6.3.1 Formation and evolution of the atmosphere

The early atmosphere probably came from volcanoes

1. Earth formed about 4.6 billion years ago, and for much of its first billion years its surface was covered by active volcanoes.
   1. Gases released by these volcanoes built up around the planet to form the early atmosphere.
2. The early atmosphere is thought to have been mainly carbon dioxide, with water vapour and small amounts of nitrogen, methane and ammonia.
3. It contained little or no oxygen, so it was very different from the air today.
4. Nobody could measure these gases, so the explanation rests on indirect evidence and describes what is thought to have happened.

Evidence for the early atmosphere

1. Gases from present-day volcanoes are mainly water vapour and carbon dioxide, with smaller amounts of sulfur dioxide and nitrogen.
   1. If early volcanoes released similar gases, the early atmosphere would have been rich in water vapour and carbon dioxide.
2. Venus and Mars formed in a similar way to Earth, and their atmospheres are about 95% carbon dioxide.
   1. Neither planet has any known life to change its atmosphere, so they may show what Earth's atmosphere was like before life began.
3. Rocks such as limestone and fuels such as coal hold huge amounts of carbon, which suggests that far more carbon dioxide was once in the atmosphere.
4. The evidence is incomplete, because very few rocks survive from Earth's first few hundred million years and no sample of the early air exists.

Cooling formed oceans that dissolved carbon dioxide

1. As the Earth cooled, water vapour in the atmosphere condensed into liquid water.
   1. The water fell as rain and collected in hollows on the surface to form the oceans.
2. Carbon dioxide from the air dissolved in the oceans, lowering the amount left in the atmosphere.
   1. Some of the dissolved carbon dioxide reacted to form insoluble carbonates, which settled on the sea floor as sediment.
3. Over millions of years these sediments were compressed into rocks such as limestone, locking the carbon away.

Photosynthesis slowly added oxygen to the atmosphere

1. The first organisms able to photosynthesise were algae and bacteria living in the oceans, about 2.7 billion years ago.
2. Photosynthesis takes in carbon dioxide and water and gives out oxygen: $$6{CO}_2+6H_{2}O\to C_6{H}_{12}{O}_6+6O_2$$
3. At first, most of the oxygen reacted with dissolved iron compounds in the oceans, forming iron oxides that settled as layers of rock.
   1. These striped layers, called banded iron formations, record the time when oxygen was first being produced in the oceans.
4. Once the dissolved iron had been used up, oxygen escaped into the air and its percentage gradually rose.
5. Land plants evolved much later and added more oxygen, until the atmosphere reached about 21% oxygen.

Locked-away carbon and unreactive nitrogen shaped today's atmosphere

1. Photosynthesis removed carbon dioxide from the air, and the carbon became part of living things.
   1. When organisms died and were buried, some of this carbon was slowly turned into fossil fuels such as coal, crude oil and natural gas.
2. Sea creatures used dissolved carbon dioxide to build shells of calcium carbonate, which formed limestone after they died.
3. Nitrogen is unreactive and does not dissolve well, so it stayed in the air while carbon dioxide and water vapour were removed.
   1. Its percentage therefore rose, and nitrogen now makes up about 78% of the atmosphere.
4. The rest of today's air is about 21% oxygen, 0.9% argon and 0.04% carbon dioxide, and the carbon cycle now keeps this composition almost constant.

6.3.2 The greenhouse effect and climate change

In the greenhouse effect, gases absorb and re-emit infrared

1. The Sun gives out mainly visible light and ultraviolet radiation, which have short wavelengths.
   1. Most of this radiation passes through the atmosphere, although some is reflected back into space by clouds, ice and the atmosphere.
2. The land and oceans absorb the radiation that reaches them and warm up.
3. The warm surface then gives out energy as infrared radiation, which has a longer wavelength.
4. Molecules of carbon dioxide, methane and water vapour absorb some of this infrared radiation, which makes their bonds vibrate more.
   1. The molecules then re-emit infrared radiation in all directions, including back towards the surface, so the lower atmosphere stays warmer.
5. Nitrogen and oxygen, which make up about 99% of the air, do not absorb infrared radiation, so they are not greenhouse gases.
6. The natural greenhouse effect keeps the Earth's average surface temperature at about $15{}^{\circ }C$ instead of about $-18{}^{\circ }C$.

