Environmental chemistry: air

Class 11 Chemistry, unit 13, book pages 251–270

Practise 48 MCQs Take the unit quiz Review 30 cards

Environmental chemistry: air covers the composition and layers of the atmosphere and the pollutants that human and natural sources add to it. It explains how these pollutants produce smog, acid rain, ozone depletion and global warming, and how air quality is measured, regulated and improved, together with the economic, social and political issues linked to air quality.

Learning outcomes (14)

Identify the properties and composition of the atmosphere (include the concept of the four layers of the atmosphere and their composition).
Describe the factors that affect air quality.
Describe the sources and understand the effects of air pollution (this can include both natural and human caused pollutants, including ozone, lead (Pb), mercury (Hg), polycyclic aromatic hydrocarbons (PAHs), persistent organic pollutants (POPs), greenhouse gases (such as carbon dioxide, methane and nitrous oxide), chlorofluorocarbons (CFCs) and other ozone-depleting substances, volatile organic compounds (VOCs), and heavy metals such as mercury and cadmium).
Familiarise yourself with the methods and techniques used to measure and monitor air quality.
Describe the impact of human activities on the atmosphere, including the effects of burning fossil fuels and deforestation.
Identify the chemical reactions and processes that occur in the atmosphere (some examples include the formation of smog and acid rain).
Identify the laws and regulations related to air quality and the measures used to control air pollution.
Analyse data and interpret air quality measurements and trends.
Explain the link between air quality and human health.
Evaluate the potential health risks associated with air pollution.
Explain the technologies and strategies used to reduce air pollution and improve air quality, such as emissions control and renewable energy sources.
Design experiments and collect data to test hypotheses about air quality.
Identify the global scale problems of air pollution, such as global warming and the greenhouse effect.
Analyse the economic, social and political issues related to air pollution and air quality management, and demonstrate this through answers.

13.1Air pollution

The environment is made up of four components that interact and influence one another: the atmosphere (air), the hydrosphere (water bodies), the lithosphere (land) and the biosphere (living organisms). Changes in one component can have far-reaching effects on the others, which shows how delicate the balance of the Earth's environment is.

Atmosphere: The envelope of gases that surrounds the Earth. It is mainly nitrogen (about 78%) and oxygen (about 21%), with traces of carbon dioxide, water vapour and noble gases. It regulates temperature and weather patterns and protects the planet from hazardous solar radiation.
Hydrosphere: All of the Earth's water in its various forms: oceans, rivers, lakes, glaciers and groundwater. Water influences climate, weather and ecosystems, and its movement distributes heat and nutrients around the planet.
Lithosphere: The solid outermost layer of the Earth, including the continents, ocean beds and the Earth's crust. It is made of rocks, minerals and soils, and geological processes such as earthquakes and volcanic activity occur in it.
Biosphere: All living organisms and their interactions with each other and with their environment, including plants, animals, microorganisms and ecosystems.

13.1.1Atmosphere and its layers

The Earth's atmosphere is divided into four main layers: the troposphere, the stratosphere, the mesosphere and the thermosphere. The thermosphere is itself divided into the ionosphere and the exosphere.

Figure 13.1: The four layers of the atmosphere, from the surface upwards. The ozone layer lies in the stratosphere (highlighted).

The four layers of the atmosphere (book text, Section 13.1.1).
Layer	Position	Temperature	Main features
Troposphere	Lowest layer, from the surface up to about 12 km	Generally decreases with altitude	Most of the clouds and water vapour; weather keeps changing; contains $N X_{2}$ , $O X_{2}$ and trace amounts of $CO X_{2}$
Stratosphere	Above the troposphere	Increases with altitude, because UV radiation is absorbed	Contains the ozone layer, which absorbs and scatters the Sun's UV radiation; higher ozone concentration than the troposphere
Mesosphere	Above the stratosphere	Extremely low	Meteors burn up here; auroras occur in this layer
Thermosphere	Above the mesosphere	High, because of absorption of intense solar radiation	Contains the ionosphere (electrically charged particles used in radio communication) and the exosphere (very thin mix of gases, mainly hydrogen and helium, that merges into space)

Worked example: sketching how temperature changes with height

Sketch the variation of temperature with altitude through the four layers of the atmosphere, and explain the shape of the curve in each layer.

Temperature against altitude (a sketch, not to scale). The stratosphere (highlighted) gets warmer because the ozone layer absorbs UV radiation.

Troposphere (lowest layer, up to about $12 km$ ): the temperature generally decreases with altitude, so the line slopes towards the left.
Stratosphere: the temperature increases with altitude, because the ozone layer absorbs UV radiation from the Sun. The line turns back to the right.
Mesosphere: the temperature falls again and becomes extremely low. This is the coldest part of the curve.
Thermosphere: the temperature is high because of the absorption of intense solar radiation, so the line swings far to the right.
Mark the layer boundaries with dashed lines and label the axes. The curve changes direction at each boundary: down, up, down, up.

decrease, increase, decrease, increase through the four layers

13.1.2Air pollutantsOften examined

Air pollutants come from vehicles, industrial processes, power plants and natural sources such as wildfires and dust storms. They are classed as primary pollutants, which are emitted directly, or secondary pollutants, which form through chemical reactions in the atmosphere.

Primary pollutant: A pollutant that is emitted directly from its source.
Secondary pollutant: A pollutant that forms through chemical reactions in the atmosphere.
Particulate matter (PM): Tiny particles suspended in the air. They come from combustion, construction and natural sources, and can enter the respiratory system, causing respiratory and cardiovascular problems.
Nitrogen oxides ( $N O_{x}$ ): Produced from burning fossil fuels, especially in vehicles and industrial processes. They cause smog and respiratory problems and contribute to acid rain.
Sulphur dioxide ( $SO X_{2}$ ): Emitted mainly by burning coal and oil. It causes respiratory problems, and when combined with water vapour it forms acid rain that damages ecosystems.
Volatile organic compounds (VOCs): Released from vehicle emissions, industrial processes and household products. They contribute to the formation of ground-level ozone, a major component of smog that harms lung function.
Carbon monoxide ( $CO$ ): Produced by incomplete combustion of carbon-containing fuels. It reduces the blood's ability to carry oxygen, which is a threat to cardiovascular health.
Ground-level ozone ( $O X_{3}$ ): Forms when VOCs and $N O_{x}$ react in the presence of sunlight. It irritates the respiratory system, worsens asthma and damages crops and ecosystems.
Heavy metals: Released from industrial processes, including lead ( $Pb$ ), mercury ( $Hg$ ) and cadmium ( $Cd$ ). They contaminate soil and water, enter the food chain and cause various health issues.
Ammonia ( $NH X_{3}$ ): Emitted from agricultural activities and livestock waste. It contributes to the formation of particulate matter and affects air and water quality.
Polycyclic aromatic hydrocarbons (PAHs): Organic compounds made of multiple fused aromatic rings, formed during incomplete combustion of coal, oil and wood. Some PAHs are environmental pollutants because of their toxicity and carcinogenic properties; they are found in vehicle emissions, industrial processes and even grilled foods.
Chlorofluorocarbons (CFCs): Synthetic compounds used in refrigeration, aerosol propellants and foam production. They reach the upper atmosphere, where they break down ozone molecules and thin the ozone layer. This lets more harmful UV radiation from the Sun reach the Earth's surface.

