28th April 2021 by Lucy Bell-Young

So far, Earth is the only known planet that supports life because of the many factors that are considered to be ideal for biological life to develop, evolve, and flourish. The chemistry of the atmosphere is one of the most important factors that determine the type of life, or any life at all that the planet can sustain. For billions of years, the atmospheric chemistry has changed, but remains relatively stable for long periods of time.

Developing the Atmosphere

At the beginning of the formation of the Earth around 4.6 billion years ago, the planet was a molten ball of rock. There was barely any atmosphere because of the instability of the surface. As the planet cooled, an atmosphere developed from erupting volcanoes. It took about half a billion years for it to sufficiently cool down and allow the collection of liquid water on its surface to form bodies like oceans.

Your knowledge of inorganic chemistry and organic chemistry will help you understand atmospheric chemistry. Scientists hypothesise that the early Earth’s atmosphere was mostly composed of hydrogen sulphide, methane, and two-hundred times as much carbon dioxide compared to today’s atmosphere. Oxygen only began to become a significant part of the atmosphere when photosynthetic organisms (plants, algae, bacteria, etc.) came into existence.

Based on paleontological records and ice core records, Earth’s current atmospheric composition has been stable for the past 200 million years. The current atmospheric gas proportions by volume (excluding water vapour) are as follow:

Nitrogen – 78.084%
Oxygen – 20.946%
Argon – 0.9340%
Carbon dioxide – 0.0407%
Neon – 0.001818%
Helium – 0.000524%
Methane – 0.00018%
Krypton – 0.000114%
Hydrogen – 0.000055%

The proportions of gases in the atmosphere fluctuate slightly because of human and industrial activities and biological processes. Scientists are particularly concerned about the proportions of greenhouse gases, primarily carbon dioxide, in the atmosphere because they have a significant impact on our climate and weather patterns. Most of the greenhouse gases are pollutants generated by power plants, factories, and vehicles.

Atmospheric gas molecules infographic diagram

For billions of years, the carbon dioxide in the atmosphere steadily decreased while the oxygen levels increased. Initially, most of the carbon dioxide in the atmosphere dissolved in water as the oceans were formed. Here, they became carbonate compounds. These compounds then precipitated as sedimentary rocks, and the remaining carbon dioxide was utilised in photosynthesis.

Carbon dioxide in the atmosphere is relatively stable because of photosynthesis. The carbon is absorbed by plants and other photosynthetic organisms and becomes part of their cells as proteins, carbohydrates, lipids, and genetic materials. Oxygen is released as a waste byproduct of photosynthesis. The carbon remains in the biosphere through the food cycle.

For the past two hundred years, however, the amount of carbon dioxide in the atmosphere has steadily increased, mainly because of the use of fossil fuels in industries and in transportation. These fossil fuels are made from ancient remains of flora and fauna that are buried underground for millions of years. When fossil fuels are burned, carbon dioxide, carbon monoxide, and other types of pollutants are released into the atmosphere.

Based on various historical climate studies, human activities have significantly raised the amount of carbon dioxide in the atmosphere by 48% since the pre-industrial levels in 1850. This is a far greater and more rapid increase compared to the natural increase that took place for a period of 20,000 years, i.e. from the last Ice Age until 1850. The latest measured level of carbon dioxide in the atmosphere is 416 ppm. Now compare that to the levels recorded during the 20,000-year period, where the level of CO₂went from 185 ppp tp 280 ppm in 1850.

Anthropogenic (human) contribution to climate change is undeniable based on the evidence. Although many international treaties have been signed to reduce our carbon footprint, the efforts are still not effective or efficient enough to slow climate change. The world economy still needs to be weaned from its dependence on fossil fuels.

Polluting the Atmosphere

Pollution affects the climate and can disrupt the protective function of the atmosphere. Many human activities produce pollutants, particularly carbon dioxide from the burning of fossil fuels.

The Earth’s atmosphere is relatively thin compared to its diameter. The atmosphere is approximately 480 kilometres in thickness, with the diameter 12,742 kilometres at the equator. This means that the thickness of the atmosphere is only 3.8% of the Earth’s diameter.

Despite how proportionally thin the atmosphere is, it is crucial in supporting life. Without the atmosphere, even microbial life would probably not exist on our planet. Additionally, the ionising radiation from the sun and the cosmic rays wouldn’t permit the formation of complex chemicals like DNA. The crucial roles of the atmosphere are the following:

Protection against radiation: The atmosphere, especially the ozone layer, protects against ionising radiation like ultraviolet (UV) rays
Temperature regulation: The average temperature doesn’t radically fluctuate because of the greenhouse effect
Physical barrier against space debris: Meteors and other space debris are burned when they enter the atmosphere
Climate and weather: Convection current, lightning, pressure and temperature differences, and cloud formation wouldn’t be possible without the atmosphere
Water cycle: Water vapour condenses and precipitates as rain. Water from the ground evaporates as water vapour, forming clouds
Gas cycles: Various types of gases like nitrogen, carbon dioxide, and oxygen are recycled in the atmosphere. For example, nitrogen becomes available to the soil as a nutrient because of lightning
Pressure maintenance: Humans and most other animals wouldn’t survive without the normal atmospheric pressure

Human activities, such as industrial productions and transportation, have a serious impact in terms of polluting the atmosphere. Aside from making the air less healthy for life, pollution also affects the climate and causes damage to the protective ozone layer.

Some of the major pollutants are:

Carbon dioxide: While this is biologically produced by animals through the respiration process, the largest source is the complete burning of fossil fuels. CO₂is the major greenhouse gas that causes global warming
Carbon monoxide: This is a highly poisonous gas and a byproduct of the incomplete combustion of fossil fuels
Methane: Aside from crude oil mines, methane is also significantly produced by cattle farming and animal domestication. A large amount of methane is actually from cow flatulence and the decomposition of animal dung. The high demand for meat means that there is a lot of cattle, and therefore more methane
Sulphur dioxide: Fossil fuels, plastics, and other materials that have sulphur impurities are among the sources of this pollutant. Volcanic eruptions are also a major source. This can cause acid rain as it combines with water vapour
CFCs and HCFCs: These are refrigerants that become pollutants when released into the atmosphere. They cause damage on the ozone layer

Further GCSE Chemistry revision support: Chemistry of the Atmosphere

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