Constraining the complex refractive index of black carbon particles using the complex forward-scattering amplitude
Constraining the complex refractive index of black carbon particles using the complex forward-scattering amplitude
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DOI：
10.1080/02786826.2023.2202243
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发表时间：
2023-04
影响因子：
5.2
通讯作者：
N. Moteki;S. Ohata;Atsushi Yoshida;K. Adachi
中科院分区：
文献类型：
--
作者：
N. Moteki;S. Ohata;Atsushi Yoshida;K. Adachi
摘要 黑碳是当前大气中全球气溶胶短波吸收的最大贡献者，是重要的积极气候力量。对于大气黑碳材料，复折射率 m = m r + im i 是每单位体积颗粒材料吸收和散射能量的主要决定因素。由于黑碳的波长尺度不规则性以及聚集体形状、颗粒尺寸分布以及与其他气溶胶化合物的混合的可变性，很难获得可见波长下的准确值。在这里，我们提出了一种从观察到的复杂前向散射幅度 S(0°) 的分布中约束黑碳的合理 (m r, m i) 域的方法。这种方法抑制了由于上述复杂性而产生的偏差。将大气气溶胶收集到水中后，通过在水介质中进行单粒子 S(0°) 测量来获得黑碳的 S(0°) 分布。我们演示了在 λ = 0.633 μm 下运行的方法，用于限制西北太平洋边界层黑碳气溶胶的折射率。根据与观察到的 S(0°) 分布和报告的质量吸收截面范围一致的合理 (m r, m i) 域，我们保守地选择 1.95 + 0.96i 作为未涂层炭黑在可见波长下的折射率的推荐值。 m i 的推荐值比当前气溶胶气候模型中广泛使用的值 1.95 + 0.79i 大 0.17，这意味着当前气候模拟中黑碳气溶胶的短波吸收被低估了约 16%。
Abstract Black carbon is the largest contributor to global aerosol’s shortwave absorption in the current atmosphere and is an important positive climate forcer. The complex refractive index, m = m r + im i, the primary determinant of the absorbed and scattered energies of incident radiation per unit volume of particulate material, has not been accurately known for atmospheric black carbon material. An accurate value at visible wavelengths has been difficult to obtain due to the black carbon’s wavelength-scale irregularity and variability of aggregate shape, distribution in particle size, and mixing with other aerosol compounds. Here, we present a method to constrain a plausible (m r, m i) domain for black carbon from the observed distribution of the complex forward-scattering amplitude S(0°). This approach suppresses the biases due to the above-mentioned complexities. The S(0°) distribution of black carbon is acquired by performing single particle S(0°) measurements in a water medium after collecting atmospheric aerosols into water. We demonstrate the method operating at λ = 0.633 μm for constraining the refractive index of black carbon aerosols in the north-western Pacific boundary layer. From the plausible (m r, m i) domain consistent with the observed S(0°) distributions and the reported range of mass absorption cross-section, we conservatively select 1.95 + 0.96i as a recommendable value of the refractive index for uncoated black carbon at visible wavelengths. The recommendable value is 0.17 larger in m i than the widely used value 1.95 + 0.79i in current aerosol-climate models, implying a ∼16% underestimate of shortwave absorption by black carbon aerosols in current climate simulations.