Sources, sinks, and trends of ozone precursors and their impact on ozone in northern Colorado
Date
2017
Authors
Abeleira, Andrew Joseph, author
Farmer, Delphine K., advisor
Henry, Charles, committee member
Fischer, Emily, committee member
Crans, Debbie, committee member
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Abstract
Ozone is a structurally simple molecule that plays immensely important roles in Earth's atmosphere. In the troposphere, ozone is vital in maintaining the oxidative capacity of the lower atmosphere. However, unlike the chemical structure, the formation and lifecycle of ozone in the troposphere is anything but simple. The role of ozone in severe air pollution episodes, and the negative human and ecosystem health impacts of ozone were first established in the United States during the "smog" pollution episodes of the early 1950s in the Los Angeles basin. Since then, understanding the formation and impacts of ozone has been an air quality research priority in the United States. The primary source of tropospheric ozone is the photochemically initiated oxidation of anthropogenic or biogenic volatile organic compounds in the presence of nitrogen oxides. The production of ozone relies on the interplay between two catalytic cycles that share initiation and termination reactions. The linkage of the ozone catalytic cycles, via those initiation and termination reactions, leads to the non-linear nature of the chemical production of ozone. The urbanization of the United States in the 1950s-1970s led to increased frequency of severe ozone events in urban areas from increased ozone precursor emissions – specifically emissions of NO and NO2 from automobiles and coal-fired electricity generating power plants. These high ozone events, coupled with results from ozone epidemiologic, exposure, and toxicology studies, prompted the U.S. Congress to establish the Clean Air Act of 1970. The Clean Air Act authorized the U.S. Environmental Protection Agency to establish the National Air Quality Standards for six criteria air pollutants – including ozone. The goal of this new standard was to systematically reduce ambient ozone concentrations by targeting major ozone precursor emission sources. Near 50 years later high ozone events are still occurring in densely populated urban and suburban regions in the United States. Herein, an in-depth study of the sources and sinks of ozone precursors, and the impact of precursor reductions on long-term ozone trends in Northern Colorado is presented. Chapter 1 of this dissertation provides relevant historical context regarding ozone in the United States, pertinent tropospheric ozone chemistry for urban and suburban regions, ozone precursor trends in the United States, and other important processes that affect regional and global ozone. Chapter 2 examines long-term (15 year) trends in ozone and ozone precursors in Northern Colorado with a focus on day of week ozone and NO2 trends that suggest Northern Colorado is transitioning from a NOx-saturated to peak ozone production region. Additionally, the impact of severe drought on the ozone/temperature relationship is addressed. Chapter 3 details the seasonal sources of a suite of volatile organic compounds measured during two 8-week periods in spring and summer 2015 at a ground site in Northern Colorado, and demonstrates the impact of drought on the local isoprene and reactive carbon budget. The reduction in isoprene emissions during drought is tied back to the suppression of the ozone/temperature relationship in the region. In the fourth chapter, the sources and sinks of alkyl nitrates, a key ozone precursor sink, are investigated using a simple sequential production-destruction reaction model. The final chapter highlights the need for long-term ozone and ozone precursor monitoring in Northern Colorado as population, energy demands, and ozone precursor emissions change.
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Subject
Northern Colorado
volatile organic compounds
ozone
alkyl nitrates