Abstract:  China is facing the critical situation of the high occurrence of haze. The enhancement in secondary inorganic species in PM2.5 is largely responsible for the haze formation, reflecting the complex pollution with coal-burning and traffic emissions. Both particle size distribution and refractive indices are remarkably changed after deliquescence and hygroscopic growth of secondary aerosols in atmospheric environment. Aerosol hygroscopicity is largely dependent on chemical composition as well as size effect. The aerosols may exist as liquid droplet under rather lower relative humidity due to the drag hysteresis. The urban aerosols contain in China nearly-hydrophobic, less-hygroscopic and more-hygroscopic mode particles, varying content of each mode among cities. Hygroscopic growth may contribute 60% to the visibility decrease during haze episode, exceeding extinction effect of dry aerosols themselves. Hygroscopic growth can also provide a carrier for heterogeneous reaction with gas pollutants. This process favors the formation of sulfate and nitrate, which further deteriorate the visibility.

Key words:  fine particulate matter pollution, hygroscopicity, visibility deterioration