The Reactive–Diffusive Length of OH and Ozone in Model Organic Aerosols

Recent advances in mass spectrometry techniques for atmospheric chemistry research on molecular-level

The Earth's atmosphere is composed of an enormous variety of chemical species associated with trace gases and aerosol particles whose composition and chemistry have critical impacts on the Earth's climate, air quality, and human health. Mass spectrometry analysis as a powerful and popular analytical technique has been widely developed and applied in atmospheric chemistry for decades. Mass spectrometry allows for effective detection, identification, and quantification of a broad range of organic and inorganic chemical species with high sensitivity and resolution. In this review, we summarize recently developed mass spectrometry techniques, methods, and applications in atmospheric chemistry research in the past several years on molecular-level. Specifically, new developments of ion-molecule reactors, various soft ionization methods, and unique coupling with separation techniques are highlighted. The new mass spectrometry applications in laboratory studies and field measurements focused on improving the detection limits for traditional and emerging volatile organic compounds, characterizing multiphase highly oxygenated molecules, and monitoring particle bulk and surface compositions.

When Does Multiphase Chemistry Influence Indoor Chemical Fate?

Human chemical exposure often occurs indoors, where large variability in contaminant concentrations and indoor chemical dynamics make assessments of these exposures challenging. A major source of uncertainty lies in the rates of chemical transformations which, due to high surface-to-volume ratios and rapid air change rates relative to rates of gas-phase reactions indoors, are largely gas-surface multiphase processes. It remains unclear how important such chemistry is in controlling indoor chemical lifetimes and, therefore, human exposure to both parent compounds and transformation products. We present a multimedia steady-state fugacity-based model to assess the importance of multiphase chemistry relative to cleaning and mass transfer losses, examine how the physicochemical properties of compounds and features of the indoor environment affect these processes, and investigate uncertainties pertaining to indoor multiphase chemistry and chemical lifetimes. We find that multiphase reactions can play an important role in chemical fate indoors for reactive compounds with low volatility, i.e., octanol-air equilibrium partitioning ratios (Koa) above 108, with the impact of this chemistry dependent on chemical identity, oxidant type and concentration, and other parameters. This work highlights the need for further research into indoor chemical dynamics and multiphase chemistry to constrain human exposure to chemicals in the built environment.

Coupled Interfacial and Bulk Kinetics Govern the Timescales of Multiphase Ozonolysis Reactions

Chemical transformations in aerosols impact the lifetime of particle phase species, the fate of atmospheric pollutants, and both climate- and health-relevant aerosol properties. Timescales for multiphase reactions of ozone in atmospheric aqueous phases are governed by coupled kinetic processes between the gas phase, the particle interface, and its bulk, which respond dynamically to reactive consumption of O₃. However, models of atmospheric aerosol reactivity often do not account for the coupled nature of multiphase processes. To examine these dynamics, we use new and prior experimental observations of aqueous droplet reaction kinetics, including three systems with a range of surface affinities and ozonolysis rate coefficients (trans-aconitic acid (C₆H₆O₆), maleic acid (C₄H₄O₄), and sodium nitrite (NaNO₂)). Using literature rate coefficients and thermodynamic properties, we constrain a simple two-compartment stochastic kinetic model which resolves the interface from the particle bulk and represents O₃ partitioning, diffusion, and reaction as a coupled kinetic system. Our kinetic model accurately predicts decay kinetics across all three systems, demonstrating that both the thermodynamic properties of O₃ and the coupled kinetic and diffusion processes are key to making accurate predictions. An enhanced concentration of adsorbed O₃, compared to gas and bulk phases is rapidly maintained and remains constant even as O₃ is consumed by reaction. Multiphase systems dynamically seek to achieve equilibrium in response to reactive O₃ loss, but this is hampered at solute concentrations relevant to aqueous aerosol by the rate of O₃ arrival in the bulk by diffusion. As a result, bulk-phase O₃ becomes depleted from its Henry's law solubility. This bulk-phase O₃ depletion limits reaction timescales for relatively slow-reacting organic solutes with low interfacial affinity (i.e., trans-aconitic and maleic acids, with k_rxn ≈ 10³-10⁴ M^-1 s^-1), which is in contrast to fast-reacting solutes with higher surface affinity (i.e., nitrite, with k_rxn ≈ 10⁵ M^-1 s^-1) where surface reactions strongly impact the observed decay kinetics.

