Aerosol lidar intercomparison in the framework of the EARLINET project. 1. Instruments

In the framework of the European Aerosol Research Lidar Network to Establish an Aerosol Climatology (EARLINET), 19 aerosol lidar systems from 11 European countries were compared. Aerosol extinction or backscatter coefficient profiles were measured by at least two systems for each comparison. Aerosol extinction coefficients were derived from Raman lidar measurements in the UV (351 or 355 nm), and aerosol backscatter profiles were calculated from pure elastic backscatter measurements at 351 or 355, 532, or 1064 nm. The results were compared for height ranges with high and low aerosol content. Some systems were additionally compared with sunphotometers and starphotometers. Predefined maximum deviations were used for quality control of the results. Lidar systems with results outside those limits could not meet the quality assurance criterion. The algorithms for deriving aerosol backscatter profiles from elastic lidar measurements were tested separately, and the results are described in Part 2 of this series of papers [Appl. Opt. 43, 977-989 (2004)]. In the end, all systems were quality assured, although some had to be modified to improve their performance. Typical deviations between aerosol backscatter profiles were 10% in the planetary boundary layer and 0.1 x 10(-6) m(-1) sr(-1) in the free troposphere.     

Aerosol lidar intercomparison in the framework of the EARLINET project. 2. Aerosol backscatter algorithms

An intercomparison of aerosol backscatter lidar algorithms was performed in 2001 within the framework of the European Aerosol Research Lidar Network to Establish an Aerosol Climatology (EARLINET). The objective of this research was to test the correctness of the algorithms and the influence of the lidar ratio used by the various lidar teams involved in the EARLINET for calculation of backscatter-coefficient profiles from the lidar signals. The exercise consisted of processing synthetic lidar signals of various degrees of difficulty. One of these profiles contained height-dependent lidar ratios to test the vertical influence of those profiles on the various retrieval algorithms. Furthermore, a realistic incomplete overlap of laser beam and receiver field of view was introduced to remind the teams to take great care in the nearest range to the lidar. The intercomparison was performed in three stages with increasing knowledge on the input parameters. First, only the lidar signals were distributed; this is the most realistic stage. Afterward the lidar ratio profiles and the reference values at calibration height were provided. The unknown height-dependent lidar ratio had the largest influence on the retrieval, whereas the unknown reference value was of minor importance. These results show the necessity of making additional independent measurements, which can provide us with a suitable approximation of the lidar ratio. The final stage proves in general, that the data evaluation schemes of the different groups of lidar systems work well.     

Raman lidar monitoring of extinction and backscattering of African dust layers and dust characterization

Results on the monitoring of strong African dust outbreaks at Lecce in the southeastern corner of Italy (40 degrees 20' N, 18 degrees 6' E) during May 2001 are presented. This activity has been performed in the framework of the European Aerosol Research Lidar Network (EARLINET). The lidar station of Lecce is located on a flat rural area that is approximately 800 km from the northern Africa coast. So it is closer to Africa than most of all other EARLINET stations and allow monitoring African dust transport early in its life cycle, at all levels in the plume. An elastic-backscatter Raman lidar based on a XeF excimer laser (351 nm) has been used to monitor the time evolution and vertical structure of the dust layers and get independent measurements of the aerosol extinction and backscatter coefficients. The findings are presented in terms of vertical profiles of the extinction and backscatter coefficients and of the lidar ratio. A quite deep dust layer extending between 2 and 6 km and characterized by a backscatter coefficient of approximately 0.0016 (km sr)(-1), a lidar ratio of approximately 50 sr, and an aerosol optical depth of 0.26 was observed on 17 May 2001 between 18:55 and 20:07 UT. The layer persisted for approximately five days. Dust layers of lower optical thickness and shorter persistence time have generally been monitored at the lidar site during African dust outbreaks. Results on the chemical and morphological characterization of the dust collected at the lidar station are also given to further support the origin of the monitored aerosol layers.     

