Chapter 10Environmental Chemistry

The objective of environmental education is to enlighten the people, particularly students, about the importance of protection and conservation of our environment. It teaches how pollutants harm living and non-living things, and what each person can do to reduce pollution. The need for it, both formal and non-formal, is keenly felt at the national level.

Carbon monoxide (CO) is far more poisonous. It is an extremely toxic gas that stops red blood cells from carrying the oxygen the body needs, causing suffocation and death even in small amounts; it has no colour or smell. CO₂ is not poisonous at normal levels (we breathe it out); it is harmful as a greenhouse gas.

Acid rain makes the soil more acidic, dissolves and washes away the nutrients plants need, and dissolves toxic aluminium and mercury. Trees weaken, lose leaves and die. Tree roots hold the soil on hills and mountains; when trees are destroyed the soil is washed away and new plants cannot grow.

When coal and oil burn, the sulphur in them forms sulphur dioxide: S + O₂ → SO₂. SO₂ irritates the eyes and causes breathing difficulties. In the air it is oxidised to SO₃ (helped by oxides of nitrogen), which forms sulphuric acid with moisture and falls as acid rain, damaging soil, lakes, forests and buildings; it also gives haze.

Wind turns the blades of wind turbines, which drive generators and produce electricity without burning fuel, so no CO₂, SO₂, NOₓ or smoke is released. It is renewable (it will not run out) and cheap to run once installed. Pakistan has wind farms in the Jhimpir-Gharo wind corridor of Sindh. Wind is also used to pump water and to sail boats.

Late at night and early in the morning, before sunrise. The air is coolest then, so it can hold the least water vapour and its relative humidity is highest; that is when dew and fog form. Humidity is lowest in the hot afternoon.

Carbon dioxide (CO₂). It is dissolved under pressure and comes out as bubbles when the pressure is released.

Dust and soot settle on leaves, blocking sunlight and the stomata, so less CO₂ is absorbed in photosynthesis; SO₂, NOₓ and ozone damage leaves; acid rain makes soil acidic and washes out nutrients; smog inhibits plant growth.

SO₂ gives sulphurous acid H₂SO₃ (and, after oxidation to SO₃, sulphuric acid H₂SO₄). NO₂ gives nitrous acid HNO₂ and nitric acid HNO₃.

Tree roots hold the soil together on hills and mountains. If the trees are destroyed, the soil is washed away (soil erosion) and new plants cannot grow there.

All living things add CO₂ by respiration (and decay of dead matter); humans add more by burning fuels. Plants remove CO₂ by photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.

Methane (CH₄). It is produced by bacteria in the waterlogged soil of rice fields, where vegetation decays without air, and escapes through the plants.

Methane (CH₄), along with carbon dioxide; landfill gas is mostly these two.

About 78 % nitrogen, 21 % oxygen, and the remainder a mixture of noble gases (mainly argon, 0.93 %) and carbon dioxide (0.04 %). Water vapour varies with humidity.

Any substance (solid, liquid or gas) in the air which has an adverse effect on human health, quality of life and the natural functioning of the ecosystem.

Parts per million: one part of pollutant per million parts of the mixture. CO₂ at 0.04 % is 400 ppm.

CO₂, CO, NOₓ, SOₓ, hydrocarbons (methane), particulates and ozone. Together they account for more than 90 % of air pollution.

At the very high temperature inside the engine, nitrogen and oxygen of the air combine: N₂ + O₂ → 2NO; then 2NO + O₂ → 2NO₂.

Smoke mixed with fog. In winter cold, still, humid air gives fog, and smoke from vehicles, industry, brick kilns and crop burning stays trapped near the ground, so Lahore and other cities are covered by smog.

When heat and sunlight cause chemical reactions between oxides of nitrogen and volatile organic compounds (hydrocarbons), mostly from car exhausts. It is a part of photochemical smog.

It has no colour, smell or taste, so it cannot be noticed; it stops red blood cells from carrying oxygen, causing headache, sleepiness, suffocation and death, often from faulty gas heaters in closed rooms.

They damage lungs, irritate the eyes and damage vegetation. They cause acid rain, which affects buildings of limestone, and photochemical smog.

It irritates the eyes, causes breathing difficulties and acid rain, and produces haze.

It irritates the eyes and causes severe breathing problems for people with asthma, increases the risk of respiratory diseases and cancer, dirties clothes and reduces visibility.

It leads to allergies, asthma and lung infections. It also inhibits the growth of plants by reducing the amount of CO₂ absorbed during photosynthesis.

Rain water with pH between 4.2 and 4.4 is known as acid rain. In 1852 Robert Angus Smith first showed its relationship with atmospheric pollution in Manchester; he is called the "Father of Acid Rain".

NO₂ oxidises SO₂ to SO₃ and becomes NO, which is oxidised back to NO₂ by air, ready to act again: NO₂ + SO₂ → NO + SO₃; 2NO + O₂ → 2NO₂. So NOₓ speeds up the formation of H₂SO₄, besides forming HNO₃ directly.

