Harvesting atmospheric gases

A quick thought that seems intriguing, but not sure it would be feasible---Would it be worth it to harvest atmospheric gases in low Martian orbit?

The goal, of course, is to not have to lift propellants and oxidizers out of deep gravity wells.  Such would be the case for the Earth, the Moon, or for Mars.

There may be ways to lift these things from the Moon and Mars--- the QuickLaunch system.  The downside of that is that you have to build the thing on the ground, and those facilities do not exist, nor will they be easy or cheap to implement.  However,  an atmospheric harvester may be feasible for any atmosphere of a planet that has one--- another example could be Venus.

Could it be possible to get hydrogen from the Venusian atmosphere, as contains sulfuric acid as well as carbon dioxide?  If so, then you have oxidizers and fuel.  You could even use a nuclear thermal rocket to power the thing, which is not what you would want on Earth.