Bias in quantification of light absorption enhancement of black carbon aerosol coated with low-volatility brown carbon
Bias in quantification of light absorption enhancement of black carbon aerosol coated with low-volatility brown carbon
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DOI：
10.1080/02786826.2021.1873909
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发表时间：
2021-03
影响因子：
5.2
通讯作者：
Nishit Shetty;P. Beeler;T. Paik;F. Brechtel;R. Chakrabarty
中科院分区：
文献类型：
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作者：
Nishit Shetty;P. Beeler;T. Paik;F. Brechtel;R. Chakrabarty
摘要棕碳（BrC）是一类光学性质明确的有机碳（OC），它对可见光和紫外线A（UVA）有很强的吸收。光吸收和非吸收OC都被发现包覆在黑碳（BC）上，并且可以调节BC的光学性质。热稀释器通常与测量颗粒光吸收的仪器（例如光声光谱仪）结合使用，以参数化由于OC而增强的BC光吸收（Eabs）。这种方法可能会引入未量化的测量伪像，因为热稀释器在其通常操作的温度下并不去除低挥发性化合物，并且由于仪器内颗粒的温度依赖性损失。这些伪影可能导致准确估计涂覆的BC颗粒的Eabs的误差。Eabs的值也可以通过将涂覆的BC气溶胶的质量吸收截面（MAC）与基于文献的未涂覆的BC的MAC值进行比较来估计。在这项研究中，我们使用两个集成的光声浊度仪光谱仪，分别在375 nm和532 nm的波长下工作，以量化与使用这两种方法测量Eabs相关的差异和误差。我们的研究结果表明，使用这两种方法在375 nm处测量的Eabs的差异范围从25%到65%。我们观察到很少或没有增强在任何波长使用热扩散法。相比之下，使用基于文献的MAC方法，在375 nm处的Eabs值随着OC/BC比率而增加。这两种方法之间的差异归因于低挥发性的棕色碳的存在下，这些结果得到证实，使用离散偶极子近似计算。理论计算预测，这些低挥发性OC在375 nm波长处的折射率虚部值可高达0.32，这与先前发现低挥发性有机物的大吸收的研究一致。版权所有© 2021美国气溶胶研究协会
Abstract Brown carbon (BrC) is an optically defined class of organic carbon (OC) which strongly absorbs light at shorter visible and ultraviolet-A (UVA) wavelengths. Both light absorbing and non-absorbing OC have been found to coat black carbon (BC) and could modulate the optical properties of BC. Thermodenuders are conventionally used in conjunction with instruments measuring particle light absorption, such as photoacoustic spectrometers, to parametrize enhanced BC light absorption (Eabs) due to OC. This method can introduce unquantified measurement artifacts because thermodenuders do not remove low-volatility compounds at the temperatures they are commonly operated at and due to temperature-dependent loss of particles within the instrument. These artifacts could lead to errors in accurately estimating Eabs for coated BC particles. The value of Eabs can also be estimated by comparing the mass absorption cross-section (MAC) of coated BC aerosol to literature-based MAC values of uncoated BC. In this study, we use two integrated photoacoustic-nephelometer spectrometers, operating at wavelengths of 375 nm and 532 nm, respectively, to quantify the differences and errors associated with measuring Eabs using these two methods. Our results indicate differences ranging from 25% to 65% for Eabs measured using both methods at 375 nm. We observed little to no enhancement at either wavelength using the thermodenuder method. In contrast, the Eabs value increased with the OC/BC ratio at 375 nm using the literature-based MAC method. The difference between the two methods was attributed to the presence of low-volatility brown carbon, and these results were corroborated using discrete dipole approximation calculations. Theoretical calculations predicted that these low-volatility OC could have values of the imaginary part of the refractive index up to 0.32 at a wavelength of 375 nm, which is in line with previous studies that found large absorption by low-volatility organics. Copyright © 2021 American Association for Aerosol Research