China uses PM 2.5 in weather alert system

China's meteorological authority has introduced a new weather warning system for smog which will use readings of PM 2.5, a particle pollutant.

The announcement comes as thick haze chokes many cities.

The National Meteorological Center (NMC) has introduced a three-tier color-coded weather warning system to alert the public to the severity of smog. The system uses yellow to imply moderately smoggy weather, orange for severe conditions and red for extremely severe levels of smog.

The system not only measures the density of PM 2.5, a major pollutant that triggers smog, but also takes into account visibility and relative humidity to better reflect air quality conditions, according to the center.

The smog that has blanketed many Chinese cities in recent days has affected a total area of 1.3 million square kilometers, according to the Ministry of Environmental Protection.

The municipalities of Beijing and Tianjin, together with the provinces of Hebei, Shandong and Henan, will remain the hardest-hit regions on Tuesday evening through Wednesday daytime.

However, rain and snow that is expected to fall on Wednesday night will help disperse the smog in central and eastern regions, the NMC said.

Earlier this month, several consecutive days of smoggy weather choked Beijing and other cities in north and east China, as readings for PM 2.5 far exceeded safe levels.