Light particles like helium can escape from the edge of the atmosphere. CO2 is ~11 times heavier, so you have to pump it well above the atmosphere for its orbit to have a chance of carrying it away, or else it will just fall right back down. It's probably not that difficult to maintain the pressure on the ground to keep it flowing, but the problem is you now need 1000's of km of a rigid pressure vessel, instead of a thin film or something.
Light particles like helium can escape from the edge of the atmosphere. CO2 is ~11 times heavier, so you have to pump it well above the atmosphere for its orbit to have a chance of carrying it away, or else it will just fall right back down. It's probably not that difficult to maintain the pressure on the ground to keep it flowing, but the problem is you now need 1000's of km of a rigid pressure vessel, instead of a thin film or something.