Christopher Cattrall

Variability of aerosol and spectral lidar and backscatter and extinction ratios of key aerosol types derived from selected Aerosol Robotic Network locations

Received 13 June 2004; revised 1 February 2005; accepted 3 March 2005; published 3 May 2005. [1] The lidar (extinction-to-backscatter) ratios at 0.55 and 1.02 mm and the spectral lidar, extinction, and backscatter ratios of climatically relevant aerosol species are computed on the basis of selected retrievals of aerosol properties from 26 Aerosol Robotic Network (AERONET) sites across the globe. The values, obtained indirectly from sky radiance and solar transmittance measurements, agree very well with values from direct observations. Low mean values of the lidar ratio, Sa, at 0.55 mm for maritime (27 sr) aerosols and desert dust (42 sr) are clearly distinguishable from biomass burning (60 sr) and urban/industrial pollution (71 sr). The effects of nonsphericity of mineral dust are shown, demonstrating that particle shape must be taken into account in any spaceborne lidar inversion scheme. A new aerosol model representing pollution over Southeast Asia is introduced since lidar (58 sr), color lidar, and extinction ratios in this region are distinct from those over other urban/industrial centers, owing to a greater number of large particles relative to fine particles. This discrimination promises improved estimates of regional climate forcing by aerosols containing black carbon and is expected to be of utility to climate modeling and remote sensing communities. The observed variability of the lidar parameters, combined with current validated aerosol data products from Moderate Resolution Imaging Spectroradiometer (MODIS), will afford improved accuracy in the inversion of spaceborne lidar data over both land and ocean.

Spaceborne lidar aerosol retrieval approaches based on aerosol model constraints

In the absence of auxiliary optical depth or transmittance information, or self-determination of same for specialized observing situations, aerosol backscatter and extinction profiles cannot be retrieved from lidar observations along a single direction without an assumption linking aerosol extinction and backscatter (i.e., the aerosol extinction-to-backscatter ratio, or aerosol lidar ratio, Sa). Aerosol retrievals at 532 nm for the current GLAS and upcoming CALIPSO satellite lidar missions employ/can employ a look-up table approach to select climatologically based Sa model values for these retrievals when alternate, less uncertain methods for either defining S a or providing the needed auxiliary information are unavailable. This paper addresses a revised table look-up approach that incorporates two notable revisions for improved Sa selection which, as a consequence, enable more bounded aerosol retrievals. One is a refined, more bounded set of Sa values, both for 532 nm and 1064 nm, representative of a definitive set of aerosol types/models determined from an extensive analysis of the global aerosol solar radiometer network, AERONET, data base. The other is an accompanying set of key spectral ratio parameters (i.e., dual wavelength, 532 nm to 1064 nm, ratios of backscatter, extinction and Sa) also derived from the AERONET data which offer additional ways to bound the lidar aerosol retrievals. Thus, aerosol retrievals can be obtained subject to the constraints that the lidar data yield retrievals with spectral ratio parameters consistent with a given aerosol model (or models), to confirm the model choice and better bound the retrievals. Examples of retrievals make subject to these constraints are included as a part of the paper

Ground-reference calibration results for Landsat-7 ETM+

Ground-reference techniques for the Enhanced Thematic Mapper Plus (ETM+) on Landsat 7 are described. The reflectance-based approach for vicarious calibration has been applied by the Remote Sensing Group of the Optical Sciences Center to 59 sets of ground-based data collected at large uniform test sites imaged by Landsat-7 ETM+. The results of this work coupled with the apparent stability of the ETM+ sensor shows that a one-sigma precision less than 3% is currently being achieved. Band-by-band analysis of the precision gives insight into the effects of atmospheric correction and surface reflectance uncertainties giving an understanding of the error sources in this approach. Variations in results are not seen between test sites and atmospheric effects are not the primary cause of day-to-day variability. These results are discussed with an emphasis on the current state of vicarious calibration accuracies/precision as well as areas for improvement and future accuracy expectations.

Comparison of ground-reference calibration results for Landsat-7 ETM+ for large and small test sites

Recent results of the vicarious calibration of the Landsat-7 ETM+ sensor are presented based on the reflectance-based vicarious method using results from a smaller test site in close proximity to the University of Arizona. The typical, larger test site is brighter and more spatially uniform then the smaller site. However, the location of the small test site allows for more frequent data collections and a more complete understanding of the calibration coefficients of the sensor as a function of time. The Remote Sensing Group previously reported results based on data collected from the smaller test site on seven dates. Here we report calibration values for additional dates as well as a comparison of the calibration values for the large and smaller test sites over recent years. The most recent data shows the calibration values using the smaller sites continue to agree within 3% of the large test sites in all bands.

