 |
| Titel |
Modeling organic aerosols during MILAGRO: importance of biogenic secondary organic aerosols |
| VerfasserIn |
A. Hodzic, J. L. Jimenez, S. Madronich, A. C. Aiken, B. Bessagnet, G. Curci, J. Fast, J.-F. Lamarque, T. B. Onasch, G. Roux, J. J. Schauer, E. A. Stone, I. M. Ulbrich |
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| ISSN |
1680-7316
|
| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 9, no. 18 ; Nr. 9, no. 18 (2009-09-22), S.6949-6981 |
| Datensatznummer |
250007642
|
| Publikation (Nr.) |
 copernicus.org/acp-9-6949-2009.pdf |
|
|
|
|
|
| Zusammenfassung |
| The meso-scale chemistry-transport model CHIMERE is used to assess our
understanding of major sources and formation processes leading to a fairly
large amount of organic aerosols – OA, including primary OA (POA) and
secondary OA (SOA) – observed in Mexico City during the MILAGRO field project
(March 2006). Chemical analyses of submicron aerosols from aerosol mass
spectrometers (AMS) indicate that organic particles found in the Mexico City
basin contain a large fraction of oxygenated organic species (OOA) which
have strong correspondence with SOA, and that their production actively
continues downwind of the city. The SOA formation is modeled here by the
one-step oxidation of anthropogenic (i.e. aromatics, alkanes), biogenic
(i.e. monoterpenes and isoprene), and biomass-burning SOA precursors and
their partitioning into both organic and aqueous phases. Conservative
assumptions are made for uncertain parameters to maximize the amount of SOA
produced by the model. The near-surface model evaluation shows that
predicted OA correlates reasonably well with measurements during the
campaign, however it remains a factor of 2 lower than the measured total OA.
Fairly good agreement is found between predicted and observed POA within the
city suggesting that anthropogenic and biomass burning emissions are
reasonably captured. Consistent with previous studies in Mexico City, large
discrepancies are encountered for SOA, with a factor of 2–10 model
underestimate. When only anthropogenic SOA precursors were considered, the
model was able to reproduce within a factor of two the sharp increase in OOA
concentrations during the late morning at both urban and near-urban
locations but the discrepancy increases rapidly later in the day, consistent
with previous results, and is especially obvious when the column-integrated
SOA mass is considered instead of the surface concentration. The increase in
the missing SOA mass in the afternoon coincides with the sharp drop in POA
suggesting a tendency of the model to excessively evaporate the freshly
formed SOA. Predicted SOA concentrations in our base case were extremely low
when photochemistry was not active, especially overnight, as the SOA formed
in the previous day was mostly quickly advected away from the basin. These
nighttime discrepancies were not significantly reduced when greatly enhanced
partitioning to the aerosol phase was assumed. Model sensitivity results
suggest that observed nighttime OOA concentrations are strongly influenced
by a regional background SOA (~1.5 μg/m3) of biogenic origin
which is transported from the coastal mountain ranges into the Mexico City
basin. The presence of biogenic SOA in Mexico City was confirmed by SOA
tracer-derived estimates that have reported 1.14 (±0.22) μg/m3
of biogenic SOA at T0, and 1.35 (±0.24) μg/m3 at T1, which are
of the same order as the model. Consistent with other recent studies, we
find that biogenic SOA does not appear to be underestimated significantly by
traditional models, in strong contrast to what is observed for anthropogenic
pollution. The relative contribution of biogenic SOA to predicted monthly
mean SOA levels (traditional approach) is estimated to be more than 30%
within the city and up to 65% at the regional scale which may help
explain the significant amount of modern carbon in the aerosols inside the
city during low biomass burning periods. The anthropogenic emissions of
isoprene and its nighttime oxidation by NO3 were also found to enhance
the SOA mean concentrations within the city by an additional 15%. Our
results confirm the large underestimation of the SOA production by
traditional models in polluted regions (estimated as 10–20 tons within the
Mexico City metropolitan area during the daily peak), and emphasize for the
first time the role of biogenic precursors in this region, indicating that
they cannot be neglected in urban modeling studies. |
| |
|
| Teil von |
|
|
|
|
|
|
|
|