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| Titel |
Low hygroscopic scattering enhancement of boreal aerosol and the implications for a columnar optical closure study |
| VerfasserIn |
P. Zieger, P. P. Aalto, V. Aaltonen, M. Äijälä, J. Backman, J. Hong, M. Komppula, R. Krejci, M. Laborde, J. Lampilahti, G. Leeuw, A. Pfüller, B. Rosati, M. Tesche, P. Tunved, R. Väänänen, T. Petäjä |
| 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 ; 15, no. 13 ; Nr. 15, no. 13 (2015-07-02), S.7247-7267 |
| Datensatznummer |
250119869
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| Publikation (Nr.) |
 copernicus.org/acp-15-7247-2015.pdf |
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| Zusammenfassung |
Ambient aerosol particles can take up water and thus change their optical
properties depending on the hygroscopicity and the relative humidity (RH) of
the surrounding air. Knowledge of the hygroscopicity effect is of crucial
importance for radiative forcing calculations and is also needed for the
comparison or validation of remote sensing or model results with in situ
measurements. Specifically, particle light scattering depends on RH and can
be described by the scattering enhancement factor f(RH), which is defined
as the particle light scattering coefficient at defined RH divided by its dry
value (RH <30–40 %).
Here, we present results of an intensive field campaign carried out in summer
2013 at the SMEAR II station at Hyytiälä, Finland. Ground-based and
airborne measurements of aerosol optical, chemical and microphysical
properties were conducted. The f(RH) measured at ground level by a
humidified nephelometer is found to be generally lower (e.g. 1.63±0.22 at
RH = 85 % and λ = 525 nm) than observed at other
European sites. One reason is the high organic mass fraction of the aerosol
encountered at Hyytiälä to which f(RH) is clearly anti-correlated
(R2≈0.8). A simplified parametrization of f(RH) based on the
measured chemical mass fraction can therefore be derived for this aerosol
type. A trajectory analysis revealed that elevated
values of f(RH) and the corresponding elevated inorganic mass fraction are
partially caused by transported hygroscopic sea spray particles. An optical
closure study shows the consistency of the ground-based in situ measurements.
Our measurements allow to determine the ambient particle light extinction
coefficient using the measured f(RH). By combining the ground-based
measurements with intensive aircraft measurements of the particle number size
distribution and ambient RH, columnar values of the particle extinction
coefficient are determined and compared to columnar measurements of a
co-located AERONET sun photometer. The water uptake is found to be of minor
importance for the column-averaged properties due to the low particle
hygroscopicity and the low RH during the daytime of the summer months. The
in situ derived aerosol optical depths (AOD) clearly correlate with directly
measured values of the sun photometer but are substantially lower
compared to the directly measured values (factor of ~ 2–3). The comparison
degrades for longer wavelengths. The disagreement between in situ derived and
directly measured AOD is hypothesized to originate from losses of coarse and
fine mode particles through dry deposition within the canopy and losses in
the in situ sampling lines. In addition, elevated aerosol layers (above
3 km) from long-range transport were observed using an aerosol lidar
at Kuopio, Finland, about 200 km east-north-east of Hyytiälä.
These elevated layers further explain parts of the disagreement. |
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