Air pollutants by category, examples, physical state and major sources (book table).
Pollutant or category	Examples	Physical state	Major sources
Photochemical oxidants	Ozone ( $O X_{3}$ )	Gas	Pollutant in the troposphere produced from $N O_{x}$ and volatile organic compounds such as methane or CO; stratospheric origin, where it protects from UV but is destroyed by chlorine or bromine compounds (ozone hole)
Sulphur dioxide	$SO X_{2}$	Gas	Combustion of fossil fuels, volcanoes, transformation of natural sulphur compounds
Carbon monoxide	$CO$	Gas	Incomplete combustion of carbon compounds, oxidation of volatile organic compounds (VOCs)
Nitrogen oxides ( $N O_{x}$ )	$NO$ , $NO X_{2}$	Gas	Nitrogen oxidation during combustion, thunderstorms
Polycyclic hydrocarbons	Benzene, 1,3-butadiene, formaldehyde, acids	Gas	Incomplete combustion, atmospheric chemistry, use of solvents
Mercury ( $Hg$ )	$HgO$ , mercury methyl	Gas and particles	Combustion of carbon, ore refining, incinerators, natural sources
Lead ( $Pb$ )	$Pb$	Particles	Combustion of gas with lead (now removed), incinerators, foundries and lead transformation
PM, including $P M_{10}$ and $P M_{2.5}$ , inhalable particles, suspended particles, black carbon	Inorganic ions (for example sulphate; metallic oxides; carbon materials including inorganic carbon and elementary carbon)	Particles	Wind erosion, marine sprays, combustion of fossil fuels or biomass, agriculture, pollen, spores, viruses, bacteria, gas-particle conversion
Organic carbon	Hopanes, steranes, hydrocarbons, levoglucosanes, terpenes, isoprenes	Gas and particles	Incomplete combustion of fossil combustibles or biomass, emissions by plants, oxidation of gaseous organic compounds

13.1.3Impacts of human activities on the atmosphere

Human activities add to the gases from natural sources and change the composition of the atmosphere. The book identifies the following impacts.

Greenhouse gas emissions: burning fossil fuels (coal, oil and natural gas) for energy releases large amounts of carbon dioxide and other greenhouse gases. These gases trap heat, leading to global warming and climate change.
Air pollution: emissions from vehicles, industrial processes and power plants release pollutants such as sulphur dioxide, nitrogen oxides and particulate matter. These contribute to smog, acid rain and respiratory issues.
Ozone depletion: emission of CFCs used in refrigeration and aerosol propellants has depleted the ozone layer, so harmful UV radiation reaches the Earth's surface.
Deforestation: clearing forests for agriculture, logging and urbanisation reduces the planet's ability to absorb $CO X_{2}$ . Trees act as carbon sinks, so their removal contributes to higher atmospheric $CO X_{2}$ levels.
Health impacts: poor air quality caused by pollutants can lead to respiratory and cardiovascular diseases. Heatwaves and changing disease patterns also affect human health.

13.2Smog

Smog is a type of air pollution that results from the combination of fog and smoke, or other pollutants, in the atmosphere. It has a hazy, often yellowish appearance and can dangerously reduce visibility. Smog forms when pollutants released by vehicles, industries and natural sources react in the presence of sunlight.

13.2.1Types of smogOften examined

Photochemical smog

Photochemical smog forms usually in urban areas with high levels of vehicle emissions. It is the result of reactions between nitrogen oxides ( $N O_{x}$ ) and volatile organic compounds (VOCs) in the presence of sunlight. It contains ozone, which can be harmful to human health.

Industrial smog

Industrial smog is also known as black smog. It results from burning coal or other fossil fuels and contains large amounts of particulate matter and sulphur dioxide. It is commonly associated with areas where coal is the primary source of energy.

Photochemical and industrial smog compared.
Feature	Photochemical smog	Industrial smog
Formed from	$N O_{x}$ and VOCs reacting in sunlight	Burning coal or other fossil fuels
Main components	$N O_{x}$ , VOCs, particulate matter, $SO X_{2}$ and ozone	Large amounts of particulate matter and $SO X_{2}$
Typical place	Urban areas with high levels of vehicle emissions	Areas where coal is the primary source of energy

Composition of photochemical smog

Photochemical smog is composed of various pollutants, including:

Nitrogen oxides ( $N O_{x}$ )
Volatile organic compounds (VOCs)
Particulate matter (PM)
Sulphur dioxide ( $SO X_{2}$ )
Ozone ( $O X_{3}$ )

General mechanism of formation of smog

The sequence below shows how photochemical smog forms. $R$ stands for organic fragments from VOCs, and PAN is peroxyacetyl nitrate.

Mechanism of smog formation

Nitrogen dioxide absorbs sunlight and splits into nitric oxide and an oxygen atom.
$NO X_{2} + sunlight NO + O$
The oxygen atom combines with an oxygen molecule to form ozone.
$O X_{2} + O O X_{3}$
Ozone reacts with nitric oxide to regenerate nitrogen dioxide and form oxygen.
$NO + O X_{3} NO X_{2} + O X_{2}$
Nitrogen dioxide reacts with organic fragments from VOCs to form products such as PAN.
$NO X_{2} + R PAN$
Nitric oxide reacts with ROX to form nitrogen dioxide, together with other products.
$NO + ROX NO X_{2}$

Worked example: primary and secondary pollutants in photochemical smog

In the formation of photochemical smog, nitrogen dioxide is split by sunlight but is formed again later. (a) Explain how

NO X_{2}

is regenerated. (b) Which species in the sequence is a secondary pollutant, and why is

NO X_{2}

not used up completely?

Write the first step. Sunlight splits $NO X_{2}$ into nitric oxide and an oxygen atom (step 1 in the figure).
$NO X_{2} + sunlight NO + O$
The oxygen atom is very reactive and joins an oxygen molecule to form ozone (step 2).
$O X_{2} + O O X_{3}$
Ozone reacts with the nitric oxide made in step 1. This gives back $NO X_{2}$ and an oxygen molecule (step 3), which is how $NO X_{2}$ is regenerated.
$NO + O X_{3} NO X_{2} + O X_{2}$
Classify the species. $NO$ and VOCs come directly from vehicles and industry, so they are primary pollutants. Ozone and PAN are not emitted by any source: they form by chemical reactions in the atmosphere in sunlight, so they are secondary pollutants.
Because step 3 returns $NO X_{2}$ , it acts again in step 1. Some $NO X_{2}$ leaves the cycle by reacting with organic fragments of VOCs to form PAN, which is why the smog contains PAN as well as ozone.

Figure: the

NO X_{2}

cycle in photochemical smog. (1) Sunlight splits

NO X_{2}

, (2) the oxygen atom joins

O X_{2}

to form ozone (highlighted), (3)

NO

reacts with

O X_{3}

and gives

NO X_{2}

back.

ozone and PAN are secondary pollutants; NO X_{2} is regenerated by NO + O X_{3} NO X_{2} + O X_{2}

13.2.2Hazardous effects of smog

Smog has several hazardous effects on both human health and the environment.

Hazardous effects of smog (book text, Section 13.2.2).
Effect	What smog does
Respiratory problems	Can exacerbate asthma, pharyngitis, bronchitis and other lung diseases; causes irritation, coughing and difficulty in breathing
Cardiovascular issues	Prolonged exposure is linked to heart problems, including heart attacks and arrhythmia
Eye irritation	Redness, itching and watering of the eyes
Reduced visibility	Increases the risk of accidents on roads, in air travel and in maritime navigation
Environmental impact	Damages crops, forests and aquatic ecosystems; ozone, a component of smog, harms plant growth and decreases crop yields
Global warming	Some components of smog, such as methane, are potent greenhouse gases
Aesthetic degradation	The persistent haze spoils scenic views and the overall quality of an area

13.3Acid rainOften examined

Acid rain is a type of environmental pollution in which rainfall or precipitation has a pH lower than normal, below 5.6.

Rain water does not have a pH of 7, because of slight mixing with atmospheric carbon dioxide, which forms a weak acid, carbonic acid. The carbonic acid then releases hydronium ions:

Carbon dioxide dissolves in rain water and forms carbonic acid.
$CO X_{2} + H X_{2} O H X_{2} CO X_{3}$
Carbonic acid ionises slightly, giving hydronium ions and hydrogencarbonate ions.
$H X_{2} CO X_{3} + H X_{2} O H X_{3} O X^{+} + HCO X_{3} X^{-}$

These hydronium ions make rain water slightly acidic, but this is not acid rain. Acid rain has a pH below 5.6 because of the oxides of sulphur and nitrogen in the atmosphere, which are described in the next section.

Worked example: is this rain sample acid rain?

