Numerically exact computation of the optical properties of light absorbing carbon aggregates for wavelength of 200 nm–12.2 μm M. Kahnert Swedish Meteorological and Hydrological Institute, 601 76 Norrköping, Sweden
Abstract. The optical properties of externally mixed
light absorbing carbon (LAC) aggregates are computed over the
spectral range from 200 nm–12.2 μm by use of the numerically exact
superposition T-matrix method. The spectral computations are tailored to the
14-band radiation model employed in the Integrated Forecasting System operated
at the European Centre for Medium Range Weather Forecast.
The size- and wavelength dependence of the optical properties obtained with the
fractal aggregate model differs significantly from corresponding results based
on the homogeneous sphere approximation, which is still commonly employed in
climate models. The computational results are integrated into the chemical
transport model MATCH (Multiple-scale Atmospheric Transport and
CHemistry modelling system) to compute 3-D fields of size-averaged aerosol
optical properties. Computational results obtained with MATCH are coupled to
a radiative transfer model to compute the shortwave radiative impact of LAC.
It is found that the fractal aggregate model gives a shortwave forcing estimate
that is twice as high as that obtained with the homogeneous sphere approximation.
Thus previous estimates based on the homogeneous sphere model may have
substantially underestimated the shortwave radiative impact of freshly emitted LAC.
Citation: Kahnert, M.: Numerically exact computation of the optical properties of light absorbing carbon aggregates for wavelength of 200 nm–12.2 μm, Atmos. Chem. Phys., 10, 8319-8329, doi:10.5194/acp-10-8319-2010, 2010.