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| Titel |
Comparison of ambient aerosol extinction coefficients obtained from in-situ, MAX-DOAS and LIDAR measurements at Cabauw |
| VerfasserIn |
P. Zieger, E. Weingärtner, J. Henzing, M. Moerman, G. Leeuw, J. Mikkilä, M. Ehn, T. Petäjä, K. Clémer, M. Roozendael, S. Yilmaz, U. Frieß, H. Irie, T. Wagner, R. Shaiganfar, S. Beirle, A. Apituley, K. Wilson, U. Baltensperger |
| 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. 6 ; Nr. 11, no. 6 (2011-03-18), S.2603-2624 |
| Datensatznummer |
250009510
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| Publikation (Nr.) |
 copernicus.org/acp-11-2603-2011.pdf |
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| Zusammenfassung |
| In the field, aerosol in-situ measurements are often performed under dry
conditions (relative humidity RH<30–40%). Since ambient aerosol
particles experience hygroscopic growth at enhanced RH, their
microphysical and optical properties – especially the aerosol light
scattering – are also strongly dependent on RH. The knowledge of this RH effect is
of crucial importance for climate forcing calculations or for the comparison
of remote sensing with in-situ measurements. Here, we will present results
from a four-month campaign which took place in summer 2009 in Cabauw, The
Netherlands. The aerosol scattering coefficient σsp(λ)
was measured dry and at various, predefined RH conditions between 20 and
95% with a humidified nephelometer. The scattering enhancement factor
f(RH,λ) is the key parameter to describe the effect of RH on
σsp(λ) and is defined as σsp(RH,λ)
measured at a certain RH divided by the dry σsp(dry,λ).
The measurement of f(RH,λ) together with the dry absorption
measurement (assumed not to change with RH) allows the determination of the
actual extinction coefficient σep(RH,λ) at ambient RH.
In addition, a wide range of other aerosol properties were measured in
parallel. The measurements were used to characterize the effects of RH on the
aerosol optical properties. A closure study showed the consistency of the
aerosol in-situ measurements. Due to the large variability of air mass origin
(and thus aerosol composition) a simple parameterization of f(RH,λ)
could not be established. If f(RH,λ) needs to be predicted, the
chemical composition and size distribution need to be known. Measurements of
four MAX-DOAS (multi-axis differential optical absorption spectroscopy)
instruments were used to retrieve vertical profiles of σep(λ).
The values of the lowest layer were compared to the in-situ
values after conversion of the latter ones to ambient RH. The comparison showed a
good correlation of R2 = 0.62–0.78, but the extinction coefficients from MAX-DOAS were
a factor of 1.5–3.4 larger than the in-situ values. Best agreement is
achieved for a few cases characterized by low aerosol optical depths and low
planetary boundary layer heights. Differences were shown to be dependent on the
applied MAX-DOAS retrieval algorithm. The comparison of the in-situ extinction data to a
Raman LIDAR (light detection and ranging) showed a good correlation and
higher values measured by the LIDAR (R2 = 0.82−0.85, slope of 1.69–1.76) if the Raman
retrieved profile was used to extrapolate the directly measured extinction
coefficient to the ground. The comparison improved if only nighttime
measurements were used in the comparison (R2 = 0.96, slope of 1.12). |
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