Multi-scale processes in severe haze events in China and their interactions with aerosols: Mechanisms and progresses

Abstract

Severe haze events occur frequently in China, characterized by exceedingly high concentration of fine particulate matter (smaller than 2.5 μm, or PM2.5). These extremes are caused by synthetic physical-chemical processes, including emissions, chemical formation, planetary boundary layer processes, regional circulation, weather and climate. These processes are multi-scales, ranging from nanometers to thousands of kilometers. The complex interplays among these processes make it more difficult to understand the formation of severe haze events, which also influences model development and weather forecast. Here, we review the contributions of dominant mechanisms in severe haze events, especially their roles in temporal oscillations of PM2.5 concentrations, and discuss the interplays among these atmospheric multi-scale processes. Previous studies indicate that: (1) Secondary aerosols become the dominant components in aerosols. Heterogeneous aqueous reactions play important roles in gas-to-particle conversion, especially in the late haze period; (2) PM2.5 owns extensive temporal oscillations (on daily, weekly, to monthly timescales), which are caused by multi-scale processes; (3) high aerosols in China have already influenced photochemistry, boundary layer, weather and climate processes. The complex interplays among aerosols and the above processes make it more difficult to unravel the causes, mechanisms, and trends for haze pollution. In the future, the following issues should be considered: cooperative observations of aerosols, gas pollutants, photochemistry, and meteorological variables should be strengthened, especially their vertical distribution in the troposphere; multidisciplinary research should be strengthened, especially among atmospheric physics, chemistry, weather and climate; and model simulations related to the interplays among aerosols and atmospheric physical and chemical processes should be strengthened.