Blue sky means a day when the city's air pollution is below 100 on the air pollution index (API). The API at noon yesterday was 85.

"Counting 'blue-sky' days is a tense job, for we are always nervous whether we'll be able to achieve the target," Du Shaozhong, deputy director of the Beijing municipal environmental protection bureau, said.

The measures that helped achieve the target include the temporary arrangements made for the Olympics such as allowing vehicles with odd and even numbers to run on alternate days and shutting down polluting factories, Wang Dawei, head of the air quality control division of the bureau, said.

The short- and long-term measures enabled Beijing to reduce pollution by more than 60 percent during the Olympics and Paralympics, Du said.

Though he warned that achieving a similar target next year would be a big challenge, he said: "Our campaign started for the Olympics, but will not end with it."

Good weather played a role in yesterday's API reading, too, Du said. The forecast for yesterday said the API would be above 140 during the day, but the weather improved late at night, heralding a clear morning.

More rain in spring and summer, frequent northerly winds in fall and an early winter are nature's blessings, Guo Hu, director of the Beijing Meteorological Center, said.

Beijing saw just 100 clear days in 1999, the year when the authorities launched their anti-pollution campaign. Thanks to their constant efforts, the number of blue-sky days last year rose to 246.

As part of the clean air program, the authorities shifted most of the high-polluting plants, including that of steel giant Shougang Group, out of the city, and replaced them with green facilities. Stricter fuel emission rules, too, were implemented, with gas stations revamped to curb petroleum vaporization.

Beijing will launch tougher air cleaning programs from January 1 by shifting more coal-burning industries out of the city and banning heavy polluting cars.

The success of the pollution control program can be maintained, environmental experts have said. "This year's success does underline that sustainable development can be achieved with the joint efforts of the government, industries and the public," said Zhang Jianyu, head of the US-based Environmental Defense Fund's China office.

Pollution caused by vehicles, which account for about 40 percent of the pollutants, have to be controlled and regional collaboration continued for better days, he said.