Potential of lidar backscatter data to estimate solar aerosol radiative forcing

The potential to estimate solar aerosol radiative forcing (SARF) in cloudless conditions from backscatter data measured by widespread standard lidar has been investigated. For this purpose 132 days of sophisticated ground-based Raman lidar observations (profiles of particle extinction and backscatter coefficients at 532 nm wavelength) collected during two campaigns [the European Aerosol Research Lidar Network (EARLINET) and the Indian Ocean Experiment (INDOEX)] were analyzed. Particle extinction profiles were used as input for radiative transfer simulations with which to calculate the SARF, which then was plotted as a function of the column (i.e., height-integrated) particle backscatter coefficient (beta(c)). A close correlation between the SARF and beta(c) was found. SARF-beta(c) parameterizations in the form of polynomial fits were derived that exhibit an estimated uncertainty of +/-(10-30)%. These parameterizations can be utilized to analyze data of upcoming lidar satellite missions and for other purposes. The EARLINET-based parameterizations can be applied to lidar measurements at mostly continental, highly industrialized sites with limited maritime influence (Europe, North America), whereas the INDOEX parameterizations rather can be employed in polluted maritime locations, e.g., coastal regions of south and east Asia.     

Measurements of stratospheric aerosols with a combined elastic-Raman-backscatter lidar

Improvements made to the NASA Goddard Space Flight Center Stratospheric Ozone Lidar system have extended its atmospheric-aerosol-measuring capabilities. The methods by which aerosol-scattering ratio, aerosol backscatter, and aerosol extinction are simultaneously derived from lidar data are reported, and results obtained during several intercomparison campaigns at worldwide locations are shown. The results track the evolution of the Mt. Pinatubo aerosol cloud from 1991 to 1994 and report wavelength-dependence information for aerosol backscatter between 308 and 351 nm. Two analysis techniques, a more common inversion method and a combined elastic-Raman-backscatter approach, are also compared.     

Detection of stratospheric sulfuric Acid aerosols with polarization lidar: theory, simulations, and observations

The derivation of backscatter ratio profiles from polarization lidar measurements is discussed. The method is based on differences in depolarization between molecular backscattering and backscattering from spherical aerosol particles. Simulations show that the polarization algorithms yield backscatter ratios with uncertainties comparable with those obtained by Klett's method, provided that the backscattering process is dominated by molecular scattering. The technique could be utilized for monitoring the stratospheric sulfuric acid aerosol layer during periods of background conditions. The polarization analysis method is discussed in light of simulation results and is applied to polarization lidar profiles observed during the ALBATROSS 1996 field measurement campaign.     

Analysis of lidar backscatter profiles in optically thin clouds

The solution of the lidar equation for profiles of backscatter and extinction in optically thin clouds is constrained by values of the cloud transmittance determined from the elastically scattered lidar signals below and above the cloud. The method is extended to those cases in which an aerosol layer lies below or above the cloud layer. Examples are given in both cases. An analytical expression for the average lidar ratio in the cloud is derived for those cases in which molecular scattering is significant.     

Two-wavelength lidar inversion algorithm for a two-component atmosphere

A method for the boundary-value determination of aerosol extinction profiles from backscatter lidar measurements is presented. Artificially generated lidar signals from two-component inhomogeneous model atmospheres are inverted with the information from two wavelengths (532 and 1064 nm) simultaneously. The solution for the vertical aerosol extinction profile is formulated with Klett's far-end solution. The boundary value is expressed in terms of aerosol transmission along the lidar line according to Fernald's solution of the lidar equation. The aerosol transmission is determined iteratively with a transcendental equation on the assumption that a linear relationship exists between the extinction coefficients at both wavelengths. Inversion calculations are applied to model atmospheres with range-dependent lidar ratios representing the growth of aerosol particles caused by increasing relative humidity in the planetary boundary layer. For the inversion constant lidar ratios are assumed that vary between 40 and 70 sr. The numerical procedure turns out to be stable enough to provide meaningful results even in cases of misestimated lidar ratios. The application of the method is of less use for misestimated background radiation and low aerosol concentrations.     

Retrieval of stratospheric aerosol size distributions and integral properties from simulated lidar backscatter measurements

A new approach for retrieving aerosol properties from extinction spectra is extended to retrieve aerosol properties from lidar backscatter measurements. In this method it is assumed that aerosol properties are expressed as a linear combination of backscatters at three or fewer wavelengths commonly used in lidar measurements. The coefficients in the weighted linear combination are obtained by minimization of the retrieval error averaged for a set of testing size distributions. The formulas can be used easily by investigators to retrieve aerosol properties from lidar backscatter measurements such as the Lidar In-Space Technology Experiment and Pathfinder Instruments for Clouds and Aerosols Spaceborne Observations.     

