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

by C. J. Schumacher, C. Pöhlker, P. Aalto, V. Hiltunen, T. Petäjä, M. Kulmala, U. Pöschl, J. A. Huffman
Atmospheric Chemistry and Physics ()
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Biological aerosol particles have become increasingly important for\natmospheric study, but continuous measurements at high time and size\nresolution have not been available until recently. Here we report\nseasonal cycles of fluorescent biological aerosol particles (FBAP) from\nthe boreal forest in Hyytiala, Finland (18 months) and the semi-arid\nManitou Experimental Forest, Colorado (10 months) FBAP at both locations\nwere observed to be highest in summer and lowest in winter, increasing\nby factors of 12 and 5 between these seasons, respectively. In addition\nto the low temperatures and reduced sunlight during winter, we suggest\nthat snow cover inhibited FBAP release from local terrestrial surfaces\nand that more extensive snow cover at the Finland site contributed to\nlower winter FBAP concentrations. Average size distributions at each\nsite exhibited peaks between 1.5 and 6 lam in aerodynamic diameter. The\nFinland site consistently showed a dominant, narrow FBAP peak at similar\nto 3 mu m in addition to discreet modes at similar to 1.5 and similar to\n5 mu m, whereas the Colorado site showed broader peaks at 1.5 and 5 mu\nm, suggesting different modes of biological particles at the two sites.\nFBAP concentrations in both locations were shown to correlate with daily\npatterns of relative humidity (RH) during each season. Also during\nsummer at each site, average FBAP concentration scaled with RH, but at\nthe Finland site RH values above similar to 82% led to a significant\ndecrease in FBAP concentration. We hypothesize that this is due to dew\nformation that inhibits bioparticle release. Lastly we show that rain\nduring summer at each location led to pronounced increases in both\nfluorescent and total particle concentrations with FBAP peak particle\nsize at similar to 2 mu m and concentration scaling with rain intensity.\nWe suggest that these particles are primarily fungal spores and other\nbioparticles lofted from splashing of rain droplets hitting soil and\nleaf surfaces. During the summer at the Colorado site we consistently\nobserved a mode of similar to 4 mu m particles appearing several hours\nafter rain events that we suggest are fungal spores actively emitted\nwhen ambient conditions are most advantageous for spread and\ngermination. The pronounced patterns of fluorescent bioparticles\nobserved here suggest that parameterizations of both daily and seasonal\ncycles will be important to accurately reflect bioparticle emissions in\nfuture studies of atmospheric bioaerosols and their potential effects on\nclouds and precipitation.

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