Determination of stratospheric aerosol microphysical properties from independent extinction and backscattering measurements with a Raman lidar

An algorithm that permits the retrieval of profiles of particle mass and surface-area concentrations in the stratospheric aerosol layer from independently measured aerosol (particle and Rayleigh) and molecule (Raman or Rayleigh) backscatter signals is developed. The determination is based on simultaneously obtained particle extinction and backscatter profiles and on relations between optical and microphysical properties found from Mie-scattering calculations for realistic stratospheric particle size distributions. The size distributions were measured with particle counters released on balloons from Laramie, Wyoming, between June 1991 and April 1994. Mass and surface-area concentrations can be retrieved with relative errors of 10-20% and 20-40%, respectively, with a laser wavelength of 355 nm and with errors of 20-30% and 30-60%, respectively, with a laser wavelength of 308 nm. Lidar measurements taken within the first three years after the eruption of Mt. Pinatubo in June 1991 are shown. Surface-area concentrations around 20 μm(2) cm(-3) and mass concentrations of 3 to 6 μg m(-3) were found until spring 1993.     

Extinction-to-backscatter ratio of Asian dust observed with high-spectral-resolution lidar and Raman lidar

Extinction-to-backscatter ratio or lidar ratio is a key parameter in the issue of backscatter-lidar inversions. The lidar ratio of Asian dust was observed with a high-spectral-resolution lidar and a combined Raman elastic-backscatter lidar during the springs of 1998 and 1999. The measured values range from 42 to55 sr in most cases, with a mean of 51 sr. These values are significantly larger than those predicted by the Mie computations that incorporate measured Asian dust size distributions and a range of refractive index with a typical value of 1.55-0.005i. The enhancement of lidar ratio is mostly due to the nonsphericity of dust particles, as indicated by the T-matrix calculations for spheroid particles and a number of other theoretical studies. In addition, possible contamination of urban aerosols may also contribute somewhat in optically thin cases. Mie theory, although it can well describe spherical particle scattering, will not be sufficient to represent the scattering characteristics of irregular particles such as Asian dust, especially in directions larger than approximately 90 degrees when the size parameter is large.     

Validation of the Raman lidar algorithm for quantifying aerosol extinction

To calculate aerosol extinction from Raman lidar data, it is necessary to evaluate the derivative of a molecular Raman signal with respect to range. The typical approach taken in the lidar community is to make an a priori assumption about the functional behavior of the data to calculate the derivative. It has previously been shown that the use of the chi-squared technique to determine the most likely functional behavior of the data prior to actually calculating the derivative eliminates the need for making a priori assumptions. Here that technique is validated through numerical simulation and by application to a significant body of Raman lidar measurements. In general, we show that the chi-squared approach for evaluating extinction yields lower extinction uncertainty than traditional techniques. We also use the technique to study the feasibility of developing a general characterization of the extinction uncertainty that could permit the uncertainty in Raman lidar aerosol extinction measurements to be estimated accurately without the need of the chi-squared technique.     

Particle backscatter, extinction, and lidar ratio profiling with Raman lidar in south and north China

Aerosol Raman lidar observations of profiles of the particle extinction and backscatter coefficients and the respective extinction-to-backscatter ratio (lidar ratio) were performed under highly polluted conditions in the Pearl River Delta (PRD) in southern China in October 2004 and at Beijing during a clear period with moderately polluted to background aerosol conditions in January 2005. The anthropogenic haze in the PRD is characterized by volume light-extinction coefficients of particles ranging from approximately 200 to 800 Mm(-1) and lidar ratios mostly between 40 and 55 sr (average of 47+/-6 sr). Almost clean air masses were observed throughout the measurements of the Beijing campaign. These air masses originated from arid desert-steppe-like regions (greater Gobi area). Extinction values usually varied between 100 and 300 Mm(-1), and the lidar ratios were considerably lower (compared with PRD values) with values mostly from 30 to 45 sr (average of 38+/-7 sr). Gobi dust partly influenced the observations. Unexpectedly low lidar ratios of approximately 25 sr were found for a case of background aerosol with a low optical depth of 0.05. The low lidar ratios are consistent with Mie-scattering calculations applied to ground-based observations of particle size distributions.     

