I did not mention carbon footprint. Clean air is a very different topic than carbon emissions. Catalyst or not makes no difference to carbon footprint, but having effective emissions controls makes a significant difference to air quality. And yes, private aircraft are carbon heavy, and the exhaust is not great- but there are far fewer aircraft than cars, and the exhaust output from road vehicles is a lot closer to where people breathe.
My kJet DeLorean aced its emissions test- came out cleaner than many newer vehicles- besides which it doesn't get driven all that much. Both my daily drivers are electric. That's not a lecture- it's just how I choose to spend my transportation dollars, and I'm happy to do it in an environmentally responsible and legally compliant fashion.
Best,
KL