Two rain samples are collected. Sample A has pH

4.6

and sample B has pH

6.2

. (a) Which one is acid rain? (b) How many times larger is the hydronium ion concentration of sample A than that of normal rain water of pH

5.6

Given

$A = pH = 4.6$
$B = pH = 6.2$
$normal rain = pH = 5.6$

To find

which sample is acid rain
ratio of $[H X_{3} O X^{+}]$ for A to normal rain

Recall the book's limit: rain is acid rain when its pH is below $5.6$ . Normal rain is already slightly acidic because of dissolved $CO X_{2}$ (carbonic acid), so pH $7$ is not the dividing line.
Compare each sample with $5.6$ . Sample A: $4.6 < 5.6$ , so it is acid rain. Sample B: $6.2 > 5.6$ , so it is not acid rain, although it is slightly acidic.
Use $[H X_{3} O X^{+}] = 1 0^{- pH}$ for sample A.
$[H X_{3} O X^{+}]_{A} = 1 0^{- 4.6} = 2.5 \times 1 0^{- 5} mol d m^{- 3}$
Do the same for normal rain.
$[H X_{3} O X^{+}]_{normal} = 1 0^{- 5.6} = 2.5 \times 1 0^{- 6} mol d m^{- 3}$
Divide. One pH unit is a factor of ten in concentration.
$\frac{2.5 \times 1 0 ^{- 5}}{2.5 \times 1 0 ^{- 6}} = 10$

A is acid rain (B is not); [H X_{3} O X^{+}]_{A} is 10 times that of normal rain

13.3.1Reasons for acid rainOften examined

The primary cause of acid rain is the release of sulphur dioxide ( $SO X_{2}$ ) and nitrogen oxides ( $N O_{x}$ ) into the atmosphere, mainly from burning fossil fuels such as coal and oil in power plants and vehicles. Industrial activities such as manufacturing and mining also release these pollutants.

Formation of sulphuric acid

Sulphur dioxide reacts with oxygen and water to form sulphuric acid, which then ionises in water:

Sulphur dioxide, oxygen and water react to form sulphuric acid. The hydrogen and oxygen atoms balance with two molecules of water.
$2 SO X_{2} + O X_{2} + 2 H X_{2} O 2 H X_{2} SO X_{4}$
Sulphuric acid ionises in water. Two water molecules are needed to balance the hydrogen atoms (see the note below).
$H X_{2} SO X_{4} + 2 H X_{2} O 2 H X_{3} O X^{+} + SO X_{4} X^{2 -}$

Formation of nitric acid

Nitrogen oxides react with atmospheric moisture to form nitric acid. Nitric acid then ionises in water:

Nitrogen dioxide reacts with water to form nitric acid and nitric oxide.
$3 NO X_{2} + H X_{2} O 2 HNO X_{3} + NO$
Nitric acid ionises in water, giving hydronium ions and nitrate ions.
$HNO X_{3} + H X_{2} O H X_{3} O X^{+} + NO X_{3} X^{-}$

13.3.2Impacts of acid rain

Environmental damage: acid rain harms forests, lakes, rivers and aquatic ecosystems. It damages plant foliage and aquatic life by altering the pH of soil and water.
Corrosion: it accelerates the corrosion of buildings, statues and infrastructure, causing significant damage to historical landmarks and urban structures.
Human health: inhaling pollutants associated with acid rain, such as fine particulate matter, can cause respiratory problems and cardiovascular issues.
Economic costs: increased maintenance and repair costs for infrastructure, and reduced agricultural productivity because of soil degradation.
Loss of biodiversity: lowers the pH of water bodies and forms aluminium-based gelatinous compounds that clog the gills of fish and harm amphibians. It can also lead to the decline of sensitive plant and animal species in affected areas.
Global implications: the pollutants that form acid rain can travel long distances in the atmosphere, causing transboundary pollution and affecting ecosystems in distant regions.
Control: emission control technologies, such as scrubbers in power plants, are used to combat acid rain.

The corrosion of calcium carbonate, which is found in many building and statue materials, by sulphuric acid is written as follows:

Calcium carbonate reacts with sulphuric acid to give calcium sulphate and water. Carbon dioxide is also a product and is needed for the equation to balance.
\ce{CaCO3 + H2SO4 -> CaSO4 + H2O + CO2}

Worked example: marble damaged by sulphuric acid

A marble statue (\ce{CaCO3}) is attacked by acid rain containing \ce{H2SO4}. Find the mass of \ce{CaCO3} that dissolves when 4.9\,\mathrm{g} of \ce{H2SO4} reacts completely, and the volume of \ce{CO2} released at STP. (\ce{Ca}=40, \ce{S}=32, \ce{O}=16, \ce{C}=12, \ce{H}=1.)

Given

m(\ce{H2SO4}) = 4.9\,\mathrm{g}
M(\ce{H2SO4}) = 98\,\mathrm{g\,mol^{-1}}
M(\ce{CaCO3}) = 100\,\mathrm{g\,mol^{-1}}

To find

mass of \ce{CaCO3} dissolved
volume of \ce{CO2} at STP

Write the balanced equation of the corrosion. The mole ratio \ce{CaCO3}:\ce{H2SO4}:\ce{CO2} is 1:1:1.
\ce{CaCO3 + H2SO4 -> CaSO4 + H2O + CO2}
Convert the mass of acid to moles.
n(\ce{H2SO4})=\dfrac{4.9}{98}=0.050\,\mathrm{mol}
By the 1:1 ratio, 0.050\,\mathrm{mol} of \ce{CaCO3} dissolves. Convert to mass.
m(\ce{CaCO3})=0.050\times100=5.0\,\mathrm{g}
0.050\,\mathrm{mol} of \ce{CO2} is also formed. One mole of gas occupies 22.4\,\mathrm{dm^3} at STP.
V(\ce{CO2})=0.050\times22.4=1.12\,\mathrm{dm^3}
Check by mass. Products: \ce{CaSO4} 6.8\,\mathrm{g} + \ce{H2O} 0.9\,\mathrm{g} + \ce{CO2} 2.2\,\mathrm{g}=9.9\,\mathrm{g}. Reactants: 5.0+4.9=9.9\,\mathrm{g}.

m(\ce{CaCO3})=5.0\,\mathrm{g},\qquad V(\ce{CO2})=1.12\,\mathrm{dm^3}\ \text{at STP}

13.4Air quality parameters

Analysing air quality involves various methods and measures that show the presence of pollutants and the overall health of the air. The book describes five common ones.

Air Quality Index (AQI): a standard measure that quantifies overall air quality from several pollutants, such as particulate matter (\mathrm{PM_{2.5}} and \mathrm{PM_{10}}), ground-level ozone, carbon monoxide (\ce{CO}), sulphur dioxide (\ce{SO2}) and nitrogen dioxide (\ce{NO2}).
Particulate matter (PM) monitoring: measures the concentration of fine and coarse particles in the air. \mathrm{PM_{2.5}} (particles with a diameter of 2.5\,\mu\mathrm{m} or smaller) and \mathrm{PM_{10}} (particles with a diameter of 10\,\mu\mathrm{m} or smaller) are commonly monitored because of their health impacts.
Gas monitoring: instruments such as gas chromatographs and gas sensors measure the concentrations of gases such as \ce{CO2}, \ce{CO}, \ce{SO2}, \ce{NO2} and \ce{O3}. These gases are crucial indicators of air pollution.
Remote sensing: satellites equipped with sensors provide data on air quality over large geographic areas by measuring various pollutants and aerosols in the atmosphere. It is one of the modern methods.
Health impact assessment: analysing health data, such as hospital admissions and respiratory illnesses, in relation to air quality helps to quantify the public health impact of poor air quality.

13.5Air quality laws and standards

Air quality laws and regulations vary from country to country. The book gives these examples:

Clean Air Act (United States): a comprehensive law that sets air quality standards and regulates emissions from industrial sources, vehicles and other pollution sources.
EU Ambient Air Quality Directive: a European Union directive that sets air quality standards for various pollutants, including particulate matter, \ce{NO2} and \ce{SO2}. Member states are required to take measures to maintain these standards.
National Ambient Air Quality Standards (NAAQS): many countries set their own standards, specifying acceptable levels of pollutants. Violations can lead to fines or other enforcement actions.

