Importance of size representation and morphology in modelling optical properties of black carbon: comparison between laboratory measurements and model simulations

Abstract. Black carbon (BC) from incomplete combustion of biomass or fossil fuels is
the strongest absorbing aerosol component in the atmosphere. Optical
properties of BC are essential in climate models for quantification of their impact on radiative forcing. The global climate models, however, consider BC
to be spherical particles, which causes uncertainties in their optical
properties. Based on this, an increasing number of model-based studies
provide databases and parameterization schemes for the optical properties of
BC, using more realistic fractal aggregate morphologies. In this study, the
reliability of the different modelling techniques of BC was investigated by
comparing them to laboratory measurements. The modelling
techniques were examined for bare BC particles in the first step and for BC particles with organic material in the second step. A total of six morphological
representations of BC particles were compared, three each for spherical and
fractal aggregate morphologies. In general, the aggregate representation
performed well for modelling the particle light absorption coefficient
σabs, single-scattering albedo SSA, and mass absorption
cross-section MACBC for laboratory-generated BC particles with volume mean mobility diameters dp,V larger than 100 nm. However, for modelling
Ångström absorption exponent AAE, it was difficult to suggest a
method due to size dependence, although the spherical assumption was in
better agreement in some cases. The BC fractal aggregates are usually
modelled using monodispersed particles, since their optical simulations are
computationally expensive. In such studies, the modelled optical properties
showed a 25 % uncertainty in using the monodisperse size method. It is
shown that using the polydisperse size distribution in combination with
fractal aggregate morphology reduces the uncertainty in measured σabs to 10 % for particles with dp,V between 60–160 nm. Furthermore, the sensitivities of the BC optical properties to the various
model input parameters such as the real and imaginary parts of the
refractive index (mre and mim), the fractal dimension (Df), and the primary particle radius (app) of an aggregate were investigated. When the BC particle is small and rather fresh, the change in the Df had relatively little effect on the optical properties. There was, however, a significant relationship between app and the particle
light scattering, which increased by a factor of up to 6 with increasing
total particle size. The modelled optical properties of BC are well aligned
with laboratory-measured values when the following assumptions are used in
the fractal aggregate representation: mre between 1.6 and 2, mim between 0.50 and 1, Df from 1.7 to 1.9, and app between 10 and 14 nm. Overall, this study provides experimental support for emphasizing the
importance of an appropriate size representation (polydisperse size method)
and an appropriate morphological representation for optical modelling and
parameterization scheme development of BC.
