Most planets and some large moons have sufficient mass to generate a gravitational field capable of wrapping them in a cloud of gas we call their atmosphere. The composition of an atmosphere can vary, not only depending on the interstellar body it encloses, but also around local surface features and at differing altitudes above the solid and/or liquid surface. The lowest layer of Earth's atmosphere is called the Troposphere, it extends from the surface to approximately six kilometers (4 miles) at the North and South poles, and to about 11 kilometers (7 miles) at the equator. While the temperature, density and pressure of the mixture of gases in the Troposphere changes with altitude, the composition of the gases normally does not; we call this mixture of gases "air".
Air contains nitrogen (N2), oxygen (O2), water vapor (H2O), argon (Ar), carbon dioxide (CO2), Neon (Ne), Helium (He), Krypton (Kr), hydrogen (H2) and Xenon (Xe) as its standard constituents. Additional gases that may be found, depending on locality and circumstances, include ammonia (NH3), carbon monoxide (CO), methane (CH4), nitrous oxide (NO), nitrogen dioxide (NO2), ozone (O3) and sulfur dioxide (SO2). These might be added to the mix due to natural, local biological or geological processes or from anthropogenic causes: agriculture, industry and transportation.
Some examples: if you are standing next to a heavily trafficked road, the air you are breathing will have a significant carbon monoxide component; adjacent to a herd of cows, methane will be a noticeable component of the air; and next to geothermal mud pools, sulfur dioxide will be highly noticeable to visitors, if not local inhabitants.
Of the standard components, the amount of water vapor varies quite dramatically. From next to nothing in dry deserts like the middle of the Sahara, through 0.1% at the poles, one to two percent in the spring and fall seasons of temperate regions, to five percent or more on hot, humid days, wherever they occur. Because of this, water vapor is usually ignored when giving the composition of air, the percentages of the other components are stated as though there is no water vapor; this is referred to as the standard composition of dry air. The percentages by volume are: nitrogen 78.08, oxygen 20.95, argon 0.93, carbon dioxide 0.038, neon 0.0018, helium 0.0005, krypton 0.0001, hydrogen 0.00005 and xenon 0.0000087. If water vapor was included, where H2O made up one percent of the air, nitrogen's percentage would actually be 77.30; where it was five percent, nitrogen would be at 74.18%.
While the majority of the air we breathe is not immediately relevant to our well-being, the element that composes approximately one fifth of that air is so vital that most of us are unable to refrain from breathing for more than a minute or so. That element is oxygen and in the air of the Troposphere it is predominantly in the form of the oxygen molecule, consisting of two bonded oxygen atoms, O2. That's all to the good, because it is only in this form that it is beneficial to us.
While the highly reactive, but exceedingly rare, single oxygen atom and the three atom molecule called ozone, which is also highly reactive, are significantly damaging when breathed in, the less reactive O2 is essential to most life on Earth, including all plants and animals. While the cells of our bodies can produce a small amount of the energy molecule adenine triphosphate (ATP drives all cellular chemical reactions requiring an energy input) without O2, it results in a build up of lactic or pyruvic acids. This gives us the burning sensation we feel in exercised muscles and can be damaging if O2 is not forthcoming to continue the breakdown of these acids and manufacture the bulk of the ATP producible from the original glucose molecules.
Ozone, O3, may be found around industrial complexes, but is predominantly located above the Troposphere at altitudes ranging between 15 and 30 kilometers (10 to 19 miles), where it normally combines and breaks down repeatedly in a self-sustaining atmospheric layer. Unfortunately, lighter than air chemical by-products from a number of human industrial processes over the last several decades have been rising up to the ozone layer and inhibiting the recombination of O2 molecules and O atoms that has previously maintained the O3 volume of this layer; resulting in this protective layer thinning and the infamous holes.
The bulk of the air we breathe is composed of nitrogen in the form of the diatomic molecule N2. Although nitrogen atoms are fundamental components of our living tissues, an essential element in the amino acids that make up our proteins, none of our necessary nitrogen intake comes from the vast amount we inhale and exhale while breathing. Only an exceedingly small amount of atmospheric nitrogen is fixed by either lightning or nitrite fixing bacteria to enter the biological portion of the nitrogen cycle that includes us. The vast majority remains the relatively inert gas enveloping our world. Although not one of the actual inert gases, it is sufficiently unreactive to buffer or minimize chemical reactions. Without it, fires would be explosive rather than simply burning. We do have N2 in our extracellular body fluids, the amount is dependent on external pressure, the greater the pressure, the larger the amount of N2. If we move from a high pressure area to a low pressure area too rapidly, the N2 cannot leave our bodies fast enough through normal breathing, resulting in decompression illnesses such as the Bends.
Argon, neon, krypton and xenon are inert gases, they do not chemically combine, even with themselves, so all consist solely of individual atoms. Although they are even more effective at containing chemical reactions than N2, there is insufficient in the air for them to play more than a minor role in this. We do use industrial extraction processes to obtain these gases from the air for various uses in our technology.
Helium and hydrogen are at too low levels to be significant; the hydrogen, for example, is essentially maintained at that level as a residual from the breakdown and recombination of water vapor molecules.
Carbon dioxide, however, plays a major hand as a vital component in photosynthesis. This is the process plants and phytoplankton use to harness the Sun's energy, in the form of organic molecules that enable most life as we know it, all but some of the oldest types of bacteria. CO2 also plays a role in the Earth's climate, a role that is not as clearly understood as people hearing repeatedly about it being a greenhouse gas might believe.
The current average amount of carbon dioxide in air is 0.038% or 380 parts per million on a per volume basis (ppmv). This can vary due to local events, downwind from slash and burn operations in a rainforest for example. But what is most significant are variations in CO2 levels that coincide repeatedly with global warming and cooling cycles, typically occurring over 70,000 year periods. Scientists studying "fossil" air bubbles from ice core samples drilled in Antarctica have been able to obtained a continuous record of carbon dioxide levels for the past 400,000 years. That record shows CO2 levels around 180 to 200 ppmv towards the end of periods of glaciation (ice ages) and levels around 300 ppmv towards the end of warming periods; a level reached in the mid-Twentieth Century. Since then CO2 levels have continued increasing to the current 380 ppmv, a concentration level that the Earth has probably not seen for millions of years; possibly not since the emergence of multi-cellular life.
We are in an unprecedented situation, no-one really knows what impacts the current concentration might cause, let alone what will happen if carbon dioxide continues to rise at present rates. The world's amphibian population, such as frogs, has been decreasing rapidly in the last 10 to 20 years. Amphibians can breathe through their skins and are considered indicator species. Much like canaries were used in mines, the canary dying indicated a dangerous air pocket, usually due to a high concentration of carbon dioxide. Could the amphibian die-off be related to increasing CO2 levels, and not bode well for any of us in the future?