BEIJING, May 31 (Xinhua) -- For Beijingers, this May is the clearest in nine years, with a record 25 "blue sky days" and only six "slightly polluted days", the environment watchdog said Sunday.

    This is the second time that Beijing's air quality achieved the best level during the same period of the year since 2000, said Du Shaozhong, deputy chief of Beijing municipal environmental protection bureau. In April, the city saw 23 "blue sky" days, the best figure since 2000.

    Beijing has a five-grade classification of air quality. A reading below 50 is "excellent;" from 51 to 100 "fairly good;" 101to 200 "slightly polluted;" 201 to 300 "poor;" and over 301 "hazardous." Days with excellent or fairly good air quality are counted as blue sky days.

    Two days in May were rated "excellent", and 23 days were "fairly good.", bringing the number of "blue sky days" up to 25 --12 days more than the same period of last year and 6.6 days more than the average level since 2000, said Du.

    Six days were "slightly polluted", including May 5, May 7 to 9,May 25 and May 27, Du said. "The rapid rise of temperature and stability of atmosphere made pollutants stay."

    The monthly average density of particulate matter, the top pollutant in Beijing, measured 0.117 mg per cubic meter in May, down 43 percent year on year and 24 percent compared with the average level since 2000.

    Du attributed better air quality to the city's environmental improvements.

    "We have drawn from the experience during the Beijing Olympics, and control the pollutants with ever-strengthening supervision," he said. Relative measures include restraining the high-emission vehicles, controlling dust at construction sites, and decreasing the use of coal in the city.

    Beijing has experienced 121 "blue sky days" in the first five months.