Urban Boundary Layer Height Characteristics and Relationship with Particulate Matter Mass Concentrations in Xi'an, Central China

The characteristics of the planetary boundary layer height (PBLH) and the relationship between PBLH and particular matter (PM) mass concentration in Xi'an, central China, are analyzed in this study. Three PBLH calculation methods are used in this study, namely the Holzworth, Liu, and Nozaki approaches. The daily minimum and maximum PBLHs are determined by the Holzworth method, the hourly PBLHs are calculated by the Nozaki method, and the results of the Nozaki method are evaluated by the Liu method. The PBLH characteristics of annual, seasonal, daily, and diurnal variations are based on hourly values obtained from the Nozaki method. The results show that the Nozaki method can depict typical PBLH diurnal variations, although it substantially overestimates the related values. The daily maximum PBLH occurs from 11: 00 to 16: 00 Beijing time, and the seasonal maximum PBLH is during spring, due to the annual maximum wind speed at this time. PM2.5, PM1.0 and PM10 mass concentrations are negatively correlated with PBLH on the inter-annual, annual, and seasonal time scales. The anti-phase diurnal variations of PM mass concentrations and PBLH further indicate that PBLH is one of the important factors affecting air quality. The PM2.5 in PM10, PM1.0 in PM2.5, and PM1.0 in PM10 contents show notable monthly variations, indicating that the air quality in Xi'an city is affected upwind windy and dusty weather, along with local pollution sources.