Abstract
Abstract The meteorological conditions at the Mount Kenya (station identifier MKN) tropical Global Atmosphere Watch Programme station are described. Like other stations in mountainous terrain, the site experiences thermally induced wind systems that disturb free tropospheric conditions. Therefore, the adequacy of the site for long-term background atmospheric composition measurements needs to be evaluated. Meteorological parameters for the period June 2002–June 2006 were analyzed, focusing on the development of thermally induced wind systems and boundary layer influence. Filters based on the local wind and day–night differences in specific humidity were developed for selection of times representative of undisturbed free tropospheric conditions. In addition, the convective boundary layer depth was evaluated. Throughout the whole year the station is influenced by thermally induced wind systems and the atmospheric boundary layer. The filters distinguished between thermally and synoptically influenced days. Thermally influenced days (86%) dominated. However, maxima in specific humidity were also reached in the afternoon on synoptically influenced days and were attributed to mixing in the convective boundary layer. During nighttime, downslope wind dominated that carries undisturbed free tropospheric air masses. Nevertheless, during 24% of all nights the specific humidity was also elevated, possibly indicating the presence of residual layers. It is recommended that nighttime data only (2100–0400 UTC) be used for analysis of long-term trends of the free tropospheric background while the remaining data can be used to characterize composition and trends of the regional atmospheric boundary layer. Further exclusion of apparent pollution events and residual layer influence should be considered. With these constraints, the Mount Kenya Global Atmosphere Watch site is adequate for the study of trends and budgets of background atmospheric composition.
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