The often smog-cloaked San Gabriel Mountains jutted into clear skies today, ending a landmark year in a three-decade battle to curb the air pollution that has helped make Los Angeles famous.

For the first time since scientists began to record the levels of lung-rasping ozone in 1955, the Los Angeles basin passed a year with no second-stage smog alerts. In 1978, southern Californians suffered 23 days with second-stage alerts leading to industrial shutdowns and the suspension of after-school sports.

"The achievement is extraordinary," said James Birakos, deputy executive of the area's air quality management district. He predicted further progress as the results of the state's unusually strict pollution controls continue to accumulate.

"Reduction of the peaks is the best indication of the air control efforts," Birakos said. However, because the coastal basin's odd weather and topography are always expected to trap some pollutants, the traditional Los Angeles haze remains, and the less serious first-stage alerts still plague the area.

Scientists and state regulators devoted to curtailing nationally popular jokes about Los Angeles smog say much of the credit goes to the cumulative effects of the federal Clean Air Act. When passing the act in the late 1960s, Congress approved by a narrow margin an amendment by Sen. George Murphy (R-Calif.) to give his state power to institute air pollution controls tougher than the national standard.

Lead and sulfur levels have declined sharply, as Los Angeles and much of the rest of the country have benefited from improvements in automobile exhaust systems, fuel production and management of industrial pollutants.

Earlier this year, as a sign of the still legendary status of Los Angeles air pollution, comedian Johnny Carson designated smog "the official gas of the 1984 Olympics." The two-week athletic extravaganza appears to have had a major impact on pollution levels at one of the worst times of the year, late July and early August.

Los Angeles commuters who elected to stay home, take vacations or ride buses to avoid anticipated Olympic traffic jams forced a 3 percent to 4 percent reduction in rush-hour traffic. This led to even more significant drops in ozone, the eye-watering result of nitrogen oxides and hydrocarbons from auto exhausts reacting to sunlight, Birakos said.

In March, California launched a massive program to require testing of the antismog equipment on all 25 million automobiles and light trucks in the state. As in 26 other states -- including Maryland -- where such programs have begun, California motorists grumbled at first about the cost and the difficulties of finding convenient smog-check stations. But specially built smog-check stops, in addition to the usual garages and gas stations, have sprung up to reap profits from a brand new industry.

According to Bill Sessa, press secretary for the state air resources board, the every-other-year smog device checks are required in eight California urban areas that contain 90 percent of the state's automobiles. "The cost is about $20 on the average, and the check takes about 10 to 15 minutes," Sessa said.

Los Angeles pollution includes a foul mix of carbon monoxide, sulfur dioxide, lead, particulates, sulfate aerosols and nitrogen dioxide. But the area's worst problem is ozone. This ingredient of photochemical smog is a strong irritant that attacks the respiratory system and can lead to lung damage.

The air resources board says cardiovascular disease and such respiratory ailments as asthma and bronchitis are aggravated by exposure to ozone. Even for healthy persons, high concentrations of ozone may cause nausea, dizziness, headaches, coughs or a burning sensation in the chest.

During a first-stage smog episode -- when ozone levels reach .2 parts per million for an hour -- industries are asked to curtail emissions, drivers are advised to avoid unnecessary trips, and children and persons sensitive to smog are advised to stay inside.

In a second-stage alert -- when ozone levels reach .35 parts per million for an hour -- some utilities are required to switch from coal or oil to natural gas, and some factories are required to reduce smokestack emissions. Although not required by law, schools usually cut back strenuous physical activity, such as football practice, during second-stage alerts.

According to Birakos, the area had 120 first-stage alerts in 1979 and a record low of 64 in 1982, when unusually cool, cloudy and windy weather reduced the number of hot, stagnant days that trap pollutants between the San Gabriel mountains and the sea. There were 94 first-stage alerts in 1984.

Since the record 23 second-stage alerts in 1978, the number of these more serious episodes has steadily declined. There were 17 second-stage episodes in 1979, 15 in 1980, five in 1981, two in 1982, three in 1983 and none this year, Birakos said.

Sessa said California regulators, as part of their continual struggles with Washington over defining adequate standards, are now asking the Environmental Protection Agency to tighten proposed standards for soot-like pollution from big trucks and buses. The state also is beginning to worry about pollutants from "gray-market" cars, imported in greater numbers from Europe by individual buyers without the usual factory-installed antipollution devices.