Time Dependent Uptake of Volatile Organic Compounds on Silica and the Observation and Quantification of Ambient Sesquiterpenes in Virginia
Correlation of adsorption behavior for a gas-surface system to the molecular structure of the gas-phase molecule is necessary to better constrain atmospheric modeling. Despite the variety of compounds emitted into the atmosphere, the role of molecular structure on uptake probability, γ, for a compound adsorbing onto a surface is not well understood. A custom designed flow manifold coupled to a mass spectrometer provides the means to analyze changes in γ for gases at environmentally relevant low ppb concentrations. Studies have focused on the adsorption of two classes of compounds: singly substituted benzene derivatives and several terpenoid compounds for a silica surface composed of hydroxyl groups and bridging oxygen species. Results for benzene derivatives show an enhancement of the initial γ value of all functionalized compounds relative to benzene, with this initial value influenced by both the dipole and volatility of a compound. The time-dependent behavior of γ has been shown to depend on the interactions occurring between the adsorbate and surface with species capable of hydrogen bond interactions exhibiting a greater number of adsorbed species at equilibrium. The initial γ value for terpenoid species was observed to increase from isoprene to sesquiterpenes, with several monoterpene isomers exhibiting distinct adsorption behavior based on structural differences. The sesquiterpene examined, β-caryophyllene, exhibited continuous uptake onto the silica surface and such uptake behavior could contribute in part to the low concentrations of sesquiterpenes observed in the environment. Encapsulation of the hydroxyl groups resulted in decreased uptake of several aromatic and terpenoid species examined and indicates that hydroxyl groups represent the majority of adsorption sites for the systems examined. Comparison between observed uptake behavior to adsorption models exhibited the best agreement with a model depicting monolayer formation with cooperative adsorption due to interactions between adsorbates. Formulations for predicting initial γ values based on the volatility and dipole of the compound are provided. Overall, these results represent the first step towards a better understanding of gas-surface interactions that occur in the environment. Sesquiterpenes represent one class of biogenic emissions not well constrained with regards to SOA influence due to their low volatility and concentrations relative to more abundant terpenoid species. Ambient measurements of sesquiterpenes through the Semi-Volatile Thermal desorption Aerosol Gas chromatography (SV-TAG) instrument presented total sesquiterpene concentrations that ranged from 0.8 to 2 ppt with no isomer dominating. Sesquiterpene contribution towards hydroxyl reactivity is negligible in comparison to more abundant terpenoid species while ozone reactivity was dominated by two isomers and could contribute to atmospheric reactivity during periods of high emissions. These measurements represent the first step in better constraining the contribution of sesquiterpenes towards secondary organic aerosol formation.