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Right now, as you are listening to the sound of my voice, you are breathing air.
Air is all around you all the time. When humans go into space or beneath the surface of the ocean, the one thing you absolutely have to take with you is air.
But what exactly makes up air? How did it get that way, and what was the air on Earth like millions of years ago?
Learn more about air, its composition, and its origin on this episode of Everything Everywhere Daily.
Air is probably the most ubiquitous thing we experience in our lives. Every minute of every day we are alive, we are breathing air. Over the course of a year, the average person will take between six to eight million breaths.
Without air, none of us could survive more than a few minutes.
So what exactly is air, this thing we experience every moment of our lives?
Air is simply the mixture of gasses at standard temperature and pressure that is found on Earth.
What that mixture consists of, why it is that way, and how it changed over time is what the rest of this episode will be about.
Before I get into the composition of air, I should separately address one of the important elements of air: water.
If you take the entirety of the Earth’s atmosphere, water vapor makes up about one-quarter of one percent.
The reason I’m addressing water first is that, unlike the other gases in the atmosphere, water composition can vary dramatically from place to place and over time.
Water composition will vary depending on the humidity, if you are inside a cloud, and if it is raining. There will be more water in the air in a rainforest than there will be in a desert or in a polar region.
Unlike every other gas which can be found in the air, water has a much higher boiling point and can easily precipitate out of the atmosphere.
So, everything I’m talking about going forward will be the composition of dry air, that being air without any water vapor in it because water vapor can vary so much.
I’m also not going to be considering aerosols and particulate matter because, as with water, it can vary greatly from place to place and from time to time. A city with lots of air pollution, like Beijing, will have more particulate matter in the air, and there can be more in the atmosphere after a major volcanic eruption.
With that being said, the gas which makes up the overwhelming majority of air is nitrogen.
Nitrogen comprises 78.08% of the atmosphere.
Nitrogen doesn’t float around in its atomic form but rather in its molecular form.
Molecular nitrogen is just two nitrogen atoms that are bound to each other. The bond between the atoms is an extremely strong triple bond which is very hard to break.
This makes a nitrogen molecule almost inert. It isn’t quite as inert as a nobel gas like helium, argon, or neon, but it is pretty inert. This is why nitrogen gas can serve as a cheap way to preserve items.
Why is nitrogen the most abundant gas in the atmosphere?
There are a few reasons for it. In the very early atmosphere of the Earth, nitrogen was not the most abundant element.
Nitrogen doesn’t like to go into crystal lattice structures that make up minerals. There are a few minerals that contain nitrogen, but not many, and they don’t make up much of the Earth.
Over geologic time nitrogen was added to the atmosphere through volcanism. This was mostly nitrogen which was in the Earth, but it wasn’t necessarily bound to any rock.
Nitrogen molecules are heavier than the lighter elements which made up the early Earth’s atmosphere, so they settled near the surface as lighter elements might have escaped into space.
Once nitrogen started to accumulate, it was able to remain in the atmosphere because the nitrogen molecule is so inert. As new rock formations were exposed over time, it didn’t react with the nitrogen, so it was allowed to accumulate.
There is a nitrogen cycle, but it takes special microbes to break down nitrogen to make it useful. I will leave a discussion of the nitrogen cycle for another episode.
The next most abundant element in the composition of dry air is oxygen.
Oxygen makes up 20.95% of air.
As with nitrogen, oxygen is found in molecular form. Oxygen binds to itself with a double bond to form O2.
Unlike nitrogen, oxygen is highly reactive. In addition to being the second most abundant element in air, it is also the most abundant element in the Earth’s crust.
Oxygen is so reactive that if there was no life on Earth, over time, all of the oxygen in the atmosphere would eventually be removed because it would bond with freshly exposed rock.
It is for this reason that an atmosphere of oxygen on an exoplanet may in and of itself be evidence of life.
The only reason why we have so much oxygen is due to photosynthesis.
About 2.45 billion years ago, cyanobacteria appeared on Earth and began expelling oxygen as a waste product.
Much of the first oxygen in the atmosphere didn’t last very long. It reacted with iron and other exposed rocks. It took several hundred million years for oxygen to start accumulating in the atmosphere because it took that long for oxygen to saturate the exposed rock.
As cyanobacteria spread and plants arose, the amount of oxygen in the atmosphere continued to grow.
