Atmospheric Chemistry and Physics
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1
2007
10.5194/acp-7-211-2007
http://www.atmos-chem-phys.net/7/211/2007/
http://www.atmos-chem-phys.net/7/211/2007/acp-7-211-2007.html
http://www.atmos-chem-phys.net/7/211/2007/acp-7-211-2007.pdf
211
222
2007-01-16
Hygroscopic properties of ultrafine aerosol particles in the boreal forest: diurnal variation, solubility and the influence of sulfuric acid
M. Ehn
mikael.ehn@helsinki.fi
T. Petäjä
H. Aufmhoff
P. Aalto
K. Hämeri
F. Arnold
A. Laaksonen
M. Kulmala
Division of Atmospheric Sciences, Department of Physical Sciences, P.O. Box 64, 00014 University of Helsinki, Finland
Max-Planck Institute for Nuclear Physics (MPIK), Atmospheric Physics Division, P.O. Box 103980, 69029 Heidelberg, Germany
Finnish Institute of Occupational Health, Topeliuksenkatu 41 a A, 00250 Helsinki, Finland
Department of Applied Physics, University of Kuopio, P.O. Box 1627, 70211 Kuopio, Finland
Finnish Meteorological Institute, P.O. Box 503, 00101 Helsinki, Finland
The hygroscopic growth of aerosol particles present in a boreal
forest was measured at a relative humidity of 88%. Simultaneously
the gas phase concentration of sulfuric acid, a very hygroscopic
compound, was monitored. The focus was mainly on days with new
particle formation by nucleation. The measured hygroscopic growth
factors (GF) correlated positively with the gaseous phase sulfuric
acid concentrations. The smaller the particles, the stronger the
correlation, with <I>r</I>=0.20 for 50 nm and <I>r</I>=0.50 for 10 nm
particles. The increase in GF due to condensing sulfuric acid is
expected to be larger for particles with initially smaller masses.
During new particle formation, the changes in solubility of the new
particles were calculated during their growth to Aitken mode
sizes. As the modal diameter increased, the solubility of the
particles decreased. This indicated that the initial particle
growth was due to more hygroscopic compounds, whereas the later
growth during the evening and night was mainly caused by less
hygroscopic or even hydrophobic compounds. For all the measured
sizes, a diurnal variation in GF was observed both during days
with and without particle formation. The GF was lowest at around
midnight, with a mean value of 1.12–1.24 depending on particle
size and if new particle formation occurred during the day, and
increased to 1.25–1.34 around noon. This can be tentatively
explained by day- and nighttime gas-phase chemistry; different
vapors will be present depending on the time of day, and through
condensation these compounds will alter the hygroscopic properties
of the particles in different ways.
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