Use of chemical ionization mass spectrometry for study of photochemical properties: ketone photolysis quantum yields

Abstract

Measurements of organic radicals produced during organic trace gas photolysis are critical to our understanding of radical budgets throughout the troposphere. This dissertation demonstrates the utility of chemical ionization mass spectrometry for measurements of radical quantum yields in the photolysis of organic trace gases in the laboratory setting. Chapter 2 addresses the development of a coupled chemical ionization mass spectrometer with iodide reagent ions (I-CIMS) and wide band light source instrument design, which was used to measure the quantum yield for CH3C(O) from acetone photolysis, through measurement of CH3C(O)O2. Acetone is the most abundant oxygenated organic gas in the troposphere and its photolysis can account for up to 1/3 of radical production in the upper troposphere. The results from this chapter demonstrate that the I-CIMS can be used for acetone photolysis measurements under conditions of the troposphere. In Chapter 3, the I-CIMS measurements of the CH3C(O) quantum yield in acetone photolysis are expanded to temperatures (223 to 323 K) and pressures (150 to 850 mbar) reflecting the conditions of the troposphere. The measurements are used to parameterize the quantum yield of CH3C(O) for use in models of tropospheric radical production. The parameterization shows that acetone photolysis near the tropopause may be up to 1.4 times slower than previously expected. These are the only measurements of acetone photolysis under tropospheric conditions based on the detection of the dominant radical product, CH3C(O)O2. Chapter 4 explores a new, multiple-reagent ion system with Cl2- as the primary reagent ion (Cl2-CIMS). Cl2-CIMS provides higher sensitivity for small acyl peroxy radicals than achieved with I-CIMS. However, the higher background of Cl2-CIMS leads to higher limits of detection and the uncertainty on multiple reagent ion chemistries makes this system unsuitable for ambient measurements. Cl2-CIMS could be further improved through larger changes to the instrument design than those discussed here, and other novel reagent ion chemistries may be accessible using the multi-step ionization mechanism that produces Cl2-. Chemical ionization mass spectrometry is well-suited to fast, speciated measurements of radicals and is thus useful for measurements of complex photolysis mechanisms, like that of acetone in the troposphere. Further instrument development could improve CIMS sensitivities and limits of detection to organic radicals, expanding its utility to more photochemical systems and ambient measurements of radicals as well.