BEIJING, Jan. 29 (Xinhua) -- Thirty percent of official cars in Beijing should not be used when there is "extremely bad" pollution, traffic authorities confirmed Tuesday.

The order was implemented on Monday evening to help combat the fourth round of heavy smog to hit the capital city in four weeks, Beijing Traffic Management Bureau said.

Traffic authorities will monitor, take pictures and trace official cars that violate the government order. Whatever department the vehicles belong to will be punished.

Beijing has a list of official cars that should not be used when there is heavy pollution.

The capital categorizes air pollution days as "bad," "seriously bad" and "extremely bad" depending on indexes that include measurements of both fine and coarse air particles and ozone levels.

Beijing's air quality has worsened again since Monday, with a significant thickening of the PM 2.5, or airborne particles measuring less than 2.5 micrometers in diameter.

At 10 a.m. Tuesday, the average concentration of PM 2.5 was tested at 354 micrograms per cubic meter.

To curb pollution, the government has introduced a number of measures, including moving heavy polluters away from urban districts.