BEIJING — Weary of waiting for the authorities to alert residents to the city’s most pernicious air pollutant, citizen activists last May took matters here into their own hands: they bought their own $4,000 air-quality monitor and posted its daily readings on the Internet.

That began a chain reaction. Volunteers in Shanghai and Guangzhou purchased monitors in December, followed by citizens in Wenzhou, who are selling oranges to finance their device. Wenzhou donated $50 to volunteers in Wuhan, 140 miles inland. Officials have claimed for years that the air quality in fast-growing China is constantly improving. Beijing, for example, was said to have experienced 286 “blue sky” days in 2011, a statistic belied by the heavy smog smothering the city for much of the year.

But faced with an Internet-led brush fire of criticism, the edifice of environmental propaganda is collapsing. The government recently reversed course and began to track the most pernicious measure of urban air pollution — particulates 2.5 microns in diameter or less, or PM 2.5. It decreed that about 30 major cities must begin monitoring the particulates this year, followed by about 80 more next year.

The Ministry of Environmental Protection also promised to set health standards for such fine particulates “as soon as possible.” Last week, after years of concealing its data on such pollutants, Beijing began publishing hourly readings from one monitoring station.