Season-Specific Diurnal Patterns of Total and Respirable Bacterial Aerosols

This study examined seasonal and diurnal variability in culturable bacterial aerosol (BA) concentrations, aerodynamic size distribution, and meteorological conditions during spring, summer, autumn, and winter measurement campaigns. BA was collected for 10 min at 28.3 L/min with a six-stage Andersen cascade impactor at approximately 09:00, 12:00, 15:00, 18:00, and 20:00 during spring, summer, autumn, and winter campaigns. These times spanned daytime changes in human activity, solar radiation and boundary-layer evolution. Flow calibration, sampler disinfection, and sterile controls were used for quality assurance. Fine BA fraction was operationally defined as combined stages 5 and 6 (0.65–1.1 and 1.1–2.1 µm). Associations with six meteorological variables (temperature, relative humidity, atmospheric pressure, wind speed, wind direction, and solar radiation) were assessed using two-sided Spearman rank correlations and p < 0.05 were adjusted using the Benjamini-Hochberg correction (BH).

Mean total BA concentrations were highest in summer (324.2 CFU/m³), followed by spring (300.1 CFU/m³), winter (186.1 CFU/m³), and autumn (175.6 CFU/m³). The fine respirable fraction accounted for 17.6–24.6% of total BA. Peak concentrations occurred at 15:00 in spring and winter, 12:00 in summer, and 18:00 in autumn. The fine fraction did not always follow the total BA profile, potentially reflecting size-dependent sources, resuspension, removal, and atmospheric persistence.

In summer, 20 of 54 associations remained significant after BH correction. The strongest was between stage 2 BA (4.7–7 µm) and temperature (R = 0.920). Total BA correlated positively with temperature (R = 0.748) and negatively with relative humidity (RH; R = −0.708). In winter, 10 of 54 associations were significant. The fine respirable fraction (R = −0.829) and total BA (R = −0.803) were strongly negatively associated with RH. No associations remained significant in spring or autumn. Although 42 of 54 associations were significant in the complete dataset, these results were considered descriptive because seasonal differences may confound BA–meteorology relationships. The findings underscore the need for season-specific exposure assessments and highlight that size-selective sampling is critical for accurately evaluating respiratory health risks.