The book gives the recommended concentrations under NAAQS 2009 as follows:

Recommended concentrations as per NAAQS 2009 (book table).
Pollutant	Averaging time	Recommended concentration
\mathrm{PM_{2.5}} (\mu\mathrm{g\,m^{-3}})	Annual	40
\mathrm{PM_{2.5}} (\mu\mathrm{g\,m^{-3}})	24-hour	60
\mathrm{PM_{10}} (\mu\mathrm{g\,m^{-3}})	Annual	60
\mathrm{PM_{10}} (\mu\mathrm{g\,m^{-3}})	24-hour	100
\ce{O3} (\mu\mathrm{g\,m^{-3}})	8-hour	100
\ce{NO2} (\mu\mathrm{g\,m^{-3}})	Annual	40
\ce{NO2} (\mu\mathrm{g\,m^{-3}})	24-hour	80
\ce{SO2} (\mu\mathrm{g\,m^{-3}})	Annual	50
\ce{SO2} (\mu\mathrm{g\,m^{-3}})	24-hour	80
\ce{CO} (\mathrm{mg\,m^{-3}})	8-hour	2

Worked example: comparing a city with the NAAQS 2009 limits

A monitor in a city gives: \mathrm{PM_{2.5}} 24-hour value 72\,\mu\mathrm{g\,m^{-3}}; \mathrm{PM_{10}} annual value 55\,\mu\mathrm{g\,m^{-3}}; \ce{NO2} 24-hour value 75\,\mu\mathrm{g\,m^{-3}}; \ce{SO2} annual value 56\,\mu\mathrm{g\,m^{-3}}; \ce{CO} 8-hour value 1800\,\mu\mathrm{g\,m^{-3}}. Which pollutants exceed the recommended concentrations in the book's table?

Match each reading with the limit for the same pollutant and the same averaging time. An annual value is compared only with the annual limit, a 24-hour value with the 24-hour limit.
\mathrm{PM_{2.5}} (24-hour): limit 60, reading 72. This exceeds the limit by 12\,\mu\mathrm{g\,m^{-3}}.
72>60
\mathrm{PM_{10}} (annual): limit 60, reading 55. Within the limit. (100 is the 24-hour limit and is not used here.)
55<60
\ce{NO2} (24-hour): limit 80, reading 75. Within the limit.
75<80
\ce{SO2} (annual): limit 50, reading 56. This exceeds the limit by 6\,\mu\mathrm{g\,m^{-3}}.
56>50
\ce{CO} (8-hour): the limit is in milligrams, 2\,\mathrm{mg\,m^{-3}}. Change the reading to the same unit before comparing: 1800\,\mu\mathrm{g}=1.8\,\mathrm{mg}. Within the limit.
1.8<2

Readings compared with the NAAQS 2009 table (units \mu\mathrm{g\,m^{-3}} except \ce{CO}).
Pollutant	Reading	Limit	Result
\mathrm{PM_{2.5}}, 24-hour	72	60	exceeds
\mathrm{PM_{10}}, annual	55	60	within
\ce{NO2}, 24-hour	75	80	within
\ce{SO2}, annual	56	50	exceeds
\ce{CO}, 8-hour	1.8 mg	2 mg	within

\mathrm{PM_{2.5}}\ \text{(24-hour) and } \ce{SO2}\ \text{(annual) exceed the limits}

13.5Measures to control air pollutionOften examined

The book lists the following measures used to control air pollution:

Emission standards: governments set emission limits for specific pollutants from industries and vehicles.
Emission trading schemes: some regions run cap-and-trade systems in which companies can buy and sell pollution allowances. This gives companies an incentive to reduce their emissions.
Vehicle emissions testing: regulations require vehicles to meet specific emission standards, and periodic testing of cars and trucks helps to ensure compliance. A catalytic converter, shown in Figure 13.2, is one way of meeting these standards.
Clean energy initiatives: promoting renewable energy sources and reducing reliance on fossil fuels can significantly improve air quality.
Public awareness and education: governments run campaigns to educate the public about air quality issues and encourage behaviours that reduce pollution.
Industrial best practices: cleaner production methods and process technologies reduce emissions at source.
Green building codes: regulations may require environmentally friendly construction practices and materials to reduce emissions from buildings.

Figure 13.2: Catalytic converter, showing the engine emissions (HC, CO and NOₓ) entering on the left and the tailpipe emissions (H₂O, CO₂ and N₂) leaving on the right.

13.6Measuring particulate matter

Particulate matter in the air is commonly detected with instruments known as particulate matter monitors. Monitors are designed for different sizes of particulate matter, such as \mathrm{PM_{10}} (particles with a diameter of 10 micrometres or smaller) and \mathrm{PM_{2.5}} (particles with a diameter of 2.5 micrometres or smaller).

Beta attenuation monitor (BAM): Measures particulate matter by analysing the attenuation of beta radiation as it passes through a filter with collected particles. The degree of attenuation is proportional to the mass concentration of particulate matter.
Gravimetric method: Collects particulate matter on a filter over a specified period. The filter is weighed before and after sampling to find the mass of the collected particles, which indicates the concentration.
Optical particle counter: Uses light scattering or laser diffraction to count and size the particles in the air. The amount of scattered light is related to the particle size, which allows the particle concentration to be determined.

13.6Measuring carbon monoxide

Carbon monoxide (\ce{CO}) in the air is commonly detected using various types of gas sensors and analysers. The book describes three methods.

Electrochemical sensors: Commonly used for detecting \ce{CO} in indoor environments and personal safety devices. A chemical reaction occurs at an electrode when \ce{CO} interacts with an electrolyte, generating an electrical current proportional to the \ce{CO} concentration.
Infrared (IR) absorption spectroscopy: A light source emits infrared radiation through the air sample, and a detector measures the amount of infrared light absorbed by \ce{CO} molecules, giving a concentration reading.
Chemical colour change detectors: Chemical detectors that change colour in the presence of \ce{CO}. They are used for qualitative assessments: less precise than quantitative methods, but they give a visible indication that \ce{CO} is present.

13.7Greenhouse effect and global warmingOften examined

The greenhouse effect is a natural phenomenon that plays a crucial role in regulating the Earth's temperature. Certain gases in the atmosphere trap radiant energy from the Sun, especially infrared (IR) radiation, and prevent it from escaping back into space. This trapped heat warms the planet and creates a stable and habitable climate. However, human activities have intensified the greenhouse effect, which has given rise to global warming, a pressing environmental concern.

Figure 13.3: The greenhouse effect. Gases in the atmosphere trap part of the infrared radiation that the Earth emits, so the heat is held near the surface.

Greenhouse gases: the primary greenhouse gases are carbon dioxide (\ce{CO2}), methane (\ce{CH4}), nitrous oxide (\ce{N2O}) and water vapour (\ce{H2O}). Human activities such as burning fossil fuels, deforestation and industrial processes have significantly increased the concentration of these gases in the atmosphere, leading to enhanced heat trapping.

13.7.2Causes of global warming

Burning fossil fuels: the combustion of fossil fuels such as coal, oil and natural gas releases large quantities of carbon dioxide and water vapour into the atmosphere.
Deforestation: the removal of forests reduces the planet's capacity to absorb \ce{CO2}, as trees are its major users.
Industrial processes: certain industrial activities emit greenhouse gases, including methane and nitrous oxide, which contribute to global warming.

13.7.3Consequences and mitigation of global warmingOften examined

Rising temperatures: global warming increases the average global temperature, causing heatwaves and changing weather patterns.
Melting ice and rising sea levels: the warming climate accelerates the melting of polar ice caps and glaciers, leading to rising sea levels.
Extreme weather events: more frequent and severe hurricanes, droughts, floods and wildfires may be linked to global warming.
Ecosystem disruption: climate change disrupts ecosystems, threatening biodiversity and the survival of many plant and animal species.
Human health impact: heat-related illnesses, the spread of diseases, and food and water shortages can harm human populations.
Mitigation: reducing greenhouse gas emissions through policies and actions such as transitioning to renewable energy sources, improving energy efficiency and promoting sustainable land use.

