title: Humidity plays an important role in the PM₂.₅ pollution in Beijing.
abstract: Heavily-polluted PM₂.₅ (fine particulate matter) episodes frequently impacting Beijing, especially during winter, have become a substantial concern. We found that during winter, the daily variation of PM2.5 in Beijing tracked the pattern of relative humidity (RH). With the increase of PM₂.₅ (or RH), water-soluble components (especially inorganic ions) became more abundant, and the water-soluble organic carbon to organic carbon ratios increased. The nitrate to sulfate ratios also exhibited dependence on RH, and were higher than those measured about a decade ago, consistent with the increasing trend of nitrogen oxides emissions. Surprisingly, the ratios of water-insoluble organic carbon to elemental carbon showed significant increase at high RH levels, presumably indicating the formation of secondary organic aerosol that is not soluble in water. In addition, humid winters were occasionally identified during 1996-2013 which are expected to be favorable for the formation of air pollution episodes with high PM₂.₅ concentrations.
authors.affiliation: State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing, China. Electronic address: ycheng@mail.tsinghua.edu.cn.
authors.forename: Yuan
authors.initials: Y
authors.lastname: Cheng
authors.affiliation: State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing, China; State Environmental Protection Key Laboratory of Sources and Control of Air Pollution Complex, Beijing, China; Collaborative Innovation Center for Regional Environmental Quality, Beijing, China. Electronic address: hekb@tsinghua.edu.cn.
authors.forename: Ke-Bin
authors.initials: KB
authors.lastname: He
authors.affiliation: State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing, China.
authors.forename: Zhen-Yu
authors.initials: ZY
authors.lastname: Du
authors.affiliation: College of Environmental Sciences and Engineering, Peking University, Beijing, China.
authors.forename: Mei
authors.initials: M
authors.lastname: Zheng
authors.affiliation: State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing, China.
authors.forename: Feng-Kui
authors.initials: FK
authors.lastname: Duan
authors.affiliation: State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing, China.
authors.forename: Yong-Liang
authors.initials: YL
authors.lastname: Ma
mesh_terms: D000336:Aerosols / Q000032:analysis; D000393:Air Pollutants / Q000032:analysis*; D000397:Air Pollution / Q000706:statistics & numerical data; D002244:Carbon / Q000032:analysis; D002681:China; D002947:Cities; D004784:Environmental Monitoring; D006813:Humidity; D007477:Ions / Q000032:analysis; D009566:Nitrates / Q000032:analysis; D010316:Particle Size; D052638:Particulate Matter / Q000032:analysis*; D012621:Seasons; D014867:Water
chemical_list: D000336:Aerosols; D000393:Air Pollutants; D007477:Ions; D009566:Nitrates; D052638:Particulate Matter; D014867:Water; D002244:Carbon
keywords: Aqueous-phase reaction; Chemical composition; Meteorological parameter; PM(2.5); SOA