Development and Application of Advanced Mass Spectrometry Techniques for Determination of Reactive Aerosol Constituents
Creators: Gautam, Tania
Publisher: name: University of Alberta Library
Year: 2024

<p>Reactive aerosol constituents, including organic peroxides (H2O2, ROOR and ROOH), organosulfates (OS), and organonitrates (ON), are integral components of atmospheric chemistry, climate dynamics, and human health. However, their intricate chemical compositions and dynamic behaviours present formidable challenges for accurate identification and quantification. These reactive species emerge from complex photochemical reactions involving volatile organic compounds (VOCs), nitrogen oxides (NOx), and various other precursors emitted from both biogenic and anthropogenic sources. Peroxides, as reactive oxygen species, significantly contribute to respiratory illnesses, while OS and ON, derivatives of sulfur and nitrate species, influence climate dynamics by altering water uptake properties and facilitating cloud formation. Collectively, these reactive aerosol constituents exacerbate air quality issues by promoting the formation of secondary organic aerosols (SOA) and imposing health burdens on human populations. Despite extensive research, several aspects of<br>
their formation, transformation, and atmospheric fate remain ambiguous. Challenges in their characterization stem from the lack of sensitive techniques offering molecular specificity, biases prone to matrix effects, and the chemical complexity arising from<br>
structural diversity. In this thesis, we employ advanced mass spectrometry techniques to delve into the fundamental mechanisms underlying the formation of peroxides in<br>
aqueous environments and to identify sulfur/nitrate enriched species in ambient samples under the influence of meteorological conditions. <br>
Chapter 2 introduces a novel pathway to investigate the occurrence of aqueous-phase autoxidation as a crucial reaction mechanism facilitating the formation of hydroperoxides (ROOHs). Leveraging linear unsaturated organic acids, a selective chemical assay assisted (i.e., iodometry) liquid chromatography-mass spectroscopy (LC-MS)<br>
technique is utilized to systematically study the formation of highly oxygenated molecules (HOMs) containing up to two - OOH groups. The empirical dependence of these ROOHs on wavelengths (UVA, UVB and UVC) and oxidant/precursor concentrations is explored to discern the conditions favourable to their formation. While our findings support the feasibility of aqueous ROOHs from various water-soluble organic precursors, distinguishing the specific mechanism solely from offline iodometry-assisted LC-MS technique remains challenging due to lack of online measurements for rapid formation of intermediate compounds. <br>
In Chapter 3, I have investigated the experimental limits of the conventional iodometry method, which is known to undergo interferences from reducing agents such as olefins. This halogen chemistry has been widely used in food chemistry to determine<br>
the degree of unsaturation. Our results show that linear unsaturated compounds can react with halogen species such as I2 - a key intermediate in iodometry. However, this underestimation in peroxide content will occur during extended periods of bench reaction and relatively higher concentrations (&gt;500 µM) of olefinic compounds. I have determined that in the case of atmospheric samples including complex mixtures<br>
of SOA, it is unlikely that olefinic concentrations will reach the level of causing interference with the conventional iodometry approach.<br>
In Chapter 4, I have adopted a broader approach to understanding the role of ROS and sulfate/nitrate enriched particle-bound species in ambient aerosol samples. Compared<br>
to separation-based techniques utilized in Chapters 2 and 3, here I have adopted a far more robust analytical approach, i.e., nano-DESI-HRMS. Through this study, I have determined that sulfate/nitrate enriched species are more episodic during day-to-day comparisons, with meteorological factors such as wind direction playing a determining role in the emergence of OSs. Furthermore, photochemical processing may be alluding to the dominance of ONs. There could likely be potential CHO compounds with peroxy functionality, however, the application of chemical derivatization techniques (e.g., iodometry) to resolve the molecular ambiguity is difficult in complex matrices<br>
with high salt concentrations.<br>
Overall, this thesis adopts a complementary functional approach and robust analytical methodology to pursue investigation into elusive reactive aerosol constituents, thereby providing crucial insights into their formation and critical dependence on<br>
meteorological parameters.</p>