NDSC Stratospheric Ozone-Temperature-Aerosol Lidar

The Jet Propulsion Laboratory (JPL) lidar system, which measures stratospheric profiles of ozone, temperature, and aerosols for the Network for the Detection of Stratospheric Change (NDSC), was installed at the Mauna Loa Observatory, Hawaii (MLO) in July 1993. Since then it has been making regular nighttime measurements of these parameters, averaging more than 100 nights per year. The incidence of cirrus clouds and high winds at the observatory are the primary limiting factors on the number of measurements obtained.

A brief description of the original lidar system was given in CMDL Summary Report No. 22. Some modifications have been carried out since the initial installation to increase the altitude range over which the profiles can be measured. The impetus for these changes was the increased need to obtain ozone measurements in the upper troposphere in addition to the stratospheric measurements. To enable the increased altitude range of these measurements, the field-of-view of the telescope was increased so that the laser and telescope would overlap at lower altitudes. These changes were carried out just prior to the Stratospheric Ozone Profile Intercomparison (MLO3) NDSC intercomparison campaign (see below) and it turned out the increased field-of-view caused a saturation in the receiver at low altitudes resulting in incorrect measurements in this region. Following MLO3, the detectors in the receiver were changed to correct this problem, and since the beginning of 1996 high-quality ozone profiles are routinely obtained over the altitude range from ~13-km to >55-km. Temperature profiles, from the combined Rayleigh and Raman returns, typically cover the altitude range from just below the tropopause to about the mesopause, ~15-km to >80-km. Enhanced strato-spheric aerosols from the eruption of Mt. Pinatubo are no longer observable in our lidar observations and the level of aerosols in the Junge layer of the lower stratosphere only just exceeds our detection limit indicating that this region has returned, at least, to the pre-Pinatubo levels. Since we were not operating at MLO prior to the Mt. Pinatubo eruption, we cannot comment on the suggestion that the aerosol loading is even lower than before except to say that the aerosol levels we are currently observing are very low.

During the winter of 1994-1995 the CMDL Dobson spectrophotometer indicated very low ozone column content dipping below 200 Dobson Units (DU) for the first time in the measurement record. The lidar profiles are routinely integrated to obtain stratospheric column amounts and these can be compared with the Dobson results. Also the profiles can be studied to determine where the losses are actually occurring. Changes in the integrated lidar profile for both daily measurements and for monthly mean column amounts agreed very well with the Dobson data. Compared to the winters of 1993-1994 and 1995-1996 the period of low ozone in 1994-1995 extended from about September through June. Inspection of the ozone profiles showed the maximum ozone reduction occurred at approximately 30-km altitude and extended from roughly 25-km to 35-km.

During August 1995 a formal NDSC intercomparison of ozone profiling instruments was carried out at MLO and refereed by a National Aeronautic and Space Administration-Goddard Space Flight Center (NASA-GSFC) scientist. The participating instruments included the JPL lidar, the Millitech-NASA/LaRC microwave radiometer, the NASA/GSFC mobile lidar, and CMDL electrochemical concentration cell (ECC) balloon sondes (launched from Hilo). Additionally, satellite data from the Stratospheric Aerosol and Gas Experiment (SAGE) II and the Upper Atmosphere Research Satellite (UARS) MLS were provided for intercomparison. The results from the campaign are still being evaluated and will be published sometime in the near future. Here, we just consider the preliminary conclusions for the JPL lidar. From 20-km to the maximum altitudes reported (i.e., up to 60-km) the JPL lidar results agreed very well with the consensus profile in this region. However, below 20-km the JPL lidar profile showed a positive deviation from the correct profile, increasing as the altitude decreased. As indicated earlier, this was determined to be caused by a saturation effect caused by increasing the field-of-view of the telescope. Following the campaign, this problem was thoroughly investigated and a new, different design, photomultiplier tube was installed in all of the receiver channels. The correct operation of the modified system was then verified in several informal intercomparisons with the GSFC lidar and the Millitech-NASA/LaRC microwave radiometer over the period up to the end of 1995. We are now confident that the ozone results from the JPL lidar are accurate over the range from ~13-km to >55-km.

An evaluation of the performance of the temperature measurement capabilities of the lidars is also underway and will compare the results obtained by JPL, GSFC, CMDL lidar systems, and the CMDL balloon sondes during MLO3.

Acknowledgments. The work described here was carried out by the Jet Propulsion Laboratory, California Institute of Technology, under an agreement with the National Aeronautics and Space Administration. We are grateful to the staff at MLO for continuing support of this program.

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