Beijing authorities said Sunday they had met their target of "blue sky" days for 2011, amid growing public criticism that officials are underplaying the pollution problem in the Chinese capital.

The city had 274 days of "grade one or two" air quality compared with 252 days in 2010, according to a statement on the Beijing government's official news portal.

"Beijing has seen an overall decline in the concentration of various pollutants in 2011," said Zhuang Zhidong, the deputy director of the Beijing Municipal Environmental Protection Bureau.

But Zhuang admitted that Beijing also experienced "several days of poor air quality as a result of bad weather conditions".

China uses a five-grade classification system to rate its air quality, with one being the best and five the worst.

But the environment ministry is under pressure to change the way it measures air quality after thick smog blanketed Beijing earlier this month, forcing the cancellation of hundreds of flights and triggering a surge in face mask sales.

Public anger over heavy pollution has been compounded by official data showing air quality is good, or only slightly polluted, when smog is visible and figures published by the US embassy rank it as "very unhealthy".

Chinese authorities currently use a method known as PM10, focusing on larger particles in the air.

But the environment ministry has proposed adopting the system favoured by the US embassy, which measures the smallest and most dangerous airborne pollution, known as PM2.5.