The Development of Novel Spectroscopic Tools to Probe Free Radical Chemistry in the Troposphere

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The Development of Novel Spectroscopic Tools to Probe Free Radical Chemistry in the Troposphere

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Title: The Development of Novel Spectroscopic Tools to Probe Free Radical Chemistry in the Troposphere
Author: Hannun, Reem A.
Citation: Hannun, Reem A. 2016. The Development of Novel Spectroscopic Tools to Probe Free Radical Chemistry in the Troposphere. Doctoral dissertation, Harvard University, Graduate School of Arts & Sciences.
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Abstract: The oxidizing capacity of the atmosphere regulates both the longwave and shortwave radiation budgets holding our planet in equilibrium. As planetary conditions continue to shi , insights into the oxidation pathways determining both greenhouse gas lifetimes and aerosol formation become paramount. In addition, oxidation and photochemistry act in conjunction to cleanse the troposphere of pollutants, which impact surface ozone levels and particulate ma er content. Both of these have implications for human health and quality of life.

A few free radical species dominate the oxidation chemistry of the atmosphere – HOx, halogen radicals XOx (X≡Cl, Br, I), and NOx – with the hydroxyl radical playing a central role. In this thesis, I will discuss the development of laser-based spectroscopic techniques designed to target and quantify two chemical species: iodine monoxide and the hydroxyl radical. Due to their high reactivity and inhomogeneous chemical distributions, both IO and OH remain challenging to measure, with atmospheric lifetimes on the order of < 1 second and mean mixing ratios in the part per trillion (ppt) range. e spectroscopic methods discussed serve as the foundations upon which high sensitivity, high speci city instruments are developed for in situ chemical detection.

In Chapter 2, I will discuss the eld validation of the Harvard IO instrument, which exploits a laser-induced uorescence (LIF) detection technique to quantify in situ mixing ratios of iodine monoxide. e Harvard IO instrument was deployed to Shoals MarineLab on Appledore Island, ME in August and September of 2011. An overview of the instrument will be detailed as well as results from the eld deployment and their implications for IO chemistry.

Chapter 3 will continue in the vein of laser spectroscopy to discuss the development of a novel light source in the mid-infrared. I will explore the use of a nonlinear optical technique, Optical Parametric Generation (OPG), to create high-power pulsed radiation, which can be adapted to varied spectroscopic methods that require pulsed laser sources. e development of this light source allows for the detection of several trace species relevant to climate that exhibit fundamental vibrations in the overlapping spectral window.

In Chapter 4, I will focus on extending the OPG laser system to target the vibrational bands of OH. e vibrational excitation serves as the rst step in a two-photon LIF (TP-LIF) detection technique for hydroxyl radicals in the troposphere. Several interferences, both known and unknown, plague the current measurements of OH, and TP-LIF provides an alternative detection scheme to signi cantly improve measurement accuracy.

Finally, I will assess the implications for future measurements of OH and its coupling with halogen free radical species to mediate oxidation in the troposphere, which is essential to a be er understanding of the intersection between chemistry and climate.
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