The Parallel Transformations of Polycyclic Aromatic Hydrocarbons in the Body and in the Atmosphere

BACKGROUND

Polycyclic aromatic hydrocarbons (PAHs) emitted from combustion sources are known to be mutagenic, with more potent species also being carcinogenic. Previous studies show that PAHs can undergo complex transformations both in the body and in the atmosphere, yet these transformation processes are generally investigated separately.

OBJECTIVES

Drawing from the literature in atmospheric chemistry and toxicology, we highlight the parallel transformations of PAHs that occur in the atmosphere and the body and discuss implications for public health. We also examine key uncertainties related to the toxicity of atmospheric oxidation products of PAHs and explore critical areas for future research.

DISCUSSION

We focus on a key mode of toxicity for PAHs, in which metabolic processes (driven by cytochrome P450 enzymes), leads to the formation of oxidized PAHs that can damage DNA. Such species can also be formed abiotically in the atmosphere from natural oxidation processes, potentially augmenting PAH toxicity by skipping the necessary metabolic steps that activate their mutagenicity. Despite the large body of literature related to these two general pathways, the extent to which atmospheric oxidation affects a PAH's overall toxicity remains highly uncertain. Combining knowledge and promoting collaboration across both fields can help identify key oxidation pathways and the resulting products that impact public health.

CONCLUSIONS

Cross-disciplinary research, in which toxicology studies evaluate atmospheric oxidation products and their mixtures, and atmospheric measurements examine the formation of compounds that are known to be most toxic. Close collaboration between research communities can help narrow down which PAHs, and which PAH degradation products, should be targeted when assessing public health risks. https://doi.org/10.1289/EHP9984.

Experimental evidence that halogen bonding catalyzes the heterogeneous chlorination of alkenes in submicron liquid droplets

A key challenge in predicting the multiphase chemistry of aerosols and droplets is connecting reaction probabilities, observed in an experiment, with the kinetics of individual elementary steps that control the chemistry that occurs across a gas/liquid interface. Here we report evidence that oxygenated molecules accelerate the heterogeneous reaction rate of chlorine gas with an alkene (squalene, Sqe) in submicron droplets. The effective reaction probability for Sqe is sensitive to both the aerosol composition and gas phase environment. In binary aerosol mixtures with 2-decyl-1-tetradecanol, linoleic acid and oleic acid, Sqe reacts 12-23× more rapidly than in a pure aerosol. In contrast, the reactivity of Sqe is diminished by 3× when mixed with an alkane. Additionally, small oxygenated molecules in the gas phase (water, ethanol, acetone, and acetic acid) accelerate (up to 10×) the heterogeneous chlorination rate of Sqe. The overall reaction mechanism is not altered by the presence of these aerosol and gas phase additives, suggesting instead that they act as catalysts. Since the largest rate acceleration occurs in the presence of oxygenated molecules, we conclude that halogen bonding enhances reactivity by slowing the desorption kinetics of Cl2 at the interface, in a way that is analogous to decreasing temperature. These results highlight the importance of relatively weak interactions in controlling the speed of multiphase reactions important for atmospheric and indoor environments.

Molecular Properties and Chemical Transformations Near Interfaces

The properties of bulk water and aqueous solutions are known to change in the vicinity of an interface and/or in a confined environment, including the thermodynamics of ion selectivity at interfaces, transition states and pathways of chemical reactions, and nucleation events and phase growth. Here we describe joint progress in identifying unifying concepts about how air, liquid, and solid interfaces can alter molecular properties and chemical reactivity compared to bulk water and multicomponent solutions. We also discuss progress made in interfacial chemistry through advancements in new theory, molecular simulation, and experiments.

