Effects of meteorology and secondary particle formation on visibility during heavy haze events was analyzed in this work based on a comprehensive measurement,including planetary boundary layer(PBL),aerosol composition and concentration,together with some important meteorological parameters,such as visibility,relative humidity(RH),wind speed & direction.In haze events,the visibility could decrease from >20 kilometers to a few kilometers in 1-2 days,with a minimum visibility of 667 meters.In this process,the PBL height and horizontal wind speed decreased substantially,indicating that both vertical and horizontal dispersion of pollutants were strongly depressed.As a result,concentration of particle matter(PM2.5)increases sharply.