Climate research: excimer laser eyes the stratosphere

To investigate long-term temperature changes, climate researchers on Germany's highest mountain Zugspitze can for the first time measure the distribution of the most important greenhouse gas in climate-relevant air layers using a high-performance laser system.

Powerful global networks of lidar systems for remote sensing of natural and man-made atmospheric trace gases in our atmosphere are becoming increasingly important against a background of global warming. A new generation of lidar systems is the prerequisite for predicting the climatic influence of greenhouse gases and their transport processes more accurately in the future by creating meaningful concentration and temperature profiles over a wide range of altitudes. The Raman lidar method is used to explore the important greenhouse gas water vapor. A UV-laser pulse is emitted into the atmosphere and its resulting backscattering signal, which is influenced by the water molecules, is captured by a collecting mirror. The signal is measured in a time-resolved manner, so that the height from which the signal originates can be determined. The Raman scattering intensity decreases strongly with increasing height, which is why laser technologies delivering considerably lower UV output which have been in use up to now could only eye the troposphere, i.e. the atmospheric layer which merely determines our weather.

Climate research benefits from optical high technology

With the aim to extend the height profile of the water vapor concentration for the first time into the stratosphere and thus to investigate its possible influence on global warming, the scientists of the environmental research station ‘Schneefernerhaus’ on the ‘Zugspitze’ mountain in the German Alps opted for a particularly powerful 350-W UV-excimer laser from Coherent. They modified the excimer laser to generate linear-polarized UV pulses with small line width and reduced beam divergence. The remaining narrow-band laser emission with an average power of 180 W is still 10 times that of a powerful UV-Nd:YAG laser system. In combination with four times larger collection optics, a 40 times improved signal-to-noise ratio could be achieved compared to the Raman lidar systems available so far. This means that, for the first time, the greenhouse gas water vapor can now be detected more accurately, quantitatively and by a factor of 10 faster and further into the atmosphere than ever before, namely up to a height of over 22 km.

The unique Zugspitze lidar thus makes an important contribution to climate research. Additional high-performance lidar stations in other parts of the world are needed to understand possible transport processes of water vapor into the stratosphere and the resulting temperature-increasing feedback mechanisms on a global scale.

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