Representation of Multiphase OH Oxidation of Amorphous Organic Aerosol for Tropospheric Conditions

Organic aerosol (OA) is ubiquitous in the atmosphere and, during transport, can experience chemical transformation with consequences for air quality and climate. Prediction of the chemical evolution of OA depends on its reactivity with atmospheric oxidants such as the OH radical. OA particles undergo amorphous phase transitions from liquid to solid (glassy) states in response to temperature changes, which, in turn, will impact its reactivity toward OH oxidation. To improve the predictability of OA reactivity toward OH oxidation, the reactive uptake coefficients (γ) of OH radicals reacting with triacontane and squalane serving as amorphous OA surrogates were measured at temperatures from 213-293 K. γ increases strongest with temperature when the organic species is in the liquid phase, compared to when being in the semisolid or solid phase. The resistor model is applied, accounting for the amorphous phase state changes using the organic species' glass transition temperature and fragility, to evaluate the physicochemical parameters of the temperature dependent OH uptake process. This allows for the derivation of a semiempirical formula, applicable to models, to predict the degree of oxidation and chemical lifetime of the condensed-phase organic species for typical tropospheric temperature and humidity when OA particle viscosity is known.

The reaction of oleic acid monolayers with gas-phase ozone at the air water interface: the effect of sub-phase viscosity, and inert secondary components

Organic films that form on atmospheric particulate matter change the optical and cloud condensation nucleation properties of the particulate matter and consequently have implications for modern climate and climate models. The organic films are subject to attack from gas-phase oxidants present in ambient air. Here we revisit in greater detail the oxidation of a monolayer of oleic acid by gas-phase ozone at the air-water interface as this provides a model system for the oxidation reactions that occur at the air-water interface of aqueous atmospheric aerosol. Experiments were performed on monolayers of oleic acid at the air-liquid interface at atmospherically relevant ozone concentrations to investigate if the viscosity of the sub-phase influences the rate of the reaction and to determine the effect of the presence of a second component within the monolayer, stearic acid, which is generally considered to be non-reactive towards ozone, on the reaction kinetics as determined by neutron reflectometry measurements. Atmospheric aerosol can be extremely viscous. The kinetics of the reaction were found to be independent of the viscosity of the sub-phase below the monolayer over a range of moderate viscosities, , demonstrating no involvement of aqueous sub-phase oxidants in the rate determining step. The kinetics of oxidation of monolayers of pure oleic acid were found to depend on the surface coverage with different behaviour observed above and below a surface coverage of oleic acid of ∼1 × 1018 molecule m-2. Atmospheric aerosol are typically complex mixtures, and the presence of an additional compound in the monolayer that is inert to direct ozone oxidation, stearic acid, did not significantly change the reaction kinetics. It is demonstrated that oleic acid monolayers at the air-water interface do not leave any detectable material at the air-water interface, contradicting the previous work published in this journal which the authors now believe to be erroneous. The combined results presented here indicate that the kinetics, and thus the atmospheric chemical lifetime for unsaturated surface active materials at the air-water interface to loss by reaction with gas-phase ozone, can be considered to be independent of other materials present at either the air-water interface or in the aqueous sub-phase.

