In July 2002, A new DOAS instrument was installed on the UNEP building in Nairobi (1°S, 36.5°E), Kenya. Nairobi is a tropical high altitude (1798 m) site that is strongly affected by local urban pollution (Nairobi has more than 2.5 million inhabitants) and also by transport of air masses from biomass burning regions.
Unlike the meteorological station at nearby Mount Kenya, this site is not ideal for stratospheric measurements, but rather has a focus on tropospheric pollution in tropical regions.
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The DOAS instrument in Nairobi is located in a temperature controlled office room inside the UNEP building. It consists of a temperature stabilized grating spectrometer equipped with a cooled CCD detector. The instrument is connected to a telescope which is located outside the building with a quartz fibre bundle and a number of electrical and control connections. The telescope has two viewing ports: One in zenith direction, the other one towards the horizon in the south (off-axis). In the off-axis direction, the horizon can be observed sequentially in the range of 0° to 30° elevation by means of a motorized mirror. Both line-lamp and white light calibration lamps are integrated in the telescope box and are used for daily calibration measurements. In January 2004, a second spectrometer covering the visible part of the spectrum was added to the instrument.
Due to hardware problems and frequent power outages, the Nairobi data set has many gaps.
The instrument is fully automated and controlled remotely over an internet connection to Bremen. More on the instrument and measurement principle can be found on our MAXDOAS page.
In the figure, an example of the measurements in Nairobi is shown. Daily averages of tropospheric NO2 columns, averaged over months have been computed for several years using the geometric approximation in combination with a criterion requiring similar results for the 30° and 15° directions. The results show a clear seasonality of the tropospheric NO2 columns with low values in winter and high values in summer. As can be seen from the error bars which give the standard deviation of all data within a month, NO2 variability is also much larger in summer.
If you have any requests, please contact Andreas Richter.
If you are interested in more information or would like to have access to our data, please contact Andreas Richter.