Abstract. The Aethalometer model been used widely for estimating the contributions of fossil fuel emissions and biomass burning to equivalent black carbon (eBC). The calculation is based on measured absorption Ångström exponents (α_abs). The interpretation α_abs is ambiguous since it is well-known that it not only depends on the dominant absorber but also on the size and internal structure of the particles, core size and shell thickness. In this work the uncertainties of the Aethalometer-model-derived apparent fractions of absorption by eBC from fossil fuel and biomass burning are evaluated with a core-shell Mie model. Biomass-burning fractions (BB(%)) were calculated for pure and coated single BC particles, for lognormal unimodal and bimodal size distributions of BC cores coated with ammonium sulfate, a scattering-only material. BB(%) was very seldom 0 % even though BC was the only absorbing material in the simulations. The shape of size distribution plays an important role. Narrow size distributions result in higher α_abs and BB(%) values than wide size distributions. The sensitivity of α_abs and BB(%) to variations in shell volume fractions is the highest for accumulation mode particles. This is important because that is where the largest aerosol mass is. For the interpretation of absorption Ångström exponents it would be very good to measure BC size distributions and shell thicknesses together with the wavelength dependency of absorption.

中文翻译：

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涂有光散射壳的黑碳颗粒的模拟源分配

摘要。Aethalometer模型已被广泛用于估算化石燃料排放量和生物质燃烧对当量黑碳（eBC）的贡献。该计算基于测得的吸收Ångström指数（_αabs）。解释_αabs模棱两可是不明确的，因为众所周知，它不仅取决于主要的吸收剂，而且取决于颗粒的尺寸和内部结构，核的尺寸和壳的厚度。在这项工作中，使用核-壳Mie模型评估了由Eethalometer模型得出的eBC从化石燃料和生物质燃烧中吸收的表观分数的不确定性。计算了纯的和包衣的单个BC颗粒的生物量燃烧分数（BB（％）），以及涂有硫酸铵（仅用于散射的材料）的BC芯的对数正态单峰和双峰尺寸分布。即使BC是模拟中唯一的吸收材料，BB（％）也很少为0％。尺寸分布的形状起着重要作用。窄的尺寸分布导致较高的_abs和BB（％）值，而不是较大的尺寸分布。对于堆积模式粒子，_αabs和BB（％）对壳体积分数变化的敏感性最高。这很重要，因为那是最大的气溶胶质量所在的地方。为了解释吸收Ångström指数，最好测量BC尺寸分布和壳厚度以及吸收的波长依赖性。