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J. Aerosol Sci. Vol. 28, Suppl. 1, pp. $655-$656, 1997 © 1997 Elsevier Science Ltd. All fights reserved Printed in Great Britain 0021-8502/97 S17.00+O.00
REGIONAL DIRECT RADIATIVE FORCING BY THE AIRBORNE MINERAL AEROSOLS
I.N.Sokolik and O.B.Toon LASP/PAOS, University of Colorado at Boulder, Campus Box 392, Boulder CO, 80309, USA KEYWORDS airborne mineral aerosols, optical properties, radiative effects INTRODUCTION Several recent studies focused on estimation of the direct forcing by natural and anthropogenic components of the mineral aerosols have pointed out that there are large uncertainties in the assessments of both solar and infrared forcings (Sokolik and Toon, 1996a; Sokolik and Toon, 1996b; Tegen et a1.1996). In this paper we explore the importance of varying mineralogical composition of airborne mineral aerosols for an assessment of dust regional and global radiative forcings.
We examined the available measurements of the microphysical and optical properties
of dust aerosols from various geographical locations, and related them to the dust mineralogical composition. Based on empirical data and our modeling we estimated the uncertainties in solar and infrared forcings due to varying regional optical properties of the mineral aerosols in specific geographical regions. REGIONAL DUST OPTICAL PROPERTIES AND DUST COMPOSITION The scattering and absorbing properties of dust aerosol depend on its refractive index and particle size spectrum. We examined the measurements of refractive indices of dust samples from various geographical locations (Sokolik et a1.1993, Sokolik et a1.1997), and related data on particle size distributions. Analysis of available data on complex refractive indices of the mineral aerosols collected at various geographical locations revealed a great deal of variation in the values of imaginary, k, and real, n, parts of the refractive index. These discrepancies are due both to differences in the methods used to measure refractive indices and differences in the mineralogical composition of dust originating from various geographical locations. In particular, the dust absorption in the infrared wavelengths is dominated by absorption bands of quartz, clays ( mainly illite, kaolin and montmorillonite groups), calcites (calcite, dolomite) and sulfates (gypsum).
abundance of these materials can vary from place to place resulting in the large discrepancies in positions and magnitudes of peaks in n(~.) and k()~). For instance, in region X = 8-12 p.m we found that n can vary from 0.7 to 3.0, and k - from 0.02 to 1.0. By employing Mie theory we calculated dust spectral extinction and absorption coefficients, single scattering albedo and asymmetry factor using measured refractive indices of the dust samples collected at various locations ( Sahara dust collected at Barbados and Niger; Negev desert dust; dust collected in the Afghanistan-Tadzhikistan region; dust from Texas, USA;
Abstracts of the 1997 European Aerosol Conference
non-desert dust collected over Germany). Various models of size spectra were considered assuming spherical and non-spherical dust particles. Our calculations show that the existing variations in refractive indices can cause large variations of the major aerosol optical characteristics. For instance, single scattering albedo can vary from 0.1 to 0.94 in the region ~. = 8-12 p.m, and from 0.6 to 0.95 at ~, = 0.5 I~m.
REGIONAL DUST RADIATIVE FORCING Based on our dust optical modeling and empirical data on dust Ioadings, we calculated the direct radiative forcing and changes in radiative fluxes at the surface under various atmospheric conditions. To perform these calculations we used a 1-D radiative transfer code, based on the correlated K-distribution technique incorporated into two-stream scheme (Bergstrom et a1.1996). It allows us to accurately compute radiative fluxes accounting for absorption by gases along with absorption and scattering by aerosols. Our simulations gave a wide range of results for varying optical models of the mineral aerosols from the geographical locations considered. We estimated that for a 'low dust loading' scenario the changes in IR downward flux at the surface ('dust' minus 'clean' condition) are in a range from 7 to 14 W/m 2, and the IR forcing is in a range from 2 to 7 W/m 2. In turn, for a 'high dust loading' scenario the calculated changes in IR flux at the surface vary from 50 to 80 W/m 2, and the IR forcing - from 15 to 25 W/m 2. In all cases dust aerosols caused the IR heating (positive direct forcing) of the land-atmosphere column in contrast to either cooling or heating effect in the solar wavelengths. While both solar and infrared forcings have large range of uncertainties it is a complicated task to estimate net (solar plus infrared) forcing which is of main interest. Therefore we conclud that incorporation of regionally and temporally varying dust mineralogical composition into GCM and regional models could be beneficial for decreasing the currently large uncertainties in assessment of radiative forcing by the natural and anthropogenic components of the airborne mineral aerosols. REFERENCES Bergstrom R.W., Kinne S., Sokolik I.N., Toon O.B., Mlawer E., Clough T. and T.Ackerman, 3ARM: a fast, accurate radiative transfer model for use in climate studies. Proceedings from International Radiation Symposium, Fairbanks, Alaska, August 19-24, 1996. Sokolik I.N., Andronova A.V. and T.C.Johnson, Complex refractive index of atmospheric dust aerosols. Atmos. Environ. 27A, 2495-25-2, 1993. Sokolik I.N. and G.S. Golitsyn, Investigation of optical and radiative properties of atmospheric dust aerosols. Atmos. Environ. 27A, 2509-2517, 1993. Sokolik, I.N., and O.B.Toon, Direct radiative forcing by anthropogenic mineral aerosols. Nature 381,681-683, 1996a. Sokolik I.N. and O.B. Toon. Direct radiative forcing by airborne mineral dust. J. Aerosol Sci. 27, Supplement 1, $11,1996b. Sokolik I.N., Bergstrom R. W. and O.B.Toon. Modeling of optical and radiative characteristics of the airborne mineral aerosol in infrared region. Proceedings from the AMS Ninth Conference on Atmospheric Radiation, Long Beach, California, Feb. 2-7, p.189-190, 1997. Tegen I., Lacis A.A. and I.Fung, The influence on climate forcing of mineral aerosols from disturbed soils. Nature 380, 419-422, 1996.