11 November 2014 Last updated at 03:30 ET
Chinese women cover their face with scarf as they walk past the Central Business District in Beijing, China Tuesday, Nov. 11, 2014
Chinese authorities have asked sites and apps which monitor air pollution in Beijing to stop using data provided by the US embassy, say media reports.
Beijing is currently hosting a high-profile gathering of world leaders from the Asia-Pacific bloc of countries.
In an attempt to have clearer skies for the summit, factories have been shut down and cars barred from the roads.
Many Beijingers use apps and sites to check pollution readings, including levels of dangerous PM2.5 particles.
The city, which is often engulfed in a thick blanket of haze reaching well beyond healthy levels, had clear blue skies for the days leading up the summit.
However on Monday as world leaders met for bilateral talks, our correspondents reported there was smog in the capital again.
Most apps and sites provide a daily reading from both the city government and the US embassy reading. The US reading is considered by many to be more reliable and often shows a higher pollution count.
Beijing-air.com posted a notice on its website saying it had "received instructions from authorities saying that this month's air quality readings will be based only on what is released by the Beijing environmental protection bureau".
Fresh-Ideas Studio, which is the developer of a popular pollution-tracking app, told AFP news agency that Beijing city officials had asked that US embassy data be removed.
'Priority of priorities'
The gathering of the Asia-Pacific Economic Co-operation (Apec) bloc of 21 nations along the Pacific Rim includes US President Barack Obama and Russian leader Vladimir Putin.
