Topic 5 Synopsis:

Organic Speciation Effects on Regional and Global Scale Atmospheric Chemistry and Climate

Mark Jacobson (Topic Leader)

Topic 5a: How does organic carbon, particularly its individual components, affect atmospheric chemistry, aerosol scattering and absorption, ultraviolet radiation, and climate?

Martin Schnaiter
Institute of Meteorology and Climate Research
Forschungzentrum Karlsruhe GmbH
Germany

Anthropogenic organic carbon (OC) is emitted by fossil fuel and biomass burning. In these sources OC is accompanied by black carbon (BC) aerosol which represents the absorbing component of the released carbonaceous aerosol. The BC and OC are usually assumed to be internally mixed (Penner et al., 1998). This aerosol is further aged in the atmosphere by different atmospheric processes (e.g. condensation, coagulation, chemistry) to form complex internal mixtures with other aerosol components (e.g. secondary inorganic and organic aerosol matter, SOA). Due to its complexity it is difficult or even impossible to calculate the optical properties of these mixtures. This is a serious problem for the assessment of the aerosol influence on the solar radiation field and, hence, the impact of anthropogenic aerosol emissions on regional and global climate. There are several recent studies investigating the climate impact of aerosols. Many of these three-dimensional global climate models assume the aerosol to be externally mixed to simplify their optics (Haywood and Ramaswamy, 1998, Cooke et al., 1999). On the basis of such general circulation models (GCM) global mean direct radiative forcings of +0.2 Wm-2 and –0.1 Wm-2 with uncertainties of 200% and 300 % were estimated for fossil fuel BC and OC, respectively (IPCC, 2001). For biomass burning carbonaceous aerosol a negative forcing of –0.2 Wm-2 was estimated with an uncertainty of 300 %. These high uncertainties are not solely due to the uncertainties in the atmospheric aerosol burden but reflect gaps in the knowledge of the optics of internally mixed aerosols and the hygroscopic behaviour especially of OC aerosol compounds.

Jacobson, 2001 used a GCM which accounts for a wide range of atmospheric aerosol processes (e.g. condensation, coagulation, and nucleation). The optical properties of the internally mixed BC were calculated with a concentric core shell model. He found that within five days more than 60 % per mass of BC obtain a non-BC coating. This coating is a complex mixture formed of sulfates, nitrates, and OC which are all weak or non-absorbing. Since BC is a strong absorber for visible solar radiation, its absorption efficiency is therefore influenced by the mixing state of BC. Positive direct radiative forcings of +0.27 Wm-2 and +0.54 Wm-2 were estimated for the external and internal mixture, respectively. Thus, OC can affect significantly the absorption efficiency of BC and, hence, its direct climate warming potential.

Different optical models for the internal mixture between BC and a non-absorbing aerosol component (e.g. sulfate or water) have been applied, ranging from effective medium theories (Videen et al., 1994, Chylek et al., 1988) to concentric and eccentric core shell models (Fuller et al., 1999, Videen et al., 1995, Chylek et al., 1995). It was found that depending on the mass mixing ratio and refractive indices of the both materials the specific BC absorption cross section of internally mixed BC is enhanced by factors of about 2.5-4 compared to the external mixed case. Based on effective medium calculations and a simple radiative model, Lesins et al., 2002 estimated the effect of internal BC mixing on the globally averaged clear-sky direct radiative forcing. They stated that for specific internal mixing assumptions nearly all of the cooling effect predicted for the external mixture is counterbalanced by the BC absorption enhancement.

Although these investigations have been performed for internal mixtures of BC with sulfates or water, similar effects on the optical properties of BC are expected in case of OC and SOA as mixing partners. This is because OC and SOA are mostly refractive without a significant absorption. However, since BC has a complex fractal-like morphology the question is how good represent the above optical models the optical properties of real atmospheric BC.

This question was the basis of experimental investigations performed at the aerosol facility AIDA (Aerosol Interactions and Dynamics in the Atmosphere) of Forschungszentrum Karlsruhe, Germany. The exhaust of a commercial Diesel engine was cleaned for water and volatile organic compounds. The remaining BC aerosol was added to the aerosol chamber which was pre-filled with synthetic air with a relative humidity of 20 to 25 %. Subsequently, the BC was internally mixed with SOA mass by the in situ condensation of low and semi volatile organic compounds produced by the ozonolysis of 50 ppb a-pinene (see Saathoff et al., 2003a for a description of the chamber and the aerosol generation). This coating procedure was repeated several times, thus decreasing the BC mixing ratio. A comprehensive set of optical parameters were measured; the spectral extinction coefficient (200 – 1000 nm), and the integrated scattering and hemispheric backscattering coefficients at 450, 500, and 700 nm (Schnaiter et al., 2003). In addition to the optical measurements the aerosol was characterized with respect to number concentration, size distribution, EC/OC chemical composition, as well as volatility and hygroscopic growth (Saathoff et al., 2003b).