Human activities add carbon dioxide and methane

1. Burning fossil fuels in power stations, vehicles and homes releases carbon dioxide that was locked away for millions of years.
2. Deforestation increases carbon dioxide in two ways.
   1. Trees that are burnt or left to rot release the carbon they contained as carbon dioxide.
   2. Fewer trees remain to take in carbon dioxide by photosynthesis.
3. Methane comes from cattle as they digest food, from rice grown in flooded fields and from waste rotting in landfill sites.
   1. Leaks from natural gas wells and pipes also release methane, because natural gas is mainly methane.
4. There is far less methane than carbon dioxide in the air, but each methane molecule has a much stronger warming effect.

Measurements, ice cores and models link emissions to warming

1. Direct measurements since 1958 show carbon dioxide in the air rising from about 315 to over 420 parts per million.
   1. Air bubbles trapped in ice cores show that carbon dioxide stayed at about 280 parts per million for thousands of years before about 1800.
2. The rise began with the Industrial Revolution, when large-scale burning of coal started.
3. Temperature records show that the global average temperature has risen by more than $1{}^{\circ }C$ since the late 1800s.
4. Carbon dioxide levels and temperature have risen together, and the greenhouse effect explains how extra carbon dioxide causes warming.
5. Climate models that include human emissions match the observed warming, while models that include only natural causes, such as changes in the Sun's output and volcanic eruptions, do not.

Uncertainties mostly affect the size and speed of changes

1. Temperature records before about 1900 come from fewer places, mostly on land in Europe and North America, so early global averages are less certain.
2. Ice cores and other indirect records estimate past conditions rather than measuring them directly.
3. Natural factors, such as changes in the Sun's output, volcanic eruptions and ocean currents, also change the climate, so the human share has to be separated out carefully.
4. Climate models have to simplify processes such as cloud formation, so different models predict a range of future warming.
5. Future warming also depends on how much greenhouse gas people will emit, which cannot be known in advance.

Cutting emissions can limit future climate change

1. Higher levels of carbon dioxide and methane strengthen the greenhouse effect and raise the global average temperature.
2. Sea levels rise as warmer ocean water expands and ice on land melts, flooding low-lying coasts.
3. Weather patterns change, with more frequent heatwaves, droughts, heavy rainfall and floods in different regions.
   1. Changing temperatures and rainfall can destroy habitats and lower crop yields in some areas.
4. Using renewable energy and nuclear power instead of fossil fuels cuts carbon dioxide emissions.
5. Carbon capture and storage collects carbon dioxide from power stations and stores it underground.
6. Planting trees and stopping deforestation keeps more carbon dioxide out of the air.
7. Collecting methane from landfill sites and reducing waste and cattle numbers cuts methane emissions.

6.3.3 Sources and problems of atmospheric pollutants

Carbon monoxide forms when fuels burn in little oxygen

1. With plenty of oxygen, a hydrocarbon fuel undergoes complete combustion to form carbon dioxide and water.
2. With a limited supply of oxygen, some of the carbon forms carbon monoxide instead: $$2{CH}_4+3O_2\to 2CO+4H_{2}O$$
3. The main sources are vehicle engines and faulty gas boilers, fires and heaters with blocked air vents or flues.
4. Carbon monoxide is toxic, because it binds to haemoglobin in red blood cells more strongly than oxygen does.
   1. This reduces the amount of oxygen the blood can carry, causing headaches, drowsiness and, at high levels, death.
5. It is colourless and odourless, so homes with gas appliances need carbon monoxide alarms to detect it.

Particulates come from incomplete combustion and damage the lungs

1. When oxygen is very limited, a fuel burns to form tiny carbon particles, or soot: $${CH}_4+O_2\to C+2H_{2}O$$
2. Diesel engines, coal fires and wood burners are major sources, releasing soot and droplets of unburnt fuel.
3. Fine particulates are breathed deep into the lungs, where they cause and worsen breathing problems such as asthma.
   1. They also increase the risk of heart and lung disease.
4. Soot blackens buildings and makes them costly to clean.
5. Particulates in the air reflect sunlight back into space, so less sunlight reaches the ground, an effect called global dimming.

