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10.5194/acpd-7-17769-2007
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http://www.atmos-chem-phys-discuss.net/7/17769/2007/acpd-7-17769-2007.pdf
17769
17824
2007-12-10
Representativeness and climatology of carbon monoxide and ozone at the global GAW station Mt. Kenya in equatorial Africa
S. Henne
stephan.henne@empa.ch
J. Klausen
W. Junkermann
J. M. Kariuki
J. O. Aseyo
B. Buchmann
Empa, Swiss Federal Laboratories for Materials Testing and Research, Dübendorf, Switzerland
Forschungszentrum Karlsruhe, Institut für Meteorologie und Klimaforschung IFU, Garmisch-Partenkirchen, Germany
Kenyan Meteorological Department, Nairobi, Kenya
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 programme aimed to close this gap with the installation of the
Mt. Kenya baseline station. Here, the first continuous measurements
of carbon monoxide (CO) and ozone O<sub>3</sub> at this site 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
O<sub>3</sub>-CO relationships. Despite its location in 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
were in general representative of the free troposphere, while
day-time measurements were influenced by atmospheric boundary layer
air. 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%). 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 and August and with minima in April and less pronounced in
November. O<sub>3</sub> showed a weaker annual cycle with a minimum in
November and a broad summer maximum. Inter-annual variations were
explained with variations in southern African biomass burning and in
transport patterns. The measurements at MKN were representative of
air masses with little photochemical activity as indicated by weak
O<sub>3</sub>-CO correlations, underlining the baseline character of
the site. Future extensions of the measurement programme will
provide better understanding of the atmospheric chemistry of this
globally important region.
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