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Carbon Units

A Random Walk Through the Air

Is CO2 evenly distributed?

You could reasonably think that the random Brownian motion associated with any molecules in any gas would totally distribute all the components evenly.

In the long run under idealized conditions this is true, but of course the real world is not ideal.

In many ways the atmosphere behaves like a fluid, or more specifically a solution – a mix of fluids, solids and of course gases. We know that when the ingredients of a solution are first combined, they are completely unevenly distributed. Then as the components mix together, whether from stirring, gravity, or circulation within the fluids, they move toward whatever mix their associated physics dictate, and eventually reach a stable state. Oil and water for example, will usually separate almost completely as the heavier water molecules settle to the bottom, and the lighter oil floats to the top.

Oil and water don’t mix.

CO2 molecules are slightly heavier than either oxygen or nitrogen molecules, so does CO2 accumulate and become more concentrated at lower altitudes? Or since CO2 is often emitted as part of a stream of gases that is warmer than its surroundings, does it rise? Does the exhaust from combustion processes float toward the top of the atmosphere before it cools? And how does this impact our efforts to measure CO2 levels at the earth’s surface?

oil and water
Is this what mixing looks like?

Science says that in the absence of external factors, a mix of gases will gradually move toward equilibrium throughout a given volume. If there were no gravity, or wind, or the earth wasn’t rotating, or the atmosphere wan’t constantly going through heating and cooling cycles, we would expect over time that all gases would be evenly distributed at all altitudes and locations. Of course we actually do have gravity, wind, and a rotating Earth and we know from observation that CO2 levels vary considerably from location to location, and by time of day, among other factors.

The physics of this global combination of solution and suspension are complex, and indeed there does appear to be a slightly higher concentration of CO2 at lower altitudes. The difference noted in one of the more detailed studies is less than 3% however. Here at Nine Wright’s sensor site in a U.S. east coast suburban location, criss-crossed by roads carrying thousands of vehicles, we typically see swings of 30% or more throughout the day. Usually the highest levels occur at morning rush hour, when vehicle emissions are highest, and photosynthesis has not yet had time to start removing carbon accumulated overnight. We have seen levels ranging from lower than 400 parts per million (ppm), to higher than 1100, with an average closer to 500 depending on season and wind direction. And to further complicate matters, the sensor we use has an accuracy of +/- 50 ppm, and also varies slightly with temperature. So where does all this leave us?

Like many other questions we have about the ecosystem, it appears that the answer is both yes and no. Globally the average levels are remarkably homogenous; consistently around 400, although steadily rising. Where more granular local level information is available we see it is constantly changing, but over time will also tend toward the global average.

Do you know the level in your area? Health experts believe numbers over 1000 can contribute to decreased mental and physical function.

3 replies on “A Random Walk Through the Air”

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