It would be interesting to see man's relative destruction of O2 by varios means. Off hand I guess most of it is burning fossil fuels, mainly at ground level. Brazil does have some sugar cane alcohol powered crop duster air planes. They benefit from slightly greater HP produced compared to gasoline, but I'm too lazy to even compare the relative weight of a full fuel tank. Certainly gasoline gives greater range /air time, but for crop duster that is not impotant as must return for more of what it is spraying on crops, and can fill fuel tank then as well. It would be interesting to know how many acres of grass lands are needed to convert the CO2 one human exhales back into O2 for one "typical" person. Also another CO2 net zero release air plane could be electric with hydrogen* fuel cell - Has any one made one? There are a few small one or two passenger battery/ electric motor planes, so I think larger capacity may be possible with H2 -fuel instead of battery. * Generated from only wind or other solar energy.