Abstract
Abstract. Black carbon fractal aggregates have complicated shapes that make the calculation of their optical properties particularly computationally expensive. Here, a method is presented to estimate fractal aggregate light scattering properties by optimising simplified models to full light scattering calculations. It is found that there are no possible spherical models (at any size or refractive index) that well represent the light scattering in the visible or near-thermal infrared. As such, parameterisations of the light scattering as a function of the number of aggregate particles is presented as the most pragmatic choice for modelling distributions of black carbon when the large computational overheads of rigorous scattering calculations cannot be justified. This parameterisation can be analytically integrated to provide light scattering properties for lognormal distributions of black carbon fractal aggregates and return extinction cross sections with 0.1% accuracy for typical black carbon size distributions. Scattering cross sections and the asymmetry parameter can be obtained to within 3%.
Highlights
Particulate products of incomplete combustion, such as black carbon (BC), have been the subject of recent intense discussion (e.g. Jacobson, 2002, 2003; Bond, 2007; Bond et al, 2013)
After a brief description of black carbon, we show that this is not possible within the bounds of reasonably sized spheres with a wide range of refractive indices and go on to present a parameterisation of Black carbon fractal aggregates (BCFAs) light scattering properties
Light scattering properties of BCFAs were calculated over a range of wavelengths, λ, from 400 nm to 15 μm using the multiple-sphere T-Matrix Fortran-90 code (MSTM) code
Summary
Particulate products of incomplete combustion, such as black carbon (BC), have been the subject of recent intense discussion (e.g. Jacobson, 2002, 2003; Bond, 2007; Bond et al, 2013). Particulate products of incomplete combustion, such as black carbon (BC), have been the subject of recent intense discussion The developed world has reduced emissions, while Asian countries have come to dominate carbon aerosol emission (Streets et al, 2003). Since BC has a high imaginary part of refractive index, it is one of the few aerosols that has a positive direct effect on radiative forcing (Jacobson, 2001; Bond, 2007; Bahadur et al, 2011). Aside from the positive radiative forcing due to BC, there are considerable negative health effects from fine particulate matter (Dockery et al, 1993; Jansen et al, 2005) and, in urban environments, issues of poor visibility (Larson et al, 1989)
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