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| Titel |
Modeling organic aerosols in a megacity: potential contribution of semi-volatile and intermediate volatility primary organic compounds to secondary organic aerosol formation |
| VerfasserIn |
A. Hodzic, J. L. Jimenez, S. Madronich, M. R. Canagaratna, P. F. DeCarlo, L. Kleinman, J. Fast |
| 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. 12 ; Nr. 10, no. 12 (2010-06-21), S.5491-5514 |
| Datensatznummer |
250008566
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| Publikation (Nr.) |
 copernicus.org/acp-10-5491-2010.pdf |
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| Zusammenfassung |
| It has been established that observed local and regional levels of secondary
organic aerosols (SOA) in polluted areas cannot be explained by the
oxidation and partitioning of anthropogenic and biogenic VOC precursors, at
least using current mechanisms and parameterizations. In this study, the 3-D
regional air quality model CHIMERE is applied to estimate the potential
contribution to SOA formation of recently identified semi-volatile and
intermediate volatility organic precursors (S/IVOC) in and around Mexico
City for the MILAGRO field experiment during March 2006. The model has been
updated to include explicitly the volatility distribution of primary organic
aerosols (POA), their gas-particle partitioning and the gas-phase oxidation
of the vapors. Two recently proposed parameterizations, those of Robinson et al. (2007) ("ROB") and Grieshop et al. (2009) ("GRI") are compared and
evaluated against surface and aircraft measurements. The 3-D model results
are assessed by comparing with the concentrations of OA components from
Positive Matrix Factorization of Aerosol Mass Spectrometer (AMS) data, and
for the first time also with oxygen-to-carbon ratios derived from
high-resolution AMS measurements. The results show a substantial enhancement
in predicted SOA concentrations (2–4 times) with respect to the previously
published base case without S/IVOCs (Hodzic et al., 2009), both within and
downwind of the city leading to much reduced discrepancies with the total OA
measurements. Model improvements in OA predictions are associated with the
better-captured SOA magnitude and diurnal variability. The predicted
production from anthropogenic and biomass burning S/IVOC represents
40–60% of the total measured SOA at the surface during the day and is
somewhat larger than that from commonly measured aromatic VOCs, especially
at the T1 site at the edge of the city. The SOA production from the
continued multi-generation S/IVOC oxidation products continues actively
downwind. Similar to aircraft observations, the predicted OA/ΔCO
ratio for the ROB case increases from 20–30 μg sm−3 ppm−1 up
to 60–70 μg sm−3 ppm−1 between a fresh and 1-day aged air
mass, while the GRI case produces a 30% higher OA growth than observed.
The predicted average O/C ratio of total OA for the ROB case is 0.16 at T0,
substantially below observed value of 0.5. A much better agreement for O/C
ratios and temporal variability (R2=0.63) is achieved with the updated
GRI treatment. Both treatments show a deficiency in regard to POA ageing
with a tendency to over-evaporate POA upon dilution of the urban plume
suggesting that atmospheric HOA may be less volatile than assumed in these
parameterizations. This study highlights the important potential role of
S/IVOC chemistry in the SOA budget in this region, and highlights the need
for further improvements in available parameterizations. The agreement
observed in this study is not sufficient evidence to conclude that S/IVOC
are the major missing SOA source in megacity environments. The model is
still very underconstrained, and other possible pathways such as formation
from very volatile species like glyoxal may explain some of the mass and
especially increase the O/C ratio. |
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