'Eating' CO2

For starters, any self propelling craft would most likely spit out more pollutants than it could possibly take in (if indeed it could take in any at all).

 

As was already mentioned, you would need a source of power for the craft that was clean (i.e. didn't make the problem worse). The biggest problem, however, is probably just the raw energy and resources required. Simply "eating" up the components of a gas isn't a simple matter...you'd have to introduce a chemical that reacted with the pollutant and formed something harmless. I'll leave it to the chem people to speculate on what such a chemical might be, but the important thing is that you'd need vast quantities of it. The total mass of the earth's atmosphere is of order 10¹⁹ kg, so that's about 10¹⁵ kg of CO2. You'd would need a comparable quantity of the reactant to get the job done.

 

Keep on dreaming....Great inventors do that ya know?

 

Whatever the treatment method used, an atmospheric-cleaning craft would just be a drop in the bucket. You would need massive swarms of such crafts to do anything to the atmosphere (and this is assuming the crafts don't generate their own CO2 from fuel consumption). It is doubtful (obviously I have not run the numbers) that you could produce enough of these crafts to overcome the CO2-input rate to the atmosphere from all human industry.

As mentioned above, the CO2 would need to go somewhere. If the crafts collect the CO2 (strip it out of the atmosphere), then you'll need a place to dump what you've collected (a huge volume that will slowly release CO2 back to the atmosphere over time). The other option...destroying CO2...would likely not be economically feasible.

There's a simpler & far cheaper solution...promote more plant growth. Plants can cover the land (a swarm in contact with the atmosphere) and capture CO2 and store it in the form of new plant cells (which will also help capture even more CO2). However, there's still the same problem of having enough plant matter to compensate for the CO2 inputs to the atmosphere (in general, we are increasing CO2 output and decreasing plant coverage).

Another solution...cut down on CO2 generation so the existing "CO2 sink" processes can be more effective.

As a comparison, it can cost hundreds of thousands of dollars cleaning up contaminants in only a few acres of a groundwater aquifer. Now apply that scale to however-many-billions of cubic miles of atmosphere there are.