Question of the day at WUWT: Why doesn't CO2 stay near the ground?

Seen at wattsupwiththat, a comment by cnxtim, who says:

March 25, 2014 at 6:07 am
I am still waiting to find out hoe CO2 generated at ground level by the burning of fossil fuels, makes it’s way to the upper atmosphere to join other molecules of CO2? , Or was I asleep in class and somehow missed that very important first step in the ‘science’ of AGW?

surely there is a pro CAGW teacher out there who could enlighten me?

I'd say cnxtim has been asleep for a long time.  Ferdinand Engelbeen was the only person to give an answer and he says it quite well. Although I'll add that most CO2 doesn't make it to the upper atmosphere.  Most of it stays in the troposphere. Here is what Ferdinand wrote:

March 25, 2014 at 7:26 am
Not that difficult: if sand grains can be transported for thousands of km from the Gobi desert in Mongolia to Arizona, while sand is about 1,000 times more heavy than air, it is no problem for air to transport CO2 which is only slightly heavier a lot of times around the earth. Further, upwards air flows are available at a lot of places and certainly in the tropics by thunderstorms and other ascending and descending air flows.

It takes some time to mix the releases and uptakes of CO2 into the bulk of the atmosphere: a few days to a few weeks for the same altitude and latitude, a few weeks to a few months for different altitudes and latitudes, and 6 months to 2 years for the transfer from the NH to the SH, as only 10% per year of air is exchanged between the hemispheres.

But in general, CO2 levels are within 2% of full scale from sealevel to 20 km height and beyond. Only near ground on land, there are a lot of local sources and sinks which makes that you can measure any level of CO2, depending of the proximity to these sources and sinks and wind/mixing speed.

One other thing I'll add is that gas molecules move.  They don't just move downwards, they move sideways and up.  And they move quickly.  Here is an extract from an education website, which is probably the class that cnxtim dozed off in.

In gases the particles move rapidly in all directions, frequently colliding with each other and the side of the container. With an increase in temperature, the particles gain kinetic energy and move faster. The actual average speed of the particles depends on their mass as well as the temperature – heavier particles move more slowly than lighter ones at the same temperature. The oxygen and nitrogen molecules in air at normal room temperature are moving rapidly at between 300 to 400 metres per second. 

Unlike collisions between macroscopic objects, collisions between particles are perfectly elastic with no loss of kinetic energy. This is very different to most other collisions where some kinetic energy is transformed into other forms such as heat and sound. It is the perfectly elastic nature of the collisions that enables the gas particles to continue rebounding after each collision with no loss of speed. Particles are still subject to gravity and hit the bottom of a container with greater force than the top, thus giving gases weight. If the vertical motion of gas molecules did not slow under gravity, the atmosphere would have long since escaped from the Earth.