An analysis of the formation and evaporation of mixed-particles containing squalane (a surrogate for hydrophobic primary organic aerosol, POA) and secondary organic aerosol (SOA) is presented. In these experiments, one material (D62-squalane or SOA from α-pinene + O3) was prepared first to serve as surface area for condensation of the other, forming the mixed-particles. The mixed-particles were then subjected to a heating-ramp from 22 to 44 °C. We were able to determine that (1) almost all of the SOA mass is comprised of material less volatile than D62-squalane; (2) AMS collection efficiency in these mixed-particle systems can be parametrized as a function of the relative mass fraction of the components; and (3) the vast majority of D62-squalane is able to evaporate from the mixed particles, and does so on the same time scale regardless of the order of preparation. We also performed two-population mixing experiments to directly test whether D62-squalane and SOA from α-pinene + O3 form a single solution or two separate phases. We find that these two OA types are immiscible, which informs our inference of the morphology of the mixed-particles. If the morphology is core–shell and dictated by the order of preparation, these data indicate that squalane is able to diffuse relatively quickly through the SOA shell, implying that there are no major diffusion limitations.

We present direct measurements of mixing between separately prepared organic aerosol populations in a smog chamber using single-particle mass spectra from the high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS). Docosane and docosane-d46 (22 carbon linear solid alkane) did not show any signs of mixing, but squalane and squalane-d62 (30 carbon branched liquid alkane) mixed on the time scale expected from a condensational-mixing model. Docosane and docosane-d46 were driven to mix when the chamber temperature was elevated above the melting point for docosane. Docosane vapors were shown to mix into squalane-d62, but not the other way around. These results are consistent with low diffusivity in the solid phase of docosane particles. We performed mixing experiments on secondary organic aerosol (SOA) surrogate systems finding that SOA derived from toluene-d8 (a surrogate for anthropogenic SOA (aSOA)) does not mix into squalane (a surrogate for hydrophobic primary organic aerosol (POA)) but does mix into SOA derived from α-pinene (biogenic SOA (bSOA) surrogate). For the aSOA/POA, the volatility of either aerosol does not limit gas-phase diffusion, indicating that the two particle populations do not mix simply because they are immiscible. In the aSOA/bSOA system, the presence of toluene-d8-derived SOA molecules in the α-pinene-derived SOA provides evidence that the diffusion coefficient in α-pinene-derived SOA is high enough for mixing on the time scale of 1 min. The observations from all of these mixing experiments are generally invisible to bulk aerosol composition measurements but are made possible with single-particle composition data.

Secondary organic aerosol formation from volatile precursors can be thought of as a succession of generations of reaction products. Here, we constrain first-generation SOA formation from the α-pinene + OH reaction and also study SOA formation from α-pinene ozonolysis carried out without an OH scavenger. SOA yields from OH oxidation of α-pinene are significantly higher than SOA yields from ozonolysis including an OH scavenger, and the SOA mass yields for unscavenged ozonolysis generally fall within the range of mass yields for α-pinene ozonolysis under various conditions. Taken together, first-generation product yields parametrized with a volatility basis set fit provide a starting point for atmospheric models designed to simulate both the production and subsequent aging of SOA from this important terpene.

This study addresses photochemical aging of secondary organic aerosol (SOA) produced from α-pinene ozonolysis. The SOA is aged via hydroxyl radical (OH) reactions with first-generation vapors and UV photolysis. OH radicals are created through tetramethylethylene ozonolysis, HOOH photolysis, or HONO photolysis, sources that vary in OH concentration and the presence or absence of UV illumination. Aging strongly influences observed SOA mass concentrations, but the behavior is complex. In the dark or with high concentrations of OH, vapors are functionalized, lowering their volatility, resulting in an increase in OA by a factor of 2–3. However, with lower concentrations of OH under UV illumination SOA mass concentrations decrease over time. We attribute this decrease to evaporation driven by photolysis of the highly functionalized second-generation products. The photolysis rates are rapid, a few percent of the NO2 photolysis frequency, and can thus be highly competitive with other aging mechanisms in the atmosphere.

To model the temperature-induced partitioning of semivolatile organics in laboratory experiments or atmospheric models, one must know the appropriate heats of vaporization. Current treatments typically assume a constant value of the heat of vaporization or else use specific values from a small set of surrogate compounds. With published experimental vapor-pressure data from over 800 organic compounds, we have developed a semiempirical correlation between the saturation concentration (C*, μg m−3) and the heat of vaporization (ΔHVAP, kJ mol−1) for organics in the volatility basis set. Near room temperature, ΔHVAP = −11 log 10C300* + 129. Knowledge of the relationship between C* and ΔHVAP constrains a free parameter in thermodenuder data analysis. A thermodenuder model using our ΔHVAP values agrees well with thermal behavior observed in laboratory experiments.

