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| Titel |
Enhanced extinction of visible radiation due to hydrated aerosols in mist and fog |
| VerfasserIn |
T. Elias, J.-C. Dupont, E. Hammer, C. R. Hoyle, M. Haeffelin, F. Burnet, D. Jolivet |
| 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. 12 ; Nr. 15, no. 12 (2015-06-16), S.6605-6623 |
| Datensatznummer |
250119829
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| Publikation (Nr.) |
 copernicus.org/acp-15-6605-2015.pdf |
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| Zusammenfassung |
The study assesses the contribution of aerosols to the extinction of visible
radiation in the mist–fog–mist cycle. Relative humidity is large in the
mist–fog–mist cycle, and aerosols most efficient in interacting with visible
radiation are hydrated and compose the accumulation mode. Measurements of
the microphysical and optical properties of these hydrated aerosols with
diameters larger than 0.4 μm were carried out near Paris, during November
2011, under ambient conditions. Eleven mist–fog–mist cycles were observed,
with a cumulated fog duration of 96 h, and a cumulated mist–fog–mist cycle
duration of 240 h.
In mist, aerosols grew by taking up water at relative humidities larger than
93%, causing a visibility decrease below 5 km. While visibility decreased down from 5 to a few kilometres, the mean size of the hydrated aerosols increased, and their
number concentration (Nha) increased from approximately 160 to
approximately 600 cm−3. When fog formed, droplets became the strongest
contributors to visible radiation extinction, and liquid water content
(LWC) increased beyond 7 mg m−3. Hydrated aerosols of the accumulation mode
co-existed with droplets, as interstitial non-activated aerosols. Their size
continued to increase, and some aerosols achieved diameters larger than 2.5 μm. The mean transition diameter between the aerosol accumulation mode
and the small droplet mode was 4.0 ± 1.1 μm. Nha also
increased on average by 60 % after fog formation. Consequently, the mean
contribution to extinction in fog was 20 ± 15% from hydrated
aerosols smaller than 2.5 μm and 6 ± 7% from larger aerosols.
The standard deviation was large because of the large variability of
Nha in fog, which could be smaller than in mist or 3 times larger.
The particle extinction coefficient in fog can be computed as the sum of a
droplet component and an aerosol component, which can be approximated by 3.5
Nha (Nha in cm−3 and particle extinction coefficient in
Mm−1. We observed an influence of the main formation process on
Nha, but not on the contribution to fog extinction by aerosols. Indeed,
in fogs formed by stratus lowering (STL), the mean Nha was 360 ± 140 cm−3, close to the value observed in mist, while in fogs formed by
nocturnal radiative cooling (RAD) under cloud-free sky, the mean Nha was
600 ± 350 cm−3. But because visibility (extinction) in fog was
also lower (larger) in RAD than in STL fogs, the contribution by aerosols to
extinction depended little on the fog formation process. Similarly, the
proportion of hydrated aerosols over all aerosols (dry and hydrated) did not
depend on the fog formation process.
Measurements showed that visibility in RAD fogs was smaller than in STL fogs
due to three factors: (1) LWC was larger in RAD than in STL fogs, (2) droplets
were smaller, (3) hydrated aerosols composing the accumulation mode were more
numerous. |
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