Well, instead of repeating myself manually, I'll paste in a comment of mine here from a past discussion on carbon capture:

It's easy to forget why there is a bit of a challenge to getting C02 out of the air: there's so little of it, comparatively.

In order, air is, broadly, made up of the following:

Nitrogen: %78.084

Oxygen: %20.946

Argon: %00.934

CO2: %00.042

The stuff is essentially beyond a rounding error - it really gives one an appreciation of the "either don't release it, or capture it at the point of release" sentiment, and for the difficulties in making carbon capture outside of these scenarios be even slightly cost-effective.