Since sunlight is one of the essential ingredients to create ozone, anything that postpones emissions to the end of the day will help reduce the problem.  Mowing your lawn after 6:00 PM gives the atmosphere a chance to disperse your mower emissions before the sun has a chance to ‘cook’ them.  If waiting is not an option, such as a 100 degree day, at least mow your lawn before 10:00 AM.

Better Yet:

If it’s a particularly hot and stagnant day (Over 100 degrees and winds less than 5 mph), wait a day or two for a cooler or breezier day.  Not only will it help reduce ozone formation, but it’ll feel a lot more comfortable.