The capital needs at least 22 such days per month from June to December to achieve its aim.

"It is indeed very tough (to have over 22 'blue sky' days a month)," said Du Shaozhong, deputy director of the Beijing Municipal Bureau for Environmental Protection.

Beijing saw only 11 "blue sky" days, or days with fairly good air quality, April, the least in five years, Du said.

From January to April, Beijing reported only 51 "blue sky" days, 16 less than the same period last year, he said.

Despite 22 clear days by Monday this month, Du showed no optimism.

"We are still confronted with a severe situation in achieving the 'blue sky' goal, and we call for joint efforts of all citizens to reduce the emission of pollutants," he said while promoting a latest city-wide campaign of "no car day" to clean up air pollution.

More than 200,000 drivers from about 100 Beijing automobile associations have been encouraged to leave their cars at home at least one day a month to ease traffic jams, reduce noise and improve air quality. Motor vehicle emission is the leading cause of the city's air pollution, Du said.

There are more than 2.6 million motor vehicles in Beijing, and increasing by more than 1,000 a day, Du said.