13.8.1Economic issues

Healthcare costs: poor air quality leads to respiratory diseases, which increase healthcare expenditure. This can strain public health systems and individual budgets.
Lost productivity: air pollution can reduce labour productivity through illness and absenteeism, affecting economic output.
Environmental costs: industries may bear the costs of environmental regulations, which affects their competitiveness. The book cites a study in the United States that estimated air pollution costs the economy billions of dollars a year in healthcare and productivity losses.

13.8.3Political issues

Regulation versus business interests: balancing environmental regulations with economic interests can be politically contentious, as industries often resist stricter air quality standards.
Public awareness: the level of public awareness and concern about air pollution can influence political will and policy decisions.
International cooperation: air pollution often crosses borders, so international agreements and cooperation are needed to address cross-border emissions.

In some countries, powerful industry lobbies work against stricter regulations. Countries with strong public awareness tend to have more stringent air quality regulations and enforcement.

13.8.4Technologies and strategies to reduce air pollutionOften examined

The book describes the following technologies and strategies for reducing air pollution and improving air quality.

Technologies and strategies to reduce air pollution (book text, Section 13.8.4).
Technology or strategy	How it reduces air pollution
Renewable energy resources	Transition to solar, wind and hydropower decreases dependence on fossil fuels
Electric vehicles	Curtail emissions from conventional automobiles, contributing to cleaner air in urban areas
Eco-friendly building designs	Energy-efficient designs curtail energy consumption and reduce the environmental footprint
Plantation	Green spaces and planted trees absorb pollutants and provide environmental health benefits
Regulating pollutant release	Enforcing and updating emission standards for industries and vehicles
Proper waste disposal	Recycling and proper disposal prevent harmful pollutants being released from waste burning or decomposition
Deploying monitoring systems	Identifies pollution sources and allows timely intervention
Clean industries	Cleaner technologies and processes cut emissions at the source
Public awareness	Awareness of the impact of individual choices leads to behavioural changes and a collective effort

13.8.5An experiment to analyse air quality

The book describes an experiment to test whether indoor plants affect air quality.

Null hypothesis (H_0): The presence of indoor plants has no significant effect on air quality.
Alternative hypothesis (H_1): Indoor plants positively impact air quality by reducing pollutants.

Materials: two identical small-sized classrooms; identical air quality monitors (simple DIY monitors can be used); four small potted plants suitable for indoor environments; a thermometer and a hygrometer.

Set-up: place two plants in one classroom (the experimental group) and keep the other classroom as the control group, without plants. Make sure both classrooms have similar initial air quality readings.

Design of the experiment, as set up in the book.
Classroom	Group	Plants
First classroom	Experimental group	Two potted plants
Second classroom	Control group	No plants

Variables: presence or absence of indoor plants; temperature and humidity.

Data collection: take daily temperature and humidity readings in both classrooms. Use simple DIY air quality indicators (for example, colour changes that indicate \ce{CO2} levels) and record the observations. Measure and record the growth of the plants over the course of the experiment.

Data analysis: compare the temperature, humidity and air quality observations between the two classrooms, and discuss the observations to interpret the results.

Replication and conclusion: have several groups of students conduct the same experiment in different classrooms to compare results. Discuss the findings and draw conclusions about the impact of plants on air quality, based on the observations.

Key terms

Air pollutant: A gas or particle added to the air by human activity, in addition to natural sources, that pollutes the air.
Primary pollutant: A pollutant that is emitted directly from its source.
Secondary pollutant: A pollutant that forms through chemical reactions in the atmosphere.
Particulate matter (PM): Tiny particles suspended in the air. \mathrm{PM_{10}} are particles of diameter 10 micrometres or smaller; \mathrm{PM_{2.5}} are particles of diameter 2.5 micrometres or smaller.
Volatile organic compounds (VOCs): Organic compounds released by vehicle emissions, industrial processes and household products, which contribute to the formation of ground-level ozone.
Troposphere: The lowest layer of the atmosphere, from the surface up to about 12 km, where weather occurs.
Stratosphere: The layer above the troposphere that contains the ozone layer, which absorbs and scatters the Sun's UV radiation.
Mesosphere: The layer above the stratosphere, with extremely low temperatures, where meteors burn up and auroras occur.
Thermosphere: The uppermost layer, with high temperatures, divided into the ionosphere and the exosphere.
Ozone layer: The layer of ozone in the stratosphere that protects the Earth from most dangerous UV radiation from the Sun.
Ozone depletion: The thinning of the ozone layer, mainly caused by CFCs, which allows more harmful UV radiation to reach the Earth's surface.
Chlorofluorocarbons (CFCs): Synthetic compounds used in refrigeration, aerosol propellants and foam production, which break down ozone molecules in the upper atmosphere.
Smog: A mixture of smoke and fog, a composite of many pollutants that are highly dangerous to human beings.
Photochemical smog: Smog formed by reactions between nitrogen oxides and VOCs in sunlight, common in urban areas with high vehicle emissions; it contains ozone.
Industrial smog: Black smog formed by burning coal or other fossil fuels, containing large amounts of particulate matter and sulphur dioxide.
Ground-level ozone: Ozone formed when VOCs and nitrogen oxides react in the presence of sunlight; it is a major component of smog.
PAN: Peroxyacetyl nitrate, a product formed in the reactions that produce photochemical smog.
Acid rain: Rainfall or precipitation with a pH lower than 5.6, caused mainly by the oxides of sulphur and nitrogen.
Transboundary pollution: Pollution that travels long distances in the atmosphere and crosses national borders.
Greenhouse effect: A natural phenomenon in which gases in the atmosphere trap infrared radiation from the Sun and the Earth, keeping the planet warm.
Global warming: The rise in average global temperature caused by the intensification of the greenhouse effect by human activities.
Carbon sink: A reservoir that absorbs carbon dioxide; trees act as carbon sinks.
Air Quality Index (AQI): A standard measure of overall air quality based on several pollutants.
NAAQS: National Ambient Air Quality Standards: the acceptable levels of pollutants that many countries set for their air.
Emission trading scheme: A cap-and-trade system in which companies can buy and sell pollution allowances.
Catalytic converter: A vehicle device through which engine emissions (HC, CO and NOx) pass before the tailpipe emissions (\ce{H2O}, \ce{CO2} and \ce{N2}) leave (Figure 13.2).
Beta attenuation monitor (BAM): An instrument that measures particulate matter by the attenuation of beta radiation passing through a filter with collected particles.
Gravimetric method: A method of measuring particulate matter by collecting it on a filter and weighing the filter before and after sampling.
Optical particle counter: An instrument that counts and sizes particles in the air using light scattering or laser diffraction.
Electrochemical sensor: A sensor in which a reaction at an electrode, when CO reaches the electrolyte, produces a current proportional to the CO concentration.

Multiple choice questionsp. 268–269

What is a primary source of air pollution?

ANatural volcanic eruptions
BIndustrial emissions
CForest fires
DAll of the aboveAnswer

Why

Primary pollutants are added to the air directly by their source. Volcanic eruptions release sulphur compounds such as \ce{SO2}, industrial emissions release \ce{SO2}, \ce{NO_x} and particulate matter, and forest fires release smoke and particulate matter. All three are sources, so the complete answer is D. The book lists natural sources such as wildfires and dust storms alongside human sources, so B and C are not wrong on their own, but neither is complete.

Note. The book prints no key. 'Primary source' is read as a source that emits pollutants directly, so all three options qualify.

Which gas is primarily responsible for the greenhouse effect?

AOxygen (\ce{O2})
BCarbon dioxide (\ce{CO2})Answer
CNitrogen (\ce{N2})
DHydrogen (\ce{H2})

Why

Carbon dioxide is the greenhouse gas the book names first, and human activities such as burning fossil fuels and deforestation raise its concentration, which increases heat trapping. \ce{O2} and \ce{N2} make up most of the air but do not absorb infrared radiation in any significant amount, so they do not trap heat. \ce{H2} is not on the book's list of greenhouse gases.

iii

Which of the following is NOT a greenhouse gas?

