GSFC Code 916:Atmospheric Chemistry and Dynamics Branch [menu bar image map]

AROTEL Instrumentation



The Airborne Raman Ozone, Temperature, and Aerosol Lidar (AROTEL) was designed and built at Goddard Space Flight Center in collaboration with Dr. Chris Hostetler of Langley Research Center. The instrument was funded by the NASA Upper Atmosphere Research Program, within the Earth Science Enterprise, for deployment on board the NASA DC-8 during the SAGE III Ozone Loss Validation Experiment (SOLVE). This mission was flown from Kiruna, Sweden, during the winter of 1999 - 2000. Because of the fact that the SAGE III instrument was not launched, the mission focused mainly on the processes involved in polar ozone loss during the Arctic Spring. The AROTEL instrument is particularly well-suited to this mission.

AROTEL has four primary data products:

So, with a single instrument, we are able to measure ozone (and thus directly measure any ozone loss), the atmospheric temperature in regions where polar stratospheric clouds form; and the clouds themselves, as well as any depolarization caused by irregular particles within the clouds. As mentioned above these parameters are measured at altitudes above the aircraft flying at about 12 km.

As with any lidar instrument, AROTEL is made up of three major components: the transmitter, the receiver and the data acquisition system. The transmitter in the AROTEL instrument comprises two different lasers: a XeCl excimer laser, transmitting 308 nm; and a Nd-YAG laser transmitting at 1064, 532, and 355 nm.

The primary receiver is a 16" Newtonian telescope, with a variable iris to define the field-of-view; typically the instrument was operated at 2.0 mRad. The visible and infra-red radiation is split from the UV wavelengths, and then further separated according to wavelength. In addition, the 532 nm radiation which is polarized 90° to the transmitted 532 radiation is also collected. There are separate channels for near-field and far-field returns, and data is recorded in an analog mode.

On the UV side, returns are separated using dichroic beamsplitters into four wavelengths: the two transmitted wavelengths at 308 and 355 nm, as well as the Raman scattered return from atmospheric N2 at 332 and 387 nm. Because of the intense signals in the near field, each of these returns is split into detectors of varying sensitivity, so that linear returns can be obtained.

Ozone is retrieved using a technique called Differential Absorption Lidar (DIAL). In this technique, two wavelengths are used the deduce ozone. One, at 308 nm, is absorbed by ozone, and the other, at 355 is not, thereby providing an atmospheric reference. Ozone can then be deduced from the difference between the slopes of the two return signals. When there are significant amounts of aerosols present, the Raman wavelengths can be used in this same way to retrieve ozone. Raman scattering from nitrogen does not contain any aerosol scattering signature and so errors introduced from aerosol scattering when the transmitted wavelengths are used to retrieve ozone are greatly diminished.

Atmospheric temperature is retrieved from the 355 and 387 nm returns. A relative measure of the atmospheric density can be constructed using these returns, normalizing at a point with a known density - either from a sonde or the NCEP data. We use the NCEP data at the 10 mbar level for this normalization. The temperature profile is computed from the relative density profile with the assumption that the atmosphere is in hydrostatic equilibrium, i.e. there is negligible vertical motion in the atmosphere. The temperature profile is then initialized to a model temperature at an altitude where the error in the density profile reaches 5%. For the AROTEL instrument in nighttime conditions, this is about 60 km. Using the ideal gas law and the hydrostatic equation, temperature below this initialization height can be computed. After about two scale heights the retrieval is no longer sensitive to the initialization temperature. In this manner, a temperature profile can be constructed nearly down to the aircraft. Since Raman scattering is a purely molecular process, the Raman return at 387 nm can be used to extract temperature when there are thin clouds or aerosols present.

The aerosol backscatter ratio can be computed using the ratio of the 355 nm return to the 387 nm return. When aerosols are present, the return at 355 includes both Rayleigh scattering (molecules) and Mie scattering (aerosols), whereas the Raman scattered return is due purely to molecular scattering. From this data it is also possible to compute the extinction due to the cloud.

More information on all these measurements can be found in the following publications and the references contained therein. Algorithms for each of the three data products discussed above were adapted from those discussed in the references. 


      "STROZ LITE:  NASA Goddard Stratospheric Ozone Lidar Trailer Experiment",
      T. J. McGee, D. Whiteman, R. Ferrare, J. J. Butler, and J. F. Burris,
      Optical Engineering, 30, 31-39, 1991.

      "Raman DIAL Measurements of Stratospheric Ozone in the Presence of Volcanic
      Aerosols," T. J. McGee, M. Gross, R. Ferrare, W. S. Heaps, and U. Singh,
      Geophys. Res. Lett. , 20, 955-958, 1993.

      "An Improved Stratospheric Ozone Lidar", T. J. McGee, M. Gross, U.N. Singh,
      J. J. Butler, and P. Kimvilakani, Opt. Engin., 34, 1421-1430, 1995

      "Measurements of Stratospheric Aerosols with a Combined Rayleigh/Raman Lidar",
      M. Gross, T. J. McGee, U. N. Singh, and P. Kimvilakani , Appl. Opt., 34,
      6915-6924, 1995.

      "Temperature Measurements Made with a Combined Rayleigh-Mie/Raman Lidar,"
      M. R. Gross, T. J. McGee, R. A. Ferrare, U. Singh, and P. Kimvilikani,
      Applied Optics, 24, 5987-5995, 1997.

      "Lidar Temperature Measurements During the TOTE/VOTE Mission," J. Burris,
      W. Heaps, B. Gary, W. Hoegy, L. Lait, T. McGee, M. Gross, U. Singh,
      J. Geophys. Res., 103, 3505-3510, 1998.

Images of AROTEL (under construction)


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Last Updated: 6/01/2001
Web Curator: Grant Sumnicht (Science Systems and Applications, Inc.) (sumnicht@code916.gsfc.nasa.gov)
Responsible NASA organization/officials: Dr. John Burris and Dr. Thomas McGee, Atmospheric Chemistry and Dynamics Branch