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The urbanized region ofJing(Beijing)-Jin(Tianjin)-Ji (alias of Hebei province) and its nearby surroundingregion (3JNS) isbecoming China's most polluted area by haze, exceeding even the Yangtze andPearl river deltas. Aside from pollutant emission, the meteorology of theplanetary boundary layer (PBL) is the most important factor affecting hazepollution. Focusing on July 2008, the aerosol optical properties and PBLmeteorology features closely related to haze formation were simulated in the3JNS region using an online atmospheric chemical transport model. Therelationship between regional PBL meteorology, PM2.5, and haze isdiscussed. Model results accurately simulated the aerosol optical depth(AOD), single scattering albedo (SSA) and asymmetry parameter (ASY),validated by comparison with observations from the MODerate ResolutionImaging Spectroradiometer (MODIS), the China Aerosol Remote Sensing NETwork(CARSNET) and the Aerosol Robotic NETwork (AERONET). Modeled PBL wind speedsshowed reasonable agreement with those from the National Centers forEnvironmental Prediction (NCEP) Reanalysis 2. A monthly mean AOD value ashigh as 1.2 was found from both model and observations, with a daily meanlarger than 2.0 during haze episodes in the 3JNS region. Modeled and observedSSA values of 0.90–0.96 and ASY values of 0.72–0.74 demonstrated the highscattering characteristic of summer aerosols in this region. PBL wind speedsfrom modeled and NCEP data both showed a reversing trend of PM2.5variation, illustrating the importance of the "PBL window shadow" in hazeformation. Turbulence diffusion and PBL height had opposite phases to surfacePM2.5, indicating that lower PBL height and weaker PBL turbulencediffusion are essential to haze formation. It is noted that homogeneous airpressure does not occur at the surface, but at an 850–950 hPa height duringthe haze episode. The momentum transmitting downward of the cold air fromabove the PBL to the low PBL and surface lead to an increase in surface windspeeds and haze dispersal

atmospheric chemistry and physics

Mesoscale modeling study of the interactions between aerosols and PBL meteorology during a haze episode in Jing–Jin–Ji (China) and its nearby surrounding region – Part 1: Aerosol distributions and meteorological features