AMethane (\ce{CH4})
BNitrous oxide (\ce{N2O})
CSulphur dioxide (\ce{SO2})Answer
DWater vapour (\ce{H2O})

Why

The book lists carbon dioxide, methane, nitrous oxide and water vapour as the primary greenhouse gases, so A, B and D are all greenhouse gases. Sulphur dioxide is an air pollutant that causes acid rain and respiratory problems; it is not one of the greenhouse gases in the book.

What is the main component of smog that poses health risks to humans?

AOxygen (\ce{O2})
BCarbon monoxide (\ce{CO})
CSulphur dioxide (\ce{SO2})
DNitrogen dioxide (\ce{NO2})Answer

Why

Oxygen is not a pollutant. Carbon monoxide reduces the blood's ability to carry oxygen, but it is not a main component of smog. Nitrogen dioxide is the main nitrogen oxide in photochemical smog, which the book lists first among the components of smog (\ce{NO_x}, VOCs, particulate matter, \ce{SO2} and ozone), and it causes respiratory problems. Sulphur dioxide is the main component of industrial (black) smog, which is why the answer depends on the type of smog.

Note. The book prints no key. Sulphur dioxide is the main component of industrial smog. D is chosen because the book's composition list for photochemical smog starts with nitrogen oxides.

Acid rain is primarily caused by the emission of:

ACarbon dioxide (\ce{CO2})
BAmmonia (\ce{NH3})
CSulphur dioxide (\ce{SO2})Answer
DOxygen (\ce{O2})

Why

The book says the primary cause of acid rain is the release of sulphur dioxide and nitrogen oxides from burning fossil fuels. Sulphur dioxide reacts with oxygen and water to form sulphuric acid. Carbon dioxide forms only weak carbonic acid, which the book says is not acid rain. Ammonia contributes to particulate matter, and oxygen is not a pollutant.

Which of the following is a short-term effect of air pollution on human health?

AIncreased cancer risk
BRespiratory problemsAnswer
CLong-term memory loss
DNone of the above

Why

Respiratory problems such as coughing, irritation and difficulty in breathing appear soon after exposure to smog and other pollutants. Cancer risk is a long-term effect of exposure to carcinogenic compounds such as PAHs, and long-term memory loss is not an effect the book describes. D is wrong because B is a short-term effect.

vii

What is the main consequence of global warming?

ARising sea levelsAnswer
BDecreased carbon dioxide levels
CIncreased ozone layer thickness
DReduced rainfall

Why

Global warming raises average temperatures, which melts polar ice caps and glaciers and so raises sea levels. The book lists this among its main consequences, after rising temperatures. B is the reverse of what happens, since human activities increase carbon dioxide. C is wrong because ozone depletion, not global warming, thins the ozone layer. D is not the main consequence the book describes; droughts are listed under extreme weather.

Note. The book prints no key. 'Main consequence' is read as the consequence the book identifies in its list: rising temperatures lead to melting ice and rising sea levels.

viii

Which of these is NOT an adverse effect of air pollution on the environment?

ADisruption of ecosystems
BIncreased agricultural productivityAnswer
CDamage to buildings and monuments
DLoss of biodiversity

Why

Disruption of ecosystems, damage to buildings and monuments (through acid rain corrosion) and loss of biodiversity are all adverse environmental effects described in the book. Air pollution reduces crop yields, for example through ozone, so increased agricultural productivity is not an effect of air pollution.

The ozone layer depletion is primarily caused by:

AGreenhouse gases
BVolcanic eruptions
CChlorofluorocarbons (CFCs)Answer
DDeforestation

Why

CFCs are synthetic compounds used in refrigeration, aerosol propellants and foam production. They reach the upper atmosphere, where they break down ozone molecules and thin the ozone layer. Greenhouse gases warm the lower atmosphere, volcanic eruptions and deforestation do not destroy ozone, and the book's 'Do you know?' box links the ozone hole to CFCs.

What can individuals do to help reduce air pollution?

ADrive gasoline-powered cars
BUse single-use plastics
CReduce energy consumptionAnswer
DBurn waste materials openly

Why

Reducing energy consumption cuts the fossil fuel burning that releases \ce{SO2}, \ce{NO_x} and carbon dioxide. A and B add to pollution, and D releases pollutants directly into the air. The book's public awareness point says that individual choices lead to behavioural changes that reduce pollution.

Short questionsp. 269

ii3 marks

What are main sources of air pollution?

Solution

Air pollutants come from natural sources and from human activities. The book's natural sources are volcanic eruptions, wildfires, dust storms, sea sprays, pollen, spores and the decomposition of organic matter by bacteria and other microorganisms.

Burning fossil fuels (coal, oil and natural gas) in vehicles, power plants and for domestic use, which releases \ce{SO2}, \ce{NO_x} and \ce{CO}.
Industrial processes such as manufacturing, mining, ore refining and foundries, which release particulate matter, heavy metals and volatile organic compounds.
Deforestation, which removes trees that absorb carbon dioxide (\ce{CO2}).
Agriculture and livestock waste, which release ammonia (\ce{NH3}).
Refrigerants and aerosol propellants, which release chlorofluorocarbons (CFCs) that destroy ozone.
Burning of waste and the use of household products, which release volatile organic compounds.

iii3 marks

Describe photochemical smog, its ingredients and health effects.

Solution

Photochemical smog forms in urban areas with heavy vehicle traffic. Nitrogen oxides (\ce{NO_x}) and volatile organic compounds (VOCs) from vehicles react in the presence of sunlight. Its ingredients are nitrogen oxides, volatile organic compounds, particulate matter, sulphur dioxide (\ce{SO2}) and ozone (\ce{O3}), and ozone is the component that harms the lungs and plants.

\begin{gathered} \ce{NO2 + sunlight -> NO + O} \\ \ce{O2 + O -> O3} \\ \ce{NO + O3 -> NO2 + O2} \\ \ce{NO2 + R -> products such as PAN} \\ \ce{NO + ROX -> NO2 + other products} \end{gathered}

Health effects: smog irritates the eyes and airways, causes coughing and difficulty in breathing, and worsens asthma, pharyngitis and bronchitis. Prolonged exposure is linked to heart attacks and arrhythmia. Smog also reduces visibility, which raises the risk of accidents, and it damages crops and forests. Here R is an organic fragment from VOCs, and PAN is peroxyacetyl nitrate.

iv3 marks

Write down the reactions which are involved in the formation of acid rain.

Solution

Acid rain forms when the oxides of sulphur and nitrogen released by burning fossil fuels react with water and oxygen in the air. The acids dissolve in rain and lower its pH below 5.6. Carbon dioxide alone forms only weak carbonic acid, so it does not make acid rain.

\begin{gathered} \ce{2SO2 + O2 + 2H2O -> 2H2SO4} \\ \ce{H2SO4 + H2O -> H3O+ + HSO4-} \\ \ce{3NO2 + H2O -> 2HNO3 + NO} \\ \ce{HNO3 + H2O -> H3O+ + NO3-} \end{gathered}

Sulphur dioxide forms sulphuric acid, and nitrogen dioxide forms nitric acid. Both acids ionise in water to give hydronium ions, which make the rain acidic.

Note. Three equations in the book are misprinted. The book writes \ce{NO2 + H2O -> HNO3}, which is not balanced, so the balanced form \ce{3NO2 + H2O -> 2HNO3 + NO} is used here. It writes \ce{H2SO4 + H2O -> H3O+ + SO4^2-}, which has the wrong charge, and the first ionisation gives \ce{HSO4-}. The book also gives \ce{CaCO3 + H2SO4 -> CaSO4 + H2O} with no \ce{CO2} product; that equation is not part of this answer.

v3 marks

Describe the phenomenon of greenhouse effect.

Solution

Sunlight passes through the atmosphere and warms the Earth's surface. The warm Earth radiates the energy back as infrared (IR) radiation. Greenhouse gases such as carbon dioxide (\ce{CO2}), methane (\ce{CH4}), nitrous oxide (\ce{N2O}) and water vapour absorb this infrared radiation and re-emit it in all directions. Part of it returns to the surface, so heat is trapped and the average temperature of the Earth stays suitable for life.

The greenhouse effect is a natural phenomenon. Human activities such as burning fossil fuels, deforestation and industrial processes add more greenhouse gases, which trap more heat and cause global warming.

viii3 marks

Suggest some measures to reduce air pollution.

