Leonid Sokoletsky 1 , Hezi Gildor 1 , Emmanuel Boss 2

1. ; ; ; ; OS361-04Bio-Optical Properties of the Dead Sea  the poster as it is about model of bio-optical properties. I see no IOPs presented anywhere. Leonid Sokoletsky1, Hezi Gildor1, Emmanuel Boss2 1Department of Environmental Sciences & Energy Research, The Weizmann Institute of Science, 76100 Rehovot, Israel; e-mails: leonid.sokoletsky@weizmann.ac.il and hezi.gildor@weizmann.ac.il 2School of Marine Sciences, The University of Maine, 5741 Libby Hall, Orono, ME 04469-5741, ME United States; e-mail: emmanuel.boss@maine.edu INCOMING SOLAR AND SKY IRRADIANCE Global (i.e., solar direct and sky diffuse) radiation incoming on the Dead Sea was estimated from the measurements of the upwelling radiance of a near-perfectly diffuse surface (Spectralon reference standard) and using simple model for the Dead Sea developed by Sokoletsky [3]; for the total irradiance (appr. 0.2 to 4 mm spectral interval; units are W m-2) it have a form: Ed, total (0+) = 1027f(Cosqatm)1.565 , (3) where f is a factor taking the variations in the Earth-Sun distance into account. The conversion from the total to spectral irradiance performed using spectral extraterrestrial irradiance values by Thuillieret al. [2]. The historical data by Anati were compared directly with a model expressed by Eq. (3) in the Fig. 3. INTRODUCTION The aim of our study is to get a better understanding of bio-geo-optical properties of the Dead Sea, thesaltiest lake in the world (a salinity of about 30%). Being such a highly saline terminal lake, the Dead Sea is characterized by extreme conditions including low pH values, restricted biology, and thus represents a unique "environmental laboratory" for the aquatic optical studies. Only the unicellular flagellate green alga (Dunaliella parva), several types of the halophilic Archaea (archaeobacteria) and eubacteria (bacteria) exist, and usually in very low concentrations. The first regular optical measurements (vertical profiles of the downwelling irradiance in the photosynthetically active region, PAR) have been carried out during the period from the 1960s to 1980s by Dr. David Anati. We present results of processing of these historical data in the form of a vertical gradient of downwelling attenuation coefficient, and show that there is an essential relation of this optical property with the Dunaliella parva content in the lake. Recent optical observations carried out in April 2004 in the Dead Sea allow the developmentof a complete bio-geo-optical model of the lake. The input parameters for the model are spectral values of vertical absorption and attenuation coefficients (measured at 9 wavelengths using a WETLabs ac-9 instrument), vertical volume scattering function (measured at 660 nm using a WETLabs three-angle backscattering meter ECO-VSF), and the above-water downwelling irradiance and reflectance factor (measured over the whole short-wave spectrum by the FieldSpec Pro ASD spectroradiometer). Measurements of absorption and attenuation coefficients were performed with and without size filters. These measurements give a possibility to estimate a variety of optical abd biogeochemicalproperties. Additionally, a simple atmospheric transmittance model above the Dead Sea developed before, has been verified with the current observations. A success of such a verification along with the applying the reciprocity principle for transmittance make potentially possible a development of atmospheric correction algorithm. Such an algorithm along with the in situ relationships derived can be used for monitoring the lake and ecological forecasting. Figure 7.Vertical profiles of the particulate relative refractive indices. Numbers in the legend represent the Dead Sea samples just as in Fig. 5. Figure 4.Observed spectra of reflectance factor (FieldSpec Pro ASD spectroradiometer); Dead Sea, 13-14 April 2004. DIFFUSE ATTENUATION COEFFICIENT FOR DOWNWELLING IRRADIANCE Figure 2.The same as Fig. 1, but for the recent observations. Figure 5.Comparison between the modelled plane albedo (PA) and the measured reflectance factor (RF) values. Numbers in the legend represent the samples from the Dead Sea data collection, 13-14 April 2004. The stations 001 and 003 are shallow (up to 25 m) and the stations 005-026 are deep (up to 300 m). Figure 8.Vertical profiles of the TSM concentration. Numbers in the legend represent the Dead Sea samples just as in Fig. 5. • CONCLUSIONS AND FUTURE WORK • A comparison between the historical and recent diffuse attenuation observations indicates the current lack of the phytoplankton in the lake  I have not seen data to show that they were there in the 80’s. • A comparison between the measured and modelled global irradiance shows that a simple model based on the solar zenith angle knowledge only is appropriate for the Dead Sea. Such a model should be examined for further remote sensing aims. • A comparison between the measured and modelled reflectance properies shows a good correlation, what will be used for solution of the optical closure problem: estimation of inherent optical properties from reflectance What??? • The physical properties of solid particles (mean sizes, refractive indices, concentrations and densities) were predicted.  