Friday will be an inversion-fighting day of free transit fares to promote ridership on one of the season’s busiest shopping days.

“Free Fare Friday” is a one-off demonstration to promote public awareness, Salt Lake City and County officials and the Utah Transit Authority announced Tuesday.

The brainchild of outgoing City Council Chairman Stan Penfold, it will waive the $2.50 fare on all buses and trains in the UTA’s entire six-county service area. The City Council, Salt Lake County mayor’s office and UTA are underwriting the effort with $70,000 that covers the average weekday take on cash fares.

“All day this Friday your ride is on us,” Penfold said at a news conference at the North Temple Bridge/Guadalupe TRAX station.

On Friday, bus drivers will not accept fares, and signs at rail stations will tell riders no fare is required. Riders with electronic passes or FAREPAY cards will not have to “tap” on or off when boarding or exiting a bus or train. Users of the mobile ticketing app won’t need to use one of their tickets.

A wintertime temperature inversion traps cold, stale and polluted air at the surface below a warmer air layer in low-lying basins or valleys. According to the state Department of Environmental Quality, Utah typically sees five to six multiday inversions each winter, with 18 days when airborne levels of fine particulate matter — the pollution generated by vehicles — exceeds national air quality standards.

The Salt Lake City metro area experienced its first such inversion of the season last week. A thick haze sits over the valley on inversion days with no circulating winds to clear the air.

“Because of the unique geography of our valley, we will not be able to prevent inversions from happening, but all of us can reduce the amount of pollution that we pump into the air,” Penfold said. “And we can do our part to in particular prevent red air days from happening by leaving our cars at home and riding transit as much as possible.”

Though only a one-day effort, the idea is in keeping with the goals and policies of the new transit master plan the council approved earlier this month. Pollution from vehicles accounts for 57 percent of wintertime air pollution. Organizers noted Tuesday that a full UTA bus can carry 45 passengers and take that many single-occupant vehicles off the road; a full TRAX light rail car has 150 riders, and a FrontRunner commuter train carries up to 350.

“I’ll acknowledge that one day of free fares is not going to improve our air overnight, but what we’re hoping will happen is people will have conversations,” Salt Lake County Mayor Ben McAdams said. Instead of driving his kids to school Friday, “We’re gonna figure out the train schedule. the bus schedule, what it takes to get them to school and then for me to get to work and then home, and how we’re going to get home together.”

UTA President and CEO Jerry Benson said fare revenue is too critical to make free fare days a routine transit incentive.

“All of us here today know that underwriting fares for just one day won’t fix bad air, but it’s an important gesture that highlights the issue and ways that we can address it,” Benson said. “Partnerships like this are critical to making a difference.”