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| Titel |
Absorbing aerosols at high relative humidity: linking hygroscopic growth to optical properties |
| VerfasserIn |
J. Michel Flores, R. Z. Bar-Or, N. Bluvshtein, A. Abo-Riziq, A. Kostinski, S. Borrmann, I. Koren, I. Koren, Y. Rudich |
| 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 ; 12, no. 12 ; Nr. 12, no. 12 (2012-06-25), S.5511-5521 |
| Datensatznummer |
250011282
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| Publikation (Nr.) |
 copernicus.org/acp-12-5511-2012.pdf |
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| Zusammenfassung |
One of the major uncertainties in the understanding of Earth's climate
system is the interaction between solar radiation and aerosols in the
atmosphere. Aerosols exposed to high humidity will change their chemical,
physical, and optical properties due to their increased water content. To
model hydrated aerosols, atmospheric chemistry and climate models often use
the volume weighted mixing rule to predict the complex refractive index (RI)
of aerosols when they interact with high relative humidity, and, in general,
assume homogeneous mixing. This study explores the validity of these
assumptions. A humidified cavity ring down aerosol spectrometer (CRD-AS) and
a tandem hygroscopic DMA (differential mobility analyzer) are used to
measure the extinction coefficient and hygroscopic growth factors of
humidified aerosols, respectively. The measurements are performed at 80%
and 90%RH at wavelengths of 532 nm and 355 nm using size-selected
aerosols with different degrees of absorption; from purely scattering to
highly absorbing particles. The ratio of the humidified to the dry
extinction coefficients (fRHext(%RH, Dry)) is measured and compared to
theoretical calculations based on Mie theory. Using the measured hygroscopic
growth factors and assuming homogeneous mixing, the expected RIs using the
volume weighted mixing rule are compared to the RIs derived from the
extinction measurements.
We found a weak linear dependence or no dependence of fRH(%RH, Dry) with
size for hydrated absorbing aerosols in contrast to the non-monotonically
decreasing behavior with size for purely scattering aerosols. No discernible
difference could be made between the two wavelengths used. Less than 7%
differences were found between the real parts of the complex refractive
indices derived and those calculated using the volume weighted mixing rule,
and the imaginary parts had up to a 20% difference. However, for
substances with growth factor less than 1.15 the volume weighted mixing rule
assumption needs to be taken with caution as the imaginary part of the
complex RI can be underestimated. |
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