Atmospheric Chemistry and Physics www.atmos-chem-phys.net 1680-7316 1680-7324 10 3 2010 10.5194/acp-10-1403-2010 http://www.atmos-chem-phys.net/10/1403/2010/ http://www.atmos-chem-phys.net/10/1403/2010/acp-10-1403-2010.html http://www.atmos-chem-phys.net/10/1403/2010/acp-10-1403-2010.pdf 1403 1416 2010-02-08 Modelling the optical and radiative properties of freshly emitted light absorbing carbon within an atmospheric chemical transport model M. Kahnert michael.kahnert@smhi.se Swedish Meteorological and Hydrological Institute, 601 76 Norrköping, Sweden Light absorbing carbon (LAC) aerosols have a complex, fractal-like aggregate structure. Their optical and radiative properties are notoriously difficult to model, and approximate methods may introduce large errors both in the interpretation of aerosol remote sensing observations, and in quantifying the direct radiative forcing effect of LAC. In this paper a numerically exact method for solving Maxwell's equations is employed for computing the optical properties of freshly emitted, externally mixed LAC aggregates. The computations are performed at wavelengths of 440 nm and 870 nm, and they cover the entire size range relevant for modelling these kinds of aerosols. The method for solving the electromagnetic scattering and absorption problem for aggregates proves to be sufficiently stable and fast to make accurate multiple-band computations of LAC optical properties feasible. The results from the electromagnetic computations are processed such that they can readily be integrated into a chemical transport model (CTM), which is a prerequisite for constructing robust observation operators for chemical data assimilation of aerosol optical observations. A case study is performed, in which results obtained with the coupled optics/CTM model are employed as input to detailed radiative transfer computations at a polluted European location. It is found that the still popular homogeneous sphere approximation significantly underestimates the radiative forcing at top of atmosphere as compared to the results obtained with the aggregate model. Notably, the LAC forcing effect predicted with the aggregate model is less than that one obtains by assuming a prescribed mass absorption cross section for LAC. Andersson, C., Langner, J., and Bergström, R.: Interannual variation and trends in air pollution over Europe due to climate variability during 1958–2001 simulated with a regional CTM coupled to the ERA40 reanalysis, Tellus B, 59, 77–98, 2007. Bond, T C. and Bergstrom, R W.: Light absorption by carbonaceous particles: an investigative review, Aerosol Sci. Technol., 40, 27–67, 2006. Bond, T C., Habib, G., and Bergstrom, R W.: Limitations in the enhancement of visible light absorption due to mixing state, J. Geophys. Res., 111, D20211, \doi10.1029/2006JD007315, 2006. Borghese, F., Denti, P., Saija, R., Toscano, G., and Sindoni, O I.: Use of group theory for the description of electromagnetic scattering from molecular systems, J. Opt. Soc. Am. A, 1, 183–191, 1984. Bruggemann, D A G.: Berechnung verschiedener physikalischer Konstanten von heterogenen Substanzen. 1. Dielektrizitätskonstanten und Leitfähigkeiten der Mischkörper aus isotropen Substanzen, Ann. Phys., 24, 636–664, 1935. Chang, H. and Charalampopoulos, T T.: Determination of the wavelength dependence of refractive indices of flame soot, Proc. R. Soc. Lond. A, 430, 577–591, 1990. Ch\'ylek, P., Videen, G., Geldart, D J W., Dobbie, J S., and Tso, H C W.: Effective medium approximations for heterogeneous particles, in: Light scattering by nonspherical particles, edited by: Mishchenko, M I., Hovenier, J W., and Travis, L D., 274–308, Academic Press, San Diego, USA, 2000. Dubovik, O., Holben, B N., Lapyonok, T., Sinyuk, A., Mishchenko, M I., Yang, P., and Slutsker, I.: Non-spherical aerosol retrieval method employing light scattering by spheroids, Geophys. Res. Lett., 29(10), 1415, doi:10.1029/2001GL014506, 2002. Färnlund, J., Holman, C., and Kågeson, P.: Emissions of ultrafine particles from different types of light duty vehicles, Tech. Rep. 2001:10, Swedish National Road Administration, Borlänge, 2001. Foltescu, V., Pryor, S C., and Bennet, C.: Sea salt generation, dispersion and removal on the regional scale, Atmos. Environ., 39, 2123–2133, 