[Reeses]

The average CO2 content of our atmosphere is one third of one percent.

I think you are off by a factor of 10 times too high. 350ppm / 1,000,000 = 0.035%, one third of one tenth of one percent.

[Halgr]

http://encarta.msn.com/encyclopedia_761559991/atmosphere.html

http://www.nationalcenter.org/TSR032204.html

.03 is one third of one percent