[Lewis Thomson (OP)]

Donald has been pondering this question and presented it to The Naked Scientists.

"How fast does the Earth's atmosphere mix? Certainly from volcanic eruptions and atmospheric nuclear tests, and the constant satellite surveillance must give a estimate. How long does it take for a molecule exhaled by Antarctic penguin is inhaled by a polar bear in the Arctic?"

What answers have you found? Leave them in the comments below...

[chiralSPO]

This is a tricky question, in part because each molecule has a practically infinite number of possible paths it could take from once part of the atmosphere to another, and they will all take different amounts of time (depending on how fast the molecule is traveling throughout its journey, how many other molecules or surfaces it bumps into, and how direct of a path it ends up taking). The average speed of a molecule depends on the kinetic energy it has (which is typically closely related to the temperature of its environment) and its mass (heavier molecules move more slowly for a given amount of kinetic energy).

The kinetic theory of gases is fairly math heavy, but will give you an idea of the models available for thinking about this on a molecule by molecule basis (a bottom up approach to answering the question): https://en.wikipedia.org/wiki/Kinetic_theory_of_gases

For a top down approach to answering this question, we can look at experimental data. The first example that comes to my mind is the annual variation in carbon dioxide concentration observed at Mauna Loa (https://keelingcurve.ucsd.edu/)

If we look over the entirety of the record (from 1958 to present day), we can see a very clear and consistent sinusoidal pattern (superimposed over the roughly exponential increase due to burning of fossil fuels):

mlo_full_record.png (162.17 kB . 1000x600 - viewed 1481 times)

If we zoom in and look at just the last year, we can see a maximum value at roughly in mid to late May, and a minimum in late September or early October.


mlo_one_year.png (119.03 kB . 1000x600 - viewed 1529 times)

This seasonal variation is due to the fact that there are many more deciduous trees in the northern hemisphere than in the southern hemisphere. So during the late spring and summer months, the trees have leaves and are absorbing carbon dioxide and converting it to organic compounds using photosynthesis, and then the leaves fall off in the autumn (and decay), so the deciduous trees stop absorbing carbon dioxide. We can think about the lag time between when the trees are absorbing the carbon dioxide around the world, and when the effects are detected in Hawaii as an approximation of how quickly gases are distributed. (and, by my cursory analysis, it looks like maybe a few weeks between when I would expect maximum and minimum concentrations due to just rates of absorption and when the maximum and minimum are detected.)

[alancalverd]

Major weather systems move around the globe at about 30 knots west to east, rotating at about 10 - 20 kt at low level. I would guess a molecule might drift from the north pole to the equator in 1 - 10 thousand hours, but there is very little transfer between tropical latitudes so crossing the equator is less probable.

It is notable that albatrosses mostly occupy a limited range of arctic or antarctic latitudes because they soar on a strong prevailing wind, but arctic terns use powered flight to migrate between the polar regions.