Effects of misestimated far-end boundary values on two common lidar inversion solutions

Comparison of aerosol properties derived from sampling and near-horizontal lidar measurements using Mie scattering theory

Aerosol optical properties are measured near the surface level using sampling instruments and a near-horizontal lidar. The values of the aerosol extinction coefficient inside the instruments are derived from nephelometer and aethalometer data, while the ambient values are measured from the lidar. The information on aerosol size distribution from optical particle counters is used to simulate extinction coefficients using the Mie scattering theory, with corrections on the humidity growth of hygroscopic particles. By applying this method to the continuous data obtained from November to December 2018 at Chiba, Japan, we elucidate the temporal variations of near-surface aerosol properties, including the complex refractive index, single scattering albedo, and Angstrom exponent. The results indicate how aerosol particles change their properties between the dry, instrumental conditions and relatively humid setting of the ambient atmosphere.

Surface Aerosol Properties Studied Using a Near-Horizontal Lidar

Studying near-surface aerosol properties is of importance for a better assessment of the aerosol effect on radiative forcing. We employ the data from a near-horizontal lidar to investigate the diurnal behavior of aerosol extinction and single scattering albedo (SSA) at 349 nm. The response of these parameters to ambient relative humidity (RH) is examined for the data from a one-month campaign conducted in Chiba, Japan, during November 2017, a transition period from fall to winter. The Klett method and adaptive slope method are used in deriving the aerosol extinction coefficient from the lidar data, while the SSA values are retrieved using an aethalometer. Also, a visibility-meter is used to examine the aerosol loading inside the atmospheric boundary layer. It is found that the aerosol growth during the deliquescence phase is more readily observed than the contraction in the efflorescence phase. The decrease of SSA before the deliquescence RH is found for approximately 46% of the deliquescence cases, presumably representing the particle shrinkage of soot particles.

Practical analytical backscatter error bars for elastic one-component lidar inversion algorithm

We present an analytical formulation to compute the total-backscatter range-dependent error bars from the well-known Klett's elastic-lidar inversion algorithm. A combined error-propagation and statistical formulation approach is used to assess inversion errors in response to the following error sources: observation noise (i.e., signal-to-noise ratio) in the reception channel, the user's uncertainty in the backscatter calibration, and in the (range-dependent) total extinction-to-backscatter ratio provided. The method is validated using a Monte Carlo procedure, where the error bars are computed by inversion of a large population of noisy generated lidar signals, for total optical depths tau < or = 5 and typical user uncertainties, all of which yield a practical tool to compute the sought-after error bars.

Quasi-analytical determination of noise-induced error limits in lidar retrieval of aerosol backscatter coefficient by the elastic, two-component algorithm

The elastic, two-component algorithm is the most common inversion method for retrieving the aerosol backscatter coefficient from ground- or space-based backscatter lidar systems. A quasi-analytical formulation of the statistical error associated to the aerosol backscatter coefficient caused by the use of real, noise-corrupted lidar signals in the two-component algorithm is presented. The error expression depends on the signal-to-noise ratio along the inversion path and takes into account "instantaneous" effects, the effect of the signal-to-noise ratio at the range where the aerosol backscatter coefficient is being computed, as well as "memory" effects, namely, both the effect of the signal-to-noise ratio in the cell where the inversion is started and the cumulative effect of the noise between that cell and the actual cell where the aerosol backscatter coefficient is evaluated. An example is shown to illustrate how the "instantaneous" effect is reduced when averaging the noise-contaminated signal over a number of cells around the range where the inversion is started.

Effects of noise on lidar data inversion with the backward algorithm

The lidar data-inversion algorithm widely known as the Klett method (and its more elaborate variants) has long been used to invert elastic-lidar data obtained from atmospheric sounding systems. The Klett backward algorithm has also been shown to be robust in the face of uncertainties concerning the boundary condition. Nevertheless electrical noise at the photoreceiver output unavoidably has an impact on the data-inversion process, and describing in an explicit way how it affects retrieval of the atmospheric optical coefficients can contribute to improvement in inversion quality. We examine formally the way noise disturbs backscatter-coefficient retrievals done with the Klett backward algorithm, derive a mathematical expression for the retrieved backscatter coefficient in the presence of noise affecting the signal, and assess the noise impact and suggest ways to limit it.

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.

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.

Errors in the retrieval of thin-cloud optical parameters obtained with a two-boundary algorithm

The effect of various errors on the retrieval of optical depth, integrated backscatter, and extinction-to-backscatter ratio in optically thin clouds by use of a two-boundary algorithm is discussed. Uncertainties regarding aerosol loading at the cloud base and top lead to relevant errors that are often larger than those produced by signal noise. Formulas expressing the errors in the lidar-derived optical quantities dependent on optical depth and aerosol uncertainties at the base and top are derived for different fitting procedures. A method for the reduction of errors in the case of consecutive cloud measurements is explored, consisting of the fitting of the retrieved optical-depth-integrated backscatter data to obtain a correct extinction-to-backscatter ratio.

Multiply scattered aerosol lidar returns: inversion method and comparison with in situ measurements

A novel aerosol lidar inversion method based on the use of multiple-scattering contributions measured by a multiple-field-of-view receiver is proposed. The method requires assumptions that restrict applications to aerosol particles large enough to give rise to measurable multiple scattering and depends on parameters that must be specified empirically but that have an uncertainty range of much less than the boundary value and the backscatter-to-extinction ratio of the conventional single-scattering inversion methods. The proposed method is applied to cloud measurements. The solutions obtained are the profiles of the scattering coefficient and the effective diameter of the cloud droplets. With mild assumptions on the form of the function, the full-size distribution is estimated at each range position from which the extinction coefficient at any visible and infrared wavelength and the liquid water content can be determined. Typical results on slant-path-integrated optical depth, vertical extinction profiles, and fluctuation statistics are compared with in situ data obtained in two field experiments. The inversion works well in all cases reported here, i.e., for water clouds at optical depths between ~0.1 and ~4.

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.