title: Effects of meteorology and secondary particle formation on visibility during heavy haze events in Beijing, China.
abstract: The causes of haze formation in Beijing, China were analyzed based on a comprehensive measurement, including PBL (planetary boundary layer), aerosol composition and concentrations, and several important meteorological parameters such as visibility, RH (relative humidity), and wind speed/direction. The measurement was conducted in an urban location from Nov. 16, 2012 to Jan. 15, 2013. During the period, the visibility varied from >20 km to less than a kilometer, with a minimum visibility of 667 m, causing 16 haze occurrences. During the haze occurrences, the wind speeds were less than 1m/s, and the concentrations of PM2.5 (particle matter with radius less than 2.5 μm) were often exceeded 200 μg/m(3). The correlation between PM2.5 concentration and visibility under different RH values shows that visibility was exponentially decreased with the increase of PM2.5 concentrations when RH was less than 80%. However, when RH was higher than 80%, the relationship was no longer to follow the exponentially decreasing trend, and the visibility maintained in very low values, even with low PM2.5 concentrations. Under this condition, the hygroscopic growth of particles played important roles, and a large amount of water vapor acted as particle matter (PM) for the reduction of visibility. The variations of meteorological parameters (RH, PBL heights, and WS (wind speed)), chemical species in gas-phase (CO, O3, SO2, and NOx), and gas-phase to particle-phase conversions under different visibility ranges were analyzed. The results show that from high visibility (>20 km) to low visibility (<2 km), the averaged PBL decreased from 1.24 km to 0.53 km; wind speeds reduced from 1m/s to 0.5m/s; and CO increased from 0.5 ppmv to 4.0 ppmv, suggesting that weaker transport/diffusion caused the haze occurrences. This study also found that the formation of SPM (secondary particle matter) was accelerated in the haze events. The conversions between SO2 and SO4 as well as NOx to NO3(-) increased, especially under high humidity conditions. When the averaged RH was 70%, the conversions between SO2 and SO4 accounted for about 20% concentration of PM2.5, indicating that formation of secondary particle matter had important contribution for the haze occurrences in Beijing.
authors.affiliation: Beijing Weather Modification Office, Beijing, China.
authors.forename: Qiang
authors.initials: Q
authors.lastname: Zhang
authors.affiliation: Beijing Weather Modification Office, Beijing, China.
authors.forename: Jiannong
authors.initials: J
authors.lastname: Quan
authors.affiliation: SKLLQG and Key Laboratory of Aerosol Chemistry and Physics, Institute of Earth Environment, Chinese Academy of Sciences, Xian China; National Center for Atmospheric Research, Boulder, CO, USA. Electronic address: xxtie@ucar.edu.
authors.forename: Xuexi
authors.initials: X
authors.lastname: Tie
authors.affiliation: Beijing Weather Modification Office, Beijing, China.
authors.forename: Xia
authors.initials: X
authors.lastname: Li
authors.affiliation: Beijing Weather Modification Office, Beijing, China.
authors.forename: Quan
authors.initials: Q
authors.lastname: Liu
authors.affiliation: Beijing Weather Modification Office, Beijing, China.
authors.forename: Yang
authors.initials: Y
authors.lastname: Gao
authors.affiliation: Beijing Weather Modification Office, Beijing, China.
authors.forename: Delong
authors.initials: D
authors.lastname: Zhao
mesh_terms: D000336:Aerosols / Q000032:analysis; D000393:Air Pollutants / Q000032:analysis*; D002681:China; D004784:Environmental Monitoring; D006813:Humidity; D008685:Meteorological Concepts; D055904:Meteorology; D010316:Particle Size; D052638:Particulate Matter / Q000032:analysis*; D014919:Wind
chemical_list: D000336:Aerosols; D000393:Air Pollutants; D052638:Particulate Matter
keywords: Beijing Hazes; PBL; PM(2.5); Secondary particle formation; Visibility