Heterogeneous Ozonolysis of Tetrahydrocannabinol: Implications for Thirdhand Cannabis Smoke

Thirdhand smoke (THS) deposits to surfaces following smoking events and is a source of chemical exposure to humans. However, the evolution of THS in indoor environments is not well understood. Cannabis THS is a chemically distinct and prevalent form of THS, which has not been studied. The heterogeneous reaction of Δ⁹-tetrahydrocannabinol (THC), a major component of cannabis smoke, with ozone was examined as a pure compound and within cannabis smoke. Oxidative decay via ozonolysis and product formation were monitored by liquid chromatography-tandem mass spectrometry. Epoxide, dicarbonyl, and secondary ozonide THC reaction products were detected from both pure THC and cannabis experiments, with the product ratios dependent on relative humidity. The observed reaction kinetics for loss of THC on glass and cotton surfaces are consistent with a relatively short loss lifetime, which will be strongly dependent on the film thickness, ozone mixing ratio, and ozone reactivity of the surface substrate. The low volatility of THC and its oxidation products suggest that their contributions to thirdhand cannabis smoke will be less significant than the role that nicotine plays in thirdhand tobacco smoke.

Interfacial Dimerization by Organic Radical Reactions during Heterogeneous Oxidative Aging of Oxygenated Organic Aerosols

Oxidative aging of atmospheric organic aerosols (OA) substantially modifies their chemical compositions, physical properties, and hence the various environmental impacts. Here, we report observations of a previously unrecognized process leading to dimer formation during heterogeneous ^•OH-initiated oxidative aging of oxygenated OA. Isomer-resolved ion mobility mass spectrometry measurements and reaction-diffusion kinetic simulations are in good agreement, elucidating new mechanisms of dimerization by organic radical (i.e., peroxy and alkoxy radicals) cross reactions using glutaric acid as a surrogate oxygenated OA. These radical reactions are predicted to occur more prominently near the gas-particle interface following oxidation, especially in diffusion-limited viscous OA particles. Chemical structure analysis shows that esters dominate the detected dimers, followed by organic peroxides and ethers, highlighting the importance of acyl peroxy and acyloxy radicals. Simulations suggest that the reported dimer formation through the new interfacial mechanism could be appreciable under both laboratory and ambient conditions. Therefore, the dimers that are formed and enriched at the gas-particle interface are expected to play a crucial role in the effective reactivity, volatility, viscosity, and hygroscopicity of aged OA particles.

Kinetics and Condensed-Phase Products in Multiphase Ozonolysis of an Unsaturated Triglyceride

Ozone is an important oxidant in the environment. To study the nature of multiphase ozonolysis, an unsaturated triglyceride, triolein, of the type present in skin oil, biological membranes, and most cooking oils was oxidized by gas-phase ozone on a surface. A high-performance liquid chromatography/electrospray ionization mass spectrometry (HPLC-ESI-MS) method was developed for analyzing triolein and its oxidized products. Upon exposure to ozone, the decay of thin coatings of triolein was observed, accompanied by the formation of functionalized condensed-phase products including secondary ozonides (SOZ), acids, and aldehydes. By studying the reaction kinetics as a function of average coating thickness and ozone mixing ratio, we determined that the reactive uptake coefficient (γ) is on the order of 10^-6 to 10^-5. It is also concluded that the reaction occurs in the bulk without a major interfacial component, and the reacto-diffusive depth of ozone in the triolein coating is estimated to be between 8 and 40 nm. The specific nature of the reaction products is affected by the reactions of the Criegee intermediate formed during ozonolysis. In particular, although an increase in the relative humidity to 50% from dry conditions has no effect on the kinetics of triolein decay, the yield of SOZs is significantly depressed, indicating reactions of the Criegee intermediates to form hydroperoxides. Once formed, the SOZ products are thermally stable over periods of at least 48 h at room temperature but decomposition was observed under simulated outdoor sunlight, likely forming organic acids. From an environmental perspective, this chemistry indicates that SOZs and other oxygenates will form via ozonolysis of oily indoor surfaces and skin oil.

