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| Titel |
Formation and growth of nucleated particles into cloud condensation nuclei: Comparison of a global microphysics model with observations |
| VerfasserIn |
D. M. Westervelt, P. J. Adams, I. Riipininen, J. R. Pierce, W. Trivitayanurak |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250063610
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| Zusammenfassung |
| Aerosol nucleation occurs frequently in the atmosphere and is an important source of particle
number. Observations suggest that nucleated particles are capable of growing to sufficiently
large sizes that they act as cloud condensation nuclei (CCN), but some global models have
reported that CCN concentrations are only modestly sensitive to large changes in nucleation
rates. The connection between nucleation and CCN represents a key uncertainty in
understanding the CCN budget and its implications for Earth’s radiative balance. Here we
present a novel approach for evaluating, in observations and models, the impact of single-day
nucleation and growth events on the regional CCN budget. We also compare model-predicted
nucleation rates, diameter growth rates, condensation and coagulation sinks, and survival
probabilities to observations. This work uses the TwO-Moment Aerosol Sectional
algorithm (TOMAS) hosted by the global chemical transport model GEOS-Chem
to simulate nucleation events predicted by ternary (with a 10-5 tuning factor) or
activation nucleation over one year. To evaluate model performance, we compare
GEOS-Chem-TOMAS output in 30-minute intervals against a full year of size distribution
datasets measured at the five following locations: Pittsburgh, Hyytiälä, Atlanta, St. Louis,
and Po Valley. Results show that GEOS-Chem-TOMAS does not understate the
importance of nucleation to CCN concentrations, as most metrics are within a 50%
error in the model-measurement agreement and tend to be biased high. Median
survival probabilities to 100 nm within one day for the model and measurements
range from less than 1% to 9% across the five locations we considered. The strength
of the coagulation sink and the infrequency of strong growth events (greater than
10 nm h-1) are mainly responsible for these relatively low single-day survival
probabilies. Additionally, both observations and models suggest that single-day
nucleation and growth events contribute less than 5% to annually averaged CN100
concentrations (a proxy for CCN). Growth of nucleated particles on subsequent days means
that nucleation makes a larger contribution, but this is difficult to constrain with
available measurements. When a 50 nm CCN size limit is considered, particle survival
probabilities for single-day events range from 25-50% across the different locations, and
CN50 enhancements from nucleation are at least an order magnitude larger than
CN100. This detailed exploration of new particle formation and growth dynamics
adds support to the use of global models as tools for assessing the contribution of
microphysical processes such as nucleation to the total number and CCN budget. |
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