Wintertime Haze Formation in Beijing

Abstract:

Recent severe haze events in China have attracted significant public attention due to the severely reduced visibility and unprecedentedly high pollutant concentrations. Particular attention has been given to the high concentrations of particulate matter with a diameter less than 2.5 microns (PM_2.5), which can exceed several hundred micrograms per cubic meter over several days. During January and February of 2015, a suite of aerosol instruments was deployed in Beijing to directly measure a comprehensive set of aerosol properties, including the particle size distribution, effective density, and chemical composition. In this presentation, we will discuss the particulate matter formation mechanisms, the evolution of aerosol properties throughout the event, and how the winter formation mechanisms compare with the warmer seasons. We show that the periodic cycles of severe haze episodes in Beijing are largely driven by meteorological conditions. During haze events, stagnation typically develops as a result of a low planetary boundary layer and weak southerly wind from polluted industrial source regions. Stronger northerly winds were frequently observed during the clean period, which carry unpolluted air masses from the less populated northern mountainous areas. Nucleation consistently occurs on clean days, producing a high number concentration of nano particles. The particle mass concentration exceeding several hundred micrograms per cubic meter is attributed to the continuous size growth from the nucleation-mode particles (diameter less than 10 nm) over multiple days to produce a high concentration of larger particles (diameter greater than 100 nm). The particle chemical composition in Beijing is similar to those commonly measured in other urban centers, which is indicative of chemical constituents dominated by secondary aerosol formation. Our results reveal that the severe haze formation in Beijing during the wintertime is similar to the mechanism of haze formation during the warmer months and is likely attributable the high concentration of aerosol precursor gases and the stagnation caused by the large-scale meteorological conditions.