At the event, residents could turn in their old mowers and buy new, rechargeable electric Black & Decker mowers for a special price of $99.99. The mowers typically cost $400 plus tax.The county distributed more than 480 of the 36-volt cordless mowers on a first-come, first-serve basis as part of the 13th Annual "Mowing Down Pollution" Lawn Mower Trade-In Event.The event is held in an effort to reduce air pollution. Since the annual exchange began in 1990, people have bought nearly 6,000 discounted electric mowers. Local air pollution has been reduced by an estimated 22 tons each year, according to the county.The event started at 8 a.m. Saturday in the North Parking Lot of the County Administration Center at 1600 Pacific Highway but many lined up earlier than that.Residents had to show photo identification and prove county residency to exchange their mowers. One trade-in was allowed per household and old models had to be in working condition with all parts attached.