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Heterogeneous reactions of NO₃ and other oxidants with organic films and substrates of atmospheric relevance Gross, Simone

Abstract

Knowledge of atmospheric heterogeneous reactions between gas-phase species and aerosol particles is limited. The goal of this thesis was to contribute to the understanding of these reactions, in particular to the reactions of gas-phase oxidants (NO₃, N₂O₅, NO₂, HNO₃, and O₃) with a variety of organic substrates that served as proxies for aerosol surfaces. Organics chosen for this study represent classes of compounds found in field studies. A cylindrical flow tube reactor coupled to a chemical ionization mass spectrometer was employed to study the reactive uptake of the different gas-phase oxidants on organic substrates. These studies showed fast reaction of polycyclic aromatic hydrocarbons (PAHs) with NO₃ (uptake coefficient γ ≥ 0.059), while reactions of the different PAHs with the other gas-phase species (N₂O₅, NO₂, HNO₃, and O₃) were at or below the detection limit (γ ≤ 6.6 × 10-⁵). NO₂ and HNO₃ were identified as gas-phase products in the reactions of NO₃ with PAHs. The results show that NO₃ may be a more important sink for tropospheric PAHs than NO₂, N₂O₅, HNO₃, or O₃. The uptake of NO₃ and N₂O₅ on liquid and solid films of an alkenoic acid, an alkanoate, and a polyalcohol was measured. Uptake of NO₃ on the alkenoic acid was fast (γ > 0.07 for the liquid), approximately two orders of magnitude faster than for the other two compounds. Uptake of N₂O₅ was slower than NO₃ reaction for all three compounds. The polyalcohol had the highest uptake coefficient with N₂O₅ (γ = (4 – 8) × 10-⁴). Based on these results, the presence of NO₃ and N₂O₅ may decrease the atmospheric lifetimes of alkenoic acids and alcohols, respectively. The flow reactor enabled quantitative exposures of self assembled monolayers to controlled amounts of NO₃ in order to mimic atmospheric exposures of aerosol surfaces. These NO₃ exposed monolayers (an alkane and a terminal alkene) were subsequently analyzed with different surface-analytical tools in order to determine condensed phase reaction products and deduce reaction mechanisms. Hydroxyl, carbonyl, carboxylic acid, and N-containing functional groups were confirmed. Additionally, the atmospheric lifetime of alkenes in the presence of NO₃ was shown to be short.

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