Critical Roles of Surface-Enhanced Heterogeneous Oxidation of SO2 in Haze Chemistry: Review of Extended Pathways for Complex Air Pollution

Purpose of Review

Complex air pollution has spread in the conurbation areas of China along with chemically complicated air pollution processes. Classic secondary-pollutant formation toward high concentrations of fine particulate matter (PM_2.5) and ozone (O₃) is imperfectly understood in the currently accepted chemical mechanisms. The combustion-produced fine particles contain abundant nanosized black carbon (BC) internally mixed with transition metal ions (TMI) and contribute to the complicated oxidation pathways and products substantially. Based on current understandings of multiphase sulfate formation, we propose that the surface-enhanced heterogeneous reaction processes can play critical roles in the fast formation of “haze chemistry smog” pollution.

Recent Findings

Pathways of sulfate enhancement by BC and TMI have been identified to explain the formation mechanisms of the missing sulfate sources. Responsible for additional production of secondary gas molecules and aerosols, the heterogeneous chemistry is initiated with surface photosensitive catalysis. In addition, unidentified atmospheric oxidizing capacity is recognized as part of the heterogeneous processes. Given unique surface-specific profiles and electronically excited dismutation reactions, BC and TMI particles can steadily generate reactive oxygen species (ROS) such as hydroxyl radicals (OH) and promote the oxidation of SO₂. This phenomenon provides an extended insight into atmospheric free-radical chemistry. As such, the heterogeneous catalytic oxidation of SO₂ on aerosol surfaces accounts for up to 69.2% of sulfate formation in haze episodes.

Summary

Unlike in-cloud aqueous oxidation, representative heterogeneous reaction pathways (i.e., TMI aqueous catalysis pathway and surface catalysis pathway) enhance sulfate formation via surface radical reactions in both winter and summer. The heterogeneous processes are thought to reduce gaps between model-predicted and measured sulfate levels. The physically and chemically active BC and TMI can change the composition, morphology, hygroscopicity, and optical properties of PM in their atmospheric aging processes. Therefore, the heterogeneous pathways facilitate rapid particle growth for haze pollution and help to understand and develop a new type of air pollution chemistry (i.e., “haze chemistry” processes) in China and other developing countries.