Excimer-laser-based lidar for tropospheric ozone monitoring

The description of ozone differential absorption lidar (DIAL) based on KrF and XeCl excimer lasers for day and night-time measurements in the troposphere is presented. The XeCl laser is used as the “off” wavelength emitter and the radiation of KrF laser is Raman-shifted in a hydrogen and deuterium cell to obtain 277 nm and 292 nm “on” wavelengths. For the measurements in the range 0.5–4.5 km the pair 277/308 nm is used and for 4–10 km range the pair 292/308 nm is applied. Simultaneously with the elastic backscattering, the Raman backscatter of the XeCl laser from nitrogen and water vapor are monitored. The nitrogen Raman signal is used for the calculation of aerosol backscattering and extinction coefficients and these are compared with the results derived from XeCl elastic backscatter by the Klett method. The aerosol profiles obtained are used for correction of ozone concentration. Some examples of lidar application for the investigation of diurnal and seasonal ozone variation are given. PACS: 42.68.Rp; 93.85.+q; 94.10.Gb; 92.60.Jq The importance of long-term measurements of tropospheric ozone is now well recognized and a network of ground-based monitoring stations is in operation throughout the world. Nevertheless for a fully representative picture of the tropospheric ozone, vertical sounding methods are necessary. Studies of photochemistry and ozone transport require regular diurnal measurements from the ground level up to tropopause with a height resolution of several hundred m and an accuracy of the order of 1 ppb. The capability of the DIAL technique for such monitoring was demonstrated about 15 years ago [1– 3]. But in contrast to the stratosphere, where DIAL systems based on excimer lasers have become the routine instrument for stratospheric ozone monitoring [4–7] (see also references therein), the development of fully operational lidar systems and their application to the regular study of tropospheric ozone evolution were only started quite recently [8–13]. Such delay is related to the numerous problems to be solved before acceptable accuracy of the measurements had been achieved. The main error source is high aerosol and trace gas concentrations in the troposphere and especially in the planetary boundary layer (PBL), so the choice of DIAL wavelengths for the sounding becomes very critical. If in stratospheric measurements the choice of XeCl laser as “on” wavelength source has no serious alternative today, in troposphere at least three types of laser sources have already proved their suitability for such measurements. These are tunable dye lasers [1, 3, 8, 9, 13, 15] and fixed-frequency UV lasers such as KrF excimer [2, 11, 14] or a frequency-quadrupled Nd:YAG [10, 12, 16] in combination with Raman shifting technique. Though tunable lasers allow one to choose the optimal wavelengths for the sounding, their energy-power capabilities are very limited. The sounding errors for KrF and frequency-quadrupled Nd:YAG lasers are similar, so the choice between these lasers is related mainly to the operation convenience inside the lidar station. The advantages of frequency-quadrupled Nd:YAG laser are general for solid-state systems: compactness, reliability, and long lifetime of all laser components. Otherwise the KrF laser possesses significantly higher output power and the numerous Stokes orders may be efficiently produced to choose the optimal DIAL wavelengths for every altitude range. Especial interest for application of excimer lasers is supported by recent progress in elaboration of new high-density ceramic laser tubes with long life of discharge and optical components [17], which meet the specification of lidar radiation sources. The excimer lasers are simpler in operation when compared with solid-state lasers: short-time operation of excimer lasers sufficient for tropospheric measurements may be produced without water cooling, the radiation parameters are stable with temperature variations, and the pauses between the soundings do not influence the output energy. In this paper we present a description of a excimer laser based DIAL system designed in the Physics Instrumentation Center of General Physics Institute to provide ozone density distribution from near ground level up to the free troposphere. The lidar station is located in Troitsk, which is 20 km southwest from Moscow.