A methodology is presented, by which atmospheric aerosol retrievals from a standard, elastic-scatter, lidar can be constrained by using information from coincident measurements from a high spectral resolution lidar (HSRL) or Raman lidar at a different wavelength. As high spectral resolution or inelastic-scattering lidars are now being incorporated coaxially into instruments with traditional, elastic-scatter channels at different wavelengths, a standard approach is needed to incorporate or fuse the diversity of spectral information so as to make maximal use of the aerosol measurements made from the elastic-scatter channel or channels. The approach is evaluated through simulation and with data from the NASA Langley Research Center Airborne HSRL instrument. The generality and extensibility of the method is also explored and discussed in the context of aerosol modeling.
The Aeronet network has had a wide-ranging impact on the study of atmospheric aerosols, both temporally and geographically. This paper examines the results of measurements from the Aeronet network from a radiometer deployed in Tucson, Arizona during 1999 and 2000. Monthly averages of aerosol optical depth and Angstrom parameter values are presented. These show that a maximum in aerosol loading occurs in summer months with an average value for optical thickness of 0.11 at 670 nm compared to 0.03 during winter months. The Angstrom coefficient shows a similar trend with largest values, corresponding to smaller-sized aerosols dominating during the summer months. These results show significant differences from results obtained from similar measurements during the period of 1975-77. In addition to optical depth, aerosol extinction-to-backscatter ratio, or lidar aerosol ratio, is calculated and examined using size distribution data available from Aeronet and Mie scatter computations. This ratio varies from an average value of near 25 in March, April, and May to values near 100 for October, November, and December. Comparison of a subset of these data to those from an independent solar radiometer support these conclusions.
Lidar (extinction-to-backscatter) ratios are computed at 0.55, 1 and 10 μm, based upon a recently published summary of the physicochemical properties of climatically relevant aerosol species. The results agree very well with previously measured values in the literature, indicating that low Sa values for desert dust (15-30) and maritime (30-45) aerosols are clearly distinguishable from biomass burning (55-65) and urban/industrial pollution (55-80). The results show that most aerosol types can be discriminated by their absorption and scattering characteristics through use of spectral lidar ratios, except between biomass burning and pollution aerosols. Predictions of on- and off-axis scattering in the presence of these aerosol types illustrate the range of signal that may be expected in a bistatic lidar system in such cases, and indicate that bistatic lidar may be successfully used to detect a source lidar signal and discriminate the aerosol species present. These findings strongly suggest that a combination of passive and active remote sensing systems operating simultaneously (e.g., ground-based sky radiance and bistatic lidar), would be capable of directly measuring the absorption and scattering characteristics required to describe the optical behaviour of the aerosol with vertical resolution. This is expected to be of great utility to climate researchers or other communities interested in comprehensively measuring atmospheric optical properties.
A variation of the direct detection Doppler Lidar method known as the edge technique is discussed. This new method uses a frequency-agile laser transmitter to alternate the outgoing laser pulse between the two edges of a high resolution Fabry-Perot etalon. The difference in sign of the two slopes of the edge allows the unwanted molecular return signal to be eliminated as a background error source. This technique is similar to that of the 'double-edge' technique, with the main advantage being reduced complexity and cost for the system. By eliminating the error in the wind velocity measurement due to the molecular return, transmitter powers, within the eye-safety range, may be utilized to measure winds within the planetary boundary layer (PBL) with reasonable accuracy for many applications.
This paper addresses current design improvement issues of aerosol sensing Micro-Pulse Lidars (MPL). MPLs are designed to adhere to eye-safety restrictions while achieving acceptable signal to noise ratios (SNR). This method is realized by reducing the per pulse energy of the laser and employing a narrow receiver field-of-view (FOV). Due to the narrow FOV requirement, only a partial return signal is measured until the laser beam propagates a distance where the receiver FOV fully overlaps the laser beam. This is called the full overlap distance and is usually 4 km or more for reasonable MPL parameters. Accurate MPL measurements are typically only possible beyond this distance. The fraction of laser beam energy that is within the receiver FOV versus range is called the overlap function. The causes of the overlap function are discussed. An overlap related problem with current MPL designs is that the majority of the atmospheric aerosols are located below an altitude of 4 km to 5 km, within the partial overlap region. Another problem is that the overlap function is not thermally constant. This introduces errors in the experimentally derived overlap function and system constant factor, ultimately leading to errors in the retrieved lidar signal.
By combining the capability of a differential absorption lidar (DIAL) and the excellent characteristics of a micro pulse lidar (MPL) we have designed and tested a micro pulse DIAL system, which could be operated from the ground or airborne platform, to monitor the atmospheric water vapor mixing ratio. To maintain the compact and rugged optical frame work of an MPL it employs a diode pumped tunable Cr:LiSAF laser operating at 825 - 840 nm range, a fiber optic beam delivery system, and an APD photon counting detector. The system parameters were optimized through extensive DIAL simulations, and the design concept was tested by building a breadboard lidar system. Based on the results of the simulations and the performance of the breadboard lidar the Micro Pulse DIAL system design has been refined to (1) minimize scattered laser light -- the major source of signal induced bias, (2) permit near field measurements from less than 400 m, (3) produce a compact, rugged, eye-safe instrument with a day and night operating capability. The lidar system is expected to provide 150 m vertical resolution, high accuracy (approximately 5%), and 3 km range looking up from the ground.
Lidar observations from the Lidar-in Space Technology Experiment (LITE), in conjunction with European Center for Medium Range Weather Forecasting (ECMWF) and Meteosat data were used to examine the Saharan dust characteristics including its structure, evolution and optical depths over Western Africa and E. Atlantic regions. The lidar backscatter profiles reveal a complex structure of the dust layer but, in general, show a good agreement with the features depicted in the conceptual model of the dust plume. Optical depths of the Saharan dust layer derived from two independent methods were compared with those obtained from the Meteosat data. Although the LITE-derived optical depth patterns from the two methods are in good agreement with each other, they show some differences with those derived from the satellite data, particularly in the inference of heavy dust concentration over the E. Atlantic.
The design of a three-channel solar radiometer for obtaining total columnar water vapor using solar transmittance and differential absorption is presented. Water vapor transmittance is determined using a modified Langley approach and converted to columnar water vapor using a band model developed at the University of Arizona. Several cases are presented in which columnar water vapor amounts determined using the current instrument and method are compared to sounding balloon results. Tests using simulated data indicate that columnar water vapor may be retrieved with an uncertainty less than 10%.
This paper describes approaches for using the strong return signals from ground/sea reflections to improve upon the information that can be retrieved from spaceborne lidar observations. Relations are presented for computing lidar ground/sea returns, and examples are given for representative lidar parameters and surface characteristics. Various
methods are outlined for obtaining information about atmospheric transmittance, surface reflectance, and aerosol/cloud features from lidar ground/sea returns. Techniques for retrieving aerosol extinction profiles from atmospheric lidar returns are also presented, including retrieval examples.
Special Section Guest Editorial
John A. Reagan, MEMBER SPIE
University of Arizona
Department of Electrical and Computer Engineering
Tucson, Arizona 85721
This special section on lidar provides a snapshot of a variety of ongoing and future lidar applications and developments. It is by no means all inclusive because the field oflidar has grown too large in the almost 30 years since its inception to be treated comprehensively in a single issue.