Features of particulate matter and microbiota in the lowest atmospheric layer above densely populated urban areas

A comprehensive study of the air condition in a limited urban area in the Bulgarian capital Sofia during the COVID-19 quarantine measures 20202021 was conducted. The work has been focused on the systematic in situ reading of the mass concentration of particulate matter (PM) with dimensions - PM2.5&10, by sampling with Air quality sensor SDS 011 and absorber with filter collector. X-ray diffraction (XRD), Scanning electron microscopy (SEM), Energy Dispersive X-Ray Analysis (EDAX) and Mossbauer Spectroscopy (MoS) have been applied to study the collected PM. Six-stage cascade impactor was used for microbial enumeration and size distribution. Physico-chemical investigation reveals that the collected PM is mostly aggregates of a solid core and liquid surface, but it is composed of small particles including nanosized ones. SEM and microbiological analysis shows that PM > 2.5 is loaded with microbiota especially fungi. Studies show that while the peaks of PM pollution are predominantly related to car traffic, industry and heating, the change in the microbiota is seasonal and correlations between them were observed in one-year study period.