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| Titel |
Modelling the partitioning of ammonium nitrate in the convective boundary layer |
| VerfasserIn |
J. M. J. Aan de Brugh, J. S. Henzing, M. Schaap, W. T. Morgan, C. C. Heerwaarden, E. P. Weijers, H. Coe, M. C. Krol |
| 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 ; 12, no. 6 ; Nr. 12, no. 6 (2012-03-27), S.3005-3023 |
| Datensatznummer |
250010948
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| Publikation (Nr.) |
 copernicus.org/acp-12-3005-2012.pdf |
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| Zusammenfassung |
An explanatory model study is presented on semi-volatile secondary inorganic
aerosols on three clear days in May 2008 during the IMPACT campaign at the
Cabauw tower in the Netherlands. A single column model in combination with
the equilibrium aerosol model ISORROPIA is used. This model uses surface
observations from IMPACT and calculates the gas-aerosol partitioning of
ammonium nitrate. The calculated gas-aerosol equilibrium overestimates the
gas phase fraction during daytime, and overestimates the aerosol phase
fraction during night-time. This discrepancy can partly be solved when the
approach of the gas-aerosol equilibrium is forced to proceed with a delay
timescale of up to two hours. Although it is shown that the delay itself has
a small effect, the most important effect is caused by the mixing of air from
higher altitudes at which the equilibrium is shifted to the aerosol phase.
Thus, vertical mixing is shown to have a significant influence on the
calculated partitioning at the surface. On some occasions, the correspondence
to the observed partitioning improves dramatically.
Even though gas-aerosol partitioning of ammonium nitrate is not
instantaneous, observations show that a different equilibrium in the upper
boundary layer causes aerosol ammonium nitrate concentrations to increase
with altitude. Our model calculates similar vertical gradients depending on
the assumed speed of gas-aerosol equilibrium. The calculated optical
properties of the aerosol show a similar behaviour. The aerosol optical
properties depend on the aerosol size distribution both directly, because
light scattering depends on particle size, and indirectly, because the
equilibration timescale depends on the aerosol sizes. Future studies should
therefore focus on a fully size-resolved treatment of the gas-aerosol
partitioning.
Finally, coarser-resolution models may treat the gas-aerosol equilibrium of
ammonium nitrate by calculating the equilibrium with a temperature and
humidity sampled at a different altitude. We found that the equilibrium at an
altitude of 200 m (night) up to 600 m (day) is representative for the
partitioning of ammonium nitrate at the surface in the beginning of May 2008. |
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