Fluorescent biological aerosol particle concentrations and size distributions measured with an Ultraviolet Aerodynamic Particle Sizer (UV-APS) in Central Europe

Primary Biological Aerosol Particles (PBAPs), including bacteria, spores and pollen, are essential for the spread of organisms and disease in the biosphere, and numerous studies have suggested that they may be important for atmospheric processes, including the formation of clouds and precipitation. The atmospheric abundance and size distribution of PBAPs, however, are largely unknown. At a semi-urban site in Mainz, Germany we used an Ultraviolet Aerodynamic Particle Sizer (UV-APS) to measure Fluorescent Biological Aerosol Particles (FBAPs), which provide an estimate of viable bioaerosol particles and can be regarded as an approximate lower limit for the actual abundance of PBAPs. Fluorescence of non-biological aerosol components are likely to influence the measurement results obtained for fine particles (<1 mu m), but not for coarse particles (1-20 mu m). Averaged over the four-month measurement period (August-December 2006), the mean number concentration of coarse FBAPs was similar to 3 x 10(-2) cm(-3), corresponding to similar to 4% of total coarse particle number. The mean mass concentration of FBAPs was similar to 1 mu g m(-3), corresponding to similar to 20% of total coarse particle mass. The FBAP number size distributions exhibited alternating patterns with peaks at various diameters. A pronounced peak at similar to 3 mu m was essentially always observed and can be described by the following campaign-average lognormal fit parameters: geometric mean diameter 3.2 mu m, geometric standard deviation 1.3, number concentration 1.6 x 10(-2) cm(-3). This peak is likely due to fungal spores or agglomerated bacteria, and it exhibited a pronounced diel cycle (24-h) with maximum intensity during early/mid-morning. FBAP peaks around similar to 1.5 mu m, similar to 5 mu m, and similar to 13 mu m were also observed, but less pronounced and less frequent. These may be single bacterial cells, larger fungal spores, and pollen grains, respectively. The observed number concentrations and characteristic sizes of FBAPs are consistent with microscopic, biological and chemical analyses of PBAPs in aerosol filter samples. To our knowledge, however, this is the first exploratory study reporting continuous online measurements of bioaerosol particles over several months and a range of characteristic size distribution patterns with a persistent bioaerosol peak at similar to 3 mu m. The measurement results confirm that PBAPs account for a substantial proportion of coarse aerosol particle number and mass in continental boundary layer air. Moreover, they suggest that the number concentration of viable bioparticles is dominated by fungal spores or agglomerated bacteria with aerodynamic diameters around 3 mu m rather than single bacterial cells with diameters around 1 mu m.