30 Primary and Secondary Air Pollutants

Amrita Daripa

epgp books

1. Learning outcomes
2. Introduction
2.1. Definition
2.2. Classification of air pollutants
2.3. Major sources of air pollutants
2.4. The different sources of primary and secondary air pollutants and their sinks
3. Formation of primary and secondary pollutants in the troposphere
4. Smog
4.1. London type and Los Angeles (LA) type smog
4.2. Photochemical smog and their formations in the troposphere
5. Acid rain/ acid deposition
5.1. Sources of acid precipitation
5.2. pH scale and acid rain
6. Harmful effects of air pollutants on environment
6.1. Effect of carbon monoxide (CO)
6.2. Effect of sulphur dioxide (SO2)
6.3. Effect of oxides of nitrogen (NOx)
6.4. Effect of photochemical smog
6.5. Effect of acid rain
6.6. Effect of benzene
6.7. Effect of benzo (a) pyrene
6.8. Effect of ammonia (NH3)
6.9. Effect of arsenic (As)
6.10. Effect of lead (Pb)
6.11. Effect of nickel (Ni)
7. Monitoring primary and secondary air pollutants
8. Mitigation measures for combating air pollution
8.1. Source control
8.2. Control measures at industrial centers
8.3. Equipment used to control air pollution
9. Summary

1. Learning Outcomes

 

  • After studying this module, you shall be able to:
  • Understand air pollution, its types and sources
  • Know formation of primary and secondary air pollutants through natural and human interferences Know the harmful effects of air pollutants on the environment and life forms
  • Know the various methods of monitoring air pollutants
  • Know the mitigation measures for combating air pollution

 

2. Introduction

 

2.1. Definition

 

Air pollution means any solid, liquid or gaseous substance present in ambient air in such concentrations that may tend to be injurious to human beings and other living creatures, plant, property or enjoyment (Indian air amendment act, 1987).

 

2.2. Classification of air pollutants

 

a) Primary pollutants- Pollutants are directly emitted from the source. For example, Sulfur dioxide (SO2) emission from thermal power plant; Nitric oxide (NO) and nitrogen dioxide (NO2) emission from automobiles.

bSecondary pollutants- When primary pollutants are emitted from the source, they undergo various physical processes and chemical reactions in the atmosphere and form secondary pollutants. For example, Ozone (O3), Acid rain (H2SO4, HNO3).

   VOCs + NOx + Sunlight

O3 SO2 + H2O vapour + O3 H2SO4

NOx + H2O vapour HNO3

 

2.3. Major sources of air pollutants

a) Natural source – Pollutants emitted in the atmosphere through natural source. For example, dust from land with little or no vegetation; vegetation emits volatile organic carbons (VOCs) such as isoprene,smoke and carbon mono oxide (CO) from wildfires; volcanic activity produces sulfur, chlorine and ash particulates.

b) Artificial source – Human factors are involved in causing the emission. For example, Oxides of nitrogen (NOx) emitted from a vehicle; burning of fossil fuels; incinerators; solid fuel for cooking and heating etc.

c) Stationary Source – The sources of pollutants are fixed in a place. For example, thermal power plants; coal fired power plants.

d) Mobile source – The sources of pollutants are not fixed in a place and are mobile in nature. For example, automobiles on highways, aircraft, farm vehicles, boats and ships.

i) Sources and sinks of SO2 in air

A) Sources –

a) Volcanoes are the natural source of SO2. It emits about 67% of the total SO2 emitted in the atmosphere.

b) About 33% of the total SO2 is emitted through anthropogenic influence. For example, fossil fuel combustion; smelting (process in which metals are extracted from ore); manufacture of sulphuric acid (H2SO4); conversion of wood pulp to paper; incineration of refuse and production of elemental sulfur.

B) Sink – Conversion into H2SO4 in either gas or liquid phase.

ii) Sources and sinks of NOx in air

Although there are seven oxides of nitrogen known to occur, the only two that are important in the study of air pollution are nitric oxide (NO) and nitrogen dioxide (NO2).

  1. A) Sources
  2. a) Natural – Volcanoes, Oceans, biological decay, lightening strikes
  3. b) Man-made –
  • Fossil fuel combustion in stationary sources for power generation (24%) and in mobile sources for transport (49%)
  • Non combustion processes like nitric acid manufacture, welding processes, use of explosives etc.

B) Sink – Conversion into HNO3 in either gas or liquid phase

 

iii)  Sources and sinks of CO in air

 

A) Sources

 

a) Natural- Volcanic action, electrical discharge during storms, seed germination, forest fires, methane oxidation, oxidation of non- methane hydrocarbon, decay of plant matter etc.

