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 (E_abs) 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 E_abs for coated BC particles. The value of E_abs 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 E_abs using these two methods. Our results indicate differences ranging from 25% to 65% for E_abs measured using both methods at 375 nm. We observed little to no enhancement at either wavelength using the thermodenuder method. In contrast, the E_abs 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

摘要

棕碳（BrC）是光学定义的一类有机碳（OC），可以强烈吸收较短可见光和紫外A（UVA）波长的光。已经发现吸光和不吸光的OC均覆盖黑碳（BC），并且可以调节BC的光学性能。传统上，热剥蚀仪与测量颗粒光吸收率的仪器（例如光声光谱仪）结合使用，以参数化由于OC导致的增强的BC光吸收率（E _abs）。该方法可能会引入未量化的测量伪影，因为热剥蚀器无法在通常操作的温度下去除低挥发性化合物，并且由于仪器中颗粒的温度依赖性损失而导致的损失。这些伪影可能导致准确估计E时出错包被的BC颗粒的_吸收。E _abs的值也可以通过将包衣的BC气雾剂的质量吸收横截面（MAC）与未包衣的BC的基于文献的MAC值进行比较来估算。在这项研究中，我们使用两个集成的光声浊度仪，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E _ab相关的差异和误差。我们的结果表明，使用这两种方法在375 nm下测得的E _abs的差异在25％至65％之间。我们使用热剥蚀器方法观察到在任一波长下几乎没有增强。相反，E _abs使用基于文献的MAC方法，在375 nm处，OC / BC比值会增加。两种方法之间的差异归因于低挥发性棕碳的存在，使用离散偶极近似计算可以证实这些结果。理论计算预测，这些低挥发性有机化合物在375 nm波长处的虚部折射率可以达到0.32，这与以前的研究一致，即低挥发性有机物的吸收量很大。

版权所有©2021美国气溶胶研究协会