Design and Application of Novel Instrumental Techniques in Atmospheric Chemistry: From Gas-Phase Hydroxyl Radical to Levitated Particles

Abstract

Atmospheric chemistry has significant impacts on both human health and climate, due to the formation of pollutants such as secondary organic aerosol and ozone. Measurements alongside numeric model representations of the Earth's atmosphere can be used to inform policy decisions aimed at addressing these impacts. However, the complexity and trace concentrations involved in the chemical composition of the atmosphere represent a formidable analytical challenge, and progress in atmospheric chemistry often can be held back by limitations in instrumentation available for use in field or laboratory studies. In this work, two projects with the goal of enabling improved understanding of atmospheric chemistry via improvements in analytical instrumentation are described. To better understand fundamental processes that take place in atmospheric aerosol particles, a new electrodynamic balance–mass spectrometry (EDB-MS) instrument was built that can measure the composition of individually levitated droplets with high chemical specificity. The ability of the EDB-MS to measure vapor pressures was validated by measuring evaporation rates of a benchmark series of polyethylene glycol oligomers and comparing the results to literature values. The technique was then applied to the dialdehyde butenedial, resulting in an improved understanding of the effect of relative humidity and inorganic ions on butenedial's gas-particle partitioning behavior. In other work, research was performed to design a new instrument to measure the hydroxyl radical (OH), which is the most important gas-phase oxidant in the atmosphere. Both experimental work and numeric models were used to help design a new type of instrument that uses a two-photon laser-induced fluorescence (TP-LIF) technique to measure OH. In contrast with existing single-photon laser-induced fluorescence (SP-LIF) instruments, TP-LIF holds the promise of operating without a low-pressure expansion cell, which has been hypothesized to be the source of a poorly understood interference observed in existing measurements. Together, these new approaches to atmospheric chemistry measurement techniques contribute to an improved understanding of fundamental processes underlying air quality and climate effects.