BEIJING -- Beijing environment authorities released air quality data on Thursday for the week of APEC, with PM 2.5 levels, a key indicator of air pollution, dropping dramatically.

Following a special air pollution control plan for APEC meetings, the city's daily PM 2.5 density fell to 43 micrograms per cubic meter between November 1 and November 12, a 55-percent reduction compared with the same period last year, according to the Beijing Municipal Environmental Protection Bureau.

Daily average levels of PM 10, sulfur dioxide and nitrogen dioxide decreased by 44 percent, 57 percent and 31 percent respectively over the same period of 2013, it said.

"Beijing had 11 days with good air quality, while only one day suffered from mild air pollution," said Fang Li, deputy head of the bureau.

In order to tamp down on pollution for the Asia-Pacific Economic Cooperation forum, Beijing and neighboring regions imposed tight limits on car use, ordered factories and construction sites to close and declared a holiday for public-sector employees.

"The city became cleaner and cars ran more smoothly and quickly during APEC," said Song Qiang, head of Beijing Municipal Research Institute of Environmental Protection.

The tough emissions-reduction measures have greatly improved the air quality of the region. The phrase "APEC blue" was even coined by Chinese netizens to describe the blue sky in heavily polluted Beijing during the week.

A post circulating widely on China's popular messaging service WeChat said that APEC also stands for "Air Pollution Eventually Controlled." Another post defined "APEC Blue" as "something that is beautiful but transient, almost like an illusion."

With the end of APEC meetings, the emission-reduction measures have also been removed, but residents hope to retain the blue skies and clean air.

"But solving the air pollution issue is a complicated and long-term task. Everybody needs to shoulder the responsibility to cut emissions and make APEC Blue the new normal," Song said.