Objectives

The proposed research project represents an in-depth investigation of the fundamentals of aerosol photoacoustic spectroscopy through a combination of single particle, particle ensemble, and theoretical PA studies. It addresses the two main issues in the field; i.e. the particle size dependence of the PA response and light absorption PA measurements under high relative humidity conditions. We anticipate the results of these fundamental aspects to be of broad interest since they have the potential to substantially improve the understanding of current PA applications.

While PA spectroscopy is routinely applied in aerosol characterization and exhaust gas analysis, precise calibration is still a challenge, which mostly relies on gas-phase analyses. However, some studies indicate that there can be a systematic difference between the calibrations for gas and particle phase, depending on the size and kind of the aerosol particle.

The aim of this project is the development of a fundamental understanding of the complete PA signal generation on particles. On the one hand, this will eventually lead to improved calibration procedure for PA aerosol instrumentation. On the other hand, this understanding is necessary to establish Single Particle PA spectroscopy as a new tool in atmospheric science.

Method of Approach

For this objective, two different systems will be employed, one for ensemble measurements and one for the PA analysis of single particles trapped in an optical trap. While the ensemble system is more versatile and robust, thus allowing for the analysis of a greater variety of particles, the single particle PA system allows for a very precise control of the specific particle, as for instance high-precision size characterization, but also chemical composition and homogeneity (core/shell particles).

Although the two experimental setups, ensemble PA (E-PA) and single particle PA (SP-PA), will be performed with different PA cells in order to achieve maximum sensitivities for both approaches, these two cells will be designed in a way that they feature (nearly) identical acoustical resonance frequencies. This is necessary to facilitated comparison of the two PA systems in case the PA signals feature varying temporal behaviors and thus phase shifts.