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| Titel |
Boundary layer nucleation as a source of new CCN in savannah environment |
| VerfasserIn |
L. Laakso, J. Merikanto, V. Vakkari, H. Laakso  , M. Kulmala , M. Molefe, N. Kgabi, D. Mabaso, K. S. Carslaw, D. V. Spracklen, L. A. Lee, C. L. Reddington, V.-M. Kerminen |
| 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. 4 ; Nr. 13, no. 4 (2013-02-20), S.1957-1972 |
| Datensatznummer |
250017670
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| Publikation (Nr.) |
 copernicus.org/acp-13-1957-2013.pdf |
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| Zusammenfassung |
The South African savannah region is a complex environment of air pollution
and natural emissions influenced by a strong seasonal cycle in biomass
burning and strong precipitation. However, the scarcity of long-term
observations means that the knowledge of controlling aerosol processes in
this environment is limited. Here we use a recent dataset of 18 months of
aerosol size distribution observations trying to understand the annual cycle
of cloud condensation nuclei (CCN).
Our observations show that the concentration of CCN-sized particles remains,
in line with previous studies, high throughout the year with the highest
concentrations during the dry winter and the lowest during the wet summer.
During the wet season with reduced anthropogenic and biomass burning primary
emissions, this pool of CCN is partly filled by boundary layer nucleation
with subsequent growth. The enhanced importance of formation and growth
during the wet season is addressed to increased biogenic activity together
with enhanced free tropospheric removal decreasing the concentration of
pre-existing CCN. During the dry season, while frequent new particle
formation takes place, particle growth is reduced due to reduced condensing
vapour concentrations. Thus in the dry season particles are not able to grow
to sizes where they may act as CCN nearly as efficiently as during the wet
season.
The observations are compared to simulations by a global aerosol model
GLOMAP. To our surprise, the global aerosol model utilized to explain the
observations was not capable of re-producing the characteristics of particle
formation and the annual CCN cycle, despite earlier good performance in
predicting the particle concentrations in a number of diverse environments,
including the South African savannah region. While the average yearly CCN
concentrations of modelled CCN is close to observed concentrations, the
characteristics of nucleation bursts and subsequent growth are not captured
satisfactory by the model. Our sensitivity tests using different nucleation
parameterizations and condensing organic vapour production rates show that
neither of these is likely to explain the differences between observed and
modelled nucleation and growth rates.
A sensitivity study varying 28 modelling parameters indicates that the main
uncertainties in the result are due to uncertainties in biomass burning
emissions during the dry season, and anthropogenic sulphur emissions during
the wet season, both in terms or emitted mass and particle sizes. The
uncertainties appear to be mostly related to uncertainties in primary
particle emissions, including the emissions variability not captured by
monthly emission inventories. The results of this paper also highlights the
fact that deficiencies in emissions estimates may result in deficiencies in
particle production fluxes, while the end product such as modelled CCN
concentration may be in line with observations. |
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