Elucidating Severe Urban Haze Formation in Beijing

China has experienced severe haze pollution, with fine particulate matter concentrations reaching unprecedentedly high levels across many cities. During the fall of 2013, a suite of aerosol instruments was deployed in Beijing to directly measure a comprehensive set of aerosol properties, including the particle size distribution, hygroscopicity, and chemical composition. In this presentation, we will discuss the particulate matter formation mechanisms and potential mediation policies to minimize its regional to global impacts. We demonstrate that the periodic cycles of haze episodes in Beijing are largely driven by meteorological conditions and characterized by two distinct aerosol formation processes: nucleation and sustained particle growth. Nucleation consistently occurs during clean conditions, producing a high number concentration of nano particles. Accumulation of the particle mass concentration exceeding several hundred micrograms per cubic meter is attributed to a continuous size growth from the nucleation-mode particles over multiple days to yield numerous larger particles, distinctive from the aerosol formation typically observed in other regions worldwide. The particle compositions in Beijing exhibit a similarity to those commonly measured in other urban centers, which is indicative of chemical constituents dominated by secondary aerosol formation. Our results highlight that regulatory controls of gaseous emissions for volatile organic compounds and nitrogen oxides from local transportation and sulfur dioxide from regional industrial sources represent the key steps to reduce the urban PM level in China.