The roles of heterogeneous chemical processes in the formation of an air pollution complex and gray haze

Urban and regional air pollutions are characterized by high concentrations of secondary pollutants such as photo-oxidants (mainly ozone) and fine particulate matter, which are formed through chemical reactions of the primary pollutants emitted from various sources. The accumulation of these pollutants under stagnant meteorological conditions results in the formation of gray haze, reducing visibility and causing major impacts on human health and climate. In an air pollution complex, the coexistence of high concentrations of primary and secondary gaseous and particulate pollutants provides a large amount of reactants for heterogeneous reactions on the surface of fine particles; these reactions change the oxidizing capacity of the atmosphere, as well as chemical compositions along with the physicochemical and optical properties of particulate matter, thereby accelerating formation of the air pollution complex and gray haze. Using in situ technologies, such as diffuse reflectance infrared Fourier-transform spectroscopy and single-particle Raman spectroscopy, we systematically investigated the reaction kinetics and mechanisms of gaseous pollutants (i.e., NO2, SO2, O-3, and formaldehyde) on the surfaces of the major components of atmospheric particles such as CaCO3, kaolinite, montmorillonite, NaCl, sea salt, Al2O3, and TiO2. We found that the main reaction products were sulfate, nitrate, or formate, which can change the hygroscopicity and light extinction parameters of those particles significantly. By analyzing the reaction kinetics of these heterogeneous reactions, we identified synergetic mechanisms of the three ternary reaction systems, i.e., NO2-particles-H2O, SO2-particles-O-3, and organics/SO2-particles-UV illumination. These synergetic mechanisms can provide experimental and theoretical bases for understanding the feedback mechanisms and nonlinear processes in the formation of an air pollution complex and gray haze.Chemistry, MultidisciplinarySCI(E)EI6REVIEW1145-1535