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| Titel |
In situ observations of new particle formation in the tropical upper troposphere: the role of clouds and the nucleation mechanism |
| VerfasserIn |
R. Weigel, S. Borrmann, J. Kazil, A. Minikin, A. Stohl, J. C. Wilson, J. M. Reeves, D. Kunkel, M. Reus, W. Frey, E. R. Lovejoy, C. M. Volk, S. Viciani, F. D'Amato, C. Schiller, T. Peter, H. Schlager, F. Cairo, K. S. Law, G. N. Shur, G. V. Belyaev, J. Curtius |
| 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. 18 ; Nr. 11, no. 18 (2011-09-29), S.9983-10010 |
| Datensatznummer |
250010108
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| Publikation (Nr.) |
 copernicus.org/acp-11-9983-2011.pdf |
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| Zusammenfassung |
| New particle formation (NPF), which generates nucleation mode aerosol, was
observed in the tropical Upper Troposphere (UT) and Tropical Tropopause
Layer (TTL) by in situ airborne measurements over South America
(January–March 2005), Australia (November–December 2005), West Africa
(August 2006) and Central America (2004–2007). Particularly intense NPF was
found at the bottom of the TTL. Measurements with a set of condensation
particle counters (CPCs) with different dp50 (50% lower size
detection efficiency diameter or "cut-off diameter") were conducted on
board the M-55 Geophysica in the altitude range of 12.0–20.5 km and on board the DLR
Falcon-20 at up to 11.5 km altitude. On board the NASA WB-57F size distributions
were measured over Central America in the 4 to 1000 nm diameter range with a
system of nucleation mode aerosol spectrometers. Nucleation mode particle
concentrations (NNM) were derived from these measurements which allow
for identifying many NPF events with NNM in the range of thousands of
particles per cm3. Over Australia and West Africa, we identified NPF in
the outflow of tropical convection, in particular of a Mesoscale Convective
System (MCS). Newly formed particles with NNM > 1000 cm−3
were found to coexist with ice cloud particles (dp > 2 μm) as
long as cloud particle concentrations remained below 2 cm−3. The
occurrence of NPF within the upper troposphere and the TTL was generally
confined within 340 K to 380 K potential temperature, but NPF was of
particular strength between 350 K and 370 K (i.e. ~1–4 km below the
cold point tropopause). Analyses of the aerosol volatility (at 250 °C)
show that in the TTL on average 75–90% of the particles were volatile,
compared to typically only 50% in the extra-tropical UT, indicative for
the particles to mainly consist of H2SO4-H2O and possibly
organic compounds. Along two flight segments over Central and South America
(24 February 2005 and 7 August 2006, at 12.5 km altitude) in cloud free
air, above thin cirrus, particularly high NNM were observed. Recent
lifting had influenced the probed air masses, and NNM reached up to
16 000 particles cm−3 (ambient concentration). A sensitivity study
using an aerosol model, which includes neutral and ion induced nucleation
processes, simulates NNM in reasonable agreement with the in situ
observations of clear-air NPF. Based on new, stringent multi-CPC criteria,
our measurements corroborate the hypothesis that the tropical UT and the TTL
are regions supplying freshly nucleated particles. Our findings narrow the
altitude of the main source region to the bottom TTL, i.e. to the level of
main tropical convection outflow, and, by means of measurements of carbon
monoxide, they indicate the importance of anthropogenic emissions in NPF.
After growth and/or coalescence the nucleation mode particles may act as
cloud condensation nuclei in the tropical UT, or, upon ascent into the
stratosphere, contribute to maintain the stratospheric background aerosol. |
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