2005. Forster, P., Ramaswamy, V., Artaxo, P., Berntsen, T., Betts, R., Fahey, D. W., Haywood, J., Lean, J., Lowe, D C., Myhre, G., Nganga, J., Prinn, R., Raga, G., Schulz, M., and Van Dorland, R V.: Changes in atmospheric constituents and in radiative forcing, in: Climate Change 2007: The Physical Science Basis., edited by: Solomon, S., Qin, D., Manning, M., Chen, Z., Marquis, M., Averyt, K. B., Tignor, M., and Miller, H. L., Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge, 2007. Fuller, K A., Malm, W C., and Kreidenweis, S M.: Effects of mixing on extinction by carbonaceous particles, J. Geophys. Res., 104, 15941–15954, 1999. Grenfell, T C. and Warren, S G.: Representation of a nonspherical ice particle by a collection of independent spheres for scattering and absorption of radiation, J. Geophys. Res., 104, 31697–31709, 1999. Hess, M., Koepke, P., and Schult, I.: Optical properties of aerosols and clouds: The software package OPAC, B. Am. Meteor. Soc., 79, 831–844, 1998. Jacobson, M Z.: A physically-based treatment of elemental carbon optics: Implications for global direct forcing of aerosols, Geophys. Res. Lett, 27, 217–220, 2000. Jones, A R.: Light scattering in combustion, in: Light Scattering Reviews, edited by: Kokhanovsky, A., Springer, Berlin, 2006. Kahnert, F M.: Reproducing the optical properties of fine desert dust aerosols using ensembles of simple model particles, J. Quant. Spectrosc. Radiat. Transfer, 85, 231–249, 2004. Kahnert, M.: Variational data analysis of aerosol species in a regional CTM: background error covariance constraint and aerosol optical observation operators, Tellus B, 60, 753–770, 2008. Kahnert, M.: On the observability of chemical and physical aerosol properties by optical observations: Inverse modelling with variational data assimilation, Tellus B, 61, 747–755, 2009. Kahnert, M. and Kylling, A.: Radiance and flux simulations for mineral dust aerosols: Assessing the error due to using spherical or spheroidal model particles, J. Geophys. Res., 109, D09203, \doi10.1029/2003JD004318, errata: \doi10.1029/2004JD005311, 2004. Kahnert, M. and Nousiainen, T.: Uncertainties in measured and modelled asymmetry parameters of mineral dust aerosols, J. Quant. Spectrosc. Radiat. Transfer, 100, 173–178, 2006. Kahnert, M., Nousiainen, T., and Veihelmann, B.: Spherical and spheroidal model particles as an error source in aerosol climate forcing and radiance computations: a case study for feldspar aerosols, J. Geophys. Res., 110, D18S13, \doi10.1029/2004JD005558, 2005. Kahnert, M., Nousiainen, T., and Räisänen, P.: Mie simulations as an error source in mineral aerosol radiative forcing calculations, Q. J. Roy. Meteor. Soc., 133, 299–307, 2007. Khlebtsov, N G.: Orientational averaging of light-scattering observables in the T-matrix approach, Appl. Opt., 31, 5359–5365, 1992. Kocifaj, M. and Videen, G.: Optical behavior of composite carbonaceous aerosols: DDA and EMT approaches, J. Quant. Spectrosc. Radiat. Transfer, 109, 1404–1416, 2008. Liu, L. and Mishchenko, M I.: Effects of aggregation on scattering and radiative properties of soot aerosols, J. Geophys. Res., 110, D11211, doi:10.1029/2004JD005649, 2005. Liu, L., Mishchenko, M I., and Arnott, W P.: A study of radiative properties of fractal soot aggregates using the superposition T matrix method, J. Quant. Spectrosc. Radiat. Transfer, 109, 2656–2663, 2008. Mackowski, D W.: Calculation of total cross sections of multiplesphere clusters, J. Opt. Soc. Am. A., 11, 2851–2861, 1994. Mackowski, D W.: A simplified model to predict the effects of aggregation on the absorption properties of soot aggregates, J. Quant. Spectrosc. Radiat. Transfer, 100, 237–249, 2006. Mackowski, D W. and Mishchenko, M I.: Calculation of the T matrix and the scattering matrix for ensembles of spheres, J. Opt. Soc. Am. A., 13, 2266–2278, 1996. Maxwell-Garnett, J C.: Colours in metal glasses and in metallic films, Philos. Trans. R. Soc. A., 203, 385–420, 1904. Mie, G.: Beiträge zur Optik trüber Medien, speziell kolloidaler Metallösungen, Ann. Phys., 25, 377–445, 1908. Mishchenko, M I.: Light scattering by randomly oriented axially symmetric particles, J. Opt. Soc. Am. A., 8, 871–882, 1991. Mishchenko, M I., Travis, L D., and Mackowski, D W.: T-matrix computations of light scattering by nonspherical particles: a review, J. Quant. Spectrosc. Radiat. Transfer, 55, 535–575, 1996. Mishchenko, M