Measurement report: Atmospheric fluorescent bioaerosol concentrations measured during 18 months in a coniferous forest in the south of Sweden

Abstract. Biological aerosol particles affect human health, are essential for microbial- and gene dispersal, and have been proposed as important agents for atmospheric processes. However, the abundance and size distributions of atmospheric biological particles are largely unknown. In this study we used a laser-induced fluorescence instrument to measure fluorescent biological aerosol particle (FBAP) concentrations for 18 months (October 2020–April 2022) at a rural, forested site in Sweden. The aim of this study was to investigate FBAP number concentrations (N_FBAP) over time and analyze their relationship to meteorological parameters.

The N_FBAP was highest in the summer and lowest in winter, exhibiting a ~3-fold difference between these seasons. The median N_FBAP was 0.0050, 0.0025, 0.0027 and 0.0126 cm^-3 in fall, winter, spring, and summer, respectively, and constituted ~0.1–0.5 % of the total supermicron particle number. The N_FBAP were dominated by the smallest measured size fraction (1–3 µm), suggesting that the main portion of the biological particles measured were due to single bacterial cells, fungal spores, and bacterial agglomerates. The N_FBAP were significantly correlated with increasing air temperature (P<0.01) in all seasons. For most of the campaign N_FBAP was seen to increase with wind speed (P<0.01), while the relationship with relative humidity was for most part of the campaign nonsignificant (46 %) but to a large part (30 %) negative (P<0.05). Our results indicate that N_FBAP were highest during warm and dry conditions when wind speeds were high, suggesting that a major part of the FBAP in the spring and summer were due to mechanical aerosol generation and release mechanisms. In the fall, relative humidity may have been a more important factor for bioaerosol release. This is one of the longest time series of atmospheric FBAP, which are highly needed for estimates of bioaerosol background concentrations in comparable regions.