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| Titel |
Can 3-D models explain the observed fractions of fossil and non-fossil carbon in and near Mexico City? |
| VerfasserIn |
A. Hodzic, J. L. Jimenez, A. S. H. Prévôt, S. Szidat, J. D. Fast, S. Madronich |
| 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 ; 10, no. 22 ; Nr. 10, no. 22 (2010-11-25), S.10997-11016 |
| Datensatznummer |
250008909
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| Publikation (Nr.) |
 copernicus.org/acp-10-10997-2010.pdf |
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| Zusammenfassung |
| A 3-D chemistry-transport model has been applied to the Mexico City
metropolitan area to investigate the origin of elevated levels of non-fossil
(NF) carbonaceous aerosols observed in this highly urbanized region. High
time resolution measurements of the fine aerosol concentration and
composition, and 12 or 24 h integrated 14C measurements of aerosol
modern carbon have been performed in and near Mexico City during the March
2006 MILAGRO field experiment. The non-fossil carbon fraction (fNF),
which is lower than the measured modern fraction (fM) due to the
elevated 14C in the atmosphere caused by nuclear bomb testing, is
estimated from the measured fM and the source-dependent information on
modern carbon enrichment. The fNF contained in PM1 total carbon
analyzed by a US team (fNFTC) ranged from 0.37 to 0.67 at the
downtown location, and from 0.50 to 0.86 at the suburban site. Substantially
lower values (i.e. 0.24–0.49) were found for PM10 filters downtown by
an independent set of measurements (Swiss team), which are inconsistent
with the modeled and known differences between the size ranges, suggesting
higher than expected uncertainties in the measurement techniques of
14C. An increase in the non-fossil organic carbon (OC) fraction
(fNFOC) by 0.10–0.15 was observed for both sets of filters during
periods with enhanced wildfire activity in comparison to periods when fires
were suppressed by rain, which is consistent with the wildfire impacts
estimated with other methods. Model results show that the relatively high
fraction of non-fossil carbon found in Mexico City seems to arise from the
combination in about equal proportions of regional biogenic SOA, biomass
burning POA and SOA, as well as non-fossil urban POA and SOA. Predicted
spatial and temporal variations for fNFOCare similar to those in
the measurements between the urban vs. suburban sites, and high-fire vs.
low-fire periods. The absolute modeled values of fNFOC are
consistent with the Swiss dataset but lower than the US dataset. Resolving
the 14C measurement discrepancies is necessary for further progress in
model evaluation. The model simulations that included secondary organic
aerosol (SOA) formation from semi-volatile and intermediate volatility
(S/IVOC) vapors showed improved closure for the total OA mass compared to
simulations which only included SOA from VOCs, providing a more realistic
basis to evaluate the fNF predictions. fNFOC urban sources of
modern carbon are important in reducing or removing the difference in
fNF between model and measurements, even though they are often
neglected on the interpretation of 14C datasets. An underprediction of biomass burning POA by the model during
some mornings also explains a part of the model-measurement differences. The fNF of urban POA and SOA
precursors is an important parameter that needs to be better constrained by
measurements. Performing faster (≤3 h) 14C measurements in future
campaigns is critical to further progress in this area. To our knowledge
this is the first time that radiocarbon measurements are used together with
aerosol mass spectrometer (AMS) organic components to assess the performance
of a regional model for organic aerosols. |
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