Fundamentals of Aerosol Photoacoustic Spectroscopy

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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.