Abstract:The impact of meteorological factors on particle size distribution and its characteristic over Guanzhong Basin (GZB) was studied by employing the particulate matter (PM) mass concentration data released by Xi’an Environmental Protection Bureau, the meteorological station data and the particle size distribution data measured by aerodynamic particle sizer (APS) and Scanning Mobility Particle Sizer Model 3034 (SMPS). The result showed that heavy haze frequently occurring in GZB is mainly caused by complex special terrain and high levels of particulate pollutants. Significant negative correlations were found between planetary boundary layer height (PBLH) and mass concentrations of PM with aerodynamic diameters <2.5 μm (PM2.5), suggesting that a higher PBLH is favorable for the diffusion of particulate pollutants. Changes in PM2.5 in response to wind speed and wind direction was simultaneously investigated, clearly showing that easterly (northwesterly) wind is associated with the import (export) of pollutants to (from) GBZ. Furthermore, high relative humidity significantly contributes to the maintenance of stable atmospheric boundary layers and the accumulation of particulate pollutants. When relative humidity (RH) is less then 80%, the particle number concentration with diameters ranging from 150 nm to 1.0 μm increases greatly with the increase of RH, which could result in a poor visibility. The number concentration of particles within different size ranges changes variously with RH, causing different impacts on visibility. The higher the RH, the larger the contribution of humidity to the visibility reduction.