Solution

The book suggests the following measures, which act at the source, during use and by public action:

Renewable energy: replace fossil fuels with solar, wind and hydropower sources.
Electric vehicles: use electric cars to cut emissions from conventional automobiles in urban areas.
Eco-friendly building design: energy-efficient buildings reduce energy consumption and the overall environmental footprint.
Plantation: planting trees and green spaces absorbs pollutants and improves air quality.
Emission standards and testing: set and update emission limits for industries and vehicles, use catalytic converters and test vehicle emissions regularly.
Emission trading: cap-and-trade systems let companies buy and sell pollution allowances, which encourages them to reduce emissions.
Proper waste disposal and recycling: prevents harmful pollutants being released by waste burning or decomposition.
Monitoring systems: identify pollution sources so that action can be taken in time.
Clean industries and public awareness: cleaner technologies cut emissions at source, and informed choices by individuals reduce pollution.

x3 marks

Enlist some problems caused by global warming.

Solution

Global warming causes the following problems:

Rising temperatures: heatwaves and changing weather patterns.
Melting ice and rising sea levels: polar ice caps and glaciers melt, which raises sea levels.
Extreme weather events: more frequent and severe hurricanes, droughts, floods and wildfires.
Ecosystem disruption: threats to biodiversity and to the survival of many plant and animal species.
Human health impacts: heat-related illnesses, the spread of diseases, and food and water shortages.

Long questionsp. 269

vi6 marks

What is heavy metal pollution? Explain health problems caused by it.

Solution

Heavy metal pollution is the contamination of air, soil and water by heavy metals such as lead (Pb), mercury (Hg) and cadmium (Cd). These metals are released mainly by industrial processes.

Sources

Metal	Main sources
Lead (\ce{Pb})	Combustion of petrol containing lead (now removed), incinerators, foundries and lead transformation
Mercury (\ce{Hg})	Combustion of coal, refining of ores, incinerators and natural sources
Cadmium (\ce{Cd})	Industrial processes

How they cause health problems

Heavy metals do not break down in the environment. They contaminate soil and water, enter the food chain and build up in the bodies of plants and animals. People are exposed through contaminated food and water and by breathing dust and fumes. The main health problems are:

Lead: damage to the nervous system and the brain, especially in children, and anaemia.
Mercury: damage to the nervous system and kidneys.
Cadmium: damage to the kidneys and bones, and linked with lung damage.

Heavy metal pollution is therefore a long-term health hazard, which is why emissions from industry are controlled and lead has been removed from petrol.

vii6 marks

What is catalytic converter? Explain its working.

Solution

A catalytic converter is a device fitted in the exhaust system of a vehicle. It contains a catalyst, usually platinum, palladium and rhodium, on a honeycomb support. The catalyst converts harmful exhaust gases (carbon monoxide, unburnt hydrocarbons and nitrogen oxides) into less harmful gases before they leave the tailpipe.

Working

The hot exhaust gases pass over the catalyst surface. The gases are adsorbed on the surface, their bonds weaken and they react to form new products. The catalyst is not used up, so it can work continuously.

Carbon monoxide is oxidised to carbon dioxide.
\ce{2CO + O2 -> 2CO2}
Unburnt hydrocarbons are oxidised to carbon dioxide and water.
\ce{C_xH_y + (x + y/4)O2 -> xCO2 + (y/2)H2O}
Nitrogen oxides are reduced to nitrogen.
\ce{2NO + 2CO -> N2 + 2CO2}

Catalytic converter: the catalyst changes HC, CO and NOx from the engine into H2O, CO2 and N2 before the gases leave the tailpipe.

The products \ce{H2O}, \ce{CO2} and \ce{N2} leave the tailpipe, which is why the converter reduces the pollution from the vehicle without stopping the engine.

ix6 marks

What are different protocols and accords being followed in the world to reduce air pollution?

Solution

Countries reduce air pollution through laws, standards and international cooperation. The book describes the following:

Laws and standards

Clean Air Act (United States): a comprehensive law that sets air quality standards and regulates emissions from industrial sources, vehicles and other sources.
EU Ambient Air Quality Directive: sets standards for particulate matter, \ce{NO2}, \ce{SO2} and other pollutants, and member states must take measures to maintain them.
National Ambient Air Quality Standards (NAAQS): national limits for acceptable pollutant levels. The NAAQS 2009 limit for \mathrm{PM_{2.5}} is 40\,\mathrm{\mu g\,m^{-3}} as an annual average and 60\,\mathrm{\mu g\,m^{-3}} over 24 hours, and violations can lead to fines or enforcement action.

Measures and cooperation

Emission standards and emission trading: limits for industries and vehicles, and cap-and-trade schemes in which companies buy and sell pollution allowances.
Vehicle emissions testing: periodic testing and catalytic converters.
Clean energy and green building codes: renewable energy initiatives (in Pakistan the Alternate Energy Development Board works on these) and environmentally friendly construction.
Public awareness and industrial best practices: campaigns that change behaviour and cleaner production methods.

International cooperation: air pollution crosses borders, so countries sign international agreements to control cross-border emissions. Stronger regulation depends on public awareness, because powerful industries may lobby against stricter rules.

Note. The book does not name any international protocol or accord. The answer uses the laws, standards and cooperation the book describes.

xi6 marks

Evaluate the human activities that contribute to environmental pollution.

Solution

Human activities add gases and particles to the air, soil and water. The table shows the main activities, the pollutants they release and their effects.

Activity	Main pollutants	Main environmental effect
Burning fossil fuels in vehicles, power plants and homes	\ce{SO2}, \ce{NO_x}, \ce{CO}, \ce{CO2}, particulate matter	Smog, acid rain and global warming
Industrial processes (manufacturing, mining, refining, foundries)	Particulate matter, \ce{SO2}, heavy metals, VOCs	Smog, acid rain and contamination of soil and water
Deforestation	Less \ce{CO2} absorbed from the air	Higher atmospheric \ce{CO2} and global warming
Agriculture and livestock waste	Ammonia (\ce{NH3})	Particulate matter and water pollution
Refrigeration and aerosol propellants	CFCs	Ozone depletion and more harmful UV radiation
Burning waste and household products	VOCs, polycyclic aromatic hydrocarbons	Ground-level ozone, smog and cancer-causing compounds

Evaluation

Burning fossil fuels is the most serious activity because it causes smog, acid rain and global warming at the same time. Industrial and vehicle emissions harm people and ecosystems mainly near their source, while CFCs and carbon dioxide affect the whole planet. Many of these activities bring benefits, such as energy, food and goods, so the aim is to reduce their pollution, through cleaner technologies, emission standards and monitoring, rather than to stop them.

xii6 marks

Analyse the role of environmental chemistry in climate change.

Solution

Environmental chemistry studies the gases in the atmosphere, how they trap heat, where they come from and how they can be reduced. It explains the causes of climate change, provides the methods to measure the gases that cause it, and supports the measures to reduce them.

The greenhouse effect

Sunlight warms the Earth, and the Earth emits infrared radiation. Greenhouse gases (\ce{CO2}, \ce{CH4}, \ce{N2O} and water vapour) absorb infrared radiation and send part of it back to the surface, which keeps the planet warm. Human activities increase these gases, so more heat is trapped and the average temperature rises.

The greenhouse effect: sunlight passes through the atmosphere and warms the Earth, and greenhouse gases absorb the infrared radiation the Earth emits and send part of it back.

Causes of global warming

Burning fossil fuels, which releases large quantities of carbon dioxide and water vapour.
Deforestation, which reduces the absorption of carbon dioxide because trees are its major users.
Industrial processes, which emit methane and nitrous oxide.

Role of environmental chemistry

Measurement: gas chromatographs and gas sensors measure carbon dioxide, carbon monoxide, sulphur dioxide, nitrogen dioxide and ozone in the air.
Identification of greenhouse gases: infrared absorption spectroscopy measures how much infrared light a gas absorbs, which is the property that makes it a greenhouse gas.
Monitoring large areas: satellites with sensors measure gases and aerosols over large geographic areas.
Mitigation: transition to renewable energy, improving energy efficiency, sustainable land use and tree plantation reduce the emissions that cause global warming.

