Celebrating the constant blue skies this winter, Beijing will move forward with a new phase of its environmental campaign of mixed controls on air pollution, focusing on vehicles with high exhaust levels, the acting mayor said on Wednesday.

Beijing will make every effort to maintain blue skies this year and will unveil the new action plan based on the updated analysis of pollutants, the acting mayor, Chen Jining, said.

"Vehicles with high exhaust emissions will be the major target. The current area that allows limited emissions will be expanded to cover the whole city in 2018," he said at the opening ceremony of the first session of the 15th Beijing Municipal People's Congress on Wednesday.

In the current limited emission region, vehicles with high levels of exhaust, including trucks, cannot operate on roads. The area includes the downtown areas within the Sixth Ring Road.

In addition, the capital will finish the reform of management in the environment authority, beef up enforcement and conduct more environmental inspections amid the mix of other controls on construction sites, the municipality's government work report says.

In 2017, Beijing marked a dramatic improvement in air quality, in which the average concentration of PM2.5－h(huán)azardous fine particles measuring 2.5 microns or smaller－was lowered by 35.6 percent from the 2013 level, reaching the reduction target set by the State Council, Chen said.

The number of days with good air quality－an air quality index of 100 or lower－reached 226, 50 days more than in 2013. Blue skies have remained in the capital for weeks in January.

Behind the improvement lies the tough controls, Chen said. For example, 2.16 million vehicles with high or excessive exhaust have been phased out since 2013 and 11,000 polluting companies have been shut down in the city.

In 2018, the fight for more blue skies is one of the priorities on the municipal government's work list, and Beijing will release the new action plan soon that leaves "air pollution in a constant decrease," Chen said.

Gathering under the blue sky, the city's top legislators and political advisers voiced their support for further controls to guarantee clean air.

Xu Zewei, deputy of Beijing People's Congress, suggested on Wednesday stepping up the controls, including increasing efforts to shut down unqualified or polluting enterprises.

"The government should strengthen its regular supervision to prevent some illegal discharges, like from hidden pipes or at nights," he said, adding the technology should play a bigger role.