Work to remove air quality monitoring stations from local government control is almost complete, the Ministry of Environmental Protection said after officials in northern China were accused of tampering with pollution data.

The plan is to separate all 1,436 monitoring stations nationwide from provincial, city and county bureaus and place them under the operation of companies that will report directly to the central government.

Preparation work to become independent has been completed at 92 percent of stations nationwide, while all stations in 18 areas, including Beijing and Sichuan and Henan provinces, have transferred to third-party companies, the ministry said on Monday.

The independent monitoring network will be expanded to cover water and soil by 2018, the ministry said, adding that China plans to build a comprehensive environmental monitoring and information sharing network by 2020.

The network is intended to prevent inference from local authorities and guarantee authentic data, the ministry said.

Three senior environmental protection officials in Chang'an district of Xi'an, Shaanxi province, were detained on Friday on suspicion of altering monitoring data.

The officials are accused of having a duplicate key made without authorization so they could gain access to air quality monitoring equipment. They are accused of masking the equipment with cotton yarn to filter the air and lower pollution data to avoid punitive actions, according to city police.

"It is important to keep monitoring data accurate to better serve the government to make measures to effectively control air pollution," Chen Jining, the minister of environmental protection, said earlier this year.

The ministry has had 10 inspection teams in 20 provinces since Friday. The teams are reviewing and comparing monitoring data from official, public and corporate sites as well as checking whether construction sites have the necessary permits and targeting companies that produce excessive emissions.

The ministry exposed several companies on Tuesday that had falsified monitoring data to avoid fines after discharging excessive pollutants, including Yutai Coking Co in Handan, Hebei province.

"The independent and authentic monitoring data can help the public better assist authorities in supervising companies' emissions, which is necessary for supervising pollution," said Ruan Qingyuan, an expert in monitoring at the Institute of Public and Environmental Affairs, an NGO in Beijing.

Contact the writers at zhengjinran@chinadaily.com.cn