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| Titel |
Free troposphere as a major source of CCN for the equatorial pacific boundary layer: long-range transport and teleconnections |
| VerfasserIn |
A. D. Clarke, S. Freitag, R. M. C. Simpson, J. G. Hudson, S. G. Howell, V. L. Brekhovskikh, T. Campos, V. N. Kapustin, J. Zhou |
| 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 ; 13, no. 15 ; Nr. 13, no. 15 (2013-08-05), S.7511-7529 |
| Datensatznummer |
250085607
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| Publikation (Nr.) |
 copernicus.org/acp-13-7511-2013.pdf |
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| Zusammenfassung |
Airborne aerosol measurements in the central equatorial Pacific during PASE
(Pacific Atmospheric Sulfur Experiment) revealed that cloud condensation
nuclei (CCN) activated in marine boundary layer (MBL) clouds were strongly
influenced by entrainment from the free troposphere (FT). About 65%
entered at sizes effective as CCN in MBL clouds, while ~25% entered
the MBL too small to activate but subsequently grew via gas to particle
conversion. The remaining ~10% were inferred to be sea salt
aerosol.
FT aerosols at low carbon monoxide (CO) mixing ratios (< 63 ppbv)
were mostly volatile at 360 °C with a number mode peak of around
30–40 nm dry diameter and tended to be associated with cloud outflow from
distant (3000 km or more) deep convection. Higher CO concentrations were
commonly associated with trajectories from South America and the Amazon
region (ca. ~10 000 km away) and occurred in layers indicative of combustion
sources (biomass burning season) partially scavenged by precipitation. These
had number modes near 60–80 nm dry diameter with a large fraction of CCN.2
(those activated at 0.2% supersaturation and representative of MBL clouds)
prior to entrainment into the MBL. Flight averaged concentrations of
CCN.2 were similar for measurements near the surface, below the inversion and
in the FT just above the inversion, confirming that subsidence and
entrainment of FT aerosol strongly influenced MBL CCN.2. Concurrent flight-to-flight variations of
CCN.2 at all altitudes below 3 km also imply MBL CCN.2 concentrations were
in quasi-equilibrium with the FT over a 2–3 day timescale.
The observed FT transport over thousands of kilometers indicates
teleconnections between MBL CCN and cloud-scavenged sources of both natural
and/or residual combustion origin. Nonetheless, in spite of its importance,
this source of CCN number is not well represented in most current models and
is generally not detectable by satellite because of the low aerosol
scattering in such layers as a result of cloud scavenging. In addition, our
measurements confirm nucleation in the MBL was not evident during PASE and
argue against a localized linear relation in the MBL between dimethyl sulfide
(DMS) and CCN suggested by the CLAW hypothesis. However, when the FT is not
impacted by long-range transport, sulfate aerosol derived from DMS pumped
aloft in the ITCZ (Inter-Tropical Convergence Zone) can provide a source of CCN to the boundary layer via FT
teleconnections involving more complex non-linear processes. |
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