The extraction of optical parameters of the atmosphere from the scanning-lidar signals is possible only for the horizontally stratified atmosphere. The significant issue this technique is that no reliable methods exist for determining whether the searched atmosphere is horizontally stratified, and if not, where the heterogeneous areas are located. In the paper the essence and specifics for practically locating such areas of heterogeneous, poorly stratified atmosphere, from which lidar data should be excluded from analysis, are considered. To apply the three-directional differential solution, the lidar backscatter signals measured under three elevation angles are selected and analyzed. The theoretical basis of this solution is delineated. The specifics inherent to this remote sensing method are illustrated by simulated and experimental lidar data.
Biomass fires can significantly degrade regional air quality through the emission of primary aerosols and the photochemical production of ozone and secondary aerosols. The injection height of smoke from biomass burning into the atmosphere (‘plume rise height’) is one of the critical factors in determining the impact of fire emissions on air quality. Plume rise models are used to simulate plume rise height and prescribe the vertical distribution of fire emissions for input to smoke dispersion and air quality models. While several plume rise models exist, their uncertainties, biases, and application limits when applied to biomass fires are not well characterized. The poor state of model evaluation is due in large part to a lack of appropriate observational datasets. We have initiated a research project to address this critical observation gap. In August of 2013 we performed a multi-agency field experiment designed to obtain the data necessary to improve the air quality models used by agricultural smoke managers in the northwestern United States. In the experiment, the ground-based mobile lidar, developed at the US Forest Service Missoula Fire Science Laboratory, was used to monitor plume rise heights for nine agricultural fires in the northwestern United States. The lidar measurements were compared with plume rise heights calculated with the Briggs equations, which are used in several smoke management tools. Here we present the preliminary evaluation results and provide recommendations regarding the application of the models to agricultural burning based on lidar measurements made in the vicinity of Walla Walla, Washington, on August 24, 2013.
The distortions of the inverted lidar signals can be caused by (i) the constant offset that remains in the backscatter signal after removing the background component, (ii) the multiplicative distortion component, which level is related with the lidar signal intensity, and (iii) the signal noise in the wide wavelength spectra; the latter includes lowfrequency components, which do not obey common random-noise statistics. These distortions, even minor, may yield significant distortions in the retrieved outputs obtained by the inversion of the lidar signal. Implicit and explicit premises and assumptions required for any solution of the lidar equation are additional sources of the uncertainty in the inversion results. There is no reliable way for checking whether used assumptions are valid, therefore, the lidar signal inversion can yield significantly biased results. As a result, instead some statistically mean profile of the atmospheric parameter of interest with the corresponding probability function, one obtains some qualitative estimate of the profile with unknown uncertainty, which depends on the validity of used assumptions. It means that lidar profiling is not a measurement but a result of some simulation based on past observations.
KEYWORDS: LIDAR, Backscatter, Atmospheric particles, Atmospheric monitoring, Atmospheric modeling, Mass attenuation coefficient, 3D modeling, Data modeling, Knowledge management, Signal attenuation
Basic results of a comprehensive investigation of the potential and restrictions of the remote sensing lidar technique in
smoke-polluted atmospheres made in the Missoula Fire Sciences Laboratory (FSL) are presented. The study is based on
the three-year lidar measurements of dynamics and optical characteristics of smoke plumes originated in prescribed
burns and wildfires. For the measurements, a mobile two-wavelength scanning lidar was used. The lidar operated in the
vertical scanning mode and in a combined vertical-azimuthal mode and provided detailed, range-resolved information on
the smoke particulate loading up the distances and heights of 5 - 10 km from the lidar.
The lidar was successfully used for the real-time determination of smoke plume dispersion, its top heights, and
spatial boundaries. In some cases, the measured smoke plume tops reached heights of more than 8 km above ground
level. The lidar measurements close to large wildfires also revealed numerous cases of a multilayered atmosphere with
well-defined horizontally stratified smoke layers, generally, at heights between 1 and 3 km, originating in morning
inversions and then sustained by the solar heating of the layers. The time series measurements allowed monitoring of
their temporal transformation, including the downdraft transport of the smoke particulates to ground level.
