Washington's sweltering work week - a muggy mix of more than 90 - degreee heat and choking pollution - closed on a brighter note yesterday, with slightly cooler weather and possibly fresher air forecast for the weekend.

A drier, less broiling Canadian air mass is likely to drift into the Washington area midday today, weather forecasters said. The front, which had reached Illinois, Michigan and Kansas yesterday, is expected to bring Washington's temperatures down to the high 80s Sunday.

The area's air pollution alert, which has been in effect since Tuesday, was extended through today. Weather forecasters said, however, that the cooler front will likely also reduce pollution by Sunday. The Metropolitan Washington Council of Governments urged motorists to do less driving to help cleanse Washington's air which is polluted mainly by auto exhausts.

Washington's discomfort, though considerable, set no records yesterday. High temperatures of 96 degrees wwere recorded at National Airport shortly after 2 p.m. two degrees below the record 98 degrees of July 8, 1890. The area's air quality index rose to 120 in Suitland at 3 p.m., with two other above-100 marks measured elsewhere - 110 in Silver Spring and 105 in Alexandria, both at 4 p.m. Index readings over 100 are officially considered 'very unhealthy.'

With yesterday's sultry haze, Washington's hot spell had sent thermometer to 90 degrees for higher on 12 of the past 13 days, and the people of Washington took notice.

Louis P. Robbins, principal deputy corporation counsel for the D.C. government, noticed when the air conditioner in his District Building office went on the blink again yesterday afternoon. He repeated an aphorism: 'There's nothing you can do about the weather except complain about it.' But he did not bother to complain.

In Prince George's County, youngsters lounged about air-conditioned landover Mall. Some said they would have preferred to go swimming, but county pools were closed because of suburban Maryland's water disruption. A lifeguard at a Wheaton pool said most of his coworkers had gone to the beach.

There were signs that the end of the broiling week meant a letup for government workers.

Fewer messages were being handled at the Federal Energy Administration's communications center. 'I have to admit traffic's been pretty light here,' Alan Huff, the center's chief, said. Most Fridays, he added, are busier.

In a downtown parking lot where hundreds of government employees normally leave their cars, attendant Charles N. Johnson pointed across empty spaces and unclogged passage - ways. 'On an average day, all these aisles would be full,' he said. Apparently, headded, several hundred regular customers had taken yesterday off from their jobs, gaining a long weekend.

Thousands of government employees at suburban Maryland offices were given yesterday off because of the water emergency. In the addition, several federal personnel administrators said summer Fridays usually mean slightly lower job attendance - a symptom of increased leave allotments and seasonals longings for leisure.

Though slightly cooler weather is expected here over the weekend, the respite appears unlikely to last long. Jim Hand, a forecaster for the National Weather Service, said that a mass of hot, humid air is lurking behind the cool front now heading toward the Washington area. By mid-week, he added, tropical heat and renewed pollution may return.

While Washington missed a record yesterday, Richmond made it three in a row, with 100 - degree temperatures measured at midafternoon at Byrd Airport. The previous Richmond record for the day was 97 degrees, set in 1921. Richmond's heat wave also broke records with 105-degree temperatures Wednesday and 103 degrees Thursday.