Alkene ozonolysis reactions proceed through an unstable intermediate, the primary ozonide (POZ). POZ decomposition controls the complex mechanism. We probe the kinetics of primary ozonide decomposition using temperature programmed reaction spectroscopy (TPRS), revealing primary ozonide decomposition barrier heights of 9.1 ± 0.4, 9.4 ± 0.4, and 11.9 ± 1.2 kcal mol−1 for cyclohexene, 1-methyl-cyclohexene, and methylene-cyclohexane, respectively. We compare experimental decomposition spectra with spectral predictions using density functional theory (DFT) to reveal decomposition products resembling vinyl-hydroperoxides and dioxiranes. We do not find evidence of secondary ozonides. Additional computations with DFT, scaled with the TPRS barrier height, yield barrier heights ranging from 9.4 to 12.1 kcal mol−1 for the four competing decomposition pathways of the 1-methyl-cyclohexene POZ. Entropic differences were minimal, indicating that POZ decomposition branching is controlled purely by enthalpic variations. These kinetic computations were used to calculate a hydroxyl radical yield for 1-methyl-cyclohexene ozonolysis of 0.85 at 298 K.

Citronella candles are widely used as insect repellants, especially outdoors in the evening. Because these essential oils are unsaturated, they have a unique potential to form secondary organic aerosol (SOA) via reaction with ozone, which is also commonly elevated on summer evenings when the candles are often in use. We investigated this process, along with primary aerosol emissions, by briefly placing a citronella tealight candle in a smog chamber and then adding ozone to the chamber. In repeated experiments, we observed rapid and substantial SOA formation after ozone addition; this process must therefore be considered when assessing the risks and benefits of using citronella candle to repel insects.

The heterogeneous reaction of ozone with oleic acid has been studied extensively as a simple model system for investigating the oxidation of organic compounds in atmospheric particles. In this work, we simultaneously quantify oleic acid and ozone decay during three stepwise oxidation events, allowing us to quantify reactivity of oleic acid throughout the oxidative lifetime of initially pure particles. Throughout their lifetime, uptake in these particles is driven by reaction, as evidenced by similar timescales for ozone and oleic acid decay. The oleic acid decay rate slows with increasing particle oxidation, most likely due to the continued dilution of the particles with oxidation products. However, the initial stoichiometry is as high as 3.75 oleic acid molecules destroyed per ozone molecule lost. This significantly exceeds the 2:1 ratio that can be explained by an initial ozonolysis reaction and known secondary chemistry between the Criegee intermediate and the organic acid moiety. It implies that there is additional, previously unrecognized secondary chemistry that likely involves the carbon backbone. Our understanding of reactivity, even in this simple system, remains incomplete.

We report data from real-time FTIR temperature programmed reaction spectroscopy on a cryogenic zinc selenide window revealing the intermediates from ozonation of 2,3-dimethyl-2-butene (TME). We have found convincing evidence of a 1,2,3-trioxolane (the primary ozonide, POZ), which decomposes at 185 K to yield a 1,2,4-trioxolane product (the secondary ozonide, SOZ). Computational infrared spectra confirmed the presence of the POZ and SOZ. The barrier height for POZ decomposition, determined experimentally, was found to be 13.8 ± 1.0 kcal mol−1, and the A factor calculated with RRKM theory based on density functional reactant and transition state frequencies was found to be 4.16 × 1013 s−1. The TME SOZ has not previously been observed without the presence of a polyethylene surface. SOZ formation kinetics from the reaction of the POZ decomposition products along with the competing reaction pathways were examined with computational chemistry calculations using DFT. These calculations confirm our experimental observation of SOZ formation.

The heterogeneous reaction of ozone with oleic acid has been studied extensively as a simple model system for investigating the oxidation of organic compounds in atmospheric particles. In this work, we simultaneously quantify oleic acid and ozone decay during three stepwise oxidation events, allowing us to quantify reactivity of oleic acid throughout the oxidative lifetime of initially pure particles. Throughout their lifetime, uptake in these particles is driven by reaction, as evidenced by similar timescales for ozone and oleic acid decay. The oleic acid decay rate slows with increasing particle oxidation, most likely due to the continued dilution of the particles with oxidation products. However, the initial stoichiometry is as high as 3.75 oleic acid molecules destroyed per ozone molecule lost. This significantly exceeds the 2:1 ratio that can be explained by an initial ozonolysis reaction and known secondary chemistry between the Criegee intermediate and the organic acid moiety. It implies that there is additional, previously unrecognized secondary chemistry that likely involves the carbon backbone. Our understanding of reactivity, even in this simple system, remains incomplete.