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| Titel |
Representativeness and climatology of carbon monoxide and ozone at the global GAW station Mt. Kenya in equatorial Africa |
| VerfasserIn |
S. Henne, J. Klausen, W. Junkermann, J. M. Kariuki, J. O. Aseyo, B. Buchmann |
| 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 ; 8, no. 12 ; Nr. 8, no. 12 (2008-06-23), S.3119-3139 |
| Datensatznummer |
250006221
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| Publikation (Nr.) |
 copernicus.org/acp-8-3119-2008.pdf |
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| Zusammenfassung |
| The tropics strongly influence the global atmospheric chemistry
budget. However, continuous in-situ observations of trace gases are
rare especially in equatorial Africa. The WMO Global Atmosphere
Watch (GAW) programme aimed to close this gap with the installation
of the Mt. Kenya (MKN) baseline station. Here, the first continuous
measurements of carbon monoxide (CO) and ozone (O3) at this
site covering the period June 2002 to June 2006 are presented. The
representativeness of the site was investigated by means of
statistical data analysis, air mass trajectory clustering,
interpretation of biomass burning variability and evaluation of
O3-CO relationships. Because of its location in eastern
equatorial Africa, the site was rarely directly influenced by
biomass burning emissions, making it suitable for background
observations. Located at 3678 m above sea level the night-time
(21:00–04:00 UTC) measurements of CO and O3 were in general
representative of the free troposphere, while day-time measurements
were influenced by atmospheric boundary layer air. Increased
night-time concentrations were observed in 25% of all nights and
associated with residual layers of increased CO and water vapour in
the free troposphere. Six representative flow regimes towards
Mt. Kenya were determined: eastern Africa (21% of the time),
Arabian Peninsula and Pakistan (16%), northern Africa free
tropospheric (6%), northern Indian Ocean and India (17%),
south-eastern Africa (18%) and southern India Ocean (21%) flow
regimes. The seasonal alternation of these flow regimes was
determined by the monsoon circulation and caused a distinct
semi-annual cycle of CO with maxima during February (primary) and
August (secondary, annually variable) and with minima in April
(primary) and November (secondary, annually variable). O3
showed a weaker annual cycle with a minimum in November and a broad
summer maximum. Inter-annual variations were explained with
differences in southern African biomass burning and transport
towards MKN. Although biomass burning had little direct influence on
the measurements at MKN it introduces inter-annual variability in
the background concentrations of the southern hemisphere that
subsequently reaches Kenya. The measurements at MKN were
representative of air masses with little photochemical activity as
indicated by weak O3-CO correlations, underlining the
baseline character of the site. Inter-comparison of O3 at
MKN with sounding data from Nairobi revealed a positive offset of
the sounding data, most likely due to additional photochemical
production of O3 in the Nairobi city plume. Future
extensions of the measurement programme will provide better
understanding of the atmospheric chemistry of this globally
important region. |
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