Temperature measurements made with a combined Rayleigh -Mie and Raman lidar

The NASA Goddard Space Flight Center stratospheric ozone lidar system has the capability of collecting both Rayleigh -Mie and Raman backscatter data simultaneously at a number of wavelengths. Here we report on an improved method by which temperature can be derived from a combination of the Rayleigh -Mie return at 351-nm lidar channels and the Raman nitrogen return at 382-nm lidar channels. We also examine some common techniques by which temperatures are retrieved from lidar data. Finally, we show results obtained in 1995 during two Network for the Detection of Stratospheric Change intercomparison campaigns at Lauder, New Zealand and Mauna Loa, Hawaii.     

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.     

Regularized algorithm for Raman lidar data processing

A regularized algorithm that has the potential to improve the quality of Raman lidar data processing is presented. Compared to the conventional scheme, the proposed algorithm has the advantage, which results from the fact that it is based on a well-posed procedure. That is, the profile of the aerosol backscatter coefficient is computed directly, using the explicit relationships, without numerical differentiation. Thereafter, the profile of the lidar ratio is retrieved as a regularized solution of a first-kind Volterra integral equation. Once these two steps have been completed, the profile of the aerosol extinction coefficient is computed by a straightforward multiplication. The numerical simulations demonstrated that the proposed algorithm provides good accuracy and resolution of aerosol profile retrievals. The error analysis showed that the retrieved profiles are continuous functions of the measurement errors and of the a priori information uncertainties.     

Iterative method for the inversion of multiwavelength lidar signals to determine aerosol size distribution

Two iterative methods of inverting lidar backscatter signals to determine altitude profiles of aerosol extinction and altitude-resolved aerosol size distribution (ASD) are presented. The first method is for inverting two-wavelength lidar signals in which the shape of the ASD is assumed to be of power-law type, and the second method is for inverting multiwavelength lidar signals without assuming any a priori analytical form of ASD. An arbitrary value of the aerosol extinction-to-backscatter ratio (S(1)) is assumed initially to invert the lidar signals, and the ASD determined by use of the spectral dependence of the retrieved aerosol extinction coefficients is used to improve the value of S(1) iteratively. The methods are tested for different forms of altitude-dependent ASD's by use of simulated lidar-backscatter-signal profiles. The effect of random noise on the lidar backscatter signals is also studied.     

Methodology for the independent calibration of Raman backscatter water-vapor lidar systems

We present a method for the independent calibration of Raman backscatter water-vapor lidar systems. Particular attention is given to the resolution of instrumental changes in the short and the long terms. The method reposes on the decomposition of the instrument function, which allows the lidar calibration coefficient to be re-expressed as the product of two terms, one describing the instrumental transmission and detection efficiency and the other describing the wavelength-dependent convolution of the Raman backscatter cross sections with the instrument function. The origins of changes in instrument response necessitate the experimental determination of the system detection efficiency. Two external light sources for calibration are assessed: zenith observation of diffuse sunlight and a xenon arc lamp. The results favor use of the diffuse-sunlight measurement but highlight the need for simultaneous sunphotometer measurements to constrain modeled aerosol optical properties. Quantum mechanical models of the Raman cross sections are described, and errors in determining the cross sections and their convolution with the instrument function are discussed in detail. The calibration coefficients deduced by using the independent method are compared with coefficients deduced from Vaisala H-Humicap radiosonde measurements. These results agree to within current calibration errors (15%, unconstrained aerosol parameters), and a change in calibration coefficient following instrument modification is reproduced satisfactorily. Results from modeling and intercomparison studies are extended to estimate the calibration accuracy and the precision of the diffuse-sunlight method with constrained modeled aerosol parameters. Changes in the calibration coefficient in the short and the long terms should be resolved to 4(6)% and 6(9)%, respectively, which is comparable or better than the precision of existing dependent methods of calibration. The reduction of the absolute calibration error remains an outstanding issue for all calibration methods.     

