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| Titel |
Analysis of feedbacks between nucleation rate, survival probability and cloud condensation nuclei formation |
| VerfasserIn |
D. M. Westervelt, J. R. Pierce, P. J. Adams |
| 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 ; 14, no. 11 ; Nr. 14, no. 11 (2014-06-05), S.5577-5597 |
| Datensatznummer |
250118775
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| Publikation (Nr.) |
 copernicus.org/acp-14-5577-2014.pdf |
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| Zusammenfassung |
| Aerosol nucleation is an important source of particle number in the
atmosphere. However, in order to become cloud condensation nuclei (CCN),
freshly nucleated particles must undergo significant condensational growth
while avoiding coagulational scavenging. In an effort to quantify the
contribution of nucleation to CCN, this work uses the GEOS-Chem-TOMAS global
aerosol model to calculate changes in CCN concentrations against a broad
range of nucleation rates and mechanisms. We then quantify the factors that
control CCN formation from nucleation, including daily nucleation rates,
growth rates, coagulation sinks, condensation sinks, survival probabilities,
and CCN formation rates, in order to examine feedbacks that may limit growth
of nucleated particles to CCN. Nucleation rate parameterizations tested in
GEOS-Chem-TOMAS include ternary nucleation (with multiple tuning factors),
activation nucleation (with two pre-factors), binary nucleation, and
ion-mediated nucleation. We find that nucleation makes a significant
contribution to boundary layer CCN(0.2%), but this contribution is only
modestly sensitive to the choice of nucleation scheme, ranging from 49 to 78%
increase in concentrations over a control simulation with no nucleation.
Moreover, a two order-of-magnitude increase in the globally averaged
nucleation rate (via changes to tuning factors) results in small changes
(less than 10%) to global CCN(0.2%) concentrations. To explain this,
we present a simple theory showing that survival probability has an
exponentially decreasing dependence on the square of the condensation sink.
This functional form stems from a negative correlation between condensation
sink and growth rate and a positive correlation between condensation sink
and coagulational scavenging. Conceptually, with a fixed condensable vapor
budget (sulfuric acid and organics), any increase in CCN concentrations due
to higher nucleation rates necessarily entails an increased aerosol surface
area in the accumulation mode, resulting in a higher condensation sink, which
lowers vapor concentrations and growth rates. As a result, slowly growing
nuclei are exposed to a higher frequency of coagulational scavenging for a
longer period of time, thus reducing their survival probabilities and
closing a negative feedback loop that dampens the impact of nucleation on
CCN. We confirm quantitatively that the decreases in survival probability
predicted by GEOS-Chem-TOMAS due to higher nucleation rates are in
accordance with this simple theory of survival probability. |
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