China has warned foreign embassies to stop publishing their own reports on air quality in the country, escalating its objections to a US Embassy Twitter feed that tracks pollution in Beijing.
Only the Chinese government is authorised to monitor and publish air quality information, and data from other sources may not be standardised or rigorous, Wu Xiaoqing, a vice environmental minister, told reporters.
China has long taken issue with the US Embassy's postings of hourly readings of Beijing's air quality on a Twitter feed with more than 19,000 followers since 2008, but its past objections were raised quietly.
US Embassy officials did not immediately comment, but the Twitter feed was operating normally. Its readings are based on a single monitoring station within embassy grounds, and pollution levels are rated according to a US Environmental Protection Agency standard that is more stringent than the one used by the Chinese government.
Beijing is frequently cloaked in a yellow haze, with buildings a couple of blocks away barely visible, but Beijing's official air quality index records the pollution as "light" - a reading without a numerical comparison and at odds with what many people experience.
Earlier this year, the Beijing government began reporting PM2.5 - particulate matter less than 2.5 micrometres in size - about a 30th of the width of an average human hair - after long-standing public and international criticism that Chinese standards were insufficient.
The US Embassy reported 47 micrograms of fine particulate matter in the air on Tuesday and said the level was "unhealthy for sensitive groups". Readings from Beijing Environmental Protection Bureau's 27 monitoring stations ranged between 51 to 79 micrograms, with all those levels categorised as "good".
Beijing has monitored particulate matter for years but only began releasing the results earlier this year in response to public anger over the government's lack of transparency. The government appears frustrated that there are now duelling readings for air quality and that the US readings underscore the fact that pollution levels considered unhealthy in the US are classified as good by China.
Mr Wu said it is not fair to judge Chinese air by American standards because China is a developing country and noted that US environmental guidelines have become more stringent over time.
The standard China uses "takes into account the level of our current stage of development", he said. Mr Wu also said that air quality monitoring by foreign diplomats was inconsistent with the Vienna Convention on Diplomatic Relations and urged diplomats to abide by China's laws and regulations.