Siberian Lidar Station

Siberian Lidar Station (SLS) combines lidar and spectrophotometric systems for atmospheric sensing. It has been developed in the Center of Laser Atmosphere Sensing. SLS is situated in Tomsk (56.5° N, 85.0° E). A lidar complex comprising a principal receiving telescope with a mirror 2.2-m in diameter and several receiving mirrors with smaller apertures is capable of simultaneons measuring aerosol, ozone and temperature in the troposphere and the stratosphere at altitudes higher than 50 km. Spectrophotometric measuring devices are used for monitoring of gaseous constituents of the ozone cycles. The data of many-year integrated observations are used for developing empirical regional models of the atmospheric parameters and elucidating the mechanisms that determine interconnections and dynamics of the measured parameters, in particular, estimating the contribution of photochemical and dynamic atmospheric factors to anomalous ozone layer variations. Feasibility of simultaneous integrated measurements of the most important climatic and ecological atmospheric parameters is a salient feature of investigations carried out at the SLS. It is provided by application of various methods of laser sensing of the atmosphere combined with spectrophotometric measurements.

In 1995 the Russian Ministry of Science issued the directive that the SLS was included in the List of Unique Research and Experimental Systems of National Significance (its registration number is 01-64).

Regular lidar observations of the stratospheric aerosol were started at the SLS with a lidar having a receiving mirror 1 m in diameter in 1986. Ozone measurements with this lidar were started in 1989. In 1991, measurements with a lidar having a receiving mirror 2.2 m in diameter were organized. This lidar was used for multifrequency sensing of the optical and microphysical characteristics of the stratospheric aerosol and for temperature sensing based on molecular light scattering in the stratosphere. Now the SLS comprises a lidar complex with receiving mirrors 2.2, 1, 0.5, 0.3 m in diameters and a set of laser sources generating in the wavelength range 271-1064 nm. This allows us to measure the aerosol, ozone and temperature in different altitude ranges of the troposphere and stratosphere (in case of temperature, at altitudes up to 70 km) by different sensing methods. In addition, to study mechanisms of variability of the stratospheric ozone, regular spectrophotometric measurements of the total content of ozone as well as of the total content and vertical distribution of NO2 are carried out at the SLS.

Regular sensing of the optical aerosol characteristics is carried out at a wavelenght of 532 nm. To determine the degree of asphericity of aerosol particles, the depolarization ratio of a lidar return signal is measured. Multifrequency sensing of the microphysical aerosol characteristics (of the particle size spectrum) is carried out occasionally during separate missions It is carried out at wavelengths of 353, 511, 532, 628, 683 and 1064 nm. When sensing is carried out at 683 nm, which is the first Stokes order of the 532-nm Nd:YAG laser light for stimulated Raman shifting in hydrogen, a Raman cell with hydrogen is placed upon exiting the 532-nm laser beam.The laser sources have unstable laser cavities providing the angular beam divergence no more then 0.3 mrad and can be used without additional collimation.

Lidar measurements of geometrical and optical characteristics of clouds, including sensing of high clouds (cirrus clouds) through low cloudiness are carried out at a wavelength of 1064 nm at night and in the daytime.

Some peculiarities of application of metal vapor lasers (a copper vapor laser at 511 nm and a gold vapor laser at 628 nm) to sensing of the tropospheric aerosol should be mentioned. In contrast with high-power solid-state and excimer lasers, the energy of metal vapor lasers is lower by 2-3 orders of magnitude, which eliminates completely or reduces markedly the effects of photomultiplier saturation and lidar signal distortion caused by high-power laser radiation coming from the near sensing range. Sufficiently high average radiation power of the metal vapor laser is provided by high pulse repetition frequency of laser pulses (several kHz), resulting also in small time of signal accumulation (several minutes) in the photon counting regime. We also take these advantages in sensing of the tropospheric ozone at the DIAL wavelengths 271/289 nm (with nonlinear copper vapor laser frequency conversion in b-BBO crystals.

The multichannel regime of operation of the lidar with the receiving mirror 2.2 m in diameter is realized by tilting of sounding beams at small angles (30') to the vertical. Lidar return signals are recorded in the focal plane or optical signals are transmitted through a light guide, optically glued to a phocon, to cells of spectral selection with PMT.

Spectrophotometric measurements of the total ozone content are carried out with the M-124 ozonometer. This ozonometer is the calibrated reference device of the State Meteorological Network intended for evaluation of the total ozone content in the Earth's atmosphere from the measured directly transmitted or scattered solar radiation. The data are sampled every hour for the permissible zenith distances of the sun. The measurement error does not exceed 8%.

A twilight spectrophotometer on the basis of the MDR-23 monochromator is intended for recording the scattered solar radiation spectra in the zenith for solar zenith angles between 83 and 96° to evaluate the NO2 content for slant paths. Vertical profiles of the NO2 concentration are reconstructed from the data on the NO2 content on slant paths with a spatial resolution of 5 km between 0 and 50 km and the total NO2 content is calculated.

Projects for the future development of our investigations at the station envisage the increase of the number of measurable atmospheric parameters, modernization of measurement technique and procedure, and refinement of mathematical methods for solving the inverse problems of optical sensing to increase the accuracy of measurements and to extend the sensing range. At present spectrophotometric channels are being developed for measuring NO3 and the ratio HCl/HF, which is indicative of the OH content in the atmosphere, to study in more detail the photochemical cycles of ozone formation and destruction.