Instruments

Mobile stratospheric ozone lidar (STROZ)

General

Instrument Description

The GSFC Stratospheric Ozone (STROZ) Lidar is a combined Rayleigh-Mie, vibrational Raman backscatter, and differential absorption lidar, with the instrumentation installed in a mobile trailer. Two wavelengths are transmitted from two different lasers: 308 nm is transmitted from a XeCl excimer laser, and 355 nm is transmitted from a Nd-YAG laser This YAG laser operates at a 50 Hz pulse rate, and is rated at 17.5 W at 355 nm, while the excimer laser is rated at 70 W at a 200 Hz pulse rate. Prior to transmission into the atmosphere, the two wavelengths are expanded through a pair of 5X refractive telescopes and then transmitted co-linearly approximately 1 m from the center of the 76 cm receiver telescope. Five backscattered wavelengths are detected: 308, 332, 355, 387 and 407 nm; all are recorded using fast photon counting techniques. The 332 and 387 wavelengths are Raman scattered from N2, and 407 is Raman scattered from H2O. Spectral separation is accomplished using beamsplitters and interference filters. Because of signal dynamic range issues, the signals at each wavelength are power split into as many as three individual detectors. In addition, a 10 cm telescope provides near-field return signals at 355, 378 and 408 nm. In total the lidar retrieves data in 11 photon counting channels to retrieve the archived data products.

Measurement Technique

Ozone is retrieved using the fact that O3 strongly absorbs radiation at 308 nm, and is absorbed much less so at 355 nm. The difference in the backscattered return at these two wavelengths therefore contains vertical profile information about the concentration of ozone in the atmosphere. In regions where aerosols become important, e.g. after an intense volcanic eruption, the elastic returns at 308 and 355 nm contain substantial scattering from these aerosols. The unknown wavelength dependence of scattering from these aerosols at different wavelengths renders impossible the retrieval of ozone within these regions of high aerosol loading. Since Raman scattering is a purely molecular process, aerosol backscatter is eliminated; there is however an smaller level of aerosol extinction which is significantly smaller than the backscattered component, and at very high loadings, even this differential Raman scattering technique is subject to large errors.

Temperature is retrieved using three different techniques. Stratospheric temperature above 28-30 km is retrieved from the backscattered radiation at 355 nm. Above this altitude aerosols are considered to be negligible, and so the lidar return is a function of atmospheric density and well known geometric parameters. From this data a relative density profile can be formed and from that an absolute temperature can be extracted. Below 28-30 km, aerosols become more important and prevent accurate temperature retrieval from the elastically backscattered radiation. Between 10-12 and 28-30 km, the Raman backscattered radiation at 387 is used. This signal is a purely molecular signal and minimizes the affects of aerosol scattering. This Raman return can be normalized to the elastic return at 355 nm to generate the lower altitude atmospheric density profile.

Vertical profiles of aerosol backscatter and microphysical properties can be retrieved from the ratio of the elastic backscatter and the Raman backscattered returns. The elastic signal contains both molecular and aerosol backscatter signals, while the Raman return contains only molecular scattering. The Raman signal can be normalized to the elastic signal in a region where aerosols are negligible and the ratio is equal to one.

Water vapour is a recent addition to the vertical profiles retrieved by this instrument. This is achieved from the ratio of backscattered return signals at 407 nm (Raman scattered from H2O), and 387 (Raman scattered from N2). Since 407 represents the concentration of water vapour in the atmosphere, and the 387 nm return is representative of the atmospheric density as a whole, the ratio of these two signals can be calibrated to yield the mixing ratio of water vapour in the atmosphere. This measurement is a new one within the NDACC and is not yet archived.

The primary purpose of the STROZ Lidar within the NDACC is to be a mobile calibration transfer standard among NDACC sites which are capable of receiving the 40 ft container. This lidar was a primary participant in the first such NDACC (then NDSC) intercomparison at Table Mountain in July, 1989. This intercomparison provided much of the input into what has become the NDACC Validation Protocol. Since that time the STROZ Lidar has participated in numerous campaigns at Ny Aalesund, Svalbard; Sodankyla, Finland; Hohenpeissenberg, Germany; Observatoire de Haute Provence, France; Greenbelt, MD, USA; Cannon AFB, USA; Table Mountain, CA; Mauna Loa, Hawaii; and Lauder, New Zealand.

Data Products

The table below provides an overview of the products that can be obtained with AT and their corresponding altitude ranges:

• Stratospheric temperature profile: ~ 12 km to above 75 km
• Stratospheric aerosol backscatter ratios at 355 nm: ~ 10 km to 30 km
• Tropospheric aerosol backscatter ratios at 355 nm: ~ 1 km above site to 15 km
• Tropospheric water vapour mixing ratio: ~ 1 km above site to 14 km

References

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