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It has been known for several decades that some bioaerosols, such as ice-nucleation-active (INA) bacteria, especially Pseudomonas syringae strains, may play a critical potential role in the formation of clouds and precipitation. We investigated bacterial and fungal ice nuclei (IN) in rainwater samples collected from the Hulunber temperate grasslands in North China. The median freezing temperatures (T50) for three years’ worth of unprocessed rain samples were greater than −10 °C based on immersion freezing testing. The heat and filtration treatments inactivated 7–54% and 2–89%, respectively, of the IN activity at temperatures warmer than −10 °C. We also determined the composition of the microbial community. The majority of observed Pseudomonas strains were distantly related to the verified ice-nucleating Pseudomonas strains, as revealed by phylogenetic analysis. Here, we show that there are submicron INA particles <220 nm in rainwater that are not identifiable as the known species of high-INA bacteria and fungi and there may be a new potential type of efficient submicroscale or nanoscale ice nucleator in the regional rainwater samplers. Our results suggest the need for a reinterpretation of the source of high-INA material in the formation of precipitation and contribute to the search for new methods of weather modification.

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The biological fraction of PM 2.5 is considered to be a major cause of various allergies and respiratory diseases. Nonetheless, differences in bacterial and fungal communities in PM 2.5 under different air quality conditions are not well known. In the present study, we collected PM 2.5 samples from October 15, 2014 to November 12, 2014 when several successive "AsiaPacific Economic Cooperation (APEC) blue" days were recorded, following the implementation of strict emission control measures to ensure the APEC summit held during November 5-11, 2014 in Beijing. This study analyzed bacteria and fungi in PM 2.5 samples through rRNA gene high-throughput sequencing. In total, 690 genera of bacteria and 229 genera of fungi were detected. The variations of species richness and community diversity of bacteria and fungi in PM 2.5 were not affected significantly by the emission control measures adopted during the summit and different air quality levels. The bacterial and fungal community structures in PM 2.5 collected during the summit exhibited over 83.7% and 79.6% similarities respectively, with PM 2.5 collected from air graded as "good" quality (AQI ≤ 100) before the APEC summit. Bacteria and fungi in PM 2.5 samples collected at AQI levels between 101-200 and 201-300 before the APEC summit had more than 73.4% and 76.3% community structure similarity, respectively, with PM 2.5 samples collected at AQI ≤ 100. The difference between day and night PM 2.5 samples was very small for bacterial and fungal community structures. Furthermore, most of the inhalable bacteria and fungi were nonpathogenic and no a clear relationship between air quality levels and pathogens was observed. Our results showed that bacteria and fungi in PM 2.5 were less affected by emission control measures and different air quality levels. However, due to the limited number of samples, the relationship between air pollution levels and airborne bacteria and fungi still needs further study.

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