Variations, seasonal shifts and ambient conditions affecting airborne microorganisms and particles at a southeastern Mediterranean site.

Airborne particles are known climate drivers whilst the impact of microorganisms is investigated with increasing interest. The particle number size distribution (0.012-10 µm), PM10 concentrations, bacterial communities and cultivable microorganisms (bacteria and fungi) were measured simultaneously throughout a yearly campaign at a suburban location at the city of Chania (Greece). Most of the bacteria identified belonged to Proteobacteria, Actinobacteriota, Cyanobacteria, and Firmicutes, with Sphingomonas having a dominant partition at the genus level. Statistically lower concentrations of all microorganisms and bacterial species richness during the warm season due to the direct impact of temperature and solar radiation suggested notable seasonality. On the other hand, statistically significant higher concentrations of particles <0.1 µm during the cold season was attributed to indirect seasonality with enrichment due to heating emissions. Analysis of wind direction data demonstrated that a land prevailing origin of air resulted in statistically higher microorganism concentrations, bacterial species richness and diversity, indicating the continental environment as a dominant contributor in shaping airborne microbial load (compared to a marine air origin). Likewise, statistically higher concentration of particles <0.1 µm were measured during a land prevailing air origin as a direct result of nanoparticle enrichment from anthropogenic activities. Long-range transport of both particles and biological components was evidenced by the increased concentrations of cultivable microorganisms (with a distinct contribution at sizes >1 µm), supermicron particles and bacterial species richness during Sahara dust events. Factorial analysis of the impact of 7 environmental parameters on bacterial communities profile has identified temperature, solar radiation, wind origin and Sahara dust as strong contributors. Increased correlations between airborne microorganisms and coarser particles (0.5-10 µm) suggested resuspension, especially during stronger winds and moderate ambient humidity, whereas, increased relative humidity during stagnant conditions acted as inhibitor for suspension.