In fact, about 300 million years ago, in the late carboniferous period, the amount of oxygen in the atmosphere reached about 30-35%, which is significantly higher than it is today.
Oxygen levels that high led to a radically different world. There were giant insects during that period, with dragonflies with wing spans of a meter. An insect that large would be impossible today because insects don’t have lungs.
Insects rely on a simple gas exchange system on their bodies. The only way they could possibly grow that large is if there was a whole lot of oxygen in the atmosphere.
The high levels of oxygen would have also led to an enormous number of fires. Early in the Earth’s history, fires would have been almost non-existent because there was neither fuel nor oxygen.
Once oxygen levels get above 23%, fires can start and spread rapidly. At 35%, even wet green plants like you would find in a rainforest would easily burn.
Likewise, there have been periods where oxygen levels have dropped down to about 15%.
Periods of high oxygen tended to be cooler, and periods of low oxygen tended to be warmer.
Nitrogen and oxygen make up 99% of air. The remaining one percent, despite being small, is still important
The next most abundant gas in the air is argon, coming in at 0.934%
Argon is an inert gas and doesn’t react with anything. As with most of the inert gases in the air, it is a byproduct of radioactive decay. Almost all argon comes from the decay of potassium-40, a radioactive isotope that is found mostly in the Earth’s crust but also in things like bananas, avocados, and you.
I’ve previously talked about argon in my episode on the noble gases.
These three elements constitute 99.9% of the atmosphere.
The fourth most abundant gas is Carbon Dioxide. CO2 makes up only 0.0417% of the atmosphere. CO2 is important because, even though there isn’t much of it, it insulates the Earth. Given the size of a CO2 molecule, it can absorb infrared wavelengths of light.
Carbon dioxide is also important because it is consumed by plants.
CO2’s important function as both an insulator and plant food is all done, composing less than one-tenth of one percent of the air.
Below carbon dioxide, with 0.0018 percent, or one part in 55,000, of the atmosphere, is neon.
Neon is another noble gas. Its presence in the atmosphere comes from two sources. One is primordial neon which was around when the Earth was formed. It was trapped in rocks and released into the atmosphere through volcanic outgassing.
The other source is radioactive decay. Neon is an end result of the decay chain of some isotopes of uranium.
All of the neon which is used in the world comes from the liquefaction of air.
Liquedifying air is just reducing the temperature to the point where each component gas in air becomes a liquid. As each boiling point is reached, you can separate out that component.
So, industrial oxygen and nitrogen are created through the same process that makes neon.
Below neon is a gas that is more common than is reflected in the atmosphere, methane.
Methane makes up only 1.86 parts per million of the atmosphere.
Methane is a natural bioproduct of the decay of organic matter and it is also created through geologic processes. Methane made up much more of the Earth’s atmosphere billions of years ago before life began to change the atmosphere.
The reason why there is so little methane in the atmosphere is that unlike everything else I’ve mentioned so far, it will break down over time. It will react with ozone to form carbon dioxide and water in about ten years.
So, while methane is constantly being added to the atmosphere, it is also constantly being removed.
Methane has an even greater insulating effect than CO2.
Below methane, you are talking about extremely trace gases.
Helium is 5.24 parts per million
Krypton is 1.14 parts per million
Hydrogen is 0.55 parts per million
Xenon is 0.087 parts per million
Helium and hydrogen are so light that they float away.
Krypton and Xenon are just heavy noble gases that are rare.
The last gas which makes up air that I’m going to mention is another one which is actually really important: ozone.
Ozone is just a different molecule of oxygen. Instead of two oxygen atoms, it has three.
There isn’t a lot of ozone at only 60 parts per billion. There is some ozone produced by humans, but the vast majority is made naturally. Ozone is the reason why rain smells like rain.
Ozone usually only exists in the upper region of the atmosphere, which is good because it is highly reactive. It does absorb ultraviolet radiation, which is why it is of interest to people.
The reason for the small amount of it in the atmosphere is similar to methane. It is constantly being created and destroyed.
So, if you were to say that air is made up of nitrogen, oxygen, and argon, you’d be 99.9% correct, but that remaining 0.1% is extremely important, and without it, life on Earth would cease to exist.
So the next time you take a deep breath, pause for a moment and appreciate the complex mix of gases that you are inhaling that make up air.
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