Abstract
Abstract. 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.
Highlights
Carbon monoxide (CO) and ozone (O3) are two key species in the photochemical system of the troposphere
We focus on the question of representativeness of the station in terms of vertical atmospheric layers and horizontal regions of influence, to discuss the station’s suitability to serve as a baseline site for eastern tropical Africa
The local environment of the Mt. Kenya (MKN) site is mostly free of anthropogenic emissions
Summary
Carbon monoxide (CO) and ozone (O3) are two key species in the photochemical system of the troposphere. Carbon monoxide controls the concentrations and distributions of atmospheric oxidants, such as O3, hydroperoxy (HO2) and hydroxyl (OH) radicals. About 70% of OH radicals in the background atmosphere react with CO (Crutzen and Andreae, 1990). The concentration of CO largely determines the availability of OH radicals. An increase in CO is expected to decrease OH and to increase the lifetime and abundance of these gases. CO itself has a lifetime of weeks to months and is a useful tracer for combustion of biomass and fossil fuels CO is emitted from fossil fuel and biomass burning at low temperatures and low oxygen conditions.
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