The scattering and absorption properties of the OC material were measured in homogeneous nucleation experiments of SOA without BC as seed aerosol. There was no detectable absorption in the aerosol extinction reflecting a real refractive index m. A value of m=1.5+i0.0 has been deduced from these experiments. The complex refractive index of the Diesel BC has been deduced in earlier aerosol dynamics experiments (Naumann, 2003, Schnaiter et al., 2003).

The optical properties of BC were altered significantly due to the coating with OC. Since the OC material is non-absorbing the single scattering albedo (i.e. the ratio between the scattering and the extinction coefficient) of the aerosol was increased with decreasing BC mixing ratio. A significant growth of the aerosol was accompanied with the coating process. This increase in particle size results in a decrease of the hemispheric backscattering ratio which is due to the well known size dependence of the angular scattering phase function in case of spherical particles (Mie-Theory). However, at the very beginning of the internal mixing process a decrease of the single scattering albedo as well as the backscattering ratio were observed. This peculiar behaviour can be explained by a significant structural rearrangement of the BC aggregates to form more compact particles due to the condensation of OC matter. Thus, the condensable products of the ozonolysis of a-pinene give rise to capillary forces in the small angle cavities of the aggregates. This conclusion is also in agreement with the observed increase of the Angström exponent (the wavelength dependence of the scattering coefficient) at the beginning of the coating process and was further supported by electron microscopy.

The absorption coefficient of the BC was increased with decreasing BC mixing ratio resulting in an enhancement of specific BC absorption cross section at 550 nm of a factor of 2. This amplification of the BC absorption was found to be wavelength dependent with a value of 1.8 at 450 nm and 2.1 at 700 nm.

These experimental findings were modelled with a concentric core shell model which was also used in the global climate model by Jacobson, 2001. By using the experimentally deduced refractive indices of Diesel BC and OC the measured absorption enhancement was reproduced quite well by the core shell model. The simulation was based on the evolution of the measured aerosol size distribution. In contrast to the BC absorption, the model has more problems in reproducing the single scattering albedo, the backscattering ratio, and the Angström exponent especially for BC mixing ratios near 1.0 at the beginning of the experiments. These discrepancies reflect the already mentioned strong rearrangement of the BC aggregate structure, which has a stronger impact on the scattering behaviour of the aerosol than on the absorption. It has to be investigated by applying effective medium theories whether homogeneous mixed BC/OC aerosol might a better model at high BC mixing ratios.

References

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Chylek, P., G. Videen, D. Ngo, R.G. Pinnick, and J.D. Klett, Effect of Black Carbon on the Optical-Properties and Climate Forcing of Sulfate Aerosols, Journal of Geophysical Research-Atmospheres, 100 (D8), 16325-16332, 1995.

Cooke, W.F., C. Liousse, H. Cachier, and J. Feichter, Construction of a 1 degrees x 1 degrees fossil fuel emission data set for carbonaceous aerosol and implementation and radiative impact in the ECHAM4 model, Journal of Geophysical Research-Atmospheres, 104 (D18), 22137-22162, 1999.

Fuller, K.A., W.C. Malm, and S.M. Kreidenweis, Effects of mixing on extinction by carbonaceous particles, Journal of Geophysical Research-Atmospheres, 104 (D13), 15941-15954, 1999.

Haywood, J.M., and V. Ramaswamy, Global sensitivity studies of the direct radiative forcing due to anthropogenic sulfate and black carbon aerosols, Journal of Geophysical Research-Atmospheres, 103 (D6), 6043-6058, 1998.

IPCC, Climate Change 2001: The Scientific Basis, in Intergovernmental Panel on Climate Change (IPCC), Houghton, J.T., Y. Ding, D.J. Griggs, M. Noguer, P.J. van der Linden, and D. Xiaosu eds., Cambridge University Press, Cambridge, UK, 2001.

Jacobson, M.Z., Strong radiative heating due to the mixing state of black carbon in atmospheric aerosols, Nature, 409 (6821), 695-697, 2001.

Lesins, G., P. Chylek, and U. Lohmann, A study of internal and external mixing scenarios and its effect on aerosol optical properties and direct radiative forcing, Journal of Geophysical Research-Atmospheres, 107 (D10), -, 2002.

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Saathoff, H., O. Moehler, U. Schurath, S. Kamm, B. Dippel, and D. Mihelcic, The AIDA soot aerosol characterisation campaign 1999, Journal of Aerosol Science, 34 (10), 1277-1296, 2003a.

Saathoff, H., K.H. Naumann, M. Schnaiter, W. Schock, O. Mohler, U. Schurath, E. Weingartner, M. Gysel, and U. Baltensperger, Coating of soot and (NH4)(2)SO4 particles by ozonolysis products of alpha-pinene, Journal of Aerosol Science, 34 (10), 1297-1321, 2003b.

Schnaiter, M., H. Horvath, O. Mohler, K.H. Naumann, H. Saathoff, and O.W. Schock, UV-VIS-NIR spectral optical properties of soot and soot-containing aerosols, Journal of Aerosol Science, 34 (10), 1421-1444, 2003.

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