The reaction of Cl atoms, in the presence of Cl2 and O2, with sub-micron squalane particles is used as a model system to explore how surface hydrogen abstraction reactions initiate chain reactions that rapidly transform the chemical composition of an organic particle. The heterogeneous reaction is measured in a photochemical flow tube reactor in which chlorine atoms are produced by the photolysis of Cl2 at 365 nm. By monitoring the heterogeneous reaction, using a vacuum ultraviolet photoionization aerosol mass spectrometer, the effective reactive uptake coefficient and the distributions of both oxygenated and chlorinated reaction products are measured and found to depend sensitively upon O2, Cl2, and Cl concentrations in the flow reactor. In the absence of O2, the effective reactive uptake coefficient monotonically increases with Cl2 concentration to a value of ~;;3, clearly indicating the presence of secondary chain chemistry occurring in the condensed phase. The effective uptake coefficient decreases with increasing O2 approaching a diffusion corrected value of 0.65 ? 0.07, when 20percent of the total nitrogen flow rate in the reactor is replaced with O2. Using a kinetic model it is found that the amount of secondary chemistry and the product distributions in the aerosol phase are controlled by the competitive reaction rates of O2 and Cl2 with alkyl radicals. The role that a heterogeneous pathway might play in the reaction of alkyl radicals with O2 and Cl2 is investigated within a reasonable range of reaction parameters. These results show, more generally, that for heterogeneous reactions involving secondary chain chemistry, time and radical concentration are not interchangeable kinetic quantities, but rather the observed reaction rate and product formation chemistry depends sensitively upon the concentrations and time evolution of radical initiators and those species that propagate or terminate free radical chain reactions.

While examining the heterogeneous reaction of chlorine atoms with alkenes, in the presence of Cl₂, we have observed an unexpectedly large enhancement of reactivity and the predominance of chlorinated reaction products even under high O₂ conditions, where Cl atom recycling is expected to be minimal. These observations cannot be explained by known free radical oxidation or cycling mechanisms, but rather we find evidence for the multiphase catalytic coupling of free radical oxidation with electrophilic Cl₂ addition. The mechanism entails the production of oxygenated reaction intermediates, which act as gas-liquid phase-transfer catalysts (gl-PTCs) by promoting the accommodation of gas-phase Cl₂ by the aerosol, thereby enhancing electrophilic addition. Although the majority of PTCs typically couple chemistry between two immiscible liquid phases (aqueous/organic), there are few examples of PTCs that couple gas-liquid reactions. This work shows how multiphase reaction schemes of aerosols can be reimagined for understanding catalytic reaction mechanisms.

The reaction of closed shell Cl2 molecules with sub-micron droplets composed of unsaturated molecules, oleic acid (OA), linoleic acid (LA), linolenic acid (LNA), or squalene (Sqe), are investigated in an atmospheric pressure flow tube reactor in conjunction with a vacuum ultraviolet photoionization aerosol mass spectrometer and a scanning mobility particle sizer. Cl2 is found to react with all particles, and the reactive uptake coefficients depend on the number of unsaturated reaction sites, e.g., γCl2Sqe = (0.66 ± 0.03) × 10-4 versus γCl2OA = (0.23 ± 0.01) × 10-4. In addition, the chemical evolution of squalene and its chlorinated products reveal that the reaction becomes slower for higher chlorinated products.