Visualizing reaction and diffusion in xanthan gum aerosol particles exposed to ozone

Atmospheric aerosol particles with a high viscosity may become inhomogeneously mixed during chemical processing. Models have predicted gradients in condensed phase reactant concentration throughout particles as the result of diffusion and chemical reaction limitations, termed chemical gradients. However, these have never been directly observed for atmospherically relevant particle diameters. We investigated the reaction between ozone and aerosol particles composed of xanthan gum and FeCl₂ and observed the in situ chemical reaction that oxidized Fe²⁺ to Fe³⁺ using X-ray spectromicroscopy. Iron oxidation state of particles as small as 0.2 μm in diameter were imaged over time with a spatial resolution of tens of nanometers. We found that the loss off Fe²⁺ accelerated with increasing ozone concentration and relative humidity, RH. Concentric 2-D column integrated profiles of the Fe²⁺ fraction, α, out of the total iron were derived and demonstrated that particle surfaces became oxidized while particle cores remained unreacted at RH = 0-20%. At higher RH, chemical gradients evolved over time, extended deeper from the particle surface, and Fe²⁺ became more homogeneously distributed. We used the kinetic multi-layer model for aerosol surface and bulk chemistry (KM-SUB) to simulate ozone reaction constrained with our observations and inferred key parameters as a function of RH including Henry's Law constant for ozone, H_O3, and diffusion coefficients for ozone and iron, D_O3 and D_Fe, respectively. We found that H_O3 is higher in our xanthan gum/FeCl₂ particles than for water and increases when RH decreased from about 80% to dry conditions. This coincided with a decrease in both D_O3 and D_Fe. In order to reproduce observed chemical gradients, our model predicted that ozone could not be present further than a few nanometers from a particle surface indicating near surface reactions were driving changes in iron oxidation state. However, the observed chemical gradients in α observed over hundreds of nanometers must have been the result of iron transport from the particle interior to the surface where ozone oxidation occurred. In the context of our results, we examine the applicability of the reacto-diffusive framework and discuss diffusion limitations for other reactive gas-aerosol systems of atmospheric importance.

Detecting Intermediates and Products of Fast Heterogeneous Reactions on Liquid Surfaces via Online Mass Spectrometry

One of the research priorities in atmospheric chemistry is to advance our understanding of heterogeneous reactions and their effect on the composition of the troposphere. Chemistry on aqueous surfaces is particularly important because of their ubiquity and expanse. They range from the surfaces of oceans (360 million km2), cloud and aerosol drops (estimated at ~10 trillion km2) to the fluid lining the human lung (~150 m2). Typically, ambient air contains reactive gases that may affect human health, influence climate and participate in biogeochemical cycles. Despite their importance, atmospheric reactions between gases and solutes on aqueous surfaces are not well understood and, as a result, generally overlooked. New, surface-specific techniques are required that detect and identify the intermediates and products of such reactions as they happen on liquids. This is a tall order because genuine interfacial reactions are faster than mass diffusion into bulk liquids, and may produce novel species in low concentrations. Herein, we review evidence that validates online pneumatic ionization mass spectrometry of liquid microjets exposed to reactive gases as a technique that meets such requirements. Next, we call attention to results obtained by this approach on reactions of gas-phase ozone, nitrogen dioxide and hydroxyl radicals with various solutes on aqueous surfaces. The overarching conclusion is that the outermost layers of aqueous solutions are unique media, where most equilibria shift and reactions usually proceed along new pathways, and generally faster than in bulk water. That the rates and mechanisms of reactions at air-aqueous interfaces may be different from those in bulk water opens new conceptual frameworks and lines of research, and adds a missing dimension to atmospheric chemistry.

Predicting Aerosol Reactivity Across Scales: from the Laboratory to the Atmosphere

To fully utilize the results of laboratory-based studies of the chemistry of model atmospheric aerosol reactions, it is important to understand how to relate them to the conditions found in nature. In this study, we have taken a validated reaction-diffusion mechanism for oxidation of C₃₀H₆₂ aerosol by OH under flow tube conditions and examined its predictions for another experimental regime (continuous flow stirred tank reactor) and for the atmosphere, spanning alkane aerosol viscosities from liquid to semisolid. The results show that under OH-concentration-limited and aerosol-mixing-limited conditions, it should be possible to select laboratory experimental conditions where many aspects of the particle phase and volatile product chemistry under atmospheric conditions can be revealed. If the OH collision and organic diffusion rates are comparable, however, reactivity is highly sensitive to the details of both OH concentration and internal mixing. The characteristics of the transition between limiting conditions provide key insights into which parts of the reaction mechanism dominate in the various kinetic regimes.

