Sciencerocks

So the following question came up the other day, that I was trying to do some research to solve, but I couldn't find the answer. It was: "How long does the average infrared photon take to exit the atmosphere." Of course, this depends on the density of green house gasses in the atmosphere, so I am most interested in how long it would take with pre industrial greenhouse gas levels and present day levels. I also understand that when a photon strikes a molecule and causes it to vibrate, the photon ceases to exist, and a new photon is eventually re-emmitted. However, to make this more simple, let's pretend that the photon a molecule absorbes is the same one it emmits. I need to know the probablility that a photon will encounter a green house molecule based on the ratio of that greenouse gas to other gases, (ppm), and how long a carbon dioxide molecule will vibrate when struck by 15micrometer wavelength IR radiation, and 4.3 micrometer wavelength IR radiation. Thanks!