 

b) Man-made – Incomplete combustion (internal engine), biomass burning.

 

B) Sinks

 

  1. v) Sources and sinks of SPM (suspended particulate matter) in air
  2. A) Sources- Combustion of fossil fuel mainly diesel and coal
  3. B) Sink- Dry deposition
  4. vi) Sources of Benzene

It has two sources, indoor and outdoor. The indoor sources include building materials and furniture, attached garages, heating and cooking systems, stored solvents and various human activities, tobacco smoke. Outdoor sources are petrol stations and certain industries such as those concerned with coal, oil and natural gas, chemicals and steel.

  • vii) Sources of Benzo (a) pyrene

These are also called polyaromatic hydrocarbons (PAH).

a) Natural source – forest and brush fires, volcanoes, bacterial and algal synthesis, petroleum seeps, erosion of sedimentary rocks containing petroleum hydrocarbons, and decomposition of vegetative liter fall

b) Anthropogenic source – Incomplete combustion such as incinerators and some industrial processes, automotive emissions, smoke from wood-burning stoves, jet aircraft exhausts, cigarette and cigar smoke, backyard barbecues, petroleum product spills, sewage sludge, and tarry or creosote waste materials

 

viii) Sources of ammonia (NH3)

 

a) Natural – decaying organic matter, nitrogen-fixing bacteria

b) Anthropogenic – use of fertilizers, improper waste disposal sites, industrial processes, catalytic converters in petrol cars, landfill sites, sewage works, composting of organic materials, combustion, wild mammals and birds

 

ix) Sources of Arsenic (As)In the atmosphere, arsenic exists as particulate matter, mostly less than 2 µm in diameter.

a) Natural – Volcanoes, weathering of arsenic containing minerals and ores

b) Anthropogenic – Commercial or industrial processes such as it is a byproduct of the smelting process (separation of metal from rock)

 

  1. x) Sources of lead (Pb)

Lead is used in leaded petrol as anti knocking agent. Use of it in petrol engines causes its release into the air.

  1. xi) Sources of Nickel (Ni)

Nickel in air is mainly in the particulate form, with particle sizes ranging from 0.1 to 2 μm.

  1. a) Natural – Soil dust, forest fires, particle released from vegetation and sea salt
  2. b) Anthropogenic – Fossil fuel (oil and coal) combustion, high-temperature metallurgical operations, nickel primary production operations, and municipal waste incineration. Other sources include coke ovens, cement manufacturing, asbestos mining/milling and cooling towers.

Sinks of As, Pb and Ni- Airborne particles eventually end up being precipitated with rainfall and therefore end up in water and soil.

  1. Formation of primary and secondary pollutants in the troposphere
  2. i) Carbon monoxide (CO) – It is formed inside the automobile engine or in a thermal power plant when there is incomplete burning of fossil fuel. For example, burning of methane (CH4)

CH4 + O2   CO2 + H2O (Major) + [CO+ Hydrocarbons] (traces)

ii) Nitrogen dioxide (NO2) – It is classified as

a) Thermal NOx – Air contains 78% N & 21% O2. During functioning of an automobile engine at very high temperature, combustion takes place in which air fuel mixture is introduced. At this very high temperature, N2 present in the atmospheric air combines with O2 and forms thermal NOx.

       b)   Fuel NOx – In gasoline, N is present as impurity, this N during combustion process when combines with O2 forms fuel NOx.

iii) Sulfur dioxide (SO2) – In coal, 1-6 % sulfur is present as impurity. When coal is burnt in the presence of O2 it forms SO2.

iv) Lead (Pb) – Earlier tetraethyl lead was used in petrol as anti-knocking agent. When leaded petrol is burnt, Pb is released into the atmosphere.

v) Ozone (O3) – It is a secondary pollutant. NOx and VOC are responsible for its production in the presence of sunlight.

Sunlight

  1. vi) PM10 – Particulate matter of ≤10 mm. It may be solid, liquid or gaseous pollutant.

vii) Benzene – It is produced during incomplete combustion or combustion in limited supply of O2 of coal and oil.

viii) Benzo (a) pyrene – The destructive distillation of coal into coke and coal tar, or the thermal cracking of petroleum residuals into lighter hydrocarbons are pyrolytic processes which lead to its formation. The temperature at which pyrogenic processes occur range from about 350 oC to more than 1200 oC.

