Fundamentals of Aerosol Photoacoustic Spectroscopy

气溶胶对光吸收的定量测量在大气气溶胶领域中至关重要，因为光-气溶胶相互作用改变了局地和全球大气能量平衡，并在大气变暖中发挥着重要作用。因此，全面的大气模型需要准确描述大气气溶胶对光的吸收，从而能够更好地预测未来的大气能量和温度行为。因此，气溶胶光吸收的高精度定量野外测量对于验证和改进与气溶胶相关的大气模式是必要的。在光声光谱学中，气体或气溶胶样品吸收调制光，导致气相的周期性热膨胀。由此产生的声波可以用麦克风探测到。在气溶胶吸收光的情况下，吸收的能量迅速转移到周围经历热膨胀的气体中。气溶胶PAS的优点是动态范围大，信号响应线性，并且有可能测量零本底信号。气溶胶PAS的高灵敏度使该技术在气溶胶光吸收测量中优于消光-负散射技术。此外，气溶胶PAS对光散射不敏感，因此可以用于悬浮气溶胶的现场测量，这使气溶胶PAS具有基于过滤器的气溶胶光吸收测量技术的优势，后者只能在异地应用于收集的样品，并且由于过滤器加载而容易产生测量偏差。最近，两个应用程序(TU München和ETH Zürich)首次一起提出了对单个悬浮气溶胶粒子的光声吸收测量。该项目的总体目标是评估这一新工具在光学气溶胶分析方面的适用性。一些研究表明，PA信号并不是在所有情况下都与粒子的吸收截面成正比。这一假设必须得到检验，如果得到证实，理论基础必须以一种可以纠正潜在假象的方式建立起来。常见的大气气溶胶粒子并不总是均匀的球体。将研究二元混合物，特别是包覆层对球形颗粒的影响。固体气溶胶颗粒通常是非球形的。因此，需要研究形状对功率放大器信号的影响。根据后两项研究的结果，可以尝试使用PA光谱学对单个气溶胶颗粒进行层析成像。这一步骤特别针对层状液滴的分析，其中分析球形颗粒上涂层的厚度是可能的。

The quantitative measurement of light absorption by aerosols is critically important in the field of atmospheric aerosols, since light-aerosol interactions modify the local and global atmospheric energy balance and play a significant role in the warming of the atmosphere. Hence, comprehensive atmospheric models require accurate description of the light absorption by atmospheric aerosols, thus allowing for a better prediction of future atmospheric energy and temperature behavior. Hence, high-accuracy quantitative field measurements of aerosol light absorption are necessary for validation and refinement of aerosol-related atmospheric models. In photoacoustic spectroscopy, a gas or aerosol sample absorbs modulated light causing periodic thermal expansion in the gas phase. The resulting sound waves can be detected with a microphone. In the case of absorption of light by aerosols, the absorbed energy is rapidly transferred to the surrounding gas which undergoes the thermal expansion. The advantages of aerosol PAS are a large dynamic range, linear signal response, and the potential to measure a zero-background signal. The high sensitivity of aerosol PAS makes the technique superior to extinction-minus-scattering techniques for aerosol light absorption measurements. Additionally, aerosol PAS is insensitive to light scattering, and as a result, can be applied for in situ measurements in the suspended aerosol, giving aerosol PAS an advantage over filter-based aerosol light absorption measurement techniques, which can only be applied to collected samples ex situ and are prone to measurement biases due to filter loading. Recently, the two applications (TU München and ETH Zürich) presented together for the first time photoacoustic (PA) absorption measurements on single levitated aerosol particles. The overall aim of this project is an assessment of this new tool with respect to its applicability for optical aerosol analysis. For this purpose, some key questions have to be answered.Some studies indicate that the PA signal is not under all circumstances proportional to the absorption cross section of a particle. This hypothesis has to be tested and, if verified, the theoretical basics have to be established in a way that potential artefacts can be corrected for. Common atmospheric aerosol particles are not always homogeneous spheres. The influence of binary mixtures, particularly of coating layers on spherical particles will be investigated. Solid aerosol particles are often non-spherical. Hence, the influence of the shape on the PA signal needs to be investigated. Depending on the outcome of the latter two studies, attempts can be made to use PA spectroscopy to perform tomography on single aerosol particles. This step particularly aims for the analysis of layered droplets, where the analysis of the thickness of a coating layers on a spherical particle might be possible.