LAUSR

Abstract The Guanzhong Basin (GZB) has exhibited an obvious air pollution zone in the cold season in recent years. The variability of haze days combined with dynamic circulation was investigated using meteorological observation data and ERA-5 reanalysis data. The results show that large-scale and mountain-valley circulation, local meteorological factors and mountains affected the variability of severe haze events. The GZB is surrounded by complicated mountain conditions, with the Loess Plateau in the north, the Qinling Mountains in the south, highlands in the west and a valley plain in the east. The analysis showed that the formation and development of long-lasting severe haze events rely largely on the persistent south and east winds at a large scale. Owing to the complex effects of terrain, a weak convergence-stable stratification-divergence distribution is formed in the GZB in the vertical profile from the surface to the middle troposphere. A stable stratification exists near 850 hPa with features of near-zero vertical speed and an evident inversion layer. In this atmospheric environment, air pollution worsens until the northwest wind intensifies. In addition, mountain-valley circulation plays an important role in the diurnal variability of haze. During the day, the valley breeze dissipates pollutants, decreasing the PM2.5 concentrations. At night, the mountain breeze transports pollutants into the basin, and the PM2.5 concentrations increase. This study illustrates that terrain conditions are responsible for the formation of haze in the GZB, and efforts should be made to reduce emissions in the region.