School of Environmental Science & Engineering, Nanjing University of Information Science & Technology, Nanjing, 210044, China; School of Atmospheric Physics, Nanjing University of Information Science & Technology, Nanjing, 210044, China.
School of Environmental Science & Engineering, Nanjing University of Information Science & Technology, Nanjing, 210044, China.
Environ Pollut. 2020 Jul;262:114172. doi: 10.1016/j.envpol.2020.114172. Epub 2020 Feb 19.
Field observations have suggested that particulate nitrate can promote the aging of black carbon (BC), yet the mechanisms of the aging process and its impacts on BC's light absorption are undetermined. Here we performed laboratory simulation of internal mixing of flame-generated BC aggregates with ammonium nitrate. Variations in particle size, mass, coating thickness, effective density, dynamic shape factor, and optical properties were determined online by a suite of instruments. With the development of coatings, the particle size initially decreased until reaching a coating thickness of ∼10 nm and then started increasing, accompanied by an increase in effective density and a decrease in dynamic shape factor, reflecting the transformation of BC particles from highly fractal to near-spherical morphology. This is partially attributable to the restructuring of BC cores to more compact forms. Exposing coated particles to elevated relative humidity (RH) led to additional BC morphology changes, even after drying. Particle light absorption and scattering were also amplified with ammonium nitrate coating, increasing with coating thickness and RH. For BC particles with a 17.8 nm coating, absorption and scattering were increased by 1.5- and 7.9-fold when cycled through 70% RH (5-70-5% RH), respectively. The irreversible restructuring of the BC core caused by condensation of ammonium nitrate and water altered both absorption and scattering, with a magnitude comparable to or even exceeding the effects of increased coating. Results show that ammonium nitrate is among the most efficient coating materials with respect to modifying BC morphology and optical properties compared with other inorganic and organic species investigated previously. Accordingly, mitigation of nitrate aerosols is necessary for the benefits of both air pollution control and reducing the impacts of BC on visibility impairment and radiative forcing on climate change. Our results also pointed out that the effect of BC core restructuring needs to be considered when evaluating BC's light absorption enhancement.
现场观测表明，颗粒态硝酸盐可以促进黑碳（BC）老化，但老化过程的机制及其对 BC 光吸收的影响尚不清楚。在此，我们进行了火焰生成的 BC 聚集体与硝酸铵内部混合的实验室模拟。通过一系列仪器在线测定了粒径、质量、涂层厚度、有效密度、动态形状因子和光学性质的变化。随着涂层的发展，粒径最初减小，直到达到约 10nm 的涂层厚度，然后开始增加，同时有效密度增加，动态形状因子减小，反映了 BC 颗粒从高度分形到近球形形态的转变。这部分归因于 BC 核的重构为更紧凑的形式。将涂覆的颗粒暴露于升高的相对湿度（RH）下会导致额外的 BC 形态变化，即使在干燥后也是如此。颗粒光吸收和散射也随着硝酸铵涂层的增加而放大，涂层厚度和 RH 增加。对于具有 17.8nm 涂层的 BC 颗粒，当通过 70%RH（5-70-5%RH）循环时，吸收和散射分别增加了 1.5-和 7.9 倍。硝酸铵和水的凝结导致 BC 核的不可逆转重构，改变了吸收和散射，其幅度与增加涂层的影响相当，甚至超过了增加涂层的影响。结果表明，与之前研究的其他无机和有机物质相比，硝酸铵是最有效的改性 BC 形态和光学性质的涂层材料之一。因此，为了控制空气污染和减少 BC 对能见度损害和气候变化辐射强迫的影响，有必要减少硝酸盐气溶胶。我们的结果还指出，在评估 BC 光吸收增强时，需要考虑 BC 核重构的影响。
