The Arlington County Board, reversing an earlier decision approved last night by a 3-to-2 vote the county manager's proposal to construct a temporary parking lot for 800 cars near the Pentagon City Metro station. One board member estimated that commuters could begin parking there for $1 per day in as little as one month.

Approval of the lot had been opposed by a majority of county planning commission members who said they feared its construction would increase traffic and pollution and damage the integrity of the bus-rail sytstem.

The Board's vote was greeted by loud cheers and applause by about 20 commuters, most of them Arlington residents, who testified in favor of the lot at a two-hour hearing. A smaller number of residents in the area of the lot near S. Joyce and S. Hayes Streets voiced opposition.

The crushed-stone lot, located on land owned by the Cafritz Corp., is to be used through 1980.

"I think it's in the best financial and transportation and even best neighborhood interests that we approve this parking lot," said Board Chairman Joseph B. Wholey, who sponsored the motion.

Voting with Wholey were Walter L. Frankland Jr., who along with Wholey had supported the unsuccessful proposal to permit the lot in July. They were joined by Ellen M. Bozman, who initially voted against the lot.

Board member Dorothy Grotos, who along with Vice Chairman John W. Purdy opposed the lot, said, "We may be the dumping ground for all of Northern Virginia's transportation problems" if the proposal is adopted.

Several residents of the Pentagon City area agreed. Robert Grossman, who has purchased a condominium across from the parking lot site, said. "This is going to be a complete eyesore. This was never in the original plan for the Pentagon City station. Would you want this type of facility across from your home?"

Other home owners said they feared that the lot would lower property values and exacerbate traffic congestion and pollution.

Commuters speaking in favor of the lot said it was necessary in part because of the unreliability of evening bus service. "I hate the traffic. I hate the hassle of driving and because of my irregular hours I can't car-pool," said Angie Howard, who said it has taken her as long as two hours to reach home by bus at night.

"I'll continue to drive (into town) if I'm not able to park." Glenda Miller said.

When the Board voted against authorizing a lot in July, the planning commission had approved it. In August, commuters defied the ban and parked in the lot briefly. That prompted local authorities to dig trenches in the lost to prevent parking. Commuters then began parking on residential streets surrounding the station and angered a number of residents.