Wind can carry the acidic droplets to huge distances before they fall, even into other countries.

It falls into streams, lakes and rivers and makes the water too acidic for animals to live in. Many lakes and rivers no longer have fish.

By monitoring the soil and, when necessary, adding crushed lime to neutralise the acid.

They are made of marble (CaCO₃), which reacts with acid rain: CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂. The surface dissolves and crumbles.

The progressive warming up of the Earth's surface due to the blanketing effect of man-made CO₂, methane, water vapour and other gases in the atmosphere.

Short-wave radiation from the Sun passes through them and heats the Earth. The hot surface emits long-wave radiation that is mostly absorbed by greenhouse gases, which stops it reaching space and slows the cooling of the Earth.

The thick layer of CO₂ in the lower atmosphere allows incoming solar radiation to pass but does not allow the heat to escape, just as the glass of a greenhouse lets sunlight in and keeps the warmth inside.

About half of the CO₂ released is used by plants in photosynthesis. Cutting forests diminishes the capacity of trees to remove CO₂, and burning the wood adds even more.

Higher temperature makes sea water expand, and melting ice of Antarctica and Greenland flows into the sea. Low-lying coastal areas such as the Netherlands and Bangladesh are threatened.

Long-term changes in temperature and weather patterns, mainly caused by global warming: hotter summers, shorter warmer winters, heavier rains and floods in some regions, droughts in others, and extreme events like heat waves and wildfires.

Plants absorb CO₂ and release O₂; their leaves also trap particulate matter. Planting trees is one of the best ways to clean the air.

Flue gas is the mixture of waste gases produced when fossil fuels are burnt in power plants. In desulphurisation, adsorbents like calcium oxide are added, which can remove up to 95 % of the SO₂. CaO + SO₂ → CaSO₃.

Resources that continue to exist despite being consumed over time: the Sun, wind, water, geothermal energy and biomass. Solar and wind energy make electricity without pollution.

A rating system that shows how bad the air is. A value under 50 is good; over 300 is hazardous.

Faulty fuel-burning appliances (gas heaters, geysers, stoves) can release CO, which cannot be seen or smelt. A detector sounds an alarm before the level becomes dangerous.

Cloth or dust masks may not filter out the finer particles. Well-fitted N95 masks have better filtration capabilities (they stop at least 95 % of fine particles).

The excessive use of fertilizers, insecticides and pesticides for agriculture has proved to be harmful for animals, birds and human beings.

Even 0.04 % (about 420 ppm) is a very large mass of gas over the whole atmosphere, and CO₂ is a greenhouse gas: it absorbs the long-wave radiation given out by the Earth.

So a gas can be harmful because of what it does, not because of how much of it there is.

A gas causes the greenhouse effect only if its molecules can absorb the long-wave (infrared) radiation given out by the Earth. Molecules with three or more atoms whose bonds can bend and stretch with a changing charge separation, like CO₂, CH₄, H₂O and N₂O, absorb infrared strongly.

Simple diatomic molecules of identical atoms, N₂ and O₂, which make up 99 % of air, and the single atoms of argon, do not absorb infrared, so they do not trap heat. Gases such as SO₂ and NO₂ are present in tiny amounts and are removed quickly by rain, so their warming effect is small; their main harm is acid rain.

It is free, renewable and clean, and Pakistan receives plenty of sunshine all year.

1. Carbon dioxide (CO₂)

Higher levels of carbon dioxide lead to increased global warming, which can cause ice caps to melt and oceans to warm, causing sea levels to rise. Extreme weather changes such as heat waves, heavy rains and wildfires also occur.

2. Carbon monoxide (CO)

It is an extremely poisonous gas that can cause suffocation and death. It stops the red blood cells in the blood from carrying the oxygen that the body needs.

3. Oxides of nitrogen (NOₓ)

NOₓ can damage lungs, irritate the eyes and damage vegetation. It can also cause acid rain, which affects buildings and statues made of limestone, and forms photochemical smog.

4. Oxides of sulphur (SOₓ)

SO₂ irritates the eyes and causes breathing difficulties and acid rain, and haze.

5. Hydrocarbons

They can cause pneumonia, coughing and many other breathing and lung diseases. They also cause global warming (methane).

6. Particulate matter

It irritates the eyes and can cause severe breathing problems for people with asthma. It also makes clothes dirty, and visibility is reduced because it produces haze; it increases the risk of cancer.

7. Ozone

Breathing ozone can cause a variety of health problems including chest pain, coughing, throat irritation and congestion.

Smog

Smog leads to allergies, asthma and lung infections and inhibits plant growth by reducing the CO₂ absorbed in photosynthesis.

Greenhouse effect

The progressive warming up of the Earth's surface due to the blanketing effect of man-made carbon dioxide, methane, water vapour and other gases in the atmosphere is called the greenhouse effect.

How it works

The Sun emits short-wave radiation that passes through greenhouse gases to heat the surface of the Earth. The hot Earth's surface emits long-wave radiation that is mostly absorbed by greenhouse gases. This absorption prevents the radiation from reaching space, reducing the speed at which the Earth can cool off. The higher the concentration of greenhouse gases, the greater the warming.

