints
4. Hair oils and cosmetics
Select the correct answer using the codes given below:
a) 1,2 & 3 only
b) 1 and 3 only
c) 2 & 4 only
d) 1,2,3 & 4


Carbon monoxide is produced from

1. incomplete burning of fossil fuels.
2. enteric fermentation in cattle.
3. oil refineries.
4. automobiles.
Select the correct answer using the codes given below.
a) 1, 2 and 3 only
b) 1 ,3 and 4 only
c) 2 ,3 and 4 only
d) 1 ,2, and 4 only (from our GS Prelims Test Series 2015)


3. Identifying which pollutants are emitted from a particular source


Consider the following: (asked in UPSC Prelims 2011)

1. Carbon dioxide
2. Oxides of nitrogen
3. Oxides of Sulphur
Which of the above is/are the emission/emissions from coal combustion at thermal power plants?
a) 1 only
b) 2 and 3 only
c) 1 and 3 only
d) 1,2,and 3


Which of the following are some important pollutants released by steel industry in India?

1. Oxides of sulphur
2. Oxides of nitrogen
3. Carbon monoxide
4. Carbon dioxide
Select the correct answer using the code given below.
a) 1, 3 and 4 only
b) 2 and 3 only
c) 1 and 4 only
d) 1, 2, 3 and 4  (asked in UPSC Prelims 2014)


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Please read these important articles about prelims preparation

1. How UPSC asks current affairs in GS Prelims

2. How to practice prelims MCQs for UPSC

3. Applying logic in UPSC General Studies Prelims




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