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30 CHAPTER 2. ATMOSPHERE AND REMOTE SENSING 2.1.13 Third Generation DOAS Telescopes Jens Tschritter (Andre Merten, Ulrich Platt) Abstract To get high resolution trace gases measurements in the troposphere, A. Perner and U. Platt developed an active DOAS System in 1980. Technical improvements in fiber optics now make a redesign of the classic Active Long-Path system possible in order to use smaller telescopes with easier handling yet maintaining the same optical properties and light throughput. A: Axelssons telescope projected mirror surface transmittsurface Receive surface receive surface spectrograph light source retro B: 3rd Generation telescope receive fiber projected mirror surface for transmission and receive transmission fiber spectr. spectrograph light source Figure 2.14: Lightpath of Axelssons System (left) and the Third Generation Telescope (right) Background Optical Absorption Spectroscopy is a well known laboratorial analyzing instrument. Specially for trace gas measurements in the troposphere the Differential Optical Absorption Spectroscopy (DOAS) is well established. The classic experimental set up of a active DOAS system consisting of a transmitting telescope, which sends a light beam trough the atmosphere, a retro reflector, a receiving telescope, a controlling system and a spectrometer. The telescopes main mirror is used as transmitting mirror as well as receiving mirror. This coaxial merge of a transmitting and receiving telescope as shown in the Fig. 2.14 A was first described by Axelsson et al. [1990]. The difficult handling and the size, led to a decrease in the number of measurements with Axelsson-type long path systems in the last years. Therefore passive systems, which are using the sun as light source, are used more and more for tropospheric trace gas measurements. However, the active systems allow measurements at night and with spectral ranges different from the sunlight. Thus we decided to develop a smaller long path telescope with easier handling. Methods and results Using fibers to conduct light in the telescope allows to use a defusor plate as short cut system instead of a retro reflector to reduce lamp structures and to provide uniform illumination of the spectrographs field of view, Abstract which reduces the measurements residuum. Because we use no mirrors to conduct the light on our telescopes main mirror we have no shadows, so we can use the complete main mirror for sending and receiving and we gain more light intensity in the spectrograph. Motors controlled by the measurement program automatically optimize focus and direction of our light beam and help to adjust the optical set up. we experimentally verified that the intensity coupled into the fiber at the receiving end should not depend on the focal length of our telescopes main mirror. This result of our test-measurements is the base fact for constructing a smaller telescope with the same light throughput. Outlook/Future work If we adapt the f/# of our spectrometer to that of our telescope we would be able to construct a very small telescope. This Third Generation Telescope (fig. 2.14 B) which can be carried by one person allows measurements in remote areas. New software applications help handling the optical set up. Using light emitting diodes (LED) as light source could help to further reduce the costs. Thus, we can build a global measurement network for trace gases in the troposphere which helps us to understand the atmospheric chemistry. retro
2.1. TROPOSPHERIC RESEARCH GROUP 31 2.1.14 Testing a new Near-IR Sensor for Detection of Methane Markus Woyde , Christian Frankenberg, Thomas Wagner, Ulrich Platt Abstract A new spectrometer with InGaAs linear image sensor was tested with respect to its applicability for detecting methane in the near infrared region. Measurements of CH4 cuvettes were conducted to test its feasibility as well as atmospheric observations using a hill as reflective surface for sunlight providing a new observation mode. log(I/I 0 ) expected transmission −0.1 −0.15 −0.2 −0.25 −0.3 1635 1640 1645 wavelength [nm] 1650 1655 1660 1 0.8 0.6 0.4 0.2 fitted measured Unconvolved expected transmission 0 1635 1640 1645 wavelength [nm] 1650 1655 1660 Figure 2.15: top panel: Displayed is the optical density of a cuvette filled with methane; measured data is shown in green, IMAP-DOAS fit in blue. bottom: Unconvolved expected transmision resolved in a wavelength-grid corresponding to pixels on the sensor. Background Methane is, after carbon dioxide, the second most important anthropogenic greenhouse gas, contributing directly 0.48 Wm −2 to the total anthropogenic radiative forcing of 2.43 Wm −2 by well-mixed greenhouse gases (IPCC, 2001). Its individual source strengths need to be quantified more precisely. There have already been successful satellite-based CH4 measurements by SCIAMACHY onboard ENVISAT using the DOAS-technique providing a global coverage but with a limited temporal and spatial resolution. Therefore it is desirable to compensate for these limitations by developing a ground-based instrument. Funding IUP Methods and results The sensor properties such as dark current, offset and linearity were characterized. After that measurements of CH4 cuvettes were performed, covering a spectral range from about 1600nm to 1670nm (see figure 2.15).The concentration was analyzed by means of IMAP-DOAS developed by C. Frankenberg. As one can see there is already a good agreement between data and fit. For better results a more detailed investigation of the slit-function is nescessary. Further, the sensor was tested in Heidelberg using a new observation mode. A hill at about 2.1km was used as reflective area for sunlight, the reference spectrum was obtained by observing sunlight directly reflected into the spectrograph by an aluminium plate, so that it is possible to determine the CH4 concentration along the way between spectrograph and hill. Outlook/Future work Future work will encompass improving the instrument by a better temperature stabilization and preparing it for field campaigns. This would enable us to examine the differences between modeled data and measured data by SCIAMACHY. Furthermore an interesting application might be searching for leakages in gas pipelines or measuring water vapor in the atmosphere. Main publications ?
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