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Airborne bacteria are an influential component of the Earth’s microbiomes, but their community structure and biogeographic distribution patterns have yet to be understood. We analyzed the bacterial communities of 370 air particulate samples collected from 63 sites around the world and constructed an airborne bacterial reference catalog with more than 27 million nonredundant 16S ribosomal RNA (rRNA) gene sequences. We present their biogeographic pattern and decipher the interlacing of the microbiome co-occurrence network with surface environments of the Earth. While the total abundance of global airborne bacteria in the troposphere (1.72 × 10 24 cells) is 1 to 3 orders of magnitude lower than that of other habitats, the number of bacterial taxa (i.e., richness) in the atmosphere (4.71 × 10 8 to 3.08 × 10 9 ) is comparable to that in the hydrosphere, and its maximum occurs in midlatitude regions, as is also observed in other ecosystems. The airborne bacterial community harbors a unique set of dominant taxa (24 species); however, its structure appears to be more easily perturbed, due to the more prominent role of stochastic processes in shaping community assembly. This is corroborated by the major contribution of surface microbiomes to airborne bacteria (averaging 46.3%), while atmospheric conditions such as meteorological factors and air quality also play a role. Particularly in urban areas, human impacts weaken the relative importance of plant sources of airborne bacteria and elevate the occurrence of potential pathogens from anthropogenic sources. These findings serve as a key reference for predicting planetary microbiome responses and the health impacts of inhalable microbiomes with future changes in the environment.

Air/soil Hg exchange fluxes were measured using field chamber-automated air mercury analyzer method over different land in Wanshan Hg mine area in two seasons. The results showed that the air/soil Hg exchange fluxes were very strong. The highest Hg emission flux from soil was 27 827 ng/(m2 x h), the highest Hg deposition flux from the atmosphere was 9 434 ng/(m2 x h). Because of Hg emission from anthropogenic activities and natural lands, the Hg concentrations in air in Wanshan Hg mine area are 1 - 3 orders of magnitude greater than background area. The highest average Hg concentration in air reached 1 101.8 ng/m3, and the lowest average Hg concentration in air still reached 17.8 ng/m3. These indicated that the atmosphere was polluted seriously in Wanshan Hg mine area. The Hg exchange fluxes are influenced by solar irradiation and the Hg concentrations in air. The solar irradiation accelerates the Hg emission from soil. Conversely, the Hg concentration in air restrained the Hg emission from soil, and even leads the Hg concentration depositing to soil surface. The Hg emission fluxes from uncovered soil are higher than that from covered soil by vegetations significantly. And the slag becomes net atmospheric Hg source.

In July 2002 and March 2003, the mercury exchange flux between soil and air was measured using dynamic flux chamber method in Hongfeng Reservoir region. Mercury exchange flux is (27.4 +/- 40.1) ng x (m2 x h)(-1) (n = 255) and (5.6 +/- 19.4) ng x (m2 x h)(-1) (n = 192) in summer and winter respectively. The correlation coefficient between mercury flux and solar radiation, air temperature, soil temperature is 0.74, 0.83 and 0.80 in summer, and 0.88, 0.56 and 0.59 in winter. From the data, it was found that the mercury emission is stronger in summer than that in winter, and compared to winter, mercury exchange between soil and air depends more on meteorological conditions in summer.