Here's some specifics from other citations:
Grass Cutting Beats Driving in Making Air Pollution : Cites EPA statement: 1 hour mowing=650 miles in 1992 car.
Lawn Mowers May Account for Five Percent of U.S. Air Pollution, EPA Says
Several of the sources site a swedish study saying 1-hour mowing is like a 100-mile car ride, so there are widely varying estimates. Sweden could well have been comparing to European diesel, which has different pollution properties.
If the question regarded the relative pollution of electric power plants, I don't have the research at my fingertips, but I don't remember ever having an argument about that -- the electric plants, even those burning coal or oil, have economies of scale to spend money on pollution controls that you couldn't put into every automobile.
However, I've seen some writings indicating that hybrid cars like the Prius rival oil and coal-fired plants for pollution control. I don't have an opinion on that.
I generally like using financial comparisons as a good proxy for "efficiency", but that isn't true for pollution, because individuals do not at this time pay for their pollution. Power plants do, to some degree, but not individual users. If the state imposed a "pollution tax" on products, the price of products use would be a good proxy for their pollution value.
Using this proxy, Electricity costs me about $0.12 per KWH this equates to about $5.63 for the equivalent energy in a gallon of gasoline. That's why I asked.