Jonas Lähnemann
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Research Internship on Differential Optical Absorption Spectroscopy (January-July 2005)

During my stay in Nairobi I was doing a project (for the students here it's a requirement for the bachelor, for me more of a research internship) on Differential Optical Absorption Spectroscopy (DOAS). Using this technique one can determine trace gas amounts in vertical air columns above a location using the spectrum of scattered sunlight. The absorption of UV and visible sunlight through different species allows to calculate their amount in the atmosphere out of these spectra. The University of Bremen operates such an instrument at the UNEP (UN Environmental Program) in Nairobi.

You can read the Abstract and Introduction of the report below or download the whole report as PDF-document:
Project Report: Validation of Satellite Ozone Measurements Over Nairobi Using MAX-DOAS 2003-2004 (2 MB)
Presentation: 10 slides for a 15 minutes presentation of my DOAS project at the University of Nairobi Physics Department (1.3 MB)
Poster: Poster about my DOAS project for the Physics Department at the University of Nairobi (2.2 MB)


This work presents a study of total column ozone from ground-based DOAS measurements in Nairobi and overpass data from satellite instruments (TOMS, GOME and SCIAMACHY) for the years 2003 and 2004. The long-term record of TOMS shows no depletion of total column ozone above this tropical location with an average concentration of 260 DU, besides annual and biennial oscillations between 40-60 DU. From the comparison of the DOAS record with the data from TOMS, GOME and SCIAMACHY, as well as the ozonesondes, a very good agreement in trend was concluded for all instruments, as the root-mean-square values of the deviations from DOAS are all below 3.5% and the mean deviations are even less (< 1.5%). Correlation between the different data sets is also very good for most of the time observed, with a deviation < 5%. It can be concluded that the Bremian algorithm for the analysis of SCIAMACHY data yields good results for total column ozone. After comparing the 2003-2004 period, where the lowest ozone values were recorded for the first and last months, with the long-term TOMS record, it was infered that this work covers exactly one cycle of the Quasi-Biennial Oscillations (QBO). A special feature of the MAX-DOAS instrument are off-axis viewing directions close to the horizon, showing stronger signals of tropospheric absorbers than zenith-sky observations. Ozone slant columns of different viewing directions were, however, found to be so close to each other that no information about tropospheric concentrations can be retrieved for ozone. For the verification of satellite measurements and to obtain a long-term record of different trace gases for a tropical location, every effort should be made not only to maintain operation of the MAX-DOAS instrument in Nairobi, but also to continuously evaluate the retrieved data.


Ozone (O3) is an important but toxic molecular species in the troposhere. In the stratosphere ozone absorbs most of the ultra-violet (UV) radiation, preventing this potentially harmful radiation from reaching the earth's surface and accounting for the temperature increase in this atmospheric layer and thus for the stability of the atmosphere. Ozone is formed and destroyed in complex systems of atmospheric chemical reactions, catalyzed by various other trace gases.

With increased industrialization, rising emissions of pollutant gases have led to a rise in ozone levels close to the ground. This can be part of the so-called smog. Pollution is also responsible for a decline in the stratospheric ozone concentration, especially in the antarctic region, there known as ozone hole. Currently these are still very pressing environmental issues.

Different techniques have been developed and utilized to monitor the Ozone concentrations in the atmosphere over the years. Among them are passive spectroscopic systems using sunlight; these have the advantage that they can yield total columns of atmospheric trace gases and to a certain extent information about the vertical distribution of these trace gases. One of the passive spectroscopic methods is the Differential Optical Absorption Spectroscopy (DOAS) [Perner and Platt, 1979]. In the DOAS technique the absorption profiles in the observed sun-spectra are used to calculate the concentrations of the absorbants. Besides ground based spectroscopical systems, giving information on the diurnal variation of the observed species, these methods can be deployed in aircraft and on satellites, the latter allowing for global coverage. Other techniques for monitoring ozone include chemical in situ measurements and LIDAR.

Spectroscopic measurements of atmospheric ozone have been made since the 1930s (e.g. Dobson Spectrometers [Dobson, 1968]). In the 70s the DOAS method was developed. A current development is the implementation of MAX-DOAS (Multi-Axis DOAS) in which measurements at different zenith angles with varying lightpaths in the troposphere are used to extract limited information on the vertical distribution of ozone [Wittrock et al., 2004].

Since August 2002 the University of Bremen has been operating a MAX-DOAS system at the UNEP campus in Gigiri, Nairobi. This instrument can be used to measure the columns of atmospheric trace gases, such as O3, NOx, BrO or OClO. It is part of a Bremen based network with stations in different latitudes offering possibilities for the validation of satellite based measurements of trace gases. Such measurements are for example carried out with the EP/TOMS, GOME or SCIAMACHY instruments. The Nairobi DOAS station, in continuously monitoring the atmosphere, also fills a gap of measurements in a tropical, especially urban, location. Though its main focus are total columns of trace gases, with the off-axis measurements being directed towards the city center of Nairobi, the measurements also yield some information about the smog situation in Nairobi [Wittrock et al, 2004; Adupko, 2002].

To retrieve a more detailed profile of the vertical distribution of ozone, balloon launches are being carried out in many places in the world. As measurements for the southern hemisphere, especially for the tropical regions, had been lacking, NASA's Goddard Space Flight Center (GSFC) introduced the Southern Hemisphere Additional Ozonesondes (SHADOZ) program in 1998. Nairobi, with weekly sonde launches, is one of the stations supported. [Thompson et al., 2004]

This work is aimed at comparing data from the ground-based MAX-DOAS station in Nairobi and satellite instruments (TOMS, GOME and SCIAMACHY) for SCIAMACHY validation purposes, as well as looking at the relating SHADOZ sonde data. The time period considered are the years 2003 and 2004. The focus will be on total column ozone, as this is easily retrieved from the remote sensing instruments and offers a way to compare the different data sets. With the help of the TOMS measurements, which have been going on since 1979, the time under investigation can be placed in a long-term perspective. Total columns of ozone mainly yield information about stratospheric concentrations, as 90% of atmospheric ozone reside here. With more complex algorithms it is possible to calculate tropospheric columns from the ground-based and satellite instruments: It therefore shall be determined if the multi-axis setup of the Nairobi DOAS allows the separation of stratospheric and tropospheric absorption by ozone. The sonde data, on the other hand, readily provides vertical profile information. The DOAS is also envisaged to yield information on the diurnal variation of ozone columns.

The work is structured in the following way: An overview of the ozone chemistry in the earth's atmosphere is presented in the second chapter. Besides summarizing the processes governing tropospheric and stratospheric ozone concentrations and their impact, issues special to the tropical region are considered. Then an introduction to the experimental setup of the Multi-Axis DOAS (MAX-DOAS) instrument and to the DOAS technique is given in the third chapter, followed by information on different satellite based systems (TOMS, GOME and SCIAMACHY) and ozonesonde soundings. The fourth chapter is dedicated to the data analysis and comparison. Finally the results are summarized in the fifth chapter.

University of Bremen DOAS group (with lots of links to papers)
University of Heidelberg DOAS group
Nairobi Ozonesondes

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