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.