Special measurement methodology and data processing techniques for the smoke-polluted atmospheres were
developed. This made it possible to obtain accurate vertical profiles of the optical characteristics of the smoke
particulates, such as optical depth, and the backscatter and extinction coefficients.
An improved methodology for processing scanning lidar data is considered. We demonstrate a new principle of determining vertical profiles of the particulate extinction coefficient and the lidar ratio with the Kano-Hamilton multiangle solution. This technique, which is also applicable to combined elastic/inelastic lidar measurements, computes the extinction coefficient from the backscatter term rather than from optical depth, thus avoiding numerical differentiation. The inversion is based on determining the stepwise column-integrated lidar ratios that provide the best matching of the initial profile of the optical depth to that obtained after the inversion. We explore two approaches concerning the division of the column-integrated lidar ratio into different ranges: in the first case, divisions between ranges are uniformly distributed; in the second case, divisions are located using estimated uncertainty boundaries in the inverted optical depth.
The inversion method was used to process the experimental data obtained in the vicinity of large wildfires with the Fire Sciences Laboratory lidar. Examples of the simulated and experimental data are presented, which illustrate the specifics and prospects of this data-processing methodology.
The lidar equation uncertainty, caused by the presence of two unknown functions (extinction and backscatter coefficients), is the main source of measurement errors in the elastic lidar searching of the atmosphere. The multiangle data-processing technique that applies the layer-integrated form of the angle-dependent lidar equation, allows one to avoid the a priori selection of the extinction-to-backscatter ratio. However, the practical use of the technique is impeded by atmospheric horizontal heterogeneity and distortions in real lidar data, which worsen the inversion accuracy of the retrieved extinction-coefficient profiles; in addition, the multiangle solution is extremely sensitive even to minor systematic distortions in the inverted data. A combination of the one-directional and multiangle data-processing technique improves the measurement accuracy of the retrieved data. Here no guesses are required about the vertical profile of the extinction-to-backscatter ratio. The technique was developed and tested with experimental data using a two-wavelength scanning lidar at the Fire Sciences Laboratory in Missoula, MT, USA. The specifics, advantages, and restrictions of this combined technique are discussed and illustrated by both synthetic and experimental data.
Practical inversion methods are presented to account for the multiple scattering component of lidar signals. Two variants of the lidar signal inversion for dense smoke plumes, the near-end solution and the optical depth solution, are considered. In both cases, the measured lidar signal, contaminated by multiple scattering, is transformed into a form similar to the single-scattering lidar equation for a single-component atmosphere. To achieve this, a transformation factor related to the range-dependent ratio of the multiple-to-single scattering is determined using an assumed dependence of the ratio on the smoke optical depth.
A new lidar-inversion technique is presented for the determination of the extinction-coefficient profile within a spatially restricted zone of atmospheric aerosol inhomogeneity such as a plume, thin cloud, etc. The return lidar signal is measured through the aerosol plume under investigation and also in a direction close to but outside the plume. By using the ratio of these signals, the constituent produced by the aerosol plume is separated from the aerosol background constituent. An iterative lidar- inversion technique is applied to the ratio of these signals rather than to the original signal. This technique is shown to be relatively insensitive to the assumed value of parameters used for the extinction-profile retrieval, and yields an acceptable measurement results even when the accuracy of the assumed parameters is poor.
New algorithms are developed to improve the methodology of the ozone profile extraction from the signals measured by an ultraviolet DIAL system in a turbid troposphere. A routine procedure is developed to estimate the likely boundaries of the uncertainty in the retrieved ozone concentration profile caused both by the errors in the measured signals and by an uncertainty in the atmospheric characteristics used for the ozone concentration correction (specifically, by uncertainties in the assumed aerosol backscatter-to-extinction ratio and spectral dependence of the aerosol extinction and backscattering). The algorithms are integrated into a computer program, and a preliminary verification of the new technique for the ozone concentration derivation is made with one and two pairs of the signals, measured at the on and off wavelengths of the DIAL system.
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