Chemistry of the atmosphere 2. Evidence for the early atmosphere is limited because of the time scale of 4.6 billion years. One theory suggests that during the first billion years of the Earth’s existence there was intense volcanic activity that released gases that formed the early atmosphere and water vapour that condensed to form the oceans. Earth’s atmosphere was like that of Mars and Venus today, consisting of mainly carbon dioxide and little or no oxygen gas. Volcanoes produced nitrogen, and there may have been small amounts of methane and ammonia. When the oceans formed, carbon dioxide dissolved in the water and carbonate rocks turned into sediments, absorbing carbon dioxide. 1. For 200 million years, the proportions of the different gases in the atmosphere have remained roughly the same as they are today: 80% nitrogen 20% oxygen Small proportions of other gases including CO2, water vapour and noble gases 3. Algae first produced oxygen about 2.7 billion years ago. Over the next billion years plants evolved and the percentage of oxygen gradually increased, which enabled animals to evolve. The oxygen in the Earth’s early atmosphere was produced by photosynthesis: 6CO2 + 6H2O  C6H12O6 + 6O2 Algae and plants decreased the percentage of carbon dioxide in the atmosphere by photosynthesis. Carbon dioxide was also decreased by the formation of sedimentary rocks and fossil fuels that contain carbon. Key Words: Atmosphere, theories, photosynthesis, greenhouse gases, pollutant, climate change 5. Human activity contributes to increasing levels of greenhouse gases in the atmosphere. Based on peer-reviewed evidence, many scientists believe that human activities will cause a rise in the Earth’s temperature, leading to global climate change. 6. The carbon footprint is 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 emissions of carbon dioxide and methane. 4. Greenhouse gases maintain temperatures on Earth high enough to support life. Water vapour, carbon dioxide and methane are greenhouse gases. 7. The combustion of fuels is a major source of atmospheric pollutants. Pollutants and their effects include carbon monoxide (a colourless, odourless toxic gas), sulfur dioxide (respiratory problems and acid rain), carbon particulates/soot (global dimming and health problems) and oxides of nitrogen (respiratory problems and acid rain).