I., Travis, L D., Kahn, R A., and West, R A.: Modeling phase functions for dustlike tropospheric aerosols using a shape mixture of randomly oriented polydisperse spheroids, J. Geophys. Res., 102, 16831–16847, 1997. Nousiainen, T., Kahnert, M., and Veihelmann, B.: Light scattering modeling of small feldspar aerosol particles using polyhedral prisms and spheroids, J. Quant. Spectrosc. Radiat. Transfer, 101, 471–487, 2006. Okada, Y. and Kokhanovsky, A A.: Light scattering and absorption by densely packed groups of spherical particles, J. Quant. Spectrosc. Radiat. Transfer, 110, 902–917, 2009. Okada, Y., Mukai, T., Mann, I., Nomura, H., Takeuchi, T., Sano, I., and Mukai, S.: Grouping and adding method for calculating light scattering by large fluffy aggregates, J. Quant. Spectrosc. Radiat. Transfer, 108, 65–80, 2007. Okada, Y., Mann, I., Mukai, T., and Köhler, M.: Extended calculation of polarization and intensity of fractal aggregates based on rigorous method for light scattering simulations with numerical orientation averaging, J. Quant. Spectrosc. Radiat. Transfer, 109, 2613–2627, 2008. Otto, S., Bierwirth, E., Weinzierl, B., Kandler, K., Esselborn, M., Tesche, M., Schladitz, A., Wendisch, M., and Trautmann, T.: Solar radiative effects of a Saharan dust plume observed during SAMUM assuming spheroidal model particles, Tellus B, 61, 270–296, 2009. Pilinis, C. and Li, X.: Particle shape and internal inhomogeneity effects in the optical properties of tropospheric aerosols of relevance to climate forcing, J. Geophys. Res., 103, 3789–3800, 1998. Robertson, L., Langner, J., and Enghardt, M.: An Eulerian limited-area atmospheric transport model, J. Appl. Meteorol., 38, 190–210, 1999. Rother, T., Schmidt, K., Wauer, J., Shcherbakov, V., and Gaeyt, J.-F.: Light scattering on Chebyshev particles of higher order, Appl. Opt., 45, 6030–6037, 2006. Schulz, F M., Stamnes, K., and Stamnes, J J.: Modeling the radiative transfer properties of media containing particles of moderately and highly elongated shape, Geophys. Res. Lett., 25, 4481–4484, 1998. Schulz, F M., Stamnes, K., and Stamnes, J J.: Shape-dependence of the optical properties in size-shape distributions of randomly oriented prolate spheroids, including highly elongated shapes, J. Geophys. Res., 104, 9413–9421, 1999. Segelstein, D.: The Complex Refractive Index of Water, Master thesis, University of Missouri-Kansas City, 1981. Smekens, A., Pauwels, J., Berghmans, P., and Grieken, R V.: Correlation study between the aerodynamic diameter and the number of primary particles of soot aggregates by STEM, J. Aerosol Sci., 28, 761–762, 1997. Sorensen, C M.: Light scattering by fractal aggregates: a review, Aerosol Sci. Technol., 35, 648–687, 2001. Sorensen, C M. and Roberts, G M.: The prefactor of fractal aggregates, J. Colloid. Interface Sci., 186, 447–452, 1997. Stamnes, K., Tsay, S.-C., Wiscombe, W., and Jayaweera, K.: Numerically stable algorithm for discrete-ordinate-method radiative transfer in multiple scattering and emitting layered media, Appl. Opt., 27, 2502–2509, 1988. Toon, O B. and Ackermann, T P.: Algorithms for the calculation of scattering by stratified spheres, Appl. Opt., 20, 3657–3660, 1981. van de Hulst, H C.: Light Scattering by Small Particles, Dover Publications, Inc., New York, USA, p 70., 1981. Veihelmann, B., Nousiainen, T., Kahnert, M., and van der Zande, W. J.: Light scattering by small feldspar particles simulated using the Gaussian random sphere geometry, J. Quant. Spectrosc. Radiat. Transfer, 100, 393–405, 2006. Vignati, E., Wilson, J., and Stier, P.: M7: an efficient size-resolved aerosol microphysics module for large-scale aerosol transport models, J. Geophys. Res., 109, D22202, \doi10.1029/2003JD004485, 2004. Worringen, A., Ebert, M., Trautmann, T., Weinbruch, S., and Helas, G.: Optical properties of internally mixed ammonium sulfate and soot particles – a study of individual aerosol particles and ambient aerosol populations, Appl. Opt., 47, 3835–3845, 2008. Xu, Y. and Gustafson, B. Å. S.: A generalized multiparticle Mie-solution: further experimental verification, J. Quant. Spectrosc. Radiat. Transfer, 70, 395–419, 2001. Zhao, Y. and Ma, L.: Assessment of two fractal scattering models for the prediction of the optical characteristics of soot aggregates, J. Quant. Spectrosc. Radiat. Transfer, 110, 315–322, 2009.