Seasonal Variability of Ambient Bioaerosol Concentrations over Densely Populated Central Area in Sofia, Bulgaria: Focus on Spring and Summer

In terms of air quality, Sofia, Bulgaria, is one of Europe's most polluted capitals. The city represents a unique pollution challenge due to the combination of specific environmental and orographic features. Until now, no studies have been conducted to monitor outdoor bioaerosol concentrations. This pioneering research presents a systematic study of the seasonal cycle of bacterial and fungal aerosol concentrations in Sofia. Although the overall study extends over a full year, this paper focuses specifically on results from the spring and summer seasons. Airborne microbial samples were collected from the 1st of March 2024 to the 31st of August 2024 at an open-air location situated in a central urban area. The findings reveal a clear seasonal distribution pattern for bacterial and fungal aerosols. The mean bioaerosol concentrations during the spring-summer period were ranked in descending order as follows: a) bacterial aerosols: summer (258.0 CFU/m3) > spring (197.0 CFU/m3); (b) fungal aerosols: summer (987.0 CFU/m3) > spring (228.0 CFU/m3). This data on bioaerosol levels can be used to support future aeromicrobiology studies, policymaking, and urban planning in Sofia.