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| Titel |
Modeling the formation and aging of secondary organic aerosols in Los Angeles during CalNex 2010 |
| VerfasserIn |
P. L. Hayes, A. G. Carlton, K. R. Baker, R. Ahmadov, R. A. Washenfelder, S. Alvarez, B. Rappenglück, J. B. Gilman, W. C. Kuster, J. A. de Gouw, P. Zotter, A. S. H. Prévôt, S. Szidat, T. E. Kleindienst, J. H. Offenberg, P. K. Ma, J. L. Jimenez |
| 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 ; 15, no. 10 ; Nr. 15, no. 10 (2015-05-26), S.5773-5801 |
| Datensatznummer |
250119755
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| Publikation (Nr.) |
 copernicus.org/acp-15-5773-2015.pdf |
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| Zusammenfassung |
Four different literature parameterizations for the formation and evolution
of urban secondary organic aerosol (SOA) frequently used in 3-D models are
evaluated using a 0-D box model representing the Los Angeles metropolitan
region during the California Research at the Nexus of Air Quality and Climate Change (CalNex) 2010 campaign. We constrain the model
predictions with measurements from several platforms and compare predictions
with particle- and gas-phase observations from the CalNex Pasadena ground
site. That site provides a unique opportunity to study aerosol formation
close to anthropogenic emission sources with limited recirculation. The
model SOA that formed only from the oxidation of VOCs (V-SOA) is insufficient to
explain the observed SOA concentrations, even when using SOA
parameterizations with multi-generation oxidation that produce much higher
yields than have been observed in chamber experiments, or when increasing
yields to their upper limit estimates accounting for recently reported
losses of vapors to chamber walls. The Community Multiscale Air Quality
(WRF-CMAQ) model (version 5.0.1) provides excellent predictions of secondary
inorganic particle species but underestimates the observed SOA mass by a
factor of 25 when an older VOC-only parameterization is used, which is
consistent with many previous model–measurement comparisons for pre-2007
anthropogenic SOA modules in urban areas.
Including SOA from primary semi-volatile and intermediate-volatility organic
compounds (P-S/IVOCs) following the parameterizations of Robinson et al. (2007), Grieshop et al. (2009), or Pye and
Seinfeld (2010) improves model–measurement agreement for
mass concentration. The results from the three parameterizations show large
differences (e.g., a factor of 3 in SOA mass) and are not well constrained,
underscoring the current uncertainties in this area. Our results strongly
suggest that other precursors besides VOCs, such as P-S/IVOCs, are needed to
explain the observed SOA concentrations in Pasadena. All the recent
parameterizations overpredict urban SOA formation at long photochemical
ages
(≈ 3 days) compared to observations from multiple sites, which
can lead to problems in regional and especially global modeling. However,
reducing IVOC emissions by one-half in the model to better match recent IVOC
measurements improves SOA predictions at these long photochemical ages.
Among the explicitly modeled VOCs, the precursor compounds that contribute
the greatest SOA mass are methylbenzenes. Measured polycyclic aromatic
hydrocarbons (naphthalenes) contribute 0.7% of the modeled SOA mass. The
amounts of SOA mass from diesel vehicles, gasoline vehicles, and cooking
emissions are estimated to be 16–27, 35–61, and 19–35%,
respectively, depending on the parameterization used, which is consistent
with the observed fossil fraction of urban SOA, 71(±3) %. The
relative contribution of each source is uncertain by almost a factor of 2
depending on the parameterization used. In-basin biogenic VOCs are predicted
to contribute only a few percent to SOA. A regional SOA background of
approximately 2.1 μg m−3 is also present due to the long-distance
transport of highly aged OA, likely with a substantial contribution from
regional biogenic SOA. The percentage of SOA from diesel vehicle emissions
is the same, within the estimated uncertainty, as reported in previous work
that analyzed the weekly cycles in OA concentrations (Bahreini et al.,
2012; Hayes et al., 2013). However, the modeling work presented here
suggests a strong anthropogenic source of modern carbon in SOA, due to
cooking emissions, which was not accounted for in those previous studies
and which is higher on weekends.
Lastly, this work adapts a simple two-parameter model to predict SOA
concentration and O/C from urban emissions. This model successfully predicts
SOA concentration, and the optimal parameter combination is very similar to
that found for Mexico City. This approach provides a computationally
inexpensive method for predicting urban SOA in global and climate models. We
estimate pollution SOA to account for 26 Tg yr−1 of SOA globally, or
17% of global SOA, one-third of which is likely to be non-fossil. |
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