When comparing power for power, a two cycle is dirtier than a four cycle.
If you want to improve air quality especially in the Smoky Mountains, cut down all the pine trees.
Those Smoky Mountains — What Makes the Blue Ridge Blue? Homeschool Rangers
The truth is the EPA limits for hydrocarbon emissions for a car are lower than the emissions of a full grown pine tree.
My save the planet thinking is:
Own the smallest vehicle to meet your needs (and have a motorbike as a second vehicle
Drive it sensibly
Keep it tuned
Combine trips when possible
Carpool when I can
Avoid buying disposable anything
Say "Yes Ma'am" to your mother.
And finally, have two kids or fewer, not a whole litter!
(Sorry if that was too political)