Seasonal cycles of fluorescent biological aerosol particles in boreal and semi-arid forests of Finland and Colorado

Abstract. Biological aerosol particles have become increasingly important for atmospheric study, but continuous measurements at high time and size resolution have not been available until recently. Here we report seasonal cycles of fluorescent biological aerosol particles (FBAP) from the boreal forest in Hyytiälä, Finland (18 months) and the semi-arid Manitou Experimental Forest, Colorado (10 months). FBAP at both locations were observed to be highest in summer and lowest in winter, increasing by factors of 12 and 5 between these seasons, respectively. In addition to the low temperatures and reduced sunlight during winter, we suggest that snow cover inhibited FBAP release from local terrestrial surfaces and that more extensive snow cover at the Finland site contributed to lower winter FBAP concentrations. Average size distributions at each site exhibited peaks between 1.5 and 6 μm in aerodynamic diameter. The Finland site consistently showed a dominant, narrow FBAP peak at ~ 3 μm in addition to discreet modes at ~ 1.5 and ~ 5 μm, whereas the Colorado site showed broader peaks at 1.5 and 5 μm, suggesting different modes of biological particles at the two sites. FBAP concentrations in both locations were shown to correlate with daily patterns of relative humidity (RH) during each season. Also during summer at each site, average FBAP concentration scaled with RH, but at the Finland site RH values above ~ 82% led to a significant decrease in FBAP concentration. We hypothesize that this is due to dew formation that inhibits bioparticle release. Lastly we show that rain during summer at each location led to pronounced increases in both fluorescent and total particle concentrations with FBAP peak particle size at ~ 2 μm and concentration scaling with rain intensity. We suggest that these particles are primarily fungal spores and other bioparticles lofted from splashing of rain droplets hitting soil and leaf surfaces. During the summer at the Colorado site we consistently observed a mode of ~ 4 μm particles appearing several hours after rain events that we suggest are fungal spores actively emitted when ambient conditions are most advantageous for spread and germination. The pronounced patterns of fluorescent bioparticles observed here suggest that parameterizations of both daily and seasonal cycles will be important to accurately reflect bioparticle emissions in future studies of atmospheric bioaerosols and their potential effects on clouds and precipitation.