When it is dark over Ny-Ålesund, a bright green laser beam can be seen quite often, shooting from the AWIPEV observatory towards the clear sky. This beam belongs to a LIDAR system, runned by the Alfred-Wegener-Institute in Potsdam for more than 25 years. With this instrument scientists investigate the atmosphere and look for thin clouds and aerosols, tiny particles floating in the air.
Aerosols are indispensable for the formation of clouds. Every water droplet within a cloud has accumulated around a tiny grain of “dust”. Clouds would only form at extremely high humidities if these cores did not exist. Scientists are most interested in the influence of aerosols on the cloud formation in the arctic. Clouds partly reflect sunlight as well as the outgoing heat radiation of the earth. Like this they affect the radiation balance and can lead to a temperature change on the surface. Therefore aerosols are an important factor for the understanding of our climate.
Aerosols are manifold: They can consist of soot or sulfates from volcano eruptions, bush fires or industries. Also fine sand dust, blown by winds from deserts or sea salt staying in the air after sea spray gets evaporated manifest as aerosols. In addition even bacterias or pollen grains represent aerosols. All these particles have a life time of several days or weeks depending on their size and residence altitude. During their stay in the atmosphere, particles are carried by winds far from their origin place. Whereas most particles can be found in lower air layers up to 10 km high, other aerosols from volcano eruptions can raise far up into the stratosphere and stay for months. Such an event was recorded by AWIPEV in 2009 after a volcano in Kamtschatka (eastern russia) has released a lot af ashes.
The following video made by NASA shows the concentration and movement of aerosols in the atmosphere, simulated on the base of real weather and aerosol data. The variability of their abundance is impressive:
Although air is mostly very clear around the Arctic, winds can transport particles from middle latitudes towards north. This can lead to haze situations most common in spring. Forest fires in Canada caused such an arctic haze event in July 2015. Visibility had been drastically reduced and the far mountain tops had disappeared for two days almost completely. It is remarkable when air contains that much aerosols that sun rays are visible as above a recently cut cornfield on the mainland.
In the winter research with KARL is rather focussed on polar stratospheric clouds (PSC). Such noctiluscent clouds can form during polar night in altitudes between 15 km and 25 km when a huge polar vortex keeps air trapped around the poles. This way the upper air mass can stay without sunlight for weeks and might cool down below -78 °C. Then sulfuric acid, nitric acid or water ice can condensate and form clouds. This influences the ozone chemistry and can lead to serious ozone depletion.
The working principle of a lidar is quite simple: It is pretty much the same as a radar, but laser light is used instead of radio waves. Each measurement starts with a very short light pulse, less than 10 billionth of a second long. This pulse is shooting towards the clear sky with speed of light. When it hits some aerosols, a part of it will be backscattered. It travels back and a tiny fraction of it is captured by the small telescope next to the laser soon after. It is guided through optical fibre and detected by photo multiplier tubes. The lidar system does not only register the intensity of the backradiation, but also the time between pulse emission and detection. The time is proportional to the altitude of the particles. A complete altitude profile is captured by listening to the signal at the photo detector for a while after each single pulse. All data are transferred to the Alfred Wegener Institute in Potsdam via Internet, where physicist Dr. Christoph Ritter analyzes it in detail.
Some facts about KARL:
- The laser consumes a lot of energy: 30´000 Watt are used, as much as 500 60W light bulbes. Only 50 W are leaving the light source as radiation. All the rest is heat and has to be dissipated by the cooling system.
- 50 W does not sound much, but it is a lot for a pulsed laser: If power was measured only during the very short pulse, it would be one tenth of the power of an average nuclear power plant! A commercial laser pointer must have less than 0.001 W power. As a consequence our laser beam is so strong that one single pulse would destroy the eyes retina immediately if one was looking straight into it. Therefore laser safety goggles are compulsory.
- The laser beam has originally a diameter of 1 cm. It is widened up by a telescope to a diameter of 11 cm before it leaves the observatory. With this trick it widenes up less afterwards, stays longer in the narrow field of view of the telescope and measurements during daylight become possible.
- A part of the infrared laser light is converted and when it leaves the instrument the beam consists of three wavelengths (colors): Ultraviolet (355 nm), green (532 nm) and infrared (1064 nm). Only the green color is visible to our eyes. From the relation of the backscatter in all colors the size of the aerosols can be estimated.
- The laser light is polarized, i.e. only light waves oszillating in a certain direction are allowed. The polarization properties of the backscattered light can tell us something about the shape of the aerosols.
- Thanks to the Raman effect also the nitrogen molecules of the air backscatter some light. Thereby some of the light energy is lost and the wavelength changes. KARL also takes measurements in these wavelengths. As a result, some further corrections become possible.
- As the laser beam has to be parallel to the telescopes view, it is slightly tiltable.
- KARL emits 50 pulses per second. Therefore the beam looks continuous to our eyes.
The “heart” of a LIDAR system is the YAG-Laser. The bulbs of the flash lamps wear down and have to be replaced two or three times a year. This is done by experienced engineers of IMPRES in Bremen. It is a difficult task because the laser has to be readjusted in perfection.