why should we trust these? No validation. Where are the IOP? What is so special about the Dead Sea? • A future work should include both in situ and remote sensing observations of solid particles and dissolved matter to confirm or to disprove bio-geo-optical model developed.  this is called validation, not proof. • I was under the impression that the poster will be on data not modeling. Other than the rrs and k data the rest is interpretation. The validation of the IOP – Rrs data would have been useful as well. Figure 1. Historical observations of the PAR-averaged diffuse attenuation coefficient near surface (courtesy by Dr. David Anati). These data calculated for the true solar zenith angle, SZA (blue curve) and for zenith Sun (green curve). The peaks of the attenuation coefficient (noted by circles), correspond to the periods when the flagellate green alga (Dunaliella parva) appeared near the surface (due to significant dilution of the upper water layers caused by the massive rain floods), increasing the water turbidity. The spectral diffuse attenuation coefficient Kdcurrently was calculated by the equation (derived from the radiative transfer consideration by Lee et al. [1]: Kd(l, z) = (1+0.005qatm)a(l, z)+4.18bb(l, z){1-0.52exp[-10.8a(l, z)]}, (1) where l is wavelength (in nanometers), z is the depth (in meters), qatm is the solar zenith angle in the air (in degrees), a and bb are the absorption and backscattering coefficients (both in m-1), respectively. The PAR-averaged diffuse attenuation coefficient was calculated as , (2) where Ed(l, 0+) is the incoming solar spectral irradiance taken from the standard curve by Thuillier et al. [2]. A comparison of Kd, PARobserved recently (Fig. 2),with observations carried out during 1980’s (Fig. 1) shows clearly the reduction of the lake turbidity in 2004. This is obviously evidence of a lack of phytoplankton in the lake during observations. Figure 3. Observed (blue symbols and blue fitting curve) and modelled (pink curve) total global irradiance above the Dead Sea during 1980-1986. REFERENCES 1. Lee Z.-P., Du, K.-P., and R. Arnone. 2005. A model for the diffuse attenuation coefficient of downwelling irradiance. J. Geophys. Res.110: C02016, doi: 10.1029/2004JC002275. 2. Thuillier G., M. Herse, P. C. Simon, D. Labs, H. Mandel, D. Gillotay, and T. Foujols. 2003. The solar spectral irradiance from 200 to 2400 nm as measured by the SOLSPEC spectrometer from the ATLAS 1-2-3 and EURECA missions. Solar Physics,214: 1-22. 3. Sokoletsky L., Z. Dubinsky, M. Shoshany, and N. Stambler. 2000. Non-meteorological predictive models of solar flux and atmospheric transmittance under weakly-variable climatic conditions. In: Ocean Optics XV, Proc. of Conf. Monaco, 16 – 20 October 2000, No. OO1003. 4. Kokhanovsky A. A. and L. G. Sokoletsky. 2006. Reflection of light from semi-infinite absorbing turbid media. II. Plane albedo and reflection function. Accepted by Color Res. Appl. 5. Boss E., M. Twardowski, and S. Herring. 2001. Shape of the particle beam attenuation spectrum and its inversion to obtain the shape of the particulate size distribution. Appl. Opt. 40: 4885-4893. 6. Twardowski M. S., E. Boss, J. B. Macdonald, W. S. Pegau, A. H. Barnard, and J. R. V. Zaneveld. 2001. A model for estimating bulk refractive index from the optical backscattering ratio and the implications for understanding particle composition in case I and case II waters. J. Geophys. Res. 106: 14,129-14,142. 7. Mobley C. D., L. K. Sundman, and E. Boss. 2002. Phase function effects on oceanic light fields. Appl. Opt. 41: 1035-1050. 8. Babin M., A. Morel, V. Fournier-Sicre, F. Fell, and D. Stramski. 2003. Light scattering properties of marine particles in coastal and open ocean waters as related to the particle mass concentration. Limnol. Oceanogr. 48: 843-859. REFLECTANCE COEFFICIENT (REFLECTANCE FACTOR) AND HEMISPHERICAL REFLECTANCE (PLANE ALBEDO) The spectra of the reflectance factor (RF) above the water surface for several samples collected in different locations of the Dead Sea during 13-14 April 2004 (Fig. 4). These spectra correlate well with the plane albedo (PA) just below the surface (Fig. 5) calculated by polynomial approximation [4] to the exact radiative transfer solution. Both observed and modelled (not shown) spectra have clear maximum at 500 nm, quantity of which depends primarily on the TSM concentration. PHISICAL PROPERTIES OF SOLID PARTICLES The vertical profiles of average (volume-surface) particle diameters D32, their refractive index m and TSM concentration (Figs. 6-8, respectively) were calculated by Mie theory and assuming a hyperbolic (Junge-type) particle size distribution [5-8]. Particle density r(in kg L-1) is estimated by approximate equation derived from the data of r vs. m by Babin et al. [8] as follows: r = 5.260m-3.597 gr/cm3. (4) Figure 6.Vertical profiles of the volume-surface particle diameters. Numbers in the legend represent the Dead Sea samples just as in Fig. 5. ACKNOWLEDGMENT We are grateful to David Anati, Arnon Karnieli and Alexander Goldberg for providing the transmittance and reflectace data. We are also indebted to Dr. Alexander Kokhanovsky for his assistance in radiative transfer calculations.