Abstract

We evaluate a filter-based Aethalometer and investigate a well characterized, commercially available colloid suspension as a potential material to mimic atmospheric black carbon (BC) for use in the evaluation of such filter-based BC instruments. We size selected BC surrogate particles of 300 nm in mobility diameter, determined the particle mass, m_p, and used a condensation particle counter (CPC) as a reference method for measurement of number density, Np. The number concentration was used to determine the mass concentration and to evaluate the Aethalometer as a function of particle loading, size, wavelength, and coating. For uncoated BC surrogate particles and mass concentrations less than 2.5 μg m⁻³ the CPC-derived BC and filter-based BC measurements could, after a correction was applied, be made to agree to within 5% at all Aethalometer wavelengths (370 to 950 nm). At higher concentrations shadowing occurs, necessitating a correction for loading on the filter. Pure brown carbon (BrC) particles generated with a humic acid (HA) surrogate produced only a minor signal at 880 nm (2% of an equivalent mass of BC), but a substantial signal at shorter wavelengths (up to 20% of an equivalent mass of BC at 370 nm). The Aethalometer indicated an Attenuation Angstrom Exponent, A_tAE, value near unity (0.71) for the pure BC surrogate, but much larger (3.7) for the BrC surrogate. Uncertainty in filter-based BC measurements increased substantially for BC particles coated with minimally absorbing ammonium sulfate or with the brown carbon surrogate, and varies with detection wavelength. We present information that may improve a widely used filter-based instrument for measurements of BC and absorption properties of atmospheric aerosols.

Copyright © 2021 American Association for Aerosol Research

摘要

我们评估基于过滤器的测速仪，并研究表征良好的，可商购的胶体悬浮液，作为模拟大气黑碳（BC）的潜在材料，用于评估此类基于过滤器的BC仪器。我们对选定的迁移率直径为300 nm的BC替代颗粒进行尺寸测定，确定颗粒质量m _p，并使用缩合颗粒计数器（CPC）作为测量数密度Np的参考方法。数字浓度用于确定质量浓度，并根据颗粒载量，尺寸，波长和涂层来评估溶度计。对于未涂覆的BC替代颗粒且质量浓度小于2.5μgm ^-3在应用校正后，可以将CPC衍生的BC和基于过滤器的BC测量值在所有测速仪波长（370至950 nm）下均控制在5％以内。在较高的浓度下，会产生阴影，因此必须对过滤器上的负载进行校正。由腐殖酸（HA）替代物产生的纯棕碳（BrC）颗粒仅在880 nm处产生次要信号（相当于BC等效质量的2％），而在较短波长下产生实质性信号（等效于20％的等效信号）在370 nm处的BC质量）。湿度计指示衰减埃指数，A _tAE，纯BC替代物的值接近统一（0.71），而BrC替代物的值大得多（3.7）。基于过滤器的BC测量的不确定性对于涂有吸收最小的硫酸铵或棕色碳替代物的BC颗粒大大增加，并且随检测波长而变化。我们提供的信息可能会改善广泛使用的基于过滤器的仪器，用于测量BC和大气气溶胶的吸收特性。

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