Gas-Wall Partitioning of Oxygenated Organic Compounds: Measurements, Structure–Activity Relationships, and Correlation with Gas Chromatographic Retention Factor

<div><p>Gas-wall partitioning of 50 oxygenated organic compounds was investigated by using gas chromatography to monitor time-dependent gas-phase concentrations of authentic standards added to a large Teflon environmental chamber. Compounds included C₈–C₁₄ monofunctional ketones and alcohols, C₅–C₉ monoacids, and C₄–C₁₀ diols with linear and cyclic structures. Measured time constants for reaching gas-wall partitioning equilibrium ranged from ∼10 to 100 min with an average value of ∼30 min and exhibited no obvious trend with compound structure, whereas the extent of equilibrium partitioning to the walls ranged from ∼0 to 100% and increased with increasing carbon number and with functional group composition in the order ketones < alcohols < monoacids < diols. When results were modeled using an approach analogous to one commonly used to describe absorptive gas-particle partitioning in terms of compound vapor pressure and aerosol mass loading it was determined that the absorptive properties of the Teflon film walls were equivalent to 2–36 mg m⁻³ of liquid organic aerosol particles. These results, when combined with those obtained in previous studies, indicate that most multifunctional products formed from the oxidation of atmospherically important hydrocarbons including isoprene, monoterpenes, aromatics, and alkanes have the potential to undergo significant partitioning to the walls of Teflon chambers and thus be lost from further chemical reaction and secondary organic aerosol formation as well as from gas and particle analyses. Two approaches for estimating equilibrium gas-wall partitioning in such studies are presented: one is a structure–activity relationship based on the absorptive gas-wall partitioning model and the other involves the use of observed correlations between gas-wall partitioning and compound retention on a gas chromatographic column.</p><p>Copyright 2015 American Association for Aerosol Research</p></div

Products and Mechanism of the Reaction of 1‑Pentadecene with NO₃ Radicals and the Effect of a −ONO₂ Group on Alkoxy Radical Decomposition

The linear C₁₅ alkene, 1-pentadecene, was reacted with NO₃ radicals in a Teflon environmental chamber and yields of secondary organic aerosol (SOA) and particulate β-hydroxynitrates, β-carbonylnitrates, and organic peroxides (β-nitrooxyhydroperoxides + dinitrooxyperoxides) were quantified using a variety of methods. Reaction occurs almost solely by addition of NO₃ to the CC double bond and measured yields of β-hydroxynitrate isomers indicate that 92% of addition occurs at the terminal carbon. Molar yields of reaction products determined from measurements, a proposed reaction mechanism, and mass-balance considerations were 0.065 for β-hydroxynitrates (0.060 and 0.005 for 1-nitrooxy-2-hydroxy­pentadecane and 1-hydroxy-2-nitrooxy­pentadecane isomers), 0.102 for β-carbonylnitrates, 0.017 for organic peroxides, 0.232 for β-nitrooxyalkoxy radical isomerization products, and 0.584 for tetradecanal and formaldehyde, the volatile C₁₄ and C₁ products of β-nitrooxyalkoxy radical decomposition. Branching ratios for decomposition and isomerization of β-nitrooxyalkoxy radicals were 0.716 and 0.284 and should be similar for other linear 1-alkenes ≥ C₆ whose alkyl chains are long enough to allow for isomerization to occur. These branching ratios have not been measured previously, and they differ significantly from those estimated using structure–activity relationships, which predict >99% isomerization. It appears that the presence of a −ONO₂ group adjacent to an alkoxy radical site greatly enhances the rate of decomposition relative to isomerization, which is otherwise negligible, and that the effect is similar to that of a −OH group. The results provide insight into the effects of molecular structure on mechanisms of oxidation of volatile organic compounds and should be useful for improving structure–activity relationships that are widely used to predict the fate of these compounds in the atmosphere and for modeling SOA formation and aging