The Air That We Breathe
At about 78%, Nitrogen is easily the largest component of Earth’s atmosphere, colorless and odorless to people. Essential nitrogen-containing compounds, however, come from the food we eat.
Fortunately for most living things, oxygen in the form of O2 is the next most abundant gas in the atmosphere at about 21%. It is also colorless and odorless to us, but quite essential.
The next most abundant gas in the atmosphere is Argon at 0.93%. Compared to its gassy friends, Argon is quite aloof as one of the “noble gasses”, doing very little bonding or reacting as it floats around.
Among the most variable components of the atmosphere is water vapor, the amount of which in the air can vary widely with temperature and location. Warm air holds more moisture, so water vapor can compose 4-5% of the air in the tropics, but 0.2% in the Arctic.
Carbon dioxide currently makes up about 0.4% of the atmosphere. This molecule is breathed in by plants and is essential in the carbon cycle, but is also produced by burning fossil fuels, and its heat-trapping nature now makes it a major cause of climate change.
What’s left after these bigger components are minute amounts of the trace gasses, among them helium, neon, methane, hydrogen, ozone, nitrous oxide, and krypton. Some trace gasses actually come from human activity.
Most atmospheric gasses are at greatest concentrations closer to the ground, which explains why the air seems so “thin” at great altitudes. Helium and hydrogen, being so light, can reach great heights, however.