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| Titel |
Variation and balance of positive air ion concentrations in a boreal forest |
| VerfasserIn |
U. Hõrrak, P. P. Aalto, J. Salm, K. Komsaare, H. Tammet, J. M. Mäkelä, L. Laakso, M. Kulmala  |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 8, no. 3 ; Nr. 8, no. 3 (2008-02-12), S.655-675 |
| Datensatznummer |
250005552
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| Publikation (Nr.) |
 copernicus.org/acp-8-655-2008.pdf |
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| Zusammenfassung |
| Air ions are characterized on the basis of measurements carried out in a
boreal forest at the Hyytiälä SMEAR station, Finland, during the
BIOFOR III campaign in spring 1999. The air ions were discriminated as
small ions (charged molecular aggregates of the diameter of less than 2.5 nm),
intermediate ions (charged aerosol particles of the diameter of 2.5–8 nm),
and large ions (charged aerosol particles of the diameter of 8–20 nm).
Statistical characteristics of the ion concentrations and the parameters
of ion balance in the atmosphere are presented separately for the nucleation
event days and non-event days. In the steady state, the ionization rate is
balanced with the loss of small ions, which is expressed as the product of
the small ion concentration and the ion sink rate. The widely known sinks
of small ions are the recombination with small ions of opposite polarity and
attachment to aerosol particles. The dependence of small ion concentration
on the concentration of aerosol particles was investigated applying a model
of the bipolar diffusion charging of particles by small ions. When the
periods of relative humidity above 95% and wind speed less than 0.6 m s−1
were excluded, then the small ion concentration and the theoretically
calculated small ion sink rate were closely negatively correlated (correlation coefficient −87%). However, an extra ion loss term of the
same magnitude as the ion loss onto aerosol particles is needed for a
quantitative explanation of the observations. This term is presumably
due to the small ion deposition on coniferous forest. The hygroscopic
growth correction of the measured aerosol particle size distributions was
also found to be necessary for the proper estimation of the ion sink rate.
In the case of nucleation burst events, the concentration of small positive
ions followed the general balance equation, no extra ion loss in addition to
the deposition on coniferous forest was detected, and the hypothesis of the
conversion of ions into particles in the process of ion-induced nucleation
was not proved. The estimated average ionization rate of the air at the
Hyytiälä station in early spring, when the ground was partly
covered with snow, was about 6 ion pairs cm−3 s−1. The study of
the charging state of nanometer aerosol particles (diameter 2.5–8 nm)
in the atmosphere revealed a strong correlation (correlation coefficient 88%)
between the concentrations of particles neutralized in the aerosol
spectrometer and naturally positively charged particles (air ions) during
nucleation bursts. The charged fraction of particles varied from 3% to 6% in
accordance with the hypothesis that the particles are quasi-steady state charged. |
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