Sulfur dioxide forms when fuels containing sulfur impurities burn

1. Coal and fuels made from crude oil, such as diesel and heavy fuel oil, contain small amounts of sulfur impurities.
2. When the fuel burns, the sulfur reacts with oxygen to form sulfur dioxide: $$S(s)+O_2(g)\to {SO}_2(g)$$
3. The major sources are coal-fired power stations, industrial furnaces and ships, while volcanoes are a natural source.
4. Sulfur dioxide dissolves in water droplets in clouds and reacts with oxygen to form sulfuric acid: $$2{SO}_2(g)+O_2(g)+2H_{2}O(l)\to 2H_2{SO}_4(aq)$$
   1. Rain is naturally slightly acidic, at about pH 5.6, so this extra acid gives rain with a lower pH.
5. Breathing in sulfur dioxide irritates the airways and makes asthma worse.

Oxides of nitrogen form at high engine temperatures

1. Nitrogen and oxygen in the air do not react at ordinary temperatures.
2. Inside vehicle engines and furnaces, the very high temperature makes them react to form nitrogen monoxide: $$N_2(g)+O_2(g)\to 2NO(g)$$
   1. Nitrogen monoxide then reacts with more oxygen in the air to form nitrogen dioxide: $$2NO(g)+O_2(g)\to 2{NO}_2(g)$$
3. Nitrogen dioxide dissolves in cloud water to form nitric acid, which adds to acid rain.
4. Oxides of nitrogen irritate the lungs, worsen asthma and help to form smog in busy, sunny cities.

Acid rain damages buildings, forests and lakes

1. Acid rain reacts with limestone and marble, wearing away buildings and statues: $${CaCO}_3+H_2{SO}_4\to {CaSO}_4+H_{2}O+{CO}_2$$
2. It corrodes metals, such as the iron and steel in bridges and railings.
3. It damages the leaves of trees and washes nutrients out of the soil, so whole forests can die back.
4. Lakes and rivers become more acidic, which can kill fish and other aquatic life.
5. Winds carry the pollutants for hundreds of kilometres, so acid rain often falls far from the source.

6.3.4 Producing potable water

Potable water is safe but not pure

1. Potable water can contain dissolved salts and microorganisms, as long as their levels are low enough to be safe.
   1. It therefore contains more than one substance, so it is not pure in the chemical sense.
2. In the UK, most drinking water comes from fresh water in rivers, lakes and reservoirs, or from ground water stored in rocks.
3. Where fresh water is scarce, sea water or treated waste water can be used to increase the supply.
4. A growing population and more frequent droughts mean that more places need to use these other sources.

Four treatment stages make fresh water potable

1. Screening passes the water through a mesh that removes large objects such as twigs, leaves and litter.
2. In sedimentation tanks, the water is left to stand so that particles of sand and soil settle to the bottom.
   1. Chemicals such as aluminium sulfate can be added to make fine particles clump together and settle faster.
3. Filtration through beds of sand and gravel removes the remaining small insoluble particles.
   1. Filtration does not remove dissolved substances, because they pass through the filter with the water.
4. Chlorine is added to kill harmful microorganisms, although ozone or ultraviolet light can be used instead.

Distillation removes dissolved salts from sea water

1. Sea water contains about 35 g of dissolved salts, mainly sodium chloride, in every kilogram, which is far too salty to drink.
2. Sedimentation and filtration cannot remove these salts, because dissolved ions pass through filters.
3. In distillation, sea water is heated so that the water boils off as vapour and the salts stay behind.
   1. The vapour is cooled in a condenser, and the liquid water collected is pure.
4. Boiling large volumes of water needs a lot of energy, so distillation is expensive and is used mainly where fuel is cheap and fresh water is scarce, as in parts of the Middle East.
5. Distilled water tastes flat, so small amounts of minerals are added back before it is supplied.

Reverse osmosis holds back dissolved salts

1. Sea water is first filtered to remove sand and other insoluble particles.
2. It is then pumped at high pressure against a partially permeable membrane.
   1. The membrane lets water molecules through but stops most of the dissolved ions.
3. The very salty water left behind, called brine, is returned to the sea.
4. Reverse osmosis uses less energy than distillation, because the water does not have to be boiled.
   1. The pumps still need energy, and the membranes are expensive and need regular cleaning and replacement.

Treating waste water to supply potable water

1. Waste water from homes, farms and industry contains organic matter, harmful microorganisms and sometimes harmful chemicals.
2. Screening removes large objects and grit, as it does for fresh water.
3. Sedimentation then separates the waste into solid sludge, which settles, and a liquid layer above it.
4. Bacteria break down the organic matter left in the liquid, and further filtration removes fine particles.
5. The water is sterilised before it is returned to rivers or, after more treatment, reused as potable water.
6. Treating waste water to drinking standard takes more processing than treating fresh water, so it is used mainly where water is scarce.