Improved measurements of soot using new particle morphology models

气溶胶（包括“煤烟”）对气候的影响很大，但也有很大的不确定性。 如果不能减少对气候有影响的排放，人类可能会被迫将光散射颗粒分散在大气中，以防止全球变暖失控。要进行这种“地球工程”，就需要对气溶胶在气候中的作用有精确的了解。 颗粒物是造成空气污染相关死亡的最大因素，发动机排放的炭黑可能特别有毒，但目前尚不清楚颗粒物的哪些化学或物理特性会导致这种毒性。许多设备的颗粒排放都受到管制，并且法规可能变得更加复杂，对排放测量设备提出了新的要求。颗粒结构使这些测量复杂化。 气溶胶仪器对非球形气溶胶的响应方式各不相同，这取决于测量原理（质量、迁移率、停止距离、光散射、吸收、计数），颗粒形态以及材料特性也会影响测量。几十年来，聚集气溶胶（如煤烟）已被建模为分形组成的“初级粒子”的直径dp，分形维数为Df~1.8。颗粒与短程构型之间的接触被捕获在“分形前因子”kF中。这个3参数几何模型是一个非常有用的近似，但可能已经采取了尽可能远。研究人员长期以来观察到（基于TEM），初级粒子并不具有均匀的尺寸，但尺寸变化被认为是随机分散在气溶胶群体中。最近，我们已经表明，第一次，系统的趋势，初级颗粒尺寸与聚集体尺寸。实际上，来自真实的燃烧系统的烟灰是具有非常不同的初级颗粒直径的接近理想的分形聚集体的“外部混合物”。这些变化可能很重要，因为重要的气溶胶物理特性通常是初级颗粒直径的非线性函数：在模型中使用单一平均初级颗粒直径将导致物理特性的错误预测（或气溶胶测量值的错误反演）。面临的挑战是开发一个简单的模型，可以捕捉关键的变化-拟议的研究的核心目标。我们计划使用我们广泛的TEM图像数据库，通过SMPS，CPMA，光散射和吸收测量进行相应的测量。 一个更好的结构模型为粒子形成的物理过程打开了一扇新的窗户。燃烧条件和颗粒形态之间的关系将在两个控制实验活动（模型气体耀斑和光学发动机）进行研究，并通过一个新的模拟颗粒形成解释。 拟议的资金将利用其他资金支持3个高质量的博士项目。

The effect of aerosols (including “soot”) on climate is large but also has a large uncertainty.  Failure to reduce climate-active emissions may force humanity to disperse light-scattering particles in the atmosphere to prevent runaway global warming. A precise knowledge of the role of aerosols in climate will be needed to do this “geoengineering”. Particles are the largest contributors to air-pollution-related deaths, and black carbon from engines may be particularly toxic, but it is not known which chemical or physical characteristics of particles cause this toxicity. Particle emissions from many devices are regulated and the regulations may become more sophisticated, placing new requirements on emission measurement equipment. Particle structure complicates these measurements. Aerosol instruments respond to non-spherical aerosols in different ways depending on the measurement principle (mass, mobility, stopping distance, light scattering, absorption, count), and the particle morphology as well as material properties affect the measurement. For decades, aggregate aerosols (such as soot) have been modeled as fractals composed of “primary particles” of diameter dp, with a fractal dimension of Df~1.8. The contact between particles and short range configuration is captured in the “fractal prefactor” kF. This 3-parameter geometric model has been a very useful approximation but may have been taken as far as it can go. Researchers have long observed (based on TEM) that primary particles do not have a uniform size but the size variations were assumed to be randomly dispersed through the aerosol population. Recently we have shown, for the first time,  systematic trends of primary particle size with aggregate size. Effectively, the soot from real combustion systems is an "external mixture" of nearly ideal fractal aggregates with very different primary particle diameters. These variations can be important because the important aerosol physical characteristics are typically non-linear functions of primary particle diameter: the use of a single average primary particle diameter in models will result in an incorrect prediction of physical properties (or incorrect inversion of aerosol measurements). The challenge is to develop a simple model that can capture key variations – the core objective of the proposed research. We plan to do this using our extensive TEM image database with corresponding measurements by SMPS, CPMA, light scattering and absorption measurements. A better model of structure opens a new window onto the physical processes of particle formation. The relation between combustion conditions and the particle morphology will be studied in two controlled experimental campaigns (a model gas flare and and an optical engine) and interpreted through a new simulation of particle formation.  The proposed funding will leverage other funding to support 3 high-quality PhD projects.