Chemistry: Topic 9: Chemistry of the Atmosphere

Proportions of gases in the modern atmosphere

For 200 million years, the proportions of different gases in the atmosphere have been much the same as they are today:

• approx 80% nitrogen

• approx 20% oxygen

• small proportions of various other gases, including carbon dioxide, water vapour and noble gases

Theories about the formation of the Earth’s atmosphere

Theories about what was in the Earth’s early atmosphere and how the atmosphere was formed have changed and developed over time
Evidence for the early atmosphere is limited because of the time scale of 4.6 billion years

What was Earth’s early atmosphere like (first billion years)?

• Hot when it was formed

• Intense volcanic activity released gases that formed the early atmosphere and water vapour that condensed to form the oceans

• Atmosphere may have been like the atmospheres of Mars and Venus today, consisting of mainly carbon dioxide with little or no oxygen gas

• Volcanoes also produced nitrogen which gradually built up in the atmosphere

  • There may have been small proportions of methane and ammonia

First decrease of CO₂ in the atmosphere

When oceans formed, carbon dioxide dissolved in the water
This CO2 went through a series of reactions to form carbonate precipitates that formed sediments on the seabed
This reduced the amount of carbon dioxide in the atmosphere

Increase of oxygen in the atmosphere

Algae and plants produced the atmospheric O2 by photosynthesis, represented by the equation:

6 CO2 + 6 H2O → C6H12O6 + 6 O2
Carbon dioxide + water > (+light) > glucose + oxygen

Algae first produced oxygen about 2.7 billion years ago and soon after this oxygen appeared in the atmosphere
Over the next billion years plants evolved and the percentage of oxygen gradually increased to a level that enabled animals to evolve

Second decrease of carbon dioxide in the atmosphere

• Algae and plants decreased the percentage of carbon dioxide in the atmosphere by photosynthesis

• Was also decreased by the formation of sedimentary rocks and fossil fuels that contain carbon when

Formation of fossil fuels

some of the carbon organisms took in from the atmosphere and oceans became locked up in rocks and fossil fuels after the organism died
when plants, plankton and marine animals die, they fall to the seabed and get buried by layers of sediment
they get compressed over millions of years to form sedimentary rocks, oil and gas
they trapped carbon within them, keeping the levels of carbon in the atmosphere reduced

things like coal, crude oil and natural gas are fossil fuels

How are limestone, crude oil, coal and natural gas formed?

• crude oil and natural gas - deposits of plankton

  • these fossil fuels form reservoirs under the seabed when they are trapped in rocks

• coal - sedimentary rock made from thick plant deposits

• limestone - sedimentary rock mostly made of calcium carbonate deposits from the shells and skeletons of marine organisms

Examples of greenhouse gases

• Water vapour - H2O

• Carbon dioxide - CO2

• Methane - CH4

Necessity of the greenhouse effect for life

Greenhouse gases in the atmosphere maintain temperatures on Earth high enough to support life by allowing short wavelength radiation from the sun to pass through but absorbing outgoing long wavelength radiation
It is a natural phenomenon

Greenhouse gas effect

EM radiation at most wavelengths (both long and short) from the sun passes through the Earth’s atmosphere
The Earth absorbs some radiation and thus warms up (essential for life on Earth)
But some heat is radiated from the Earth as infrared radiation
Some of this IR radiation is absorbed by an increasing amount of greenhouse gases in the atmosphere
Atmosphere warms up leading to the greenhouse effect and global warming

Human activities increasing greenhouse gases

• Carbon dioxide - driving, consuming electricity, combustion and deforestation

• Methane - increased farming and decomposition in landfills

Global climate change

Based on peer-reviewed evidence, many scientists believe that human activities will cause the temperature of the Earth’s atmosphere to increase at the surface and that this will result in global climate change
However, it is difficult to model such complex systems as global climate change
This leads to simplified models, speculation and opinions presented in the media that may be based on only parts of the evidence and which may be biased.

What is a major cause of climate change?

An increase in average global temperature

Potential effects of global climate change

• extinction of species

• raising sea levels due to the melting of polar ice caps

• migration- people will move from areas suffering drought/flooding

• decrease in crop yield for all major world crops

Carbon footprint

The total amount of carbon dioxide and other greenhouse gases emitted over the full life cycle of a product, service or event

What action can be taken to reduce a carbon footprint?

By reducing emissions of carbon dioxide and methane:

• increased use of alternative energy supplies

• energy conservation

• carbon capture and storage

• carbon taxes and licences

• carbon off-setting including tree planting

• carbon neutrality

Challenges in reducing a carbon footprint

• Scientific disagreement over climate change causes

• Lack of public information

• Lifestyle changes

• Economic considerations

  • e.g. planting trees takes away land that could be used to grow crops

• Incomplete international cooperation

• Can be more expensive

Pollution caused by combustion

Combustion of fuels is a major source of atmospheric pollutants
Most fuels, including coal, contain carbon and/or hydrogen and may also contain some sulfur
The gases released into the atmosphere when a fuel is burned may include carbon dioxide, water vapour, carbon monoxide, sulfur dioxide and oxides of nitrogen
Solid particles and unburned hydrocarbons may also be released that form particulates in the atmosphere

What are the advantages of complete combustion?

• Less soot is made

• More heat per gram of fuel is released

• Poisonous carbon monoxide is not produced

What does incomplete combustion release?

• Solid particles of soot (carbon)

• Carbon monoxide

• Unburnt fuel

What are the properties and effects of atmospheric pollutants?

• Carbon monoxide - toxic gas which is colourless and odourless so not easy to detect

• Sulfur dioxide and oxides of nitrogen - cause acid rain and respiratory problems in humans

• Particulates - cause global dimming and human health problems