Carrier with calcium carbonate at high attitude~low orbit
1)Calcium carbonate releases carbon dioxide on heating(to above 825 °C)
CaCO3 → CaO + CO2
2)down calcium oxide (CaO) to dense atmosphere to absorb CO2, then up to the carrier.
Repeat 1) & 2)...
Could be a mix of CaO and NaOH, or something better.
FYI
http://en.wikipedia.org/wiki/Calcium_carbonate
http://en.wikipedia.org/wiki/Calcium_oxide