Department of Environmental System Science, Institute for Atmospheric and Climate Science, ETH Zurich, 8092 Zurich, Switzerland.
Environ Sci Process Impacts. 2020 Apr 29;22(4):895-907. doi: 10.1039/c9em00525k.
Fresh soot particles are generally hydrophobic, however, particle hydrophilicity can be increased through atmospheric aging processes. At present little is known on how particle chemical composition and hydrophilicity change upon atmospheric aging and associated uncertainties governing the ice cloud formation potential of soot. Here we sampled two propane flame soots referred to as brown and black soot, characterized as organic carbon rich and poor, respectively. We investigated how the ice nucleation activity of these particles changed through aging in water and aqueous acidic solutions, using a continuous flow diffusion chamber operated at cirrus cloud temperatures (T ≤ 233 K). Single aggregates of both unaged and aged soot were chemically characterized by scanning transmission X-ray microscopy and near edge X-ray absorption fine structure (STXM/NEXAFS) measurements. Particle wettability was determined through water sorption measurements. Unaged black and brown soot particles exhibited significantly different ice nucleation activities. Our experiments revealed significantly enhanced ice nucleation activity of the aged soot particles compared to the fresh samples, lowering the required relative humidities at which ice formation can take place at T = 218 K by up to 15% with respect to water (ΔRHi ≈ 25%). We observed an enhanced water uptake capacity for the aged compared to the unaged samples, which was more pronounced for the black soot. From these measurements we concluded that there is a change in ice nucleation mechanism when aging brown soot. Comparison of the NEXAFS spectra of unaged soot samples revealed a unique spectral feature around 287.5 eV in the case of black soot that was absent for the brown soot, indicative of carbon with hydroxyl functionalities. Comparison of the NEXAFS spectra of unaged and aged soot particles indicates changes in organic functional groups, and the aged spectra were found to be largely similar across soot types, with the exception of the water aged brown soot. Overall, we conclude that atmospheric aging is important to representatively assess the ice cloud formation activity of soot particles.
新鲜的烟尘颗粒通常是疏水的，但通过大气老化过程可以提高颗粒的亲水性。目前，人们对烟尘在大气老化过程中化学组成和亲水性的变化以及控制烟尘成冰云潜力的相关不确定性知之甚少。在这里，我们采集了两种丙烷火焰烟尘，分别称为棕色烟尘和黑色烟尘，它们的特点是分别富含和缺乏有机碳。我们研究了这些颗粒在水和含水酸性溶液中的老化过程中冰核活性的变化，使用连续流动扩散室在卷云温度（T≤233 K）下运行。使用扫描透射 X 射线显微镜和近边 X 射线吸收精细结构（STXM/NEXAFS）测量对未老化和老化烟尘颗粒的单聚集体进行了化学表征。通过水分吸附测量确定了颗粒的润湿性。未老化的棕色和黑色烟尘颗粒表现出明显不同的冰核活性。我们的实验表明，老化后的烟尘颗粒的冰核活性显著增强，与新鲜样品相比，在 T=218 K 时可以发生冰形成的相对湿度降低了多达 15%（相对于水的ΔRHi≈25%）。我们观察到老化后的样品比未老化的样品具有更高的水分吸收能力，而黑色烟尘的情况更为明显。从这些测量中，我们得出结论，当老化棕色烟尘时，冰核机制发生了变化。对未老化烟尘样品的 NEXAFS 光谱进行比较时，发现黑色烟尘在 287.5 eV 左右有一个独特的光谱特征，而棕色烟尘则没有，这表明黑色烟尘中的碳含有羟基官能团。对未老化和老化的烟尘颗粒的 NEXAFS 光谱进行比较表明，有机官能团发生了变化，并且除了水老化的棕色烟尘外，老化后的光谱在不同类型的烟尘中基本相似。总的来说，我们得出结论，大气老化对于代表性地评估烟尘颗粒成冰云的活性非常重要。
