Multi-year (2015–2023) trend and key factors of bioaerosols in urban atmosphere: A case study in Xi'an

Bioaerosols and associated health risks are increasingly scrutinized amid improving air quality and recurrent public health crises in China. However, the response of bioaerosols to the process of improving air quality remains unclear. Consequently, multi-year (2015–2023) bioaerosol samples were collected at two sampling sites (Yanta and Weishui) and analyzed using fluorescence counting combined with high-throughput sequencing to investigate long-term trends in bioaerosol concentrations and bacterial communities. Additionally, Prophet and XGBoost-SHAP models elucidated the impact of the COVID-19 pandemic on bioaerosol concentrations and identified key drivers of bioaerosol fluctuations. Results showed that, unlike the decreasing trends in PM_2.5 concentrations, bioaerosol concentrations initially decreased from (3.71 ± 0.88) × 10⁵ cells/m³ to (0.7 ± 0.36) × 10⁵ cells/m³ during 2015–2019, but rebounded to (7.54 ± 3.75) × 10⁵ cells/m³ following the COVID-19 pandemic (2020–2022), highlighting the complex interplay between policy interventions and public health disruptions. Notably, increases in bioaerosol concentrations correlated with influenza incidence rates, reflecting its potential health risks. Bacterial diversity was higher during the Blue Sky Defense War (2018–2020) than in other periods, while the dominant phylum (Proteobacteria, relative abundance range 32 %–89 %) were relatively stable. Pseudomonas, Burkholderia, and Paucibacter were the dominant genera during the observation period, and COVID-19 pandemic changed the structure of bacterial communities. Additionally, O₃ correlated more significantly with long-term trends of bioaerosols than SO₂ and RH. These results can provide scientific basis for comprehensive evaluation and formulation of urban air quality improvement measures and prevention policies.