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| Titel |
Oxidative capacity of the Mexico City atmosphere – Part 1: A radical source perspective |
| VerfasserIn |
R. Volkamer, P. Sheehy, 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.6969-6991 |
| Datensatznummer |
250008665
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| Publikation (Nr.) |
 copernicus.org/acp-10-6969-2010.pdf |
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| Zusammenfassung |
A detailed analysis of OH, HO2 and RO2 radical sources is
presented for the near field photochemical regime inside the Mexico City
Metropolitan Area (MCMA). During spring of 2003 (MCMA-2003 field campaign)
an extensive set of measurements was collected to quantify time-resolved
ROx (sum of OH, HO2, RO2) radical production rates from day-
and nighttime radical sources. The Master Chemical Mechanism (MCMv3.1) was
constrained by measurements of (1) concentration time-profiles of
photosensitive radical precursors, i.e., nitrous acid (HONO), formaldehyde
(HCHO), ozone (O3), glyoxal (CHOCHO), and other oxygenated volatile
organic compounds (OVOCs); (2) respective photolysis-frequencies (J-values);
(3) concentration time-profiles of alkanes, alkenes, and aromatic VOCs (103
compound are treated) and oxidants, i.e., OH- and NO3 radicals,
O3; and (4) NO, NO2, meteorological and other parameters. The
ROx production rate was calculated directly from these observations;
the MCM was used to estimate further ROx production from unconstrained
sources, and express overall ROx production as OH-equivalents (i.e.,
taking into account the propagation efficiencies of RO2 and HO2
radicals into OH radicals).
Daytime radical production is found to be about 10–25 times higher than at
night; it does not track the abundance of sunlight. 12-h average daytime
contributions of individual sources are: Oxygenated VOC other than HCHO about
33%; HCHO and O3 photolysis each about 20%; O3/alkene
reactions and HONO photolysis each about 12%, other sources <3%.
Nitryl chloride photolysis could potentially contribute ~15%
additional radicals, while NO2* + water makes – if any – a very small
contribution (~2%). The peak radical production of ~7.5
107 molec cm−3 s−1 is found already at 10:00 a.m., i.e., more than
2.5 h before solar noon. O3/alkene reactions are indirectly responsible
for ~33% of these radicals. Our measurements and analysis comprise a
database that enables testing of the representation of radical sources and
radical chain reactions in photochemical models.
Since the photochemical processing of pollutants in the MCMA is radical
limited, our analysis identifies the drivers for ozone and SOA formation. We
conclude that reductions in VOC emissions provide an efficient opportunity
to reduce peak concentrations of these secondary pollutants, because (1)
about 70% of radical production is linked to VOC precursors; (2) lowering
the VOC/NOx ratio has the further benefit of reducing the radical
re-cycling efficiency from radical chain reactions (chemical amplification
of radical sources); (3) a positive feedback is identified: lowering the
rate of radical production from organic precursors also reduces that from
inorganic precursors, like ozone, as pollution export from the MCMA caps the
amount of ozone that accumulates at a lower rate inside the MCMA. Continued
VOC reductions will in the future result in decreasing peak concentrations
of ozone and SOA in the MCMA. |
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