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| Titel |
Columnar modelling of nucleation burst evolution in the convective boundary layer – first results from a feasibility study Part III: Preliminary results on physicochemical model performance using two "clean air mass" reference scenarios |
| VerfasserIn |
O. Hellmuth |
| 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 ; 6, no. 12 ; Nr. 6, no. 12 (2006-09-21), S.4231-4251 |
| Datensatznummer |
250004116
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| Publikation (Nr.) |
 copernicus.org/acp-6-4231-2006.pdf |
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| Zusammenfassung |
| In Paper I of four papers, a revised columnar high-order model
to investigate gas-aerosol-turbulence interactions in the
convective boundary layer (CBL) was proposed. In Paper II, the
model capability to predict first-, second-
and third-order moments of meteorological variables in the CBL was
demonstrated using available observational data. In the present Paper III, the
high-order modelling concept is extended
to sulphur and ammonia chemistry as well as to aerosol dynamics. Based on
the previous CBL simulation, a feasibility study is performed using
two "clean air mass" scenarios with an emission source at the
ground but low aerosol background concentration. Such scenarios synoptically
correspond to the advection of fresh post-frontal air in an
anthropogenically influenced region. The aim is to evaluate the
time-height evolution of
ultrafine condensation nuclei (UCNs)
and to elucidate the interactions between
meteorological and physicochemical variables in a CBL column. The scenarios
differ in the treatment of
new particle formation (NPF), whereas
homogeneous nucleation according to the classical nucleation theory (CNT)
is considered. The first scenario considers nucleation
of a binary system consisting of water vapour and
sulphuric acid (H2SO4) vapour,
the second one nucleation of a ternary system additionally involving
ammonia (NH3). Here, the two synthetic scenarios
are discussed in detail, whereas
special attention is payed to the role of turbulence in the formation
of the typical UCN burst behaviour, that can often be observed in
the surface layer. The intercomparison of the two scenarios
reveals large differences in the evolution of the UCN number concentration
in the surface layer as well as in the
time-height cross-sections of first-order moments and
double correlation terms. Although in both cases the occurrence of
NPF bursts could be simulated,
the burst characteristics and genesis of the bursts are completely
different. It is demonstrated, that observations from the surface layer alone
are not conclusive to elucidate the origin of newly formed
particles. This is also true with respect to the interpretation of
box modelling studies. The binary and ternary NPF bursts observed
in the surface layer differ with respect to burst
amplitude and phase. New particles simulated in the binary scenario
are formed in the forenoon in the upper part of the
growing CBL, followed by turbulence-induced top-down transport. Hence, with
respect to the burst observation site in the surface layer,
new particles are formed ex situ. In opposite to this, the ternary case
reveals a much more complex pattern. Here, NPF is initiated in the
early morning hours in the surface layer, when temperature (T) is low and
relative humidity (RH), sulphur dioxide (SO2) and NH3
concentrations are high, hence
new particles are formed in situ. Shortly after that, ex situ NPF in
the free troposphere sets in, followed by entrainment and top-down
diffusion of newly formed particles into the surface layer. Altogether, these
processes mainly contribute to the formation of a strong
burst in the morning hours in the ternary scenario. While the time-height
cross-section of the binary nucleation rate
resembles a "blob"-like evolution pattern, the ternary one
resembles a "sucking tube"-like pattern. The time-height cross-sections
of the flux pattern and double correlations
could be plausibly interpreted in terms of CBL turbulence and
entrainment/detrainment processes
both in the binary and in the ternary case. Although the present
approach is a pure conceptual one, it shows the feasibility
to simulate gas-aerosol-turbulence interactions in the CBL. Prior to a dedicated
verification/validation study,
further attempts are necessary to consider a more advanced
description of the formation and activation of
thermodynamically stable clusters
according to modern concepts proposed by Kulmala et al. (2000), Kulmala (2003) and Kulmala et al.
(2004a). |
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