Characteristics of airborne bacterial communities across different PM2.5 levels in Beijing during winter and spring

Airborne bacteria are important components of fine particulate matter (PM2.5), and have received increasing
attention because of their impacts on public health and ecological systems. However, the relationships between the bacterial community and PM2.5 pollution are poorly understood. The properties of bacterial communities in Beijing at low, medium and high PM2.5 levels were analyzed during winter (December 2015–January 2016; January 2017) and spring (March 2016–May 2016; April 2017–May 2017). Variations in bacterial concentrations, Shannon and Simpson indices and relative abundance were significantly related to the seasons. In winter, there were no significant differences in bacterial communities among three PM2.5 pollution levels. In contrast,
significant correlations between bacterial abundance and PM2.5 levels were observed in spring, and the bacterial
concentrations, community richness and diversity indices were significantly higher on heavily polluted days compared to other pollution levels. Correlation results showed that relative humidity (RH), wind speed (WS), and O3 were most closely associated with microbial community structure in winter (P < 0.05), but temperature (T), NO2, SO2, and CO in spring, while CO, NO2, O3, RH, and WS had significant relationships (|r| = 0.360–0.553, P < 0.05) with bacterial concentrations in winter, but PM2.5, O3, T, and RH (|r| = 0.281–0.527, P < 0.05) in spring. Chemical composition, especially that of secondary aerosol particles, which were mainly produced from anthropogenic sources (e.g. fossil fuel combustion, road traffic and industrial emissions), exerted the most control of bacterial community structure of PM2.5. The effects of PM2.5 levels on the bacterial community were modified by environmental conditions, heavy metal and nutrient contents of the PM2.5. The relationships described provide a foundation for further research into the environmental and PM2.5 chemical composition controls on the bacterial community and related health risk from air pollution.