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| Titel |
Seasonal cycle, size dependencies, and source analyses of aerosol optical properties at the SMEAR II measurement station in Hyytiälä, Finland |
| VerfasserIn |
A. Virkkula, J. Backman, P. P. Aalto, M. Hulkkonen, L. Riuttanen, T. Nieminen, M. Maso, L. Sogacheva, G. Leeuw, 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 ; 11, no. 9 ; Nr. 11, no. 9 (2011-05-12), S.4445-4468 |
| Datensatznummer |
250009713
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| Publikation (Nr.) |
 copernicus.org/acp-11-4445-2011.pdf |
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| Zusammenfassung |
| Scattering and absorption were measured at the Station for Measuring
Ecosystem–Atmosphere Relations (SMEAR II) station in Hyytiälä,
Finland, from October 2006 to May 2009. The average scattering coefficient
σSP (λ = 550 nm) 18 Mm−1 was about twice as much
as at the Pallas Global Atmosphere Watch (GAW) station in Finnish Lapland.
The average absorption coefficient σAP (λ = 550 nm)
was 2.1 Mm−1. The seasonal cycles were analyzed from hourly-averaged
data classified according to the measurement month. The ratio of the highest
to the lowest average σSP and σAP was ~1.8
and ~2.8, respectively. The average single-scattering albedo (ω0) was 0.86 in winter and 0.91 in summer. σSP was highly
correlated with the volume concentrations calculated from number size
distributions in the size range 0.003–10 μm. Assuming that the
particle density was 1.5 g cm−3, the PM10 mass scattering efficiency
was 3.1 ± 0.9 g m−2 at λ = 550 nm. Scattering
coefficients were also calculated from the number size distributions by
using a Mie code and the refractive index of ammonium sulfate. The linear
regression yielded σSP(modelled) = 1.046 × σSP(measured) for the data with the low nephelometer sample volume
relative humidity (RHNEPH = 30 ± 9 %) and σSP(modelled) = 0.985 × σSP(measured) when RHNEPH = 55 ± 4 %.
The effective complex refractive index was obtained by
an iterative approach, by matching the measured and the modelled σSPand σAP. The average effective complex refractive
index was (1.517 ± 0.057) + (0.019 ± 0.015)i at λ = 550 nm. The iterated imaginary part had a strong seasonal cycle, with smallest
values in summer and highest in winter. The contribution of submicron
particles to scattering was ~90 %. The Ångström exponent of
scattering, σSP, was compared with the following weighted mean
diameters: count mean diameter (CMD), surface mean diameter (SMD),
scattering mean diameter (ScMD), condensation sink mean diameter (CsMD), and
volume mean diameter (VMD). If αSP is to be used for estimating some measure of the size of particles, the best choice would be
ScMD, then SMD, and then VMD. In all of these the qualitative relationship
is similar: the larger the Ångström exponent, the smaller the
weighted mean diameter. Contrary to these, CMD increased with increasing
αSP and CsMD did not have any clear relationship with αSP. Source regions were estimated with backtrajectories and trajectory
statistics. The geometric mean σSP and σAP
associated with the grid cells in Eastern Europe were in the range 20–40 Mm−1 and 4–6 Mm−1, respectively. The respective geometric means
of σSP and σAP in the grid cells over Norwegian
Sea were in the range 5–10 Mm−1 and <1 Mm−1. The source
areas associated with high αSP values were norther than those
for σSP and σAP. The trajectory statistical
approach and a simple wind sector classification agreed well. |
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