Lawn mower engines are pretty nasty as far as emissions go.
A quick google search finds:
EPA sets tough new emissions standards for lawn, boat engines
This is good news for the air, since the EPA estimates that boat and lawn care engines cause about a quarter of the carbon monoxide emissions in the U.S. About 25 percent of the gasoline put into small engines comes out of the exhaust without being burned - and an hour-long ride on a riding mower is as dirty as 34 average cars running during that same hour - and the EPA says that these regulations will save about 190 million gallons of gasoline a year.
Soloution: Make all lawn mowers 26hp. POWAHHHHHany 25hp or under lawn mower
But seriously though a 1:34 ratio of lawn mowers to cars =/= Average summer of lawn mowing vs. car all year.
I figure I use my lawn mower one hour a week from mid-may to mid-august. 5 months for about 20 hours of lawn mower use per year. 20*34=680...welp, maybe i am wrong lol. thats like 1hr 45min of driving a day. I dont think I drive that much. Point receded
Yeah, its pretty crazy how dirty small engines are, or how clean we've made large engines... I suppose either end of the spectrum is impressive.
i try to use shell gas exclusively (I work there) and there is a big difference in running versus some other brands.