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| Titel |
Formation of semivolatile inorganic aerosols in the Mexico City Metropolitan Area during the MILAGRO campaign |
| VerfasserIn |
V. A. Karydis, A. P. Tsimpidi, W. Lei, L. T. Molina, S. N. Pandis |
| 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 ; 11, no. 24 ; Nr. 11, no. 24 (2011-12-22), S.13305-13323 |
| Datensatznummer |
250010304
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| Publikation (Nr.) |
 copernicus.org/acp-11-13305-2011.pdf |
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| Zusammenfassung |
| One of the most challenging tasks for chemical transport models (CTMs) is
the prediction of the formation and partitioning of the major semi-volatile
inorganic aerosol components (nitrate, chloride, ammonium) between the gas
and particulate phases. In this work the PMCAMx-2008 CTM, which includes the
recently developed aerosol thermodynamic model ISORROPIA-II, is applied in
the Mexico City Metropolitan Area in order to simulate the formation of the
major inorganic aerosol components. The main sources of SO2 (such as
the Miguel Hidalgo Refinery and the Francisco Perez Rios Power Plant) in the
Mexico City Metropolitan Area (MCMA) are located in Tula, resulting in high
predicted PM1 (particulate matter with diameter less than 1 μm)
sulfate concentrations (over 25 μg m-3) in that area. The average
predicted PM1 nitrate concentrations are up to 3 μg m−3 (with
maxima up to 11 μg m−3) in and around the urban center, mostly produced from local photochemistry. The presence of calcium coming
from the Tolteca area (7 μg m−3) as well as the rest of the
mineral cations (1 μg m−3 potassium, 1 μg m−3 magnesium,
2 μg m−3 sodium, and 3 μg m−3 calcium) from the Texcoco
Lake resulted in the formation of a significant amount of aerosol nitrate in
the coarse mode with concentrations up to 3 μg m−3 over these
areas. PM1−10 (particulate matter with diameter between 1 and 10 μm)
chloride is also high and its concentration exceeds 2 μg m−3 in Texcoco Lake. PM1 ammonium concentrations peak at the
center of Mexico City (2 μg m−3) and the Tula vicinity (2.5 μg m−3). The performance of the model for the major inorganic PM
components (sulfate, ammonium, nitrate, chloride, sodium, calcium, and
magnesium) is encouraging. At the T0 measurement site, located in the Mexico
City urban center, the average measured values of PM1 sulfate, nitrate,
ammonium, and chloride are 3.5 μg m−3, 3.5 μg m−3,
2.1 μg m−3, and 0.36 μg m−3, respectively. The corresponding
predicted values are 3.7 μg m−3, 2.7 μg m−3,
1.7 μg m−3, and 0.25 μg m−3. High sulfate concentrations are
associated with the transport of sulfate from the Tula vicinity, while in
periods where southerly winds are dominant; the concentrations of sulfate are
low. The underprediction of nitrate can be attributed to the underestimation
of OH levels by the model during the early morning. Ammonium is sensitive to
the predicted sulfate concentrations and the nitrate levels. The performance
of the model is also evaluated against measurements taken from a suburban
background site (T1) located north of Mexico City. The average predicted
PM2.5 (particulate matter with diameter less than 2.5 μm)
sulfate, nitrate, ammonium, chloride, sodium, calcium, and magnesium are 3.3,
3.2, 1.4, 0.5, 0.3, 1.2, and 0.15 μg m−3, respectively. The
corresponding measured concentrations are 3.7, 2.9, 1.5, 0.3, 0.4, 0.6, and
0.15 μg m−3. The overprediction of calcium indicates a
possible overestimation of its emissions and affects the partitioning of
nitric acid to the aerosol phase resulting occasionally in an overprediction
of nitrate. Additional improvements are possible by improving the performance
of the model regarding the oxidant levels, and revising the emissions and the
chemical composition of the fugitive dust. The hybrid approach in which the
mass transfer to the fine aerosol is simulated using the bulk equilibrium
assumption and to the remaining aerosol sections using a dynamic approach, is
needed in order to accurately simulate the size distribution of the inorganic
aerosols. The bulk equilibrium approach fails to reproduce the observed
coarse nitrate and overpredicts the fine nitrate. Sensitivity tests indicate
that sulfate concentration in Tula decreases by up to
0.5 μg m−3 after a 50% reduction of SO2 emissions
while it can increase by up to 0.3 μg m−3 when NOx
emissions are reduced by 50%. Nitrate concentration decreases by up to
1 μg m−3 after the 50% reduction of NOx or
NH3 emissions. Ammonium concentration decreases by up to
1 μg m−3, 0.3 μg m−3, and
0.1 μg m−3 after the 50% reduction of NH3,
NOx, and SO2 emissions, respectively. |
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