RADIOMETER-BASED ESTIMATION OF THE ATMOSPHERIC OPTICAL THICKNESS

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1 RADIOMETER-BASED ESTIMATION OF THE ATMOSPHERIC OPTICAL THICKNESS Vassilia Karathanassi (), Demetrius Rokos (),Vassilios Andronis (), Alex Papayannis () () Laboratory of Remote Sensing, School of Rural and Surveying Engineering () Lidar group, School of Physics National Technical University of Athens, Heroon Polytechniou 9, 5780 Athens, Greece Keywords: Atmospheric optical thickness, radiometer, Lidar system, radiance, reflectance, extinction coefficient, SPOT imagery. ABSTRACT Atmospheric optical thickness affects the quality of satellite imagery, especially over urban areas where aerosol concentrations (sulphates, soot, mineral dust, etc.) are high. Optical thickness is usually provided by photometer or Lidar ground station measurements and introduced in empirical atmospheric correction models. However, Lidar wavelengths do not correspond - in number and value - to those used by satellite sensors, and therefore they introduce drawbacks in atmospheric correction methods. In this paper, a methodology developed for estimating atmospheric optical thickness by the use of satellite images and ground radiometer (GER 500) is described. Lidar measurements of the optical thickness at two wavelengths, 55 nm and 5 nm, served for validation purposes. Within this framework, two major issues are investigated. The first concerns the most appropriate target in an urban that yields the most accurate atmospheric optical thickness value. The second deals with the appropriate surrounding area of the target. Evaluation of results showed that using the methodology developed, targets of olive leaves within a black artificial area produce the most accurate atmospheric optical thickness.. INTRODUCTION A relationship between atmospheric aerosols and the wavelength dependence of the extinction coefficient was first suggested by Angstrom []. Since that time, Yamamoto and Tanaka [] were the first to apply an algorithm to spectral measurements of extinction coefficient in order to determine an aerosol size distribution and indirect the optical thickness. Griggs [] estimated optical thickness over water and King et al. [] applied a formula of determining aerosol distribution and optical thickness from spectral measurements. Tanre et al. [9] and Rao et al. [7] have showed function methods that can be used to estimate the atmospheric optical depth from ground-based measurements and satellite data respectively. Wagener [0] has retrieved optical depth from AVHRR observations over oceans. Liu et al. [4] determined optical thickness from SPOT data and Tang-Huang et al. [8] improved the accuracy of these methods and applied SPOT imagery to estimate optical thickness in complex terrain. Milton et al. [5] observed relationships between spectral radiance and the variance in spectral reflectance and applied a method for precise measurements of spectral reflectance in the field [6]. In this paper a methodology developed for estimating atmospheric optical thickness by the use of SPOT XS imagery and ground radiometer (GER 500) is presented. Lidar estimations of the optical thickness at two wavelengths (55 nm and 5 nm) served for validation purposes. Within this framework, two major issues are investigated. The first concerns the most appropriate target in an urban that yields the most accurate atmospheric optical thickness value. The second deals with the appropriate surrounding area of the target.. DATA AND EQUIPMENT Experiments were carried out in Athens, Greece. The study area has been selected because it presents high concentrations of air pollutants. Radiometric and Lidar measurements simultaneous to SPOT XS satellite pass were performed. The radiometer GER 500 of Remote Sensing Laboratory of NTUA, the Lidar system of Physics Department Laser Applications of NTUA and SPOT XS imagery of 8//00 were used. The SPOT imagery was geocoded with an rms error in x and y of less than the size of a 8-m pixel.

2 Fig.. The SPOT XS imagery of the study area. METHODOLOGY AND IMPLEMENTATION The radiance of seven targets: grass (approximately 4cm high), bare soil, asphalt, bush (approximately m high), limestone, white pine, and olive leaves was measured with the radiometer GER 500. Each measurement took place between 0:0 am and :00 am and for each target the radiance of the reference surface was also measured. The reflectance of each target was consequently estimated. The height of the radiometer was kept fixed at.60m, and the diameter of the measured targets was approximately 0.40m. The atmospheric conditions during the measurements were: sunlight, temperature. o C, relative humidity 69.0% and wind speed.5 m/s. Then, for each target, the optical thickness has been estimated on the basis of GER 500 measurements and SPOT image digital values. This value was compared with the value provided by the Lidar system. Experiments have been repeated several times. Each time the targets were placed in different surrounding s consisting of natural or artificial materials. Fig.. Samples of the ground targets as they appeared in the IFOV of GER 500 (white pine, asphalt, grass and bare soil, olive leaves and bushes from left to right).

3 Fig. 4. Radiance diagrams of the ground targets (white pine, bushes and bare soil above, olive leaves, grass, and asphalt and limes tones below from left to right). The same targets were located in the SPOT imagery and their radiance was estimated for each, based on their digital numbers and the absolute calibration gains of the SPOT XS sensor. Table. Estimated radiance based on SPOT XS image and measured radiance using GER 500 radiometer ground target Digital numbers ( SPOT XS image) Estimated radiance SPOT XS image (w - m sr μm) Measured radiance GER 500 (w - m sr μm) asphalt lime stones white pines grass bushes bare soil olive leaves s For each target, based on the Bouguer equation the optical thickness I( s) = I(0) exp k( s) ds s 0 0 k( s) ds was estimated: where: Ι l (0): radiance measured on the earth surface, Ι l (s): radiance on the top of the atmosphere k : extinction coefficient, s : the satellite height from the earth surface Table. The optical thickness as measured by the Lidar and estimated by the proposed methodology measured by the Lidar system optical thickness estimated from the Bouguer equation for of SPOT image target asphalt Lime stones 0.96 white pine leaves grass 0.89 bushes bare soil 0.80 olive leaves mean value 0.76 The accuracy of optical thickness provided by the Lidar system is approximately 0% (i.e. values range from 0.4 to 0.485). The second wavelength of the Lidar system (i.e. 5nm) is included in the width of the first of the SPOT image ( nm). Thus, the mean value of the optical thickness estimated for the first of the SPOT image has been compared with the optical thickness value measured by the Lidar system at λ= 5 nm. Their difference (table ) is due to the fact that:

4 the neighboring targets contribute to the target radiance, and Lidar system measures the optical thickness for a depth of 8500m, where suspended particles are present, whereas the SPOT XS imagery is taken at m height from the earth surface. Table. Differences between the mean value of the optical thickness provided by the SPOT-radiometer method and the minimum, maximum and mean value provided by the Lidar system Lidar measurement Optical thickness Proposed methodology (Estimation for SPOT ) Difference Minimum value Maximum value Mean value The experiments have been repeated four more times. Each time the targets were placed in different surrounding s consisting of natural or artificial materials. When the targets have been placed within artificial material i.e. white or black cardboard, they produce the most accurate optical thickness values. Table 4 presents values of the optical thickness estimated for targets within a white and a black cardboard. Table 5 presents the estimated optical thickness for targets of tree leaves within different natural or artificial surrounding s. The first three s are natural and consist of grass (approximately 4cm high), bare soil, asphalt, bush (approximately m high), limestone, white pine, and olive leave surfaces in various proportion, and the last two are artificial consisting of white and black cardboard. The mean value of the coefficients produced by all the targets is also indicated. Table 4. The estimated optical thickness by the SPOT-radiometer method for targets placed within white and black cardboard Target Estimated optical thickness Artificial : black cardboard Artificial : white cardboard asphalt limestone white pines grass bushes bare soil olive leaves Mean value Table 5. Values of optical thickness in various surrounding Target Experiment (initial) Experiment Experiment estimated optical thickness Experiment 4 White cardboard Experiment 5 Black cardboard white pine leaves olive leaves Mean value

5 4. EVALUATION OF THE RESULTS AND CONCLUSIONS A method based on satellite and ground radiometer measurements for the estimation of the optical thickness has been developed. Several targets and surrounding s were tested in an urban area. Comparison with Lidar measurements (table 5) showed that targets with tree leaves produce the most accurate values ranging from 0.5 to All these values fall in the interval between which is indicated by the Lidar system. Olive leaves surrounded by black cardboard produce almost the same value of optical thickness with the Lidar system. For all the surrounding s, the mean optical thickness value estimated by the respective values of the seven targets, present relatively higher value compared to that of the Lidar system. Artificial surrounding s produce the most accurate mean values, which fall in the interval indicated by the Lidar system. The value of the white cardboard presents the highest accuracy. Concluding, accurate optical thickness can be produced by olive leaves and generally tree leaves within a black cardboard. Also, accurate values are produced by estimating the mean value of optical thickness values provided by several urban targets, each target placed separately within a white cardboard. The method has the advantage that the atmospheric optical thickness in the wavelength of a satellite image can be estimated, thus accurate atmospherically corrected remote sensing data can be produced. 5. REFERENCES []. Angstrom, A., On the atmospheric transmission of sun radiation and on dust in the air, Geogr. Ann., Vol., 5666, 99. []. Griggs, M., Measuraments of atmospheric optical thickness over water using ERTS- data, Journal air pollution control association, Vol. 5, 6-66, 975. []. King, M. D., Byrne, D. M., Herman, B., Reagan, J., Aerosol Size Distributions Obtained by Inversion of Spectral Optical Depth Measurements, Atm. Science, Vol. 5, no, 978. [4]. Liu, G. R., Lin, T. H., Chen, A. J., An improved method to determine aerosol optical depth from SPOT data, COAA 97 First International Ocean Atmosphere Conference, 8-9 Oct., Washington, USA, 977. [5]. Milton, E.J. and Goetz, F.H., Atmospheric influences on field spectroscopy: observed relationships between spectral radiance and the variance in spectral reflectance, Seventh International Symposium on Physical Measurements and Signatures in Remote Sensing (ISPRS), Courchevel, France, 09-4, 997. [6]. Milton, E.J., Emery, D.R. and Lawrance, D.J., A new dual beam technique for precise measurements of spectral reflectance in the field, International Symposium on Remote Sensing of Environment, ERIM, Michigan, USA, 000. [7]. Rao, C. R. N., McClain, E. P., Stowe, C. C., Remote sensing of aerosols over the oceans using AVHRR data theory, Practice and Application, Int. J. Remote Sensing, Vol. 0 (4-5), , 989. [8]. Tang-Huang Lin, Chen, A. J., and Gin -Rong Liu, Applying SPOT Data to Estimate the Atmospheric Aerosol Optical in Complex Terrain, ACRS 0 th Asian Conference on Remote Sensing, November -5, Hong Kong, China, 999 [9]. Tanre, D., Devaux, C., Herman, M., Santer, R., Gac, J. Y., Radiative properties of desert aerosols by optical ground based measurements at solar wavelength, J. Geophysics Res., Vol. 9 pp 4-4, 988. [0]. Wagener, R., Aerosol Optical Depth over Oceans: High Space- and Time-Resolution Retrieval and Error Budget from Satellite Radiometry, Journal of Atmospheric and Oceanic Technology, Vol. 4, American Meteorological Society, 577, 996. []. Yamamoto, G., and Tanaka, M., Determination of aerosol size distribution from spectral attenuation measurements, Appl. Opt., Vol. 8, , 969. ACKNOWLEDGMENT This work was carried out in the framework of the research program THALIS: 65/4.