LIDAR TECHNOLOGY FOR REMOTE SENSING MEASURMENTS OF OZONE CONCENTRATIONS

LIDAR TECHNOLOGY FOR REMOTE SENSING MEASURMENTS OF OZONE CONCENTRATIONS By Florence Bocquet April 2003 Table of contents Introduction…………………………………………………………………….……………… Principles of lidar…………………………………………………………….………………… Principle of the lidar and the DIAL technique…………………………….………………… 2 Description of ozone lidar systems……………………………………….………………… Description of the measurements and sampling strategy………………….………………… Ozone retrieval from the lidar signals…………………………………….………………… 5 Accuracy and vertical resolution of the measurement…………………….………………… OPAL application: experiment and results……………………………………………………… Data taking………………………………………………………………… ……………… Differences in methodology between the airborne in situ measurements and the lidar measurements……………………………………………………………………………… Figures of intercomparison…………………………………………………….…………… Statistics……………………………………………………………………….…………… Conclusion……………………………………………………………………………………… References……………………………………………………………………………………… 8 Introduction Since the mid-1960s, scientists have used lidar to study atmospheric particles and clouds It is considered to be among the most accurate data sets attainable using remote sensing processes Different types of lidars, such as Rayleigh lidar, Raman lidar, aerosol lidar or again, cirrus cloud lidar, measure different atmospheric properties To measure ozone – and aerosols-, NASA Langley scientists in particular, use a specialized lidar called the airborne Ultraviolet (UV) DIfferential Absorption Lidar (DIAL) Because ozone plays important roles in the tropospheric –and stratospheric- chemistry (in particular, ozone is the source of the hydroxyl radical, which controls the abundance of many atmospheric constituents, including greenhouse gases such as methane and HCFC’s) and is a health risk near the surface, it is being intensively researched It is known that tropospheric ozone arises both from ‘in situ’ photochemical production and downward transported from the stratosphere, but the relative importance of these two sources to the global budget is poorly understood Climate models are three-dimensional, but conventional ground-based ozone monitoring networks are only two-dimensional For this reason, using lidars will add the third dimension to the data and enable researchers to fully, or at least to better understand ground level ozone concentrations Principles of lidar are presented in the first section The second part of the report will show some data from an application and intercomparison of OPAL (Ozone Profiling 11 11 12 13 Atmospheric Lidar) ground-ozone lidar with an airborne UV ozone analyzer, and finally, a conclusive section Principles of lidar Principle of the lidar and the DIAL technique The lidar (LIght Detection And Ranging) is a remote sensing instrument that uses laser light, i.e., operating in the optical range, in much the same way that sonar uses sound, or radar uses radio waves Depending on the desired measurement, lidar systems use various light-matter interactions such as Rayleigh, Mie and Raman scattering or fluorescence Measurements of atmospheric ozone, as well as temperature and aerosol are based on the first processes Generally, a lidar measurement consists in sending into the atmosphere a laser beam; a small part of this laser radiation is scattered back to the ground, where it is collected by a telescope, detected by a photomultiplier tube and analyzed by an electronic acquisition system Range resolved measurements can be obtained using pulsed lasers In order to measure the ozone vertical distribution, the Differential Absorption Laser technique (DIAL) is used This technique requires the simultaneous emission of two laser beams characterized by a different ozone absorption cross-section (Figure 1) Figure 1: The absorption cross-section of ozone with the emitted wavelengths superimposed (wavelengths ‘on’ marked in red and ‘off’ marked in green) Fig 2: Absorption cross-section of ozone, sulfur dioxide and nitrogen dioxide in the ultraviolet The ozone absorption cross-section spectrum (Fig 1) shows three red vertical lines, which are the laser wavelengths of a conventional Raman-shifted DIAL system such as OPAL The line at 266 nm is obtained by quadrupling the output of an Nd:YAG laser, and the lines near 289 and 299 nm are obtained by stimulating Raman scattering in highpressure cells of deuterium and hydrogen The ozone cross-section at 266 nm is so high that it seriously limits the lidar range For an atmosphere at standard temperature and pressure containing 100 ppb ozone, the extinction coefficient at 266 nm is 2.4 km-1, which implies that the two-way transmittance to an altitude of km is only 0.01 On the other hand, it is best for the wavelength pair to be on the steepest part of the cross-section curve in order to minimize errors resulting from aerosol backscatter The Raman-shifted pair at 289/299 is on the tail of the curve, and in addition, it is susceptible to errors resulting from the presence of sulfur dioxide Ideally, the operating wavelengths of the ozone DIAL system should be selectable and the GTRI (Georgia Tech Research Institute) lidar group is currently developing tunable laser technologies (Fig - see conclusion) Description of ozone lidar systems A lidar system includes basically one or several laser sources with optical devices to reduce the divergence of the beam, a telescope, which collects the light scattered back by the atmosphere, an optical analyzing system with detectors such as photomultipliers to detect the optical signal, and an electronic acquisition system (Figure 3) The analyzing systems used to digitize the electronic signal provided by the photomultipliers include photon counting and/or transient analyzers In the case of the DIAL systems characterized by the emission of two laser wavelengths, the optical receiving system comprises spectral