Ambient Single Particle Composition Measurement with Laser-Induced Breakdown Spectroscopy

Abstract

Aerosol particles have a multitude of direct and indirect effects on human life and the planet. They cause millions of premature deaths annually, participate in the Earth’s water cycle through precipitation processes, affect the climate directly through absorption and scattering and indirectly acting as cloud condensation nuclei. Furthermore, the aerosol transmission of pathogens has been a cause célèbre since the unprecedentedly fast spreading of the latest coronavirus pandemic.

Measuring the composition of aerosol particles is essential to be able to append the knowledge on the factors affecting their toxicity and climate effects and to trace and monitor harmful particles and pathogens in the air. The particles in the local air may originate from a vast range of both nearby and remote sources and include a complex mixture of elements and molecules. Thus, in many applications, the composition must be measured on a single-particle level to maintain crucial information about the particles’ internal and external mixing state.

This thesis presents development toward a novel method capable of measuring the composition of aerosol particles on a single-particle basis utilizing laser-induced breakdown spectroscopy (LIBS). The technique utilizes efficient aerosol charging to enable electrodynamic focusing of particles sampled directly from the ambient air. The focusing is essential for maximizing the LIBS signal intensity acquired from individual particles. In the thesis, two novel aerosol charger designs are presented and characterized. The focusing efficiency with realistic charging states is thoroughly evaluated, and optical designs for LIBS analysis combined with the focusing systems are illustrated and characterized. Ultimately, a field-deployable instrument capable of analyzing the elemental composition of ambiently sampled aerosol particles on a single- particle basis is presented. The method’s ability to analyze multiple elements simultaneously from ambiently sampled single particles over a broad wavelength range and high resolution is unprecedented in aerosol technology.