Environmental chemistry therefore explains how climate change happens, measures the gases that cause it, and provides the evidence needed to reduce their emissions.

FBISE 2025 and 2026 questions

Questions from the FBISE HSSC-I 2026 (1st Annual) and FBISE HSSC-I 2025 (1st Annual) papers that belong to this unit, answered the way an examiner expects.

2026 Annual I, Set B, Q.2 (vii)3 marks

Explain the role of \ce{NO_x} in the formation of Peroxyacetyl nitrate (PAN) with chemical reactions.

Solution

Peroxyacetyl nitrate (PAN) is a harmful component of photochemical smog. Smog of this type forms in sunlight from nitrogen oxides (\mathrm{NO_x}, that is \ce{NO} and \ce{NO2}) and volatile organic compounds (VOCs). The steps below show the role of \mathrm{NO_x}.

**Role of \ce{NO2} as the source of oxygen atoms.** Nitrogen dioxide, released in vehicle exhaust, absorbs sunlight and splits into nitric oxide and an oxygen atom.
\ce{NO2 + sunlight -> NO + O}
The oxygen atom joins an oxygen molecule to form ozone, which is the harmful oxidant of the smog.
\ce{O2 + O -> O3}
**Role of \ce{NO}.** Nitric oxide reacts with ozone and regenerates nitrogen dioxide, so \ce{NO2} is available again for the cycle.
\ce{NO + O3 -> NO2 + O2}
Formation of PAN. Volatile organic compounds (VOCs) in the air give organic fragments R. Nitrogen dioxide reacts with them to form PAN, peroxyacetyl nitrate.
\ce{NO2 + R -> PAN}
Nitric oxide also reacts with the organic fragments (\ce{ROX}, as written in the book) to form more \ce{NO2}, which keeps the formation of PAN going.
\ce{NO + ROX -> NO2}

Peroxyacetyl nitrate (PAN) PAN, formed when \ce{NO2} combines with the peroxyacetyl fragment of the VOCs.

PAN is a harmful ingredient of photochemical smog together with ozone; it irritates the eyes and the respiratory tract. In short, \ce{NO2} supplies oxygen atoms for ozone and itself combines with the organic fragments to give PAN, while \ce{NO} recycles \ce{NO2}.

Note. The book writes the last two steps with the symbols R and \ce{ROX} for the organic fragments; the same symbols are used here.

2025 Annual I, Set D, Q.2 (vi) OR1+2 = 3 marks

What is photochemical smog? Briefly describe its formation.

Solution

What is photochemical smog? (1 mark)

Smog is a mixture of smoke and fog, or of other pollutants, in the air. Photochemical smog forms in sunlight, usually in urban areas with heavy vehicle traffic, from the reaction of nitrogen oxides (\mathrm{NO_x}) and volatile organic compounds (VOCs). It contains ozone, PAN, \mathrm{NO_x}, VOCs, particulate matter and \ce{SO2}, and it is harmful to human health.

Formation (2 marks)

**Role of \ce{NO2} as the source of oxygen atoms.** Nitrogen dioxide, released in vehicle exhaust, absorbs sunlight and splits into nitric oxide and an oxygen atom.
\ce{NO2 + sunlight -> NO + O}
The oxygen atom joins an oxygen molecule to form ozone, which is the harmful oxidant of the smog.
\ce{O2 + O -> O3}
Nitric oxide reacts with ozone and regenerates \ce{NO2}.
\ce{NO + O3 -> NO2 + O2}
Nitrogen dioxide reacts with organic fragments R from the VOCs to form PAN and other products.
\ce{NO2 + R -> PAN}

In this way sunlight changes the primary pollutants (\mathrm{NO_x} and VOCs) into the secondary pollutants ozone and PAN, which make up the smog.

2026 Annual I, Set D, Q.2 (vii)3 marks

Explain the role of \ce{NO} and \ce{NO2} in the formation of peroxyacetyl nitrate (PAN), with equations.

Solution

Nitric oxide (\ce{NO}) and nitrogen dioxide (\ce{NO2}), together called \mathrm{NO_x}, take part in the formation of peroxyacetyl nitrate (PAN), a harmful ingredient of photochemical smog, when sunlight acts on a mixture with volatile organic compounds (VOCs).

**Role of \ce{NO2} as the source of oxygen atoms.** Nitrogen dioxide, released in vehicle exhaust, absorbs sunlight and splits into nitric oxide and an oxygen atom.
\ce{NO2 + sunlight -> NO + O}
The oxygen atom joins an oxygen molecule to form ozone, which is the harmful oxidant of the smog.
\ce{O2 + O -> O3}
**Role of \ce{NO}.** Nitric oxide reacts with ozone and regenerates nitrogen dioxide, so \ce{NO2} is available again for the cycle.
\ce{NO + O3 -> NO2 + O2}
Formation of PAN. Volatile organic compounds (VOCs) in the air give organic fragments R. Nitrogen dioxide reacts with them to form PAN, peroxyacetyl nitrate.
\ce{NO2 + R -> PAN}
Nitric oxide also reacts with the organic fragments (\ce{ROX}, as written in the book) to form more \ce{NO2}, which keeps the formation of PAN going.
\ce{NO + ROX -> NO2}

Peroxyacetyl nitrate (PAN) PAN, formed when \ce{NO2} combines with the peroxyacetyl fragment of the VOCs.

Note. The book writes the last two steps with the symbols R and \ce{ROX} for the organic fragments; the same symbols are used here.

Exam focus

Explain the role of \ce{NO} and \ce{NO2} in the formation of peroxyacetyl nitrate (PAN) with equations. (short)
Asked in both 2026 sets (Set B Q.2 (vii) and Set D Q.2 (vii)).
Go to answer
What is photochemical smog? Describe its formation, ingredients and health effects. (short)
Asked in 2025 (Set D Q.2 (vi) OR) and book short question 3.
Go to answer
Write the reactions involved in the formation of acid rain and name its effects. (short)
Book short question 4 and MCQ 5; not asked in the 2025 or 2026 papers.
Go to answer
Describe the greenhouse effect and global warming, with the causes and consequences. (short)
Book short questions 5 and 10 and MCQs 2, 3 and 7; not asked in the 2025 or 2026 papers.
Go to answer
What is a catalytic converter and how does it work? (long)
Book long question 7; not asked in the 2025 or 2026 papers.
Go to answer
State the main sources of air pollution and the measures to control it. (short)
Book short questions 2 and 8; not asked in the 2025 or 2026 papers.
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Compare photochemical and industrial smog. (short)
The comparison table of the section; the 2025 and 2026 questions on smog are about photochemical smog.
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13.1Air pollution

13.1.1Atmosphere and its layers

13.1.2Air pollutantsOften examined

13.1.3Impacts of human activities on the atmosphere

13.2Smog

13.2.1Types of smogOften examined

Photochemical smog

Industrial smog

Composition of photochemical smog

General mechanism of formation of smog

13.2.2Hazardous effects of smog

13.3Acid rainOften examined

13.3.1Reasons for acid rainOften examined

Formation of sulphuric acid

Formation of nitric acid

13.3.2Impacts of acid rain

13.4Air quality parameters

13.5Air quality laws and standards

13.5Measures to control air pollutionOften examined

13.6Measuring particulate matter

13.6Measuring carbon monoxide

13.7Greenhouse effect and global warmingOften examined

13.7.2Causes of global warming

13.7.3Consequences and mitigation of global warmingOften examined

13.8.1Economic issues

13.8.2Social issues

13.8.3Political issues

13.8.4Technologies and strategies to reduce air pollutionOften examined

13.8.5An experiment to analyse air quality

Key terms

Multiple choice questionsp. 268–269

Short questionsp. 269

Long questionsp. 269

Sources

How they cause health problems

Working

Laws and standards

Measures and cooperation

Evaluation

The greenhouse effect

Causes of global warming

Role of environmental chemistry

FBISE 2025 and 2026 questions

What is photochemical smog? (1 mark)

Formation (2 marks)

Exam focus