Developing strategies for improving our understanding of the air quality impacts of dust heterogeneous chemistry
Creators: Abou-Ghanem, Maya
Publisher: name: University of Alberta Library
Year: 2022

<p>Dust is one of the leading contributors to particulate matter (PM) in the atmosphere and can be natural (i.e., mineral dust) and anthropogenic (i.e., road and industrial dust) in origin. Natural and anthropogenic dusts provide a reactive surface for the chemical transformation of pollutant gases, which can influence both the composition of the atmosphere and dust PM. To date, most heterogeneous chemistry studies exploring dust–pollutant interactions have used commercially available, single-component metal oxides as a proxy for mineral dust, and no studies have investigated the heterogeneous chemistry of anthropogenic dust. Together, this limits our ability to predict the air quality (AQ) impacts of dust PM, which will become increasingly important in the future as a result of: a) an increase in mineral dust emissions due to increasing aridity and desertification as a result of climate change and land modification, respectively and b) road dust dominating traffic-related PM as countries move towards net-zero fleet emissions. The goal of this thesis is to advance dust–pollutant interaction studies by investigating the uptake of ozone, an important atmospheric oxidant, by natural, road, and atmospherically aged dust substrates.<br>
In my first project, I found that the photoreactivity of a suite of natural titanium (Ti)-containing minerals commonly found in mineral dust display vastly different photoreactivity with ozone, even within the same mineral phase sampled from different locations. In addition, I found that the photoreactivity of natural titanium dioxide (TiO2) minerals is orders of magnitude lower than commercial TiO2, which is typically used as a photoactive proxy in studies of dust photochemistry. This work highlights the importance of considering mineral phase when assessing dust photoreactivity and provided motivation for me to explore the photochemical drivers of Icelandic volcanic dust in my second project. Here, I observed modest photochemistry of a Ti-rich sample from the Mýrdalssandur dust source region with ozone and found that most of the elemental Ti is contained in the non-photoactive glass fraction of the sample, with minor amounts present in moderately photoactive Ti-containing mineral phases. This project demonstrated the advantages of using elemental speciation for predicting and understanding the photochemistry of glass-rich volcanic dusts. <br>
In my third project, I investigated the reactivity of ozone with road dust and commercial anti-icer, a saline solution applied to roads in cold-climate regions. Interestingly, I found that ozone is more reactive towards road dust than mineral dust. In addition, I demonstrated that the interaction of ozone with anti-icer leads to production of inorganic chlorine gas, an important precursor for the reactive chlorine radical. This work demonstrates that the interaction of ozone with road dust PM and the road surface has the potential to influence the oxidative power of the urban atmosphere, and that this heterogeneous chemistry should be considered in future when making road maintenance decisions in high-latitude urban regions. <br>
My fourth project focused on the influence of atmospheric aging on dust photochemistry by exploring the photochemistry of internally mixed dusts. In this work, I discuss the design and construction of a custom-built photochemical aerosol flow tube reactor that is used to investigate the photochemical uptake ozone by organic-coated TiO2, which I used as a simple model system for atmospherically aged mineral dust. Here, I demonstrated the suppression of ozone uptake by organic coatings and observed decreasing ozone uptake with increasing coating thickness. This work is the first to demonstrate the evolution of mineral dust photochemistry during atmospheric aging, which may require us to consider the influence of organic coatings on the inherent photoreactivity of dust.<br>
This thesis provides significant advancements in dust heterogeneous chemistry studies by exploring the reactivity of naturally sourced mineral and road dust, as well as the influence of atmospheric aging on dust photochemistry. Together, this will ultimately allow us to better predict the AQ impacts of dust in the urban atmosphere.</p>