Greenhouse gases

CO₂ from burning coal, oil and gas (rising for 300 years; forests that absorb it are being cut); methane from decaying vegetation, animal digestion and rice fields; water vapour, about half of the effect.

Dangers of global warming

Definition

The air quality index (AQI) is a rating system that shows how bad the atmosphere around you is. It is calculated from the levels of the main pollutants: particulate matter (PM2.5, PM10), ozone, NO₂, SO₂ and CO.

Scale

Information it conveys

An AQI under 50 is good; an AQI over 300 is hazardous.

High-risk groups

Why more dangerous

Effects

Breathing polluted air may affect their lungs, heart and brain. Pollutants can enter their blood stream and cause coughing or itching of eyes, which may lead to poor quality of life, hospitalisation, cancer or even premature death.

Precautions

Check the AQI, stay indoors and reduce activity on bad days, wear N95 masks, keep indoor air clean, and see a doctor about worrying symptoms.

1. Planting trees

Planting trees is very helpful in removing air pollution. Plants clean the air through photosynthesis, absorbing carbon dioxide and releasing oxygen in sunlight, catalysed by chlorophyll. They also trap particulate matter on their leaves and trunks.

2. Catalytic converters and cleaner fuels

Catalytic converters are used in the exhaust of modern automobiles. A three-way converter reduces NOₓ to N₂ and O₂, oxidises CO to CO₂, and oxidises hydrocarbons to CO₂ and H₂O. The emission of SO₂ is reduced by using low-sulphur fuels or flue gas desulphurisation with calcium oxide (up to 95 % removed).

3. Renewable energy

To discourage the excessive use of fossil fuels, renewable resources (Sun, wind, water, geothermal, biomass) should be used. Solar and wind energy generate electricity without damaging the environment. Hydrogen fuel, which burns to water only, is another clean option.

Others

Sources of pollutants

Weather that traps them

Smog is caused by a combination of industrial pollution, vehicle emission and crop burning, which build up NOₓ, SO₂, particulate matter and volatile organic compounds.

Summer air is not always clean: dust storms can raise PM10, and ground-level ozone forms on sunny days, but the dangerous winter smog does not form.

Pakistan produces less than 1 % of the world's greenhouse gases but is among the countries most affected by climate change.

Composition

Earth is covered with a blanket of air called the atmosphere, made up of several layers of gases. Its constituents are divided into major and minor ones; the amounts vary slightly from place to place, season to season and day to night.

Air pollutants

Any substance in the air which has an adverse effect on human health and quality of life is an air pollutant. Seven substances make up over 90 % of air pollution:

Main factors

The complete and incomplete combustion of fossil fuels is alone responsible for most of our pollution. Rapid growth of population, urbanisation, industrialisation and transportation are the main factors.

Definition

Rain water with pH between 4.2 and 4.4 is known as acid rain.

Role of sulphur

Burning of fossil fuels releases SO₂ and SO₃, produced because of the sulphur present in fossil fuels.

Role of oxides of nitrogen

Oxides of nitrogen are produced by the direct combination of atmospheric nitrogen and oxygen in combustion, in automobile engines and by lightning. They act directly, forming nitric acid, and catalytically, helping SO₂ change to SO₃:

Fall

The gases mix with moisture to form acid droplets that wind carries to huge distances, returning as acid rain, hail, snow or fog.

Effects

Control

Catalytic converters, low-sulphur fuels, flue gas desulphurisation with CaO, and adding lime to acidic soils and lakes.

Formation of pollutants in engines

At the very high temperature of burning fuel inside the engine, nitrogen and oxygen of the air combine: N₂ + O₂ → 2NO; NO then forms NO₂ in air. Due to non-availability of enough oxygen the carbon fuel does not burn completely into CO₂ and water; toxic CO and hydrocarbons are produced.

Catalytic converter

A catalytic converter is used in the exhaust system of automobiles to reduce emissions. The catalysts are platinum, palladium and rhodium, coated on a honeycomb with a large surface.

Three functions

Result

Toxic CO, NOₓ and unburnt fuel are turned into CO₂, N₂ and water. CO₂ still comes out and adds to global warming, so fuel saving and electric vehicles are still needed.

Definition

Photosynthesis is the reaction between carbon dioxide and water to produce glucose and oxygen in the presence of chlorophyll, using energy from light.

How plants control pollution

Why deforestation is harmful

As human beings cut down forests, the capacity of trees to remove CO₂ from the air is diminished. So planting trees (such as Pakistan's tree-plantation campaigns) is a key strategy against climate change and smog.

Know the air quality

Check the air quality index (AQI): under 50 is good; above 150 unhealthy; over 300 hazardous.

Steps when air quality is bad

Extra care for high-risk people

Children, the elderly, heart and lung patients, diabetics and outdoor workers should take these steps first.

Personal habits

Do not burn rubbish; keep heaters and stoves serviced and rooms ventilated; avoid smoking; use public transport.