Simultaneous estimation of aerosol cloud concentration and spectral backscatter from multiple-wavelength lidar data

We present a sequential algorithm for estimating both concentration dependence on range and time and backscatter coefficient spectral dependence of optically thin localized atmospheric aerosols using data from rapidly tuned lidar. The range dependence of the aerosol is modeled as an expansion of the concentration in an orthonormal basis set whose coefficients carry the time dependence. Two estimators are run in parallel: a Kalman filter for the concentration range and time dependence and a maximum-likelihood estimator for the aerosol backscatter wavelength and time dependence. These two estimators exchange information continuously over the data-processing stream. The state model parameters of the Kalman filter are also estimated sequentially together with the concentration and backscatter. Lidar data collected prior to the aerosol release are used to estimate the ambient lidar return. The approach is illustrated on atmospheric backscatter long-wave infrared (CO2) lidar data.     

Significance of multiple scattering from tropospheric aerosols for ground-based backscatter lidar measurements

The influence of multiple scattering on the retrieval of extinction coefficients of tropospheric aerosols from ground-based backscatter lidar measurements is numerically modeled. In a first step, lidar returns are computed by means of a Monte Carlo code for model atmospheres with different aerosol types and different extinction coefficient profiles. In so doing, synthetic lidar signals with and without multiple scattering can be simulated. In a second step, both types of signal are inverted by the most frequently used analytical solution, which, however, is based on the single-scatter assumption. From a comparison of the results, the error of the retrieved aerosol-extinction profiles can be quantitatively determined. It was found that the contribution of multiply scattered photons to the lidar signals is typically below 10% and never exceeds 20%. The relative errors of the retrieved aerosol-extinction profile in the planetary boundary layer are still smaller; they were determined to be less than 3% for all aerosol types, even for extinction coefficients as large as 3.9 km(-1). Thus, for ground-based lidar measurements and typical meteorological conditions, errors caused by neglecting multiple scattering are by far less significant than other errors in lidar data evaluation.     

Principal component analysis applied to multiwavelength lidar aerosol backscatter and extinction measurements

The use of powerful Raman backscatter lidars enables one to measure the stratospheric aerosol extinction profile independently of the backscatter, thereby obtaining additional information to aid in retrieving the physical characteristics of the sampled aerosol. We used principal component analysis to construct a self-consistent method for the retrieval of aerosol bulk physical and optical properties from multiwavelength elastic and/or inelastic Raman backscatter lidar signals. The procedure is applied to synthetic and actual lidar signals. We found that aerosol surface area and volume can be usefully estimated and that the use of Raman-derived extinction data leads to a notable improvement in the accuracy of the estimations.     

Examination of the traditional raman lidar technique. II. Evaluating the ratios for water vapor and aerosols

In a companion paper [Appl. Opt. 42, 2571 (2003)] the temperature dependence of Raman scattering and its influence on the Raman and Rayleigh-Mie lidar equations were examined. New forms of the lidar equation were developed to account for this temperature sensitivity.Here those results are used to derive the temperature-dependent forms of the equations for the water vapor mixing ratio, the aerosol scattering ratio, the aerosol backscatter coefficient, and the extinction-to-backscatter ratio. The error equations are developed, the influence of differential transmission is studied, and several laser sources are considered in the analysis. The results indicate that the temperature functions become significant when narrowband detection is used. Errors of 5% and more can be introduced into the water-vapor mixing ratio calculation at high altitudes, and errors larger than 10% are possible for calculations of aerosol scattering ratio and thus of aerosol backscatter coefficient and of extinction-to-backscatter ratio.     

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.     

Notes on Rayleigh scattering in lidar signals

Classical and quantum formulations are used to estimate Rayleigh scattering within lidar signals. Within the classical approach, three scenarios are used to characterize atmospheric molecular composition: 2-component atmosphere (N2 and O2), 4-component atmosphere (N2, O2, Ar, and CO2), and 5-component atmosphere (N2, O2, Ar, CO2, and water vapor). First, analysis focuses on Rayleigh scattering, showing the relative difference between the three scenarios within classical approach. The relative difference in molecular scattering between 2(4)-component atmosphere and 5-component atmosphere is below ~1%. The second analysis focuses on the lidar retrieval of aerosol backscatter and extinction coefficients showing the effect of different molecular formulations. A relative difference of ±3% was found between the molecular formulation of 2-component atmosphere and the molecular formulation of 5-component atmosphere. Consideration of the Raman rotational lines blocked by the interference filter is important for the elastic channels, but of little significance in the N2 Raman channel. For lidar retrieval of aerosol profiles, the 5-component approximation is the best when the water vapor profile is known, but 2-component is still adequate and quite accurate when water vapor is only poorly known.