Chemistry of the Atmosphere

Theories about the composition of the Earth's early atmosphere and how it formed have changed and developed over time - evidence is limited, due to the time scale of 4.6 billion years

Here is the main theory:

• during the first billion years of the Earth's existence, there was instense volcanic activity - the gases released formed the early atmosphere and the water vapour released condensed to form the oceans
• during this period, the Earth's atmosphere would be like the atmospheres of Venus and Mars - mainly carbon dioxide with little/no oxygen
• volcanoes also released nitrogen, which built up in the atmosphere over time, as well as small proportions of methane and ammonia

• when the oceans formed, some carbon dioxide dissolved into them - this carbon dioxide then when through a series of chemical reactions to form carbonate precipitates which formed sediments on the seabed - this decreased the amount of carbon dioxide in the atmosphere

Algae and plants produced the oxygen that is now in our atmosphere through photosynthesis:

• 6CO₂ + 6H₂O à C₆H₁₂O₆ + 6O₂

Algae first produced oxygen about 2.7 billion years ago, and oxygen first appeared in the atmosphere soon after that

Over the next billion years, plants evolved and the percentage of oxygen in the atmosphere gradually increased to a level that enabled animals to evolve

• Carbon dioxide was removed from the early atmosphere as it was dissolved in the oceans - carbonates were then precipitated forming sediments (on the seabed)
• Green plants and algae evolved to absorb carbon dioxide for use in photosynthesis - decreasing its percentage in the atmosphere
• Later, marine animals evolved and their shells and skeletons contained carbonates from the oceans
• Some of the carbon that organisms (both plants and animals) took in from the atmosphere and oceans became looked up in sedimentary rocks and fossil fuels once they died

• When plants, plankton and marine animals die, they fall to the seabed and are buried by layers of sediment - over millions of years, they are compressed and form sedimentary rocks, oil and gas - trapping the carbon within them and reducing the percentage of carbon dioxide in the atmosphere:
  • crude oil and natural gas are formed from deposits of plankton
  • coal is a sedimentary rock formed from plant deposits
  • limestone is a sedimentary rock, mostly made from calcium carbonate deposits from the shells and skeletons of marine animals

About 200 million years ago, the atmosphere reached a composition to similar to what it is today:

• (approximately) 80% nitrogen
• 20% oxygen
• small amounts of other gases (less than 1%):
  • argon
  • carbon dioxide
  • water vapour

Greenhouse gases (like cabon dioxide, methane and water vapour) act as an insulating layer in the Earth's atmosphere - they are vital for maintaining temperatures on Earth high enough to support life

Greenhouse gases do not absorb incoming short wavelength radiation from the sun, so it passes into the Earth's atmosphere

Greenhouse gases do absorb long wavelength radiation reflected back off the Earth - they then  re-radiate it in all directions (including back towards the Earth) - longwave radiation is thermal radiation, so it results in the warming of the surface of the Earth (the greenhouse effect)

Some forms of human activity affect the amount of greenhouse gases in the atmosphere:

• deforestation - fewer trees mean that less carbon dioxide is removed from the atmosphere in photosynthesis
• burning fossil fuels - when fossil fuels are combusted, the carbon that was 'locked up' in them is released in the form of carbon dioxide
• agriculture - the increase in the number of farm animals means that more methane is produced from the animals' digestive processes
• creating waste - the increase in the number of landfill sites and more waste being produced from agriculture means that more carbon dioxide and methane are released by decomposition of waste

• Many scientists agree that the release of extra carbon dioxide into the atmosphere is due to human activity - they also believe that the increase in carbon dioxide in the atmosphere is causing the rise in temperature of the Earth's surface and that this will lead to global climate change
• Evidence for this has been peer-reviewed, meaning that the information is reliable

• However, it is hard to fully understand the Earth's climate, as it is a large, complex system with many variables - this makes it hard to make a model of the Earth that isn't oversimplified
• This leads to speculation in the media, where stories may be biased or only some of the information may be given

• an increase in global temperature could lead to the polar ice caps melting - this would cause a rise in sea levels, increasing flooding in coastal areas and coastal erosion
• changes in rainfall patterns (amount, timing, distribution) may cause soo regions to get too much or too little water - along with changes in temperature, this may affect the ability of certain regions to produce food
• the frequency and severity of storms may increase - this could cause mass migration or potential loss of life
• changes in temperature and water availability in a habitat may affect wild species, leading to differences in their distribution (new predators/prey = extinction of species)

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

• Carbon footprints can be reduced by reducing carbon dioxide and methane emissions:
  • renewable energy resources or nuclear energy can be used as an alternative to fossil fuels
  • using more efficient processes can conserve energy and cut waste - waste decomposes to release methane, so this will reduce methane emissions
  • governments can tax companies or individuals based on their greenhouse gas emissions
  • governments can put a cap on emissions of all greenhouse gases that companies produce - they can then sell licenses for emissions up to that cap
  • carbon dioxide can be trapped using new technology and stored underground rather than being released into the atmosphere
• However, actions taken to reduce carbon footprints can be limited:
  • a lot of work still needs to be done on new technology to lower carbon dioxide emissions
  • goverments worry about the effect of these changes on their economic growth
  • not everyone agrees, so it is hard to make international agreements on cutting emissions
  • individuals need to make changes to their lifestyles, but some people do not agree and some are not educated about why changes are necessary and how to make them

• The combustion of fuels is a major source of atmospheric pollution
• Fossil fuels contain hydrocarbons that are oxidised when they are combusted, releasing carbon dioxide and water vapour into the atmosphere (some fuels also contain sulphur impurities)
• At high temperatures (such as during combustion), the oxygen and nitrogen in the air may combine/react to form nitrous oxides
• In complete combustion, there is enough oxygen and all the fuel burns, so carbon dioxide and water vapour are produced (typically)
• In incomplete combustion, there is not enough oxygen, so solid particles (called particulates) of soot (carbon) and unburnt fuel are released and carbon monoxide can be produced (as well as carbon dioxide)

The effects of carbon dioxide, methane and water vapour (the greenhouse gases) have been covered on another card

• Particulates (solid particles of soot and unburnt fuel):
  • can get stuck in the lungs and cause respiratory problems when inhaled
  • in clouds, they can reflect sunlight back into space - this causes global dimming, as less light reaches the Earth
• Carbon Monoxide:
  • binds to haemonglobin in the blood (replacing oxygen), so less oxygen can be carried round the body - this can cause fainting, comas or death, making carbon monoxide toxic if inhaled
  • has no colour or smell, making it very hard to detect and even more dangerous
• Sulphur Dioxide and Nitrous Oxides:
  • can mix with clouds, forming dilute sulphuric or nitric acid - this then falls as acid rain
  • acid rain kills plants, damages buildings and statues and also makes metal corrode
  • they also cause respiratory problems if inhaled