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The integration of chemical and biological data in aerosol studies represents a new challenge in atmospheric science. In this perspective it will be possible to gain a clearer and deeper comprehension of biogeochemical cycles in the atmosphere. In this view, this study aimed to investigate the relationships occurring between bacterial populations and PM chemical composition in one of the most polluted and urbanized areas in Europe: the Po Valley (Italy). Moreover, seasonality, long- and short-range transports were also evaluated to investigate the influence on airborne bacterial communities. PM samples were collected in two cities of the Po Valley (Milan and Venice) characterized by different meteorological conditions and atmospheric pollutant sources. Samples were analysed for water-soluble inorganic ions (WSIIs) and bacterial community structure. Chemical and biological data were jointly processed by using redundancy discriminate analysis (RDA), while the influence of atmospheric circulation was evaluated by using wind ground data and back-trajectories analysis. Results showed strong seasonal shifts of bacterial community structure in both cities, while a different behaviour was observed for air mass circulation at Milan ad Venice sites: long-range transport significantly affected bacterial populations in Milan whereas local ground wind had more influence in the Venice area. Moreover, difference in taxonomic composition can be mostly addressed to the characteristics of sampling sites. This evidence could suggest that, while PM composition is influenced by long-range transport, bacterial populations are affected, besides transport, by other factors (i.e., season and sampling site location). This perspective allow to better understand and explain airborne bacterial community behaviour.

The study of spatio-temporal variability of airborne bacterial communities has recently gained importance due to the evidence that airborne bacteria are involved in atmospheric processes and can affect human health. In this work, we described the structure of airborne microbial communities in two urban areas (Milan and Venice, Northern Italy) through the sequencing, by the Illumina platform, of libraries containing the V5-V6 hypervariable regions of the 16S rRNA gene and estimated the abundance of airborne bacteria with quantitative PCR (qPCR). Airborne microbial communities were dominated by few taxa, particularly Burkholderiales and Actinomycetales, more abundant in colder seasons, and Chloroplasts, more abundant in warmer seasons. By partitioning the variation in bacterial community structure, we could assess that environmental and meteorological conditions, including variability between cities and seasons, were the major determinants of the observed variation in bacterial community structure, while chemical composition of atmospheric particulate matter (PM) had a minor contribution. Particularly, Ba, SO4 (2-) and Mg(2+) concentrations were significantly correlated with microbial community structure, but it was not possible to assess whether they simply co-varied with seasonal shifts of bacterial inputs to the atmosphere, or their variation favoured specific taxa. Both local sources of bacteria and atmospheric dispersal were involved in the assembling of airborne microbial communities, as suggested, to the one side by the large abundance of bacteria typical of lagoon environments (Rhodobacterales) observed in spring air samples from Venice and to the other by the significant effect of wind speed in shaping airborne bacterial communities at all sites.

The atmosphere can be considered an extreme environment for microorganisms. High solar radiation, low moisture, low levels of nutrients and large dispersion occurrence could represent an obstacle to survival and reproduction of microorganisms. However, the presence and relevance of airborne bacteria have been widely demonstrated. Due to this relevance, the biological fraction of PM raised a great interest in recent years. However, there is still a considerable lack of knowledge in this field. The baseline variability in the types of microbes found in the atmosphere is just beginning to be assessed. Hence, it is currently difficult to establish linkages between airborne microbes and health outcomes. Long‐term temporal analysis of microbial bioaerosol communities are needed to identify disturbances to this environment, which may be characteristic of microbial contamination events that are important to human and environmental health. Therefore, the aim of this thesis is to provide an exhaustive description of bacterial communities associated to PM of different size fractions in two Northern Italy urban sites, Milan and Venice and to put forward hypotheses about the processes underlying the assembly of the microbial community in urban atmosphere. A particular consideration will be given to address the following scientific questions: (i) which is the temporal and spatial variability of airborne microbial communities, (ii) which are, the environmental factors which drive community‐assembly processes, (iii) which are the sources of airborne bacteria.