It's concentrated near local sources of it, but otherwise it is fairly well-mixed throughout the atmosphere. Although it is heavier than nitrogen or oxygen molecules, if there is any vertical component of wind it will easily be carried upward. There is only a slightly lower volume fraction of it in the stratosphere than there is in the troposphere.

Under the right conditions (very light winds, temperature inversion, nighttime) it can remain near the ground. Those were the sorts of conditions that existed In 1986 in Cameroon when Lake Nyos "burped" a large amount of carbon dioxide that subsequently flowed along the ground and killed more than 1700 people in nearby villages.

This is an image of the globe showing co2 concentration.

http://photojournal.jpl.nasa.gov/jpegMod...

The image is for jul 2009, so the absolute values are a little higher now than shown in the legend, but the distribution pattern will still be virtually the same.

Where green trees are decreased because you may know tree takes CO2 and leaves Oxygen by photosynthesis process.

http://www.solutionsams89.blogspot.com

Thank you.

around the sources that produce the gas near the surface.

Near the ground it's one of the heaviest components of air.

On the ground, due to its weight.