Retrieval of aerosol extinction coefficient profiles from Raman lidar data by inversion method

We regard the problem of differentiation occurring in the retrieval of aerosol extinction coefficient profiles from inelastic Raman lidar signals by searching for a stable solution of the resulting Volterra integral equation. An algorithm based on a projection method and iterative regularization together with the L-curve method has been performed on synthetic and measured lidar signals. A strategy to choose a suitable range for the integration within the framework of the retrieval of optical properties is proposed here for the first time to our knowledge. The Monte Carlo procedure has been adapted to treat the uncertainty in the retrieval of extinction coefficients.     

III-posed retrieval of aerosol extinction coefficient profiles from Raman lidar data by regularization

In the analysis of Raman lidar measurements of aerosol extinction, it is necessary to calculate the derivative of the logarithm of the ratio between the atmospheric number density and the range-corrected lidar-received power. The statistical fluctuations of the Raman signal can produce large fluctuations in the derivative and thus in the aerosol extinction profile. To overcome this difficult situation we discuss three methods: Tikhonov regularization, variational, and the sliding best-fit (SBF). Three methods are performed on the profiles taken from the European Aerosol Research Lidar Network lidar database simulated at the Raman shifted wavelengths of 387 and 607 nm associated with the emitted signals at 355 and 532 nm. Our results show that the SBF method does not deliver good results for low fluctuation in the profile. However, Tikhonov regularization and the variational method yield very good aerosol extinction coefficient profiles for our examples. With regard to, e.g., the 532 nm wavelength, the L2 errors of the aerosol extinction coefficient profile by using the SBF, Tikhonov, and variational methods with respect to synthetic noisy data are 0.0015(0.0024), 0.00049(0.00086), and 0.00048(0.00082), respectively. Moreover, the L2 errors by using the Tikhonov and variational methods with respect to a more realistic noisy profile are 0.0014(0.0016) and 0.0012(0.0016), respectively. In both cases the L2 error given in parentheses concerns the second example.     

受激拉曼增益激光雷达测量CO2气体

激光雷达探测大气中的CO2气体目前还处在探索阶段,在差分吸收原理基础上,利用气体的受激拉曼散射增益效应,设计出测量CO2气体的非线性拉曼激光雷达系统,并模拟实验得到了增益开关两种状态下的光子计数信号,从而反演出CO2气体的浓度分布结果,在垂直方向上,CO2气体浓度随距离变化不大,合肥地区的浓度含量大约在400ppm左右。     

Aerosol lidar intercomparison in the framework of the EARLINET project. 3. Raman lidar algorithm for aerosol extinction, backscatter, and lidar ratio

An intercomparison of the algorithms used to retrieve aerosol extinction and backscatter starting from Raman lidar signals has been performed by 11 groups of lidar scientists involved in the European Aerosol Research Lidar Network (EARLINET). This intercomparison is part of an extended quality assurance program performed on aerosol lidars in the EARLINET. Lidar instruments and aerosol backscatter algorithms were tested separately. The Raman lidar algorithms were tested by use of synthetic lidar data, simulated at 355, 532, 386, and 607 nm, with realistic experimental and atmospheric conditions taken into account. The intercomparison demonstrates that the data-handling procedures used by all the lidar groups provide satisfactory results. Extinction profiles show mean deviations from the correct solution within 10% in the planetary boundary layer (PBL), and backscatter profiles, retrieved by use of algorithms based on the combined Raman elastic-backscatter lidar technique, show mean deviations from solutions within 20% up to 2 km. The intercomparison was also carried out for the lidar ratio and produced profiles that show a mean deviation from the solution within 20% in the PBL. The mean value of this parameter was also calculated within a lofted aerosol layer at higher altitudes that is representative of typical layers related to special events such as Saharan dust outbreaks, forest fires, and volcanic eruptions. Here deviations were within 15%.     

