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| Titel |
Clouds and aerosols in Puerto Rico – a new evaluation |
| VerfasserIn |
J. D. Allan, D. Baumgardner, G. B. Raga, O. L. Mayol-Bracero, F. Morales-García, F. García-García, G. Montero-Martínez, S. Borrmann, J. Schneider, S. Mertes, S. Walter, M. Gysel, U. Dusek, G. P. Frank, M. Krämer |
| 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 ; 8, no. 5 ; Nr. 8, no. 5 (2008-03-05), S.1293-1309 |
| Datensatznummer |
250005794
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| Publikation (Nr.) |
 copernicus.org/acp-8-1293-2008.pdf |
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| Zusammenfassung |
The influence of aerosols, both natural and anthropogenic, remains a major
area of uncertainty when predicting the properties and behaviour of clouds
and their influence on climate. In an attempt to better understand warm
cloud formation in a tropical marine environment, a period of intensive
measurements took place in December 2004 in Puerto Rico, using some of the
latest developments in online instrumentation such as aerosol mass
spectrometers, cloud condensation nuclei counters and a hygroscopicity
tandem differential mobility analyser. Simultaneous online measurements of
aerosol size distributions, composition, hygroscopicity and optical
properties were made near the lighthouse of Cape San Juan in the
north-eastern corner of the island and at the top of East Peak mountain
(1040 m a.s.l.), the two sites separated by 17 km. Additional measurements of
the cloud droplet residual and interstitial aerosol properties were made at
the mountain site, accompanied by measurements of cloud droplet size
distributions, liquid water content and the chemical composition of cloud
and rain water samples.
Both aerosol composition and cloud properties were found to be sensitive to
wind sector. Air from the east-northeast (ENE) was mostly free of
anthropogenic influences, the submicron fraction being mainly composed of
non-sea salt sulphate, while that from the east-southeast (ESE) was found to
be moderately influenced by populated islands upwind, adding smaller (<100 nm),
externally mixed, carbonaceous particles to the aerosol that increased
the number concentrations by over a factor of 3. This change in composition
was also accompanied with a reduction in the measured hygroscopicity and
fractional cloud activation potential of the aerosol. At the mountain site,
the average cloud droplet concentrations increased from 193 to 519 cm−3,
median volume diameter decreased from 20 to 14 μm and the
liquid water content increased from 0.24 to 0.31 g m−3 when the winds
shifted from the ENE to ESE. Larger numbers of interstitial particles were
recorded, most notably at sizes greater than 100 nm, which were absent
during clean conditions. The average size of the residual particles and
concentrations of cloudwater nitrate, sulphate and insoluble material
increased during polluted conditions.
Previous studies in Puerto Rico had reported the presence of a significant
non-anthropogenic organic fraction in the aerosols measured and concluded
that this was a factor controlling the in situ cloud properties. However,
this was not observed in our case. In contrast to the 1.00±0.14 μg m−3
of organic carbon measured in 1992 and 1995, the organic matter
measured in the current study of 0.17±0.35 μg m−3 is many
times lower, most of which can be attributed to anthropogenic sources.
During clean conditions, the submicron aerosol was observed to be almost
entirely inorganic, an observation supported by the hygroscopicity
measurements. This suggests that organic aerosols from marine sources may
not be completely ubiquitous (either spatially or temporally) in this
environment and requires further investigation to quantify their true extent
and implications, with more extensive, longer-term sampling in conjunction
with wind field analyses. |
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