It's in the air

Every once in a while Scienceblogs (through its publisher, Seed Magazine) gets a question from a reader that is circulated to see if one or more of the bloggers wants to take a crack at answering it. Recently a 9 year old wanted to know what is in the air we breathe (chemically speaking). On its face it seems like a pretty straightforward question, answerable by looking it up in a reference book, but it is not really so straightforward until you pin it down a little more. Let's parse the question so we can handle it better.

First, what are we talking about when we talk about "the air"? In usual speech "the air" is the substance that makes up the atmosphere, the name for the collection of gases that surrounds the earth. It's denser near the surface because it is compressed by the weight of the gases above it. This isn't much different than water pressure being greater deeper in the ocean than at the surface because of the weight of water above, except that water is essentially incompressible so doesn't get much denser. Gases, however, are compressible, hence the denser air at the surface. Because we live at the surface we usually don't think of our world as having dense air but rather we view the air as getting "thinner" as we go up. Everything's relative, I guess.

Another thing to keep in mind is that the composition of the air isn't constant. In fact the atmosphere has a vertical, layered structure with the composition being different in the different layers. The layers are more or less determined by how the temperature varies as we go up. In the layer we live in, the temperature usually decreases as we go higher. The reason this is of interest is that the change in temperature with altitude puts constraints on how easily mixable the air is. The atmosphere where we live and breathe is heated from below by the surface of the earth (which is heated by the sun), much like the air above a hotplate. The air absorbs some solar energy but not a lot because of its composition. The molecules in ordinary surface air don't absorb much light energy in the peak frequencies radiated by the sun. (They do absorb the re-radiated energy from the earth's surface, however, giving rise to the greenhouse gas problem. But that's another topic.) For our purposes the point is that the surface of the earth is warm and it warms the air in contact with it, which in turn warms the air above it, etc. Since warm air tends to rise, this creates a vertical motion which in turn causes a mixing. This constant mixing makes the chemical composition of the air we breathe much more uniform than if there was little mixing. The layer where there is good mixing is called the troposphere, so in answering the question we will confine ourselves to that layer. This is the layer where we breathe as well (unless we are flying in the stratosphere in an airplane with a pressurized cabin), so the restriction makes sense.

The fact that there is good mixing doesn't mean the composition of the air is the same everywhere because there can be specific sources of chemicals that change it just in a local area. We'll get too that in a moment. Let's talk about the background or baseline composition of dry air first. We specify dry air because air can have various amounts of water vapor ("humidity") and this would make the percentage of other components vary as well. While most air isn't bone dry, stipulating dry air standardizes things. We will also consider air at sea level and some standard temperature and express its composition in terms of the percentage of molecules of various kinds. Here's a table of the main ingredients:

Thecomposition of dry air in percent by volume, at sea level at 15°C.:

Nitrogen -- N₂ -- 78.084%
Oxygen -- O₂ -- 20.9476%
Argon -- Ar -- 0.934%
Carbon Dioxide -- CO₂ -- 0.0314%
Neon -- Ne -- 0.001818%
Methane -- CH₄ -- 0.0002%
Helium -- He -- 0.000524%
Krypton -- Kr -- 0.000114%
Hydrogen -- H₂ -- 0.00005%
Xenon -- Xe -- 0.0000087%
Ozone -- O₃ -- 0.000007%
Nitrogen Dioxide -- NO₂ -- 0.000002%
Iodine -- I₂ -- 0.000001%
Carbon Monoxide -- CO -- trace
Ammonia -- NH₃ -- trace

Reference: CRC Handbook of Chemistry and Physics, edited by David R. Lide, 1997.

You can see that most of what we breathe is nitrogen and then oxygen, followed by much smaller amounts of other naturally occurring stuff (carbon dioxide, of course, is also produced by human activity).

If you bother to add up these percentages (ignoring the "trace" category) you get 99.9991247%. That would seem to be almost everything, leaving only 0.0008753%. Now a percent means one part in a hundred ("part per hundred"). There are a lot of figures after the decimal pont here and a lot of zeroes. We don't have to use 100 as base. We could use 1000 or 1,000,000 or even 1,000,000,000 (a billion). In those terms what's left after we've accounted for all that stuff in the list is 8753 parts per billion. That's a pretty small fraction of the total molecules in a given volume (say a breath) but it's at that level that all the major air pollutants are found. For example, the federal clean air standard for ozone is 80 parts per billion (ppb). The standard for carbon monoxide, listed above as a trace, is up around 9000 ppb, so it doesn't quite fit and thus in some places would make the proportional contribution of the other components lower (not because there would be less of them but because the monoxide levels are a higher proportion than "trace"). Even so, common air pollutants are minute fractions of the total stuff in the air. We know, however, that they can do a lot of damage.

These are just the gases, however, i.e., the molecular component. There are also "rocks" floating around. We call them particulates, but they are mixtures of all sorts of stuff (droplets of things in solution, sand, dirt, lead particles, etc., etc.). Whether you "breathe" them or not (i.e., whether they get down into your lungs) depends on how big they are. Only the really small stuff (under a couple microns, or millionths of a meter in size) is respirable. The amount in the air is measured in terms of the mass (think "weight" if that's easier to visualize), at the level of tens of micrograms per cubic meter of air. How much air is a cubic meter? It's a cube of air a meter on a side. We breathe about a cubic meter an hour.

I've barely scratched the surface of the "easy" question, "What's in the air we breathe." As you can see it involves meteorology, earth science, chemistry, physics, biology, public health and a lot of other stuff. Good question. Not a quick and easy answer.