Nicholas Amendolare is a high school and middle school science teacher from Plymouth, Massachusetts. He has a bachelor's degree in environmental science from Worcester Polytechnic Institute and a master's degree in education from Harvard University. He has been a teacher for nine years, has written for TED-Ed, and is the founder of www.MrAscience.com.
Gases and Our Atmosphere
We live on a large planet with a rather thin atmosphere. The diameter of planet Earth is almost 8,000 miles and yet the troposphere -the layer of the atmosphere where we live and where weather forms - is only about 7 miles thick. Scientist Carl Sagan used to say that the thickness of Earth's atmosphere compared to the Earth itself is comparable to the thickness of a coat of varnish spread across a common globe that you might find in a geography classroom. In other words, our atmosphere is delicately thin.
Our atmosphere is mainly made up of nitrogen gas (78%) and oxygen gas (21%). These gases cling to the Earth because of gravity. They are mainly found in the troposphere but are also found in lower amounts, at lower densities, in the upper layers of the atmosphere as well. Beyond the two main ingredients, there is also argon (1%), carbon dioxide (0.04%), neon (0.0018%), helium (0.0005%), methane (0.00017%), hydrogen (0.00005%), nitrous oxide (0.00003%), and ozone (0.000004%). Additionally, there are significant amounts of water vapor in our atmosphere as well, anywhere between 0% and 4% depending on the location and the local humidity.
Natural Production of Gases
The composition of our atmosphere is not static, and it never has been. There are seasonal variations in carbon dioxide levels; for example, plants grow in the summer and absorb carbon dioxide, then drop leaves in the winter and release carbon dioxide. Below is a list of some of the most common atmospheric gases and their natural sources.
- Nitrogen: Nitrogen is removed from the atmosphere via lighting and deposited in the soil, then readded to the atmosphere through biomass combustion and denitrification.
- Oxygen: Oxygen is added to the atmosphere by plants during photosynthesis and removed by animals during respiration.
- Argon: Argon levels in the atmosphere are relatively constant. Since it is a noble gas, it does not combine with other elements. And since it is heavy, it is not easily lost to space like, for example, helium.
- Carbon Dioxide: Carbon dioxide is naturally added to the atmosphere by animals during respiration and is removed from the atmosphere by plants during photosynthesis.
- Helium: Helium is the lightest noble gas. It is unreactive; however, it can be added to the atmosphere from radiological sources, as elements like uranium and thorium decay underground. Humans also sometimes mine the helium produced by these sources.
- Methane: Methane is naturally added to the atmosphere during anaerobic decay processes. These happen in wetlands and swamps. It can also be removed from the atmosphere naturally by oxidation within the troposphere.
- Nitrous Oxide: The most common natural sources of nitrous oxide are chemical processes in soils found underneath natural vegetation, the tundra, and the oceans.
- Ozone: Ozone naturally forms in the stratosphere during chemical reactions that involve oxygen gas and ultraviolet light from the sun.
- CFCs: Chlorofluorocarbons have no natural sources.
Man-Made Production of Gases
Unlike natural sources of atmospheric gases, man-made sources have the potential to rapidly change the composition of the atmosphere. While some of these gases have short atmospheric lifetimes, other gases like carbon dioxide remain in the atmosphere for centuries. Man-made sources of greenhouse gases, in particular, are responsible for the problem of anthropogenic global warming, which threatens Earth's climate, and will likely continue to do so for the foreseeable future.
Below is a list of some of the most common atmospheric gases and their anthropogenic (man-made) sources.
- Nitrogen: There are no significant anthropogenic sources of nitrogen gas.
- Oxygen: Oxygen gas is not produced or mined from the atmosphere directly. But, oxygen levels are affected by human activities like deforestation and ocean contamination, both of which lower oxygen levels and raise carbon dioxide levels in the atmosphere.
- Argon: There are no significant anthropogenic sources of argon.
- Carbon Dioxide: Humans have added significant amounts of carbon dioxide to the atmosphere over the last few centuries. Since the industrial revolution, carbon dioxide levels have risen from about 280 ppm to about 420 ppm currently. This increase is mostly due to the combustion of fossil fuels like coal, oil, and natural gas.
- Helium: There are no significant anthropogenic sources of helium, although humans do affect helium levels when they mine natural underground sources.
- Methane: Anthropogenic sources of methane include landfills, oil and natural gas systems, coal mining, and wastewater treatment, among others.
- Nitrous Oxide: Anthropogenic sources of nitrous oxides include agricultural fertilizers, livestock manure, human sewage, as well as the combustion of fossil fuels.
- Ozone: Ozone is not directly emitted into the atmosphere. However, it is a secondary pollutant created by reactions between nitrous oxides and volatile organic compounds. The most common source is motor vehicle exhaust combined with direct sunlight, which spurs the chemical reaction.
- CFCs: The most common sources of CFCs were refrigerants and aerosol cans; however, the Montreal Protocol outlawed the use of CFCs for most purposes in 1987. However, the chemicals are still used in some fire suppression systems for airplanes.
The scientist Carl Sagan used to say that the thickness of Earth's atmosphere compared to the Earth itself is comparable to the thickness of a coat of varnish spread across a common globe that you might find in a geography classroom. In other words, our atmosphere is delicately thin. It is mainly made up of nitrogen gas and oxygen gas. However, it also contains smaller amounts of gases like carbon dioxide, methane, nitrous oxide, and ozone.
These gases are all naturally produced and removed from the atmosphere during processes like photosynthesis (which removed carbon dioxide) and anaerobic decomposition (which adds methane). However, the activities of humans can upset the natural balance and radically change the amounts of some of these gases in the atmosphere. Carbon dioxide has been added via the combustion of fossil fuels. Methane has been added from landfills and cattle ranching. Nitrous oxides are released by our fertilizers and sewage. And ozone is formed from the exhaust produced by cars. Thus, it is human activity that is the most important factor that impacts the levels of these gases in our atmosphere.
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