Spaceborne lidar aerosol retrieval approaches based on improved aerosol model constraints

Aerosol retrievals at 532 nm for the current GLAS and upcoming CALIPSO satellite lidar missions employ/will employ a table look-up approach to select climatologically based S/sub a/ model values for these retrievals when alternate, less uncertain methods for either defining S/sub a/ (the aerosol extinction-to-backscatter ratio, or aerosol lidar ratio) or providing the needed auxiliary information are unavailable. Reagan et al. (2004, 2005) addressed a revised table look-up approach that incorporated two notable revisions for improved S/sub a/ selection, which, as a consequence, enable more bounded aerosol retrievals. One is a refined, more bounded set of S/sub a/ values, both for 532 nm and 1064 nm, representative of a definitive set of aerosol types/models determined from an extensive analysis of the AERONET database. The other is an accompanying set of key spectral ratio parameters (i.e., dual wavelength, 532 nm to 1064 nm, ratios of backscatter, extinction and S/sub a/) also derived from the AERONET data which offer additional ways to bound the lidar aerosol retrievals. Thus, aerosol retrievals can be obtained subject to the constraints that the lidar data yield retrievals with spectral ratio parameters consistent with a given aerosol model (or models), to confirm the model choice and better bound the retrievals. This paper presents the simulation results made by assuming different models in support of the two-wavelength lidar constrained ratio aerosol model-fit (CRAM) retrieval approach. In addition, a performance function and multiple scattering corrections based on LITE signals are also investigated.

Exploitation of MODTRAN4 capabilities to predict at-sensor radiance

Top-of-atmosphere radiance is computed between 350 and 2500 nm for atmospheres containing one of three aerosol models (rural, maritime and dust) inherent to MODTRAN, over different surface reflectance values, and compared with those computed using a model of the same aerosol species derived from measurements by a global network of ground-based radiometers (AERONET). It is observed that even over high reflectance targets (R=0.5), care must be taken in the prescription of aerosol optical properties so as to limit uncertainty resulting from aerosols in the top-of-atmosphere radiance to less than 2%. It is found that, for grass and desert sites, using a simple power law exponent derived from measured spectral optical depths reduces uncertainty in the computed satellite radiance resulting from prescription of aerosol properties to less than approximately 1.5% for the aerosol species examined. Uncertainty in the computed top-of-atmosphere radiance during vicarious sensor calibration over desert sites that may result from this simple prescription of the aerosol size distribution is thus less than uncertainty in the TOA radiance resulting from measurements of the site reflectance. The new aerosol and multiple scattering capabilities of the most recent version of MODTRAN have made such studies possible and are promising for attempts to use MODTRAN in the vicarious calibration of airborne and spaceborne sensors.

MODTRAN-based retrieval of column water vapor from solar transmittance

The Remote Sensing Group of the Optical Sciences Center at the University of Arizona has a history of collecting ground-based atmospheric data in support of calibration/validation and for atmospheric correction. This work has included the determination of columnar water vapor based on measurements of direct solar irradiance. In the past, the conversion of these data to transmittance and then column water vapor has relied upon a modified Langley approach and two-parameter band model of absorption developed in the 1980s at the University of Arizona. More recently, the RSG has used the well-known MODTRAN code for its prediction of at-sensor radiance and atmospheric correction. This work examines the use of the same MODTRAN code for the retrieval of column water vapor to simplify the overall processing approach of the RSG, as well as providing consistency between the measurements and the predicted at-sensor radiance. The water vapor retrieval using MODTRAN follows the same basic approach as the previous method except that the water vapor absorption parameters are obtained from MODTRAN. A sensitivity analysis is performed to examine the influence of the MODTRAN input parameters on the retrieval. The results of the new method are compared with results from GPS-derived column water vapor. These preliminary results show that the MODTRAN-based values have an accuracy of 10% and agreement with the GPS-derived results is better than 10%.

Lidar aerosol ratios at 1 and 10 microns

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.

Solar radiation based calibration of a short-wave ir radiometer and a comparison of exoatmospheric solar spectral irradiance data sets

We have performed an absolute calibration comparison between the Sun and a NIST-calibrated spectral irradiance standard lamp. The comparison at seven bands ranging from 1.2 to 2.3 μm was made using a highly stable Short Wave IR Transfer Radiometer. The experiment consisted of laboratory and outdoor measurements of an irradiated diffusely reflecting panel. Outdoors, simultaneous atmospheric transmittance measurements were also obtained in order to correct for atmospheric effects. The results were used to compare three absolute exo-atmospheric solar spectral irradiance data sets currently used in various remote sensing applications to a NIST traceable radiometric calibration. The comparison with the limited set of recently produced SORCE-SIM data showed almost no differences at 1.2 and 1.6 μm; however, the comparison with three other data sets showed larger differences.