From May 18 to May 22, ozone was the biggest offender in Beijing air pollution, according to the city's environmental protection monitoring center.

Data released by China's Ministry of Environmental Protection on September 16 of last year shows that ozone pollutant levels exceeded those of PM 2.5 last August, and they also dominated the pollution for four consecutive months before that, starting in May 2015. 

Some experts believe that with the increasing efforts to control PM 2.5 and the improvement of air visibility, ozone pollution will become more and more prominent. From international experience, it is clear that tackling ozone pollution will be more difficult than curbing PM 2.5. 

The concentration of ozone has a complicated relationship with its precursors, nitrogen oxides and volatile organic compounds (VOCs). China is still in the beginning phases of controlling ozone pollution and regulating the emission of VOCs and nitrogen oxides. 

Currently, nitrogen oxides and VOCs are both included in the list of emission sources in Beijing.