High-altitude mountainous regions serve as natural laboratories for studying bioaerosol dynamics. This study investigated airborne microbial concentration and viability at a remote mountaintop site (1,690 m a.s.l.) in the Nanling Mountains, China. Using BacLight fluorescence staining of PM2.5 samples, we measured total microbial concentrations of \( (5.26 \pm 2.37) \times 10^{5} \,{\text{cells/m}}^{3} \) with low viability (8.82 ± 3.98)%. Bioaerosol dynamics exhibited substantial variations across distinct meteorological periods, specifically a pollution enhancement phase during the typhoon and a wet scavenging phase during the cloudy period. Aggregated boosted tree (ABT) analysis identified distinct environmental controls on microbial viability. Viable microbes were primarily regulated by relative humidity (RH, 37.2% relative influence), likely due to cell membrane hydrophilicity driving aqueous-phase transfer. In contrast, non-viable microbes were associated with the mass concentration of particulate matter (PM, 52.7%), reflecting the influence of accumulation and atmospheric processing via regional transport. Microbial inactivation may be driven by oxidative stress associated with photochemical aging. Concentration-weighted trajectory (CWT) analysis indicated that viable microbes originated predominantly from eastern coastal China, whereas non-viable microbes were associated with air masses from northern China, where industrial emissions and prolonged atmospheric processing during transport enhanced microbial inactivation. Episodic high microbial concentrations were attributed to the vertical uplift and low-altitude transport of near-surface microbes. These findings demonstrate that local environmental conditions and regional transport processes jointly regulate bioaerosol concentration and viability in high-altitude environments, advancing the mechanistic understanding of microbial behavior in natural atmospheric systems.
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