# Why is atmospheric composition compared by volume?

The composition of the Earth's atmosphere by its constituent gases (nitrogen, oxygen, argon...), is often given by percent volume. (See this Wikipedia page, for example). Why are these compositions given by volume? I understand that a volume composition shows the mole fractions, which is the real point of interest. Volume compositions compare the volume taken up by different amounts of gases under the same hypothetical pressure and temperature. However, in any "container," each gas occupies the same volume as another; what differs is the partial pressure of each gas. Thus, why not base compositions off of partial pressures? It is physically correct to say that oxygen accounts for 21% of the pressure of the atmosphere, rather than saying that oxygen accounts for 21% of the volume of the atmosphere (which is thankfully not true). I understand that comparing pressure under equal volume and comparing volume and equal pressure both show the same thing, but why not use pressure if it is more physically correct?

You have to consider that lay-men are interested in these numbers too. Most people can easily grasp the idea of volume percent of each gas, but to present this data to them as partial pressures they would have to become informed of the concept of partial pressures. Given that the two methods give the same number, and the one is more complicated for the lay-man, it makes complete sense to use the simple measure. This simplicity also biases the syndication of information to be represented in volume percent, not partial pressure.

The short answer is that (a) partial pressures have units of pressure, and (b) pressure changes with altitude and/or temperature.

The composition of the Earth's atmosphere by its constituent gases (nitrogen, oxygen, argon...), is often given by percent volume. (See this Wikipedia page, for example). Why are these compositions given by volume? I understand that a volume composition shows the mole fractions, which is the real point of interest.

You are right that the mole fractions are the real points of interest. You may additionally be interested to know that many techniques that measure the percent volume of a certain gas actually measure its mole fraction. Thus, reporting the results in percent volume is correct only if the gas is ideal.

Volume compositions compare the volume taken up by different amounts of gases under the same hypothetical pressure and temperature. However, in any "container," each gas occupies the same volume as another; what differs is the partial pressure of each gas. Thus, why not base compositions off of partial pressures?

Partial pressures have units of pressure. So at 1 bar of pressure at 298 K, oxygen has a partial pressure of 0.21 bar and nitrogen has a partial pressure of 0.78 bar. Let's say you had a closed rigid container of gas initially at this pressure and temperature. Let's now say that it's a hot day so the temperature is 313 K. Now the total pressure is $1\times\frac{313}{298}=1.05$ bar, and as a result the oxygen partial pressure is 0.22 bar and the nitrogen partial pressure is 0.82 bar. But meanwhile the volume fraction (or equivalently the mole fraction) of each gas is unchanged.

It is physically correct to say that oxygen accounts for 21% of the pressure of the atmosphere, rather than saying that oxygen accounts for 21% of the volume of the atmosphere (which is thankfully not true).

I don't fully understand this sentence but do not believe it is correct. Oxygen does account for 21% of the volume of the atmosphere.

I understand that comparing pressure under equal volume and comparing volume and equal pressure both show the same thing, but why not use pressure if it is more physically correct?

It is not any more physically correct than using volume fraction. It additionally is complicated that partial pressures have units and will change with temperature or total pressure.

It will be more a long comment than an answer.

The only reason I found is that the pressure and the volume are two different types of physical properties. The pressure is an intensive property whereas the volume is extensive. See here for more information.

It means that if you take one liter of each two gas separately at atmospheric pressure and if we assume there are absolutely no reactions between each others then if you mix them you'll get two liters of a gas solution at atmospheric pressure. But then the partial pressure of both gas will be divided by two.

So instead of give the partial pressures for which you need the volume to calculate them, this is enough to know the relative amount of each gas in volume in the gas phase.

Quite simply, for the unit's use as a constant in measurments and expiriments. The measurement's frame of reference only needs to apply to volume, not pressure. But exceptions can always be made.