Perched atop the U.S. Embassy in Beijing is a device about the size of a microwave oven that spits out hourly rebukes to the Chinese government.
It is a machine that monitors fine particulate matter, one of the most dangerous components of air pollution, and instantly posts the results to Twitter and a dedicated iPhone application, where it is frequently picked up by Chinese bloggers.
One day this month, the reading was so high compared with the standards set by the U.S. Environmental Protection Agency that it was listed as "beyond index." In other words, it had soared right off the chart.
"You couldn't get such a high level in the United States unless you were downwind from a forest fire," said Dane Westerdahl, an air quality expert from Cornell University.
But China's own assessment that day, Oct. 9, was that Beijing's air was merely "slightly polluted."
Not even the most fervent propagandist would call the city's air clean, but the Chinese government made great efforts to improve air quality for the 2008 Olympic Summer Games. Beijing authorities moved huge steelworks out of the capital, switched city dwellers from coal to natural gas heating, raised emissions standards for trucks, and created new subway and bus lines. The cost of the cleanup was estimated at $10 billion, not including the investment in mass transit.
Three years later, the difference between the Americans and the Chinese is at least in part about what they're measuring. And it highlights the rapid growth in the number of cars in Beijing.
Chinese monitoring stations around the capital track large particulates of up to 10 micrometers. The number of those particles has dropped as a result of reforestation programs that lessen the dust storms that blew in from deserts. The Chinese have also been successful in reducing sulfur dioxide emissions by limiting coal heating and imposing stricter emissions standards.
The U.S. monitor tracks tinier particles — less than 2.5 micrometers — that physicians say are capable of penetrating human lungs and other organs. Car and truck exhaust is a major source of fine particulate pollution, a particular problem in Beijing, where the number of registered cars has skyrocketed from to 5 million from 3.5 million in 2008.
"There was a valuable legacy from the Olympics," said Ma Jun, director of the Beijing-based Institute of Public and Environmental Affairs. "But there are big issues here. The rising population, the expansion of the city, the fast increase in the number of cars have offset the improvements."
Regardless, the Beijing Environmental Department reported 286 "blue sky" days last year, up from 274 in 2008. And the Chinese bristle at criticism of Beijing's air quality, especially from Americans.
When an American doctor at Beijing United Family Hospital recommended this month on his blog that people wear face masks, the Communist Party-affiliated Global Times newspaper ran an article rebuking him.
The newspaper quoted an anonymous doctor at Peking University People's Hospital as saying, "The suggestion to wear air masks will make trouble out of nothing, as we've had polluted air for a long time, and we shouldn't be living with an American standard."
Diplomatic cables released this year through WikiLeaks reveal that the Chinese government has asked the U.S. Embassy to stop publishing its data, which is posted hourly on Twitter at @beijingair, an account that has 9,200 followers.
In July 2009, a Foreign Ministry official complained that because the U.S. data conflicted with China's, they were causing "confusion" and undesirable "social consequences."
The embassy installed its monitor in 2008 before the Olympics to advise its personnel about air quality, but then decided it should make readings public under diplomatic rules that require that information regarding health and security risks be made available.
The measurements drew widespread attention last November, the first time a reading for fine particulate matter went above 500 micrograms per cubic meter, about seven times the U.S. standard for "acceptable" air quality.
The embassy ended up reporting that off-the-charts reading as "crazy bad." (Embassy officials say a computer programmer with a sense of humor embedded the language in the program linking the monitor to Twitter without realizing it would ever get used.)
The embassy quickly deleted the tweet and replaced it with "beyond index," but the fanciful description stuck in the imagination.
David Vance Wagner, a former advisor to the Chinese government on air quality, suggests that its problem is political as well as technical.
"Politically, it is hard to revise the standard. After 10 years of saying things are getting better and better, if you reverse that, people will be justifiably angry," Wagner said.
Wagner believes Beijing's air hasn't gotten worse since the Olympics, but that it hasn't improved either.
"China was justifiably praised for meeting its commitments to improve the air quality, but there is definitely not as much momentum or press now," Wagner said. "It is frustrating now because we are stagnating at a terrible air quality level."
At times the air is so dense it appears to be snowing. On several occasions, pollution combined with fog has been so bad that motorists have had to turn on their headlights in the middle of the day.
"The fine particulate matter is what affects visibility and makes it look like a horrible foggy day," said Cornell air quality expert Westerdahl. "It also is what most directly affects human health."
Fine particulate matter is linked to respiratory ailments, heart disease, stroke and premature death. Prominent Chinese environmentalists don't disagree with the need to focus on it.
"The Ministry of Environmental Protection realizes that PM 2.5 [particulate matter of less than 2.5 micrometers] reflects more about the effects of air pollution on human health," said Zhu Tong, a professor at Peking University who was the main air quality advisor for the Olympics, in an email response to questions. But the ministry needs money to buy monitoring machines and train staff members to make sure its statistics are accurate, he said.
Although it is government data that are published in newspapers and broadcast on television, other Chinese media are increasingly citing the U.S. figures.
"They are walking a fine line," Wagner said. "They don't want to question the validity of Chinese data, but they recognize the U.S. data is probably a more accurate scale for measuring health impact on residents."
Complaints about air quality are a staple of conversation in Beijing. From recent Chinese microblog chats came such comments as:
"Is it excrement floating in the air today? I'm almost choking to death!"
"There is no quality of life when you can't be sure you're not breathing anything poisonous."
Over the last year, interior designer Lu Weiwei and a photographer have been taking pictures of people against a backdrop of air, clear or polluted — observing that the mood of the people is often dependent on the air quality.
"You don't need sophisticated instruments to tell you what is the air quality," Lu said. "You look up at the sky and if it's clear, it is a good day and you're happy — or not."
News assistants Nicole Liu and Tommy Yang in The Times' Beijing bureau contributed to this report.