Fine-particle pollution associated with haze threatens the health of more than 1 billion people in China. Extremely high PM_2.5 concentrations are frequently observed, especially during the winter haze event in northern China. Even after accounting for aerosol–radiation–meteorology feedback and improving the emission inventory, state-of-the-art models still fail to capture the observed high PM_2.5 concentrations, suggesting that key chemistry is missing for secondary aerosol formation. To improve the prediction and control strategy of PM_2.5, we are in urgent need of a better understanding of the chemistry of secondary aerosol formation. Thus, we launch the special issue "Multiphase chemistry of secondary aerosol formation under severe haze" to promote research and discussion on this topic, which is highly relevant for both atmospheric chemists and the public.

The special issue is open for all submissions which address, but are not limited to, the following questions concerning secondary aerosol formation. What are the key oxidation pathways leading to aerosol formation under clean and polluted conditions? What is the role of multiphase chemistry versus gas-phase chemistry? Are laboratory-determined kinetic data of multiphase chemistry directly applicable for ambient conditions, and if not, how can the reaction kinetics relevant for ambient conditions be derived and determined? What is the aerosol particles’ and droplets’ pH and how does it influence the multiphase chemistry? What is the role of the RH, temperature and aerosol phase state in multiphase chemistry, and how does aerosol mixing state play a role? What is the contribution of aqueous secondary organic aerosol (SOA) formation under highly polluted conditions?