Chinese environment ministry is pulling up Beijing over its claims of drastic air clean up, from last year's Olympic Games till the present day.
Beijing spent the run-up to last year's Olympic Games claiming that the city's air had been cleaned up, but according to a report in Nature News, those measurements are now being called into question.
Using an air pollution index (API) in which a score of 100 or lower indicates air quality as 'good', all 17 days of Olympic events in Beijing made the grade.
Overall, the city hit an all-time high of 274 good air days in 2008.
APIs can be calculated in various different ways. Beijing's includes measurements of sulphur dioxide, nitrogen dioxide and particles smaller than 10 micrometres across - dubbed PM10.
Controversially, it has not previously used low-level ozone measurements to calculate APIs, and it does not report on the level of particles smaller than 2.5 micrometres across (known as PM2.5).
Both ozone and PM2.5 have potential negative impacts on health.
Now, Jian Wang of the Chinese environment ministry's pollution-prevention division has admitted that visibility in eastern cities in China is deteriorating.
He said that the cause is ozone pollution and, especially, PM2.5.
"PM2.5 is to blame for the haze," said Wang. "Vehicle exhausts that contain black carbon, sulfates, and nitrates contribute a lot to the density of PM2.5, which is more damaging to the respiratory system than PM10," he added.
He added that the ministry will soon start to include ozone and PM2.5 in its API calculation, with pilot projects to monitor the pollutants expected to start in the deltas of the Yangtze and Pearl rivers next year.
"This is quite a turnaround," said Steven Andrews, an environmental consultant who spent 12 months in Beijing from September 2006 as a Princeton-in-Asia fellow.
Andrews said that the attention to ozone and PM2.5 is welcome, but is concerned that the data might not be reported openly.
He said that the Beijing Municipal Environmental Protection Bureau previously listed ozone readings, but stopped in 2002.
Last year, Andrews had accused Beijing's environment bureau of moving some of its seven air-monitoring stations to improve the API results.
Andrews said there is now no way to prove this because data from individual monitoring sites, which had previously been available online since January 2003, have now been removed.
Despite the criticism surrounding the API, Beijing has said that it was halfway to its goal of having 260 days with excellent or fairly good air quality this year.