Raman and Rayleigh holographic lidar

We have designed a novel rotational Raman and Rayleigh lidar system that incorporates a simple holographic optical element. The hologram simultaneously disperses and focuses the backscattered signal light so that narrow spectral features can be isolated and detected with high efficiency. By measuring the relative strength of several nitrogen rotational Raman lines, we can obtain an accurate temperature of the atmosphere at a given altitude without the need for external calibration. Simultaneous photon counting of the Rayleigh backscatter signal permits temperature measurements at much higher altitudes.     

Experimental determination of the lidar overlap profile with Raman lidar

The range-dependent overlap between the laser beam and the receiver field of view of a lidar can be determined experimentally if a pure molecular backscatter signal is measured in addition to the usually observed elastic backscatter signal, which consists of a molecular component and a particle component. Two methods, the direct determination of the overlap profile and an iterative approach, are presented and applied to a lidar measurement. The measured overlap profile accounts for actual system alignment and for all system parameters that are not explicitly known, such as actual laser beam divergence and spatial intensity distribution of the laser light.     

Spaceborne profiling of atmospheric temperature and particle extinction with pure rotational Raman lidar and of relative humidity in combination with differential absorption lidar: performance simulations

The performance of a spaceborne temperature lidar based on the pure rotational Raman (RR) technique in the UV has been simulated. Results show that such a system deployed onboard a low-Earth-orbit satellite would provide global-scale clear-sky temperature measurements in the troposphere and lower stratosphere with precisions that satisfy World Meteorological Organization (WMO) threshold observational requirements for numerical weather prediction and climate research applications. Furthermore, nighttime temperature measurements would still be within the WMO threshold observational requirements in the presence of several cloud structures. The performance of aerosol extinction measurements from space, which can be carried out simultaneously with temperature measurements by RR lidar, is also assessed. Furthermore, we discuss simulations of relative humidity measurements from space obtained from RR temperature measurements and water-vapor data measured with the differential absorption lidar (DIAL) technique.     

Simultaneous measurements of particle backscattering and extinction coefficients and wind velocity by lidar with a Mach-Zehnder interferometer: principle of operation and performance assessment

The development of remote-sensing instruments that can be used to monitor several parameters at the same time is important for the study of complex processes such as those that control climate and environment. In this paper the performance of a new concept of lidar receiver that allows for the direct measurement of aerosol and cloud optical properties simultaneously with wind velocity is investigated. This receiver uses a Mach-Zehnder interferometer. Two different configurations, either with four photometric output channels or with fringe imaging on a multichannel detector, are studied. Analytical expressions of the statistical errors are given under the assumption of Gaussian signal spectra. It is shown that similar accuracies can be achieved for both configurations. Performance modeling of the retrieval of semitransparent cloud optical scattering properties and wind velocity was done at different operation wavelengths for a Nd:YAG laser source. Results for such a lidar system onboard an aircraft flying at an altitude of 12 km show that for semitransparent clouds the best results were obtained at 355 nm, with relative standard deviations of 0.5% and 5% for the backscatter and extinction coefficients, respectively, together with a velocity accuracy of 0.2 ms(-1). The accuracy of optical properties retrieved for boundary layer aerosols are comparable, whereas the velocity accuracy is decreased to 1 ms(-1). Finally, an extrapolation to a large 355-nm spaceborne lidar shows accuracies in the range from 2.5% to 5% for the backscatter coefficient and from 10% to 15% for the extinction coefficient together with a vertical wind speed accuracy of better than 0.5 ms(-1) for semitransparent clouds and boundary layer, with a vertical resolution of 500 m and a 100 shot averaging.     

Raman lidar observations of cloud liquid water

We report the design and the performances of a Raman lidar for long-term monitoring of tropospheric aerosol backscattering and extinction coefficients, water vapor mixing ratio, and cloud liquid water. We focus on the system's capabilities of detecting Raman backscattering from cloud liquid water. After describing the system components, along with the current limitations and options for improvement, we report examples of observations in the case of low-level cumulus clouds. The measurements of the cloud liquid water content, as well as the estimations of the cloud droplet effective radii and number densities, obtained by combining the extinction coefficient and cloud water content within the clouds, are critically discussed.     

