China has added another 40 medium-sized cities to the country's upgraded air quality monitoring network, according to a statement issued by the Ministry of Environmental Protection on Sunday.

The 172 monitoring spots in 40 cities, such as energy-intensive Baotou and Ordos in north China's Inner Mongolia Autonomous Region, and Linfen and Yangquan in coal-producing Shanxi Province, will begin working on Oct. 1, said the statement.

It said the spots have been upgraded in order to provide real-time data concerning sulfur dioxide, carbon monoxide, airborne particles measuring less than 2.5 microns in diameter, or "PM 2.5," among six indexes required by the country's improved air quality standard.

China has 668 monitoring spots in 114 cities under the new network so far, with data released daily through media including the Internet.

The country is facing severe air pollution challenges. It recently issued a 1.75 trillion-yuan (284.2 billion U.S. dollars) plan to tackle the worsening air, pledging to improve air quality within five years in order to decrease the number of heavily polluted days and improve the air in major city clusters.

On Sunday, Beijing was shrouded in choking smog due to adverse weather conditions locally as well as in neighboring regions, meteorological authorities said.

As of 8 a.m., 24 out of 35 monitoring spots in the capital indicated heavy air pollution with the highest level of six, according to the Beijing Municipal Environmental Monitoring Center.

The density of PM 2.5 exceeded 200 micrograms per cubic meter, the center said.