  1. Smog

 

4.1. London type and Los Angeles (LA) type smog

 

There are mainly two types of smog formation that take place in the troposphere, viz., London type and Los Angeles (LA) type smog. They derived the name from the place they first appeared. The major differences between them are explained in the table below.

 

4.2. Photochemical smog and their formations in the troposphere

 

When oxides of nitrogen, VOCs and sunlight come together, they can initiate a complex set of reactions that produce a number of secondary pollutants known as photochemical smog also called as photochemical oxidant. So, we can express the formation of photochemical smog as follows-

 

VOCs + NOx + Sunlight     O3

Ozone (O3) is the most abundant of the photochemical oxidants.

Photochemical reaction sequence –

At a very high temperature inside the automobile engine, formation of thermal NOx take place from the reaction of atmospheric N2 and O2.

 

1200 – 1700 o
N + O 2 c 2NO
2

 

Nitric oxide thus emitted can oxidize to NO2

2NO+ O2      2NO2

If sunlight is available, a photon can decompose NO2 in a process called photolysis.

NO2 + h√    NO + O

The freed atomic oxygen (O) can then combine with diatomic oxygen (O2) to form Ozone (O3).

O + O2 + M      O3 + M

M represents a molecule (usually O2 or N2, since they are most abundant in air) whose presence is necessary to absorb excess energy from the reaction.

 

O3 + NO ——> NO2+ O2

 

Dissociation of O3 by NO –

 

The general tendency for NO2 is to create O3 while NO tends to destroy O3. This set of reactions create a cycle that is represented in the following figure:.

 

  1. Acid Rain/ Acid deposition

 

Angus Smith (1852) first used the term acid rain. It refers to different ways acid falls from the atmosphere including rain, fog, hail and snow. The more accurate term for acid rain is acid deposition. There are two types of acid deposition. These are i) dry deposition – it occurs close to the emission point, e. g., SO2 or NOx and ii) wet deposition which may occur thousands of kilometers away from pollution sources, e.g., SO42- or NO3-. Sulfur compounds are the principal cause of acid deposition.

 

5.1 Sources of acid precipitation

a) Natural    SO2 – Oceans and volcanic eruptions  NOx- Lightening, volcanic eruptions and microbial activity

 

b) Man-made pollutants  SO2 – Burning of coal/ petroleum (fossil fuel) and the smelting of iron/ other metallic ores NOx- Steam from vehicle exhausts

 

fading of dyes and inks used in some textiles. Much of the damage to materials caused by NOx, such as stress – corrosion cracking of electrical apparatus, comes from secondary nitrates and nitric acid.

 

6.4. Effect of photochemical smog

 

i) Eye irritation

ii) Chest constriction and irritation of the mucous membrane

iii) Cracking of rubber products

iv) Damage to vegetation

6.5. Effect of acid rain

 

Direct photo toxicity to plants from excessive acid concentrations in the air and indirect phytotoxicity such as from Al3+ liberated from acidified soil. It affects the respiratory systems in human and other animals. Acidification of water bodies has toxic effects especially to fish fingerlings. It corrodes the exposed structures, electrical relays, equipment and ornamental materials. The hydrogen ions from the acid rain dissolve the lime stone (CaCO3) and thus cause damage to marble structures. It causes reduction of visibility by sulphate aerosols. The sulphate aerosol also influences the physical and optical properties of clouds.

 

6.6. Effect of Benzene

 

On acute exposure, benzene may cause narcosis: headache, dizziness, drowsiness, confusion, tremors and loss of consciousness. It is a moderate eye irritant and a skin irritant. On chronic exposure, it may cause cancer in humans. The International Agency for Research on Cancer has classified benzene as carcinogenic to humans (Group 1). It can lead to aplastic anaemia, chromosomal aberrations and mutations in humans.

 

6.7. Effect of Benzo (a) pyrene (BaP)

 

In humans, exposure of it leads to decreased immune function, cataracts, kidney and liver damage (e.g. jaundice), breathing problems, asthma-like symptoms and lung function abnormalities. Meanwhile, repeated contact with skin may induce redness and skin inflammation. BaP’s metabolites are mutagenic and highly carcinogenic, and it is listed as a Group 1 carcinogen.

 

6.8. Effect of Ammonia

 

It can be taken up through the leaves via stomata, increasing the potential for nutrient-N uptake which drive to the deleterious effects on terrestrial plants, effects epiphytic lichens, increase soil acidity and interfere with base cation uptake. Excess of it will lead to increased nitrification and denitrification, contributing to greenhouse gas emissions. In humans, on its exposure it causes nose & throat irritation and burns, pulmonary edema, cough, asthma, lung fibrosis, skin burns, burning sensation in the eyes, ulceration & perforation of the cornea, blindness, cataracts & glaucoma.