Combined Raman lidar for the measurement of atmospheric temperature,water vapor,particle extinction coefficient,and particle backscatter coefficient

The lidar of the Radio Science Center for Space and Atmosphere (RASC; Kyoto, Japan) make use of two pure rotational Raman (MR) signals for both the measurement of the atmospheric temperature profile and the derivation of a temperature-independent Raman reference signal. The latter technique is new and leads to significant smaller measurement uncertainties compared with the commonly used vibrational Raman lidar technique. For the measurement of temperature, particle extinction coefficient, particle backscatter coefficient, and humidity simultaneously, only four lidar signal are needed the elastic Cabannes backscatter signal, two RR signals, and the vibrational Raman water vapor signal. The RASC lidar provides RR signals of unprecedented intensity. Although only 25% of the RR signal intensities can be used with the present data-acquisition electronics, the 1-s -statistical uncertainty of nighttime temperature measurements is lower than for previous systems and is < 1K up to 11-km height for, e.g., a resolution of 500 m and 9 min. In addition, RR measurements in daytime also have become feasible.     

Determination of cloud microphysical properties by laser backscattering and extinction measurements

The extinction and backscattering of 514-nm laser radiation in polydisperse water droplet clouds have been studied in the laboratory. Three cloud size distributions with modal diameters of 0.02, 5, and 12 microm have been investigated. The relationships between the cloud optical parameters (attenuation coefficient sigma and volume backscattering coefficient beta(pi)) and the cloud water content C have been measured for each size distribution. It has been found that a linear relationship exists between sigma and C and between beta(pi) and C for cloud water content values up to 3 g/m3. The linear relationships obtained, however, have slopes which depend on the droplet size distribution. For a given water content both sigma and beta(pi) increase as the modal diameter decreases. The measured data are compared with existing theoretical analyses and discussed in terms of their application to lidar measurements of atmospheric clouds. It is concluded that the empirical information obtained can serve as a basis for quantitative lidar measurements.     

Fluorescence lidar monitoring of historic buildings

Laser-induced fluorescence spectra detected with high-spectral-resolution lidar on the facades of the Baptistery and the Cathedral in Parma are presented and discussed. The data show fluorescence features that are due to the stone materials that constitute the coating of the monuments and to photosynthetically active colonizations on their surfaces. This underlines the feasibility of a remote fluorescence analysis of historic facades. The data were also compared with the fluorescence lidar spectra obtained from similar lithotypes, sampled either in historic extraction areas or in sites exploited recently. The results open good prospects for spectral characterization of historic materials and identification of their provenance.     

Performance modeling of an airborne Raman water-vapor lidar

We have developed a sophisticated Raman lidar numerical model to simulate the performance of two ground-based Raman water-vapor lidar systems. After verifying the model using these ground-based measurements, we then used the model to simulate the water-vapor measurement capability of an airborne Raman lidar under both daytime and nighttime conditions for a wide range of water-vapor conditions. The results indicate that, under many circumstances, the daytime measurements possess comparable quality to an existing airborne differential absorption water-vapor lidar whereas the nighttime measurements have improved spatial and temporal resolution. In addition, an airborne Raman lidar can offer measurements that are difficult or impossible with the differential absorption lidar technique.     

Imaginary refractive-index effects on desert-aerosol extinction versus backscatter relationships at 351 nm: numerical computations and comparison with Raman lidar measurements

A numerical model is used to investigate the dependence at 351 nm of desert-aerosol extinction and backscatter coefficients on particle imaginary refractive index (mi). Three ranges (-0.005 < or = mi < or = -0.001, -0.01 < or = mi < or = -0.001, and -0.02 < or = mi < or = -0.001) are considered, showing that backscatter coefficients are reduced as /mi/ increases, whereas extinction coefficients are weakly dependent on mi. Numerical results are compared with extinction and backscatter coefficients retrieved by elastic Raman lidar measurements performed during Saharan dust storms over the Mediterranean Sea. The comparison indicates that a range of -0.01 to -0.001 can be representative of Saharan dust aerosols and that the nonsphericity of mineral particles must be considered.