chemistry of the atmosphere

Composition of the Earth’s Atmosphere

Proportion of Gases

  • ~80% nitrogen

  • ~20% oxygen

  • small proportion of various others - CO2 , H2O (g)

The Earth’s Early Atmosphere

  • limited evidence because over 4.6 billion years theories have changed / developed

    • intense volcanic activity released gases which formed the early atmosphere

      • at the start of this period, the atmosphere may have been similar to Mars and Venus’ today → mainly CO2 with little to no O2

      • volcanoes also produced nitrogen (N2) which gradually built up in the atmosphere and there may have been small proportions of methane (CH4) and ammonia (NH3)

    • water vapour condensed to form the oceans

      • CO2 dissolved in the water and carbonates were precipitated producing sediments reducing the amount of CO2 in the atmosphere

How Carbon Dioxide decreased

  • algae and plants decreased the % CO2 in the atmosphere by photosynthesis

  • CO2 was also decreased by the formation of sedentary rocks that contain carbon (eg. limestone and coal)

  • fossil fuels produced from the remains of dead plants and animals when they decayed

How Oxygen increased

  • algae and plants produced the O2 that is now in the atmosphere by photosynthesising

  • 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 % of oxygen gradually increased to a level which enabled animals to evolve

Greenhouse Gases

eg. H2O (g), CO2, CH4

  • maintain temperature on earth high enough to support life

  • The greenhouse gas effect:

    • electromagnetic radiation at most wavelengths (short and long) from the sun pass through the earth’s atmosphere

    • the earth absorbs some radiation and warms up (essential for life)

    • some heat is radiated from the earth as infrared radiation

    • some of the IR radiation is absorbed by greenhouse gases in the atmosphere

    • atmosphere warms up leading to greenhouse effect and global warming

Human Activities Contributing to Greenhouse Gas Emissions

  • increased levels of carbon dioxide and methane from activities such as…

    • driving (CO2) from the combustion of petrol in the engine

    • consuming electricity (CO2) from burning fossil fuels for energy

    • raising cows (CH4) gas passing

    • decay of organic waste in landfill sites (CH4)

  • scientists believe that human activity will cause the temperature of the earth’s surface to increase leading to climate change globally

    • but it’s difficult to model such complex systems so simplified models and speculations in the media may be only based on parts of evidence and are therefore biased

Global Climate Change

  • potential effects of global climate change

    • extinction of species

    • rising sea levels due to ice caps melting

    • migration - people move from areas suffering drought and flooding

    • decrease in crop yield for all major world crops

Carbon Footprint and Reduction

  • carbon footprint = the total amount of CO2 and other greenhouse gases emitted over the full life cycle of a product, service or event

  • how to reduce:

    • using less electricity (burning less fossil fuels)

    • reduce amount of travel and transportation

    • plant more trees

  • action may be limited as it can be more difficult, expensive and planting trees takes away land which could be used to grow crops

Atmospheric Pollutants and their sources

Atmospheric Pollutants from Fuels

  • combustion of fuels is a major source of atmospheric pollutants

  • most fuels contain carbon and/or hydrogen and may also contain sulfur

  • the gases released into the atmosphere when a fuel is burned may include carbon dioxide, water vapour, carbon monoxide and oxides of nitrogen (formed at high temperature)

    • solid particles and unburned hydrocarbons may also be released forming particulates in the atmosphere

    • combustion of hydrocarbon releases energy; the carbon and hydrogen are oxidised to form carbon dioxide and water

  • incomplete combustion = when there is not enough oxygen for all of the fuel to burn

    • here solid particles of soot, carbon monoxide and unburned fuel are released

Properties and Effects of Atmospheric Pollutants

  • carbon monoxide

    • toxic gas which is odourless and colourless

    • binds to haemoglobin and prevents it from binding and carrying oxygen to cells

  • sulfur dioxide and oxides of nitrogen

    • can cause acid rain and respiratory problems in humans

  • particulates

    • cause global dimming and human health problems