LAUSR

Abstract Airborne microbes are an abundant component of the atmospheric aerosol, which has important influences on atmospheric conditions and public health. However, their possible sources have not been well-studied. To identify the sources of airborne bacteria, airborne PM2.5, PM10 samples and microorganism samples from surface-soil and leaves were collected at three different sampling sites involved in the mountainous area and urban areas in Xi'an, China in summer and autumn. The samples were then analyzed by fluorescence staining and high-throughput sequencing to explore the relationship in bacterial community structures between ambient air samples and possible sources. Results show that the concentrations of airborne microbes differed among sites and between seasons. The airborne bacterial community structures also varied with seasons and sites, with seasonal differences being greater than spatial ones. In both seasons, leaf-surface represented the main local source of airborne bacteria. At the mountainous area, bacteria from surface-soil and leaf-surface contributed the most to the airborne microbial community; at both mountainous and urban areas, this contribution was more pronounced in autumn. Furthermore, back trajectories arriving at the three sampling sites showed that the airborne bacteria may come from other regions by long-distance airflow. The results of this study provide a better understanding about the sources of ambient bacteria.