title: Elucidating severe urban haze formation in China.
abstract: As the world's second largest economy, China has experienced severe haze pollution, with fine particulate matter (PM) recently reaching unprecedentedly high levels across many cities, and an understanding of the PM formation mechanism is critical in the development of efficient mediation policies to minimize its regional to global impacts. We demonstrate a periodic cycle of PM episodes in Beijing that is governed by meteorological conditions and characterized by two distinct aerosol formation processes of nucleation and growth, but with a small contribution from primary emissions and regional transport of particles. Nucleation consistently precedes a polluted period, producing a high number concentration of nano-sized particles under clean conditions. Accumulation of the particle mass concentration exceeding several hundred micrograms per cubic meter is accompanied by 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, on the other hand, exhibit a similarity to those commonly measured in many global areas, consistent with the 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.
authors.affiliation: State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing 100871, China; Departments of Atmospheric Sciences and Chemistry, Center for the Atmospheric Chemistry and the Environment, Texas A&M University, College Station, TX 77843; and.
authors.forename: Song
authors.initials: S
authors.lastname: Guo
authors.affiliation: State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing 100871, China; renyi-zhang@tamu.edu minhu@pku.edu.cn mjmolina@ucsd.edu.
authors.forename: Min
authors.initials: M
authors.lastname: Hu
authors.affiliation: Departments of Atmospheric Sciences and Chemistry, Center for the Atmospheric Chemistry and the Environment, Texas A&M University, College Station, TX 77843; and.
authors.forename: Misti L
authors.initials: ML
authors.lastname: Zamora
authors.affiliation: State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing 100871, China;
authors.forename: Jianfei
authors.initials: J
authors.lastname: Peng
authors.affiliation: State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing 100871, China;
authors.forename: Dongjie
authors.initials: D
authors.lastname: Shang
authors.affiliation: State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing 100871, China;
authors.forename: Jing
authors.initials: J
authors.lastname: Zheng
authors.affiliation: State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing 100871, China;
authors.forename: Zhuofei
authors.initials: Z
authors.lastname: Du
authors.affiliation: State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing 100871, China;
authors.forename: Zhijun
authors.initials: Z
authors.lastname: Wu
authors.affiliation: State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing 100871, China;
authors.forename: Min
authors.initials: M
authors.lastname: Shao
authors.affiliation: State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing 100871, China;
authors.forename: Limin
authors.initials: L
authors.lastname: Zeng
authors.affiliation: Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, CA 92093 renyi-zhang@tamu.edu minhu@pku.edu.cn mjmolina@ucsd.edu.
authors.forename: Mario J
authors.initials: MJ
authors.lastname: Molina
authors.affiliation: State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing 100871, China; Departments of Atmospheric Sciences and Chemistry, Center for the Atmospheric Chemistry and the Environment, Texas A&M University, College Station, TX 77843; and renyi-zhang@tamu.edu minhu@pku.edu.cn mjmolina@ucsd.edu.
authors.forename: Renyi
authors.initials: R
authors.lastname: Zhang
keywords: China; PM2.5; nucleation; pollution; secondary aerosols