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| Titel |
Nighttime observation and chemistry of HOx in the Pearl River Delta and Beijing in summer 2006 |
| VerfasserIn |
K. D. Lu, F. Rohrer, F. Holland, H. Fuchs, T. Brauers, A. Oebel, R. Dlugi, M. Hu, X. Li, S. R. Lou, M. Shao, T. Zhu, A. Wahner, Y. H. Zhang, A. Hofzumahaus |
| 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 ; 14, no. 10 ; Nr. 14, no. 10 (2014-05-21), S.4979-4999 |
| Datensatznummer |
250118727
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| Publikation (Nr.) |
 copernicus.org/acp-14-4979-2014.pdf |
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| Zusammenfassung |
| Nighttime HOx chemistry was investigated in two ground-based field
campaigns (PRIDE-PRD2006 and CAREBEIJING2006) in summer 2006 in China by
comparison of measured and modeled concentration data of OH and HO2. The
measurement sites were located in a rural environment in the Pearl River
Delta (PRD) under urban influence and in a suburban area close to Beijing,
respectively. In both locations, significant nighttime concentrations of
radicals were observed under conditions with high total OH reactivities of
about 40–50 s−1 in PRD and 25 s−1 near Beijing. For OH, the
nocturnal concentrations were within the range of (0.5–3) × 106 cm−3, implying a significant nighttime
oxidation rate of pollutants on the order of several ppb per hour. The
measured nighttime concentration of HO2 was about
(0.2–5) × 108 cm−3, containing a significant,
model-estimated contribution from RO2 as an interference. A chemical box
model based on an established chemical mechanism is capable of reproducing
the measured nighttime values of the measured peroxy radicals and
$k_{\text{OH}}$, but underestimates in both field campaigns the observed OH
by about 1 order of magnitude. Sensitivity studies with the box model
demonstrate that the OH discrepancy between measured and modeled nighttime OH
can be resolved, if an additional ROx production process (about
1 ppb h−1) and additional recycling (RO2 → HO2 → OH) with an efficiency
equivalent to 1 ppb NO is assumed. The additional recycling mechanism
was also needed to reproduce the OH observations at the same locations during
daytime for conditions with NO mixing ratios below 1 ppb. This could
be an indication that the same missing process operates at day and night. In
principle, the required primary ROx source can be explained by
ozonolysis of terpenoids, which react faster with ozone than with OH in the
nighttime atmosphere. However, the amount of these highly reactive biogenic
volatile organic compounds (VOCs) would require a strong local source, for
which there is no direct evidence. A more likely explanation for an
additional ROx source is the vertical downward transport of
radical reservoir species in the stable nocturnal boundary layer. Using a
simplified one-dimensional two-box model, it can be shown that ground-based
NO emissions could generate a large vertical gradient causing a downward flux
of peroxy acetic nitrate (PAN) and peroxymethacryloyl nitrate (MPAN).
The downward transport and the following thermal decomposition of these
compounds can produce up to 0.3 ppb h−1 radicals in the
atmospheric layer near the ground. Although this rate is not sufficient to
explain the complete OH discrepancy, it indicates the potentially important
role of vertical transport in the lower nighttime atmosphere. |
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