Journal article

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

Schumacher C, Pöhlker C, Aalto P, Hiltunen V, Petäjä T, Kulmala M, Pöschl U, Huffman J ...see all

Atmospheric Chemistry and Physics, vol. 13, issue 23 (2013) pp. 11987-12001

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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 Hyytiala, 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 lam in aerodynamic diameter. The
Finland site consistently showed a dominant, narrow FBAP peak at similar
to 3 mu m in addition to discreet modes at similar to 1.5 and similar to
5 mu m, whereas the Colorado site showed broader peaks at 1.5 and 5 mu
m, 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 similar to 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 similar to 2 mu m 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 similar to 4 mu m 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.

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  • C. J. Schumacher

  • C. Pöhlker

  • P. Aalto

  • V. Hiltunen

  • T. Petäjä

  • M. Kulmala

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