 

6.9. Effect of As

 

Respiratory tract irritation, bronchitis, effect on nervous system and cardiovascular problem; in smelter workers exposed to arsenic dust, higher incidence of Raynaud’s disease and increased constriction of blood vessels in response to cold are observed.

 

6.10. Effect of Pb

 

Lead effects nervous system, causes depression/mood changes, headache, diminished cognitive performance, diminished visual motor performance, dizziness, fatigue, forgetfulness, impaired concentration, increased nervousness, irritability, lethargy, reduced IQ scores and weakness.

 

6.11. Effect of Ni

 

It causes skin allergy leading to eczema and lichenification of the hands and other areas of the skin that contact nickel. Other health problems are asthma and conjunctivitis.

 

  1. Monitoring primary and secondary air pollutants

The Air Act (1981) provides means for the control and abatement of air pollution in India. National Ambient Air Quality Standards (NAAQS) for major air pollutants were notified by the Central Pollution Control Board (CPCB) in April 1994. Given the ongoing focus of the Air Act, most of the monitoring of emissions concentrations and effects of air pollution has been directed towards the twelve criteria pollutants viz., SO2, NOx, PM10, PM2.5, O3, Pb, CO, NH3, C6H6, Benzo(a) pyrene, As and Ni.

 

The methods of air pollution monitoring are as follows –

 

  1. A) Particulate matter
  2. High volume sampler – Most commonly used sampling method. This analysis is based on gravimetric principle. A standard high volume sampler collects particles in the size range from 0.1 – 100 m. The particulate concentration measured is referred to as the Total Suspended Particles (TSP) which is a combination of settle able particles and suspended particles. It is expressed as µg m-3 for a 24 hour period, normally as part of 6 days cycle.
  3. Nephelometers- It uses the principle of scattering of light to measure the size and number of particles in a given air sample. It is usually used to examine the amount of particulate material in the 0.1 – 2.5 m size range. Generally, this size range presents the greatest risk to human health.
  4. B) Carbon monoxide (CO)
  5. Infrared spectrophotometry – It is based on the principle that CO strongly absorbs infrared radiation at certain wavelengths. When IR radiation is passed through a long cell (100 cm) containing a trace of CO, part of the energy is absorbed by the gas. A non-dispersive infra red (NDIR) device, suitable for detection from 0-500 ppm by volume of CO is used.
  6. Gas chromatography – 10 ppm and lower levels of CO can be conveniently measured by gas chromatography with a flame ionization detector (FID). The sample is subjected to catalytic reduction by H2 over a Ni catalyst at 360 oC and measured with a FID.
  1. Jacob & Hochheiser (Na-Arsenite) method – NOx is collected over NAOH solution. The NO2 produced is allowed to react with H3PO4, sulphanil amide and N (1-naphthyl) ethylenediamine dihydrochloride. The resulting reddish purple azo dye is measured at 543 nm. The method is applicable to 0.01 to 1.5 µg NO2 per ml.
  2. D) Sulphur dioxide (SO2)

Modified West Gaeke spectrophotometric method is the standard method for monitoring of 0.0005 to 5 ppm SO2 in ambient air. SO2 is collected in a scrubbing solution containing HgCl4 2- (HgCl2 + KCl) which has a collection efficiency of around 95%. The solution is allowed to react with HCHO and then with pararosaniline hydrochloride. The absorbance of the product red-violet dye is measured at 548 nm.

 

E) Ozone (O3)

UV Photometric –

 

The UV photometer determines the concentration of Ozone (O3) in a sample gas at ambient pressure by detecting the absorption of UV radiation in a glass absorption tube. Ozone shows strong absorption of UV light at 254 nm. The analyzer uses the Beer-Lambert relationship to calculate the ozone concentration.

 

  1. Chemical Method –

Air containing O3 is drown through a midget impinger containing 10 ml of 1% potassium iodide in a neutral (pH 6.8) buffer composed of 0.1 M disodium hydrogen phosphate and 0.1M potassium dihydrogen phosphate. The iodine librated in the absorbing reagent is determined spectrophotometrically at 352 nm. The range extend from 0.01ppm to about 10 ppm.

