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| Titel |
Oxidative capacity of the Mexico City atmosphere – Part 2: A ROx radical cycling perspective |
| VerfasserIn |
P. M. Sheehy, R. Volkamer, L. T. Molina, M. J. Molina |
| 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. 14 ; Nr. 10, no. 14 (2010-07-30), S.6993-7008 |
| Datensatznummer |
250008666
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| Publikation (Nr.) |
 copernicus.org/acp-10-6993-2010.pdf |
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| Zusammenfassung |
A box model using measurements from the Mexico City Metropolitan Area study
in the spring of 2003 (MCMA-2003) is presented to study oxidative capacity
(our ability to predict OH radicals) and ROx
(ROx=OH+HO2+RO2+RO) radical cycling in
a polluted (i.e., very high NOx=NO+NO2) atmosphere.
Model simulations were performed using the Master Chemical Mechanism
(MCMv3.1) constrained with 10 min averaged measurements of major radical
sources (i.e., HCHO, HONO, O3, CHOCHO, etc.),
radical sink precursors (i.e., NO, NO2, SO2,
CO, and 102 volatile organic compounds (VOC)), meteorological
parameters (temperature, pressure, water vapor concentration, dilution), and
photolysis frequencies.
Modeled HOx (=OH+HO2) concentrations compare
favorably with measured concentrations for most of the day; however, the
model under-predicts the concentrations of radicals in the early morning.
This "missing reactivity" is highest during peak photochemical activity,
and is least visible in a direct comparison of HOx radical
concentrations. We conclude that the most likely scenario to reconcile model
predictions with observations is the existence of a currently unidentified
additional source for RO2 radicals, in combination with an additional
sink for HO2 radicals that does not form OH. The true
uncertainty due to "missing reactivity" is apparent in parameters like
chain length. We present a first attempt to calculate chain length rigorously
i.e., we define two parameters that account for atmospheric complexity, and
are based on (1) radical initiation, n(OH), and (2) radical
termination, ω. We find very high values of n(OH) in the early morning
are incompatible with our current understanding of ROx termination
routes. We also observe missing reactivity in the rate of ozone production
(P(O3)). For example, the integral amount of ozone produced could be
under-predicted by a factor of two. We argue that this uncertainty is partly
accounted for in lumped chemical codes that are optimized to predict ozone
concentrations; however, these codes do not reflect the true uncertainty in
oxidative capacity that is relevant to other aspects of air quality
management, such as the formation of secondary organic aerosol (SOA). Our
analysis highlights that apart from uncertainties in emissions, and
meteorology, there is an additional major uncertainty in chemical mechanisms
that affects our ability to predict ozone and SOA formation with confidence. |
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