The atmosphere is a mixed system of multi-pollutant and multi-media. Air pollution in China shows the characteristics of complex air pollution, in which the migration and transformation of pollutants will change. To reveal these new laws and mechanisms is not only a new challenge for atmospheric chemistry, but also a scientific basis for air pollution control in China. Haze frequently occurs with high concentrations of PM_2.5 due to the explosive growth of secondary particles. A key step in controlling pollution is revealing the formation mechanism of secondary particles under the conditions of complex air pollution in China.

At present, the prediction of PM_2.5 and its key secondary components in the atmospheric model shows large deviations from field measurements, which indicates that there are unknown mechanisms for the formation of secondary particles in the atmosphere. Based on laboratory simulation and field observation, we have revealed a new mechanism for the formation of secondary particles under the conditions of complex air pollution, and put forward the “haze chemistry” theory to explain the cause of complex air pollution in China. The main idea is that the oxidation capacity and the explosive growth of the transformation from gaseous pollutants to particulate pollutants increases under the conditions of complex air pollution. The decrease of environmental capacity has resulted in frequent haze pollution events in central and eastern China and made governance difficult. It is suggested that key pollutants such as SO₂, VOCs, NOx, black carbon and NH₃ should be controlled to improve the environmental capacity.