analyzing optics, such as interference filters or spectrometers Fig 3: Schematic view of the principle of a lidar system To monitor atmospheric ozone with the DIAL technique, the choice of the laser wavelengths depends on the altitude range of the measurement (Mégie et al., 1985) The spectral range is chosen first in the ultraviolet where the ozone absorption is more efficient (266 nm as shown in figure 1), but the selected wavelengths differ according to whether the measurement is made in the troposphere or in the stratosphere - in the troposphere, the ozone number density is small so the laser wavelengths must correspond to a strong UV absorption, while for stratospheric measurements, the objective is to reach the stratosphere and to detect the high ozone concentrations there (Browell, 1989, Papayannis et al., 1990) Description of the measurements and sampling strategy The lidar signals cover a very high dynamic range corresponding to several orders of magnitude, which is not handled by the electronic acquisition systems This requires the use of simultaneous photon counting and analogue acquisition for the low and the high signals or the separation of the optical signal in two parts (90% and 10%), corresponding to the high and the low altitudes respectively For reasons of simplicity in terms of electronic acquisition, the latter solution is the most commonly used (Harris and Hudson, 1998) Depending on the power and the repetition rate of the laser, an ozone measurement lasts typically several hours, leading to a spatial resolution of the order of 100 km, depending on the atmospheric conditions The vertical resolution ranges from several hundred meters in the lower range to several kilometers above 40 km Finally, one main caveat of the lidar measurements is the requirement of clear sky meteorological conditions - laser radiation is rapidly absorbed by clouds - and only cirrus can be tolerated for accurate stratospheric measurements Ozone retrieval from the lidar signals Assuming a monochromatic laser impulsion at wavelength λ, the received optical power corresponding to the light backscattering at altitude z, is given by (Measure, 1984): P(z) = K(λ)β(λ,z)exp[-2τ(λ,z)] (1) where K(λ) is an instrument constant involving the telescope surface area, the emitted power, the optical efficiency of the receiving system and a geometrical factor depending on the alignment of the laser and the telescope axis, β(λ,z) is the atmospheric backscatter coefficient and τ(λ,z) is the atmospheric optical depth τ(λ,z) depends on the following parameters: (2) where α(λ,z) is the atmospheric extinction coefficient, the ozone absorption cross-section, the ozone number density to be measured and the term corresponds to the extinction by other absorbers Applying this formula to the second wavelength and taking into account the background signal, one derives the ozone number density from the received lidar signals: (3) where Pbi is the background signal and (4) The laser wavelengths are chosen so that the term represents less than 10% of the measurement (Harris and Hudson, 1998) The derivation of the ozone number density from the laser signals shows thus that the DIAL technique is a self-calibrated technique, which doesn't need the evaluation of instrumental constants Accuracy and vertical resolution of the measurement The precision of a DIAL measurement is defined by the statistical error due to the random character of the detection process, which follows basically the Poisson statistics (Measures, 1984) The accuracy of the measurement depends on the approximations made in deriving the ozone number density from the received signals It depends also on the linearity of the lidar signals According to the Poisson statistics, the statistical error on ozone is given by the following formula: (5) where ΔZ is the initial range resolution of the acquisition system, Pi,j corresponds to the lidar signal at wavelength i from altitude Zj, cj are the coefficients of the low pass derivative filter used to differentiate the signals, Ni is the number of laser shots at wavelength λi and Pbi is the background radiation at wavelength λi The final statistical error ε on the measurement is the result of a compromise depending on the experimental system characteristics, the duration of the signal acquisition and the vertical resolution according to the following relation: (6) where A is the telescope receiving area, ΔZ the final range resolution, P0 the emitted power and Ta the acquisition time Most teams choose low pass filters with varying number of points as a function of the altitude The DIAL stratospheric ozone lidar profiles are thus generally characterized by a vertical resolution varying from several hundred meters in the lower stratosphere, to several kilometers around 50 km (see Figure for an example of ozone lidar vertical resolution profile) Figure 4: Precision and vertical resolution profile of an ozone measurement in the case of the OHP (Observatoire de Haute Provence –in France) lidar instrument Both the precision and the vertical resolution profile depend on the experimental configuration The precision can vary from one measurement to the other The accuracy of the measurement depends on the term (see equation 4) which corresponds to less than 10% of the value derived directly from the slope of the signals but still has to be corrected using ancillary measurements It depends also on the accuracy of the ozone absorption cross-sections and on the approximation concerning the monochromaticity of the laser radiations These error sources are summarized in Table 1, which indicates the residual error on the measurement after correction (Godin, 1987) Table 1: Error sources for lidar measurements Error source Residual Ozone absorption cross-section - Absolute value (Bass&Paur) 2% - Temperature sensitivity < 0.5% Laser line width < 0.3% Rayleigh extinction