F) Ammonia (NH3) Indophenols Blue Method-

Ammonia in the atmosphere is collected by bubbling of measured amount of air through a dilute solution of sulfuric acid to form ammonium sulphate. The ammonium sulfate formed in the sample is analyzed colorimetrically by reaction with phenol and alkaline sodium hypochlorite to produces Indophenols, a blue dye. Sodium nitropruside accelerates the reaction as a catalyst. The limit of detection of the analysis is 0.02µ NH3 ml-1.

G) Benzene (C6H6)  Adsorption and desorption followed by GC –

 

A known volume of air is drawn through a charcoal tube to trap the organic vapors present. The charcoal in the tube is transferred to a small, graduated test tube and desorbed with carbon disulphide. An aliquot of the desorbed sample is injected into a gas chromatograph. The area of the resulting peak is determined and compared with areas obtained from the injection of standards. Gas Chromatograph with a FID is used in the study.

 

H) Benzo (a) pyrene  Solvent Extraction followed by HPLC/GC –

 

It involves collection of air particulate on a fine particle (glass-fibre) filter using high volume sampler for total suspended particulate matter (TSPM) or respirable dust sampler for respirable suspended particulate matter (RSPM or PM1O) and subsequent analysis by Gas Chromatograph (GC) using FID.

 

  1. I) Metals (Pb, As and Ni)

 

Metals are associated mainly with the particulate matter, therefore, collected on EPM – 2000 cellulose membrane filter paper by any dust collecting device. The dust collecting area of filter is calculated. This filter is then digested with digestion mixture (nitric acid and perchloric acid in 6:1 ratio) and digested at1000C. Digested samples are filtered through Whatman filter paper (Grade No.1). The volume is made upto 25 ml with double distilled water and analyzed for Pb, Hg, Cu, Cd, Zn and Ni using AAS.

  1. Mitigation measures for combating air pollution

The atmosphere cleanses itself by dispersion, gravitational settling, flocculation, absorption, rain-washout etc. However, source control of contaminants is more desirable for combating air pollution problem.

 

8.1. Source control

 

Some measures that can be adopted in this direction are:

 

a) Using unleaded petrol

b) Using fuels with low sulphur and ash content

c) Encouraging people to use public transport, walk or use a cycle as opposed to private vehicles

d) Plant trees along busy streets as they remove particulates, carbon dioxide and absorb noise

e) Industries and waste disposal sites should be situated outside the city preferably on the downwind of the city.

f) Catalytic converters should be used to help control emissions of carbon monoxide and hydrocarbons

 

8.2. Control measures at industrial centers

  1. a) Emission rates should be restricted to permissible levels by each and every industry.
  2. b) Incorporation of air pollution control equipment in design of plant layout must be made mandatory
  3. c) Fuel switching – This may include: Use of low sulphur coal for combustion at thermal power plants. This may help in reducing emission of SO2  to the tune of 30 to 90 %. Use of compressed natural gas (CNG) or liquefied petroleum gas (LPG) in motor vehicles and cooking stoves.

Reduce use of fossil fuels and use of non conventional energies such as- Solar energy, Wind energy, tidal energy etc.

 

8.3. Equipment used to control air pollution

 

  1. a) Ensuring sufficient supply of oxygen to the combustion chamber and adequate temperature so that the combustion is complete thereby eliminating much of the smoke consisting of partly burnt ashes and dust.
  2. b) Use of mechanical devices post combustion in industries is required to remove pollutants. Such as wet and dry scrubbers for flue gas desulfurization; for particulate control, centrifugal cyclones (5-25 µm particulates), bag house collector, electro-static precipitators (>1 µm) and venturi scrubbers; for gaseous pollutants (such as Cl, SOx, NOx, H2S), plate column, packed column and spray column is used.
  3. c) The air pollutants collected must be carefully disposed. The factory fumes are dealt with chemical treatment.
  4. Summary
  • ü Air pollutants are of two types- primary and secondary, based on their mode of formation.
  • ü Primary pollutants are directly emitted from the source.
  • ü When primary pollutants are emitted from the source, they undergo various physical processes and chemical reactions in the atmosphere and form secondary pollutants.
  • ü Based on source, pollutants are classified as natural, artificial, stationary and mobile.
  • ü Acid rain, London type and Los Angeles (LA) type smogs are secondary pollutants.
  • ü Air pollutants are harmful to the environment and cause detrimental effects not only to humans but also crops, plants and building-infrastructures.
  • ü NAAQS for major air pollutants were notified by the CPCB, has been directed towards the twelve criteria pollutants , SO2, NOx, PM10, PM2.5, O3, Pb, CO, NH3, C6H6, Benzo (a) pyrene, As and Ni.
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