Understanding interactions of organic nitrates with the surface and bulk of organic films: implications for particle growth in the atmosphere

Understanding impacts of secondary organic aerosol (SOA) in air requires a molecular-level understanding of particle growth via interactions between gases and particle surfaces. The interactions of three gaseous organic nitrates with selected organic substrates were measured at 296 K using attenuated total reflection Fourier transform infrared spectroscopy. The organic substrates included a long chain alkane (triacontane, TC), a keto-acid (pinonic acid, PA), an amorphous ester oligomer (poly(ethylene adipate) di-hydroxy terminated, PEA), and laboratory-generated SOA from α-pinene ozonolysis. There was no uptake of the organic nitrates on the non-polar TC substrate, but significant uptake occurred on PEA, PA, and α-pinene SOA. Net uptake coefficients (γ) at the shortest reaction times accessible in these experiments ranged from 3 × 10-4 to 9 × 10-6 and partition coefficients (K) from 1 × 107 to 9 × 104. Trends in γ did not quantitatively follow trends in K, suggesting that the intermolecular forces involved in gas-surface interactions are not the same as those in the bulk, which is supported by theoretical calculations. Kinetic modeling showed that nitrates diffused throughout the organic films over several minutes, and that the bulk diffusion coefficients evolved as uptake/desorption occurred. A plasticizing effect occurred upon incorporation of the organic nitrates, whereas desorption caused decreases in diffusion coefficients in the upper layers, suggesting a crusting effect. Accurate predictions of particle growth in the atmosphere will require knowledge of uptake coefficients, which are likely to be several orders of magnitude less than one, and of the intermolecular interactions of gases with particle surfaces as well as with the particle bulk.

In Situ Chemical Analysis of the Gas-Aerosol Particle Interface

The gas-aerosol particle interface is believed to contribute to the growth of secondary organic aerosols in the atmosphere. Despite its importance, the chemical composition of the interface has not been probed directly because of a lack of suitable interface-specific analytical techniques. The preliminary result in our early work has demonstrated direct observations of molecules at the gas-aerosol particle interface with the development of second harmonic scattering (SHS). However, the SHS technique is far away from being an analytical tool of chemical compositions at the gas-aerosol particle interface. In this work, we continued to develop the interface-specific SHS for in situ chemical analysis of molecules at the gas-aerosol particle interface. As an example, we demonstrated coherent SHS signal of a new SHS probe, crystal violet (CV), from interfaces of aerosol particles. The development of the SHS technique includes: (1) Optimization for a more efficient femtosecond laser system in the generation of SHS from aerosol particles. A near 5 MHz repetition rate of a femtosecond laser was found to be optimal for the generation of SHS; (2) exploration of a more effective detector for SHS of aerosol particles. We found that both a CCD detector and a single-photon counter produce similar signal-to-noise ratios of the interfacial SHS signals from aerosol particles. The CCD detector is a more effective option for the detection of SHS and could greatly reduce sampling time of the interfacial responses; (3) combination of the optimal laser system with the CCD detector, which has greatly improved the detection sensitivity of interfacial molecules by more than 2 orders of magnitude and could potentially detect interfacial SHS from a single aerosol particle. These experimental results not only provided a thorough analysis of the SHS technique but also built a solid foundation for further development of a new vibrational sum frequency scattering (SFS) technique for chemical structures at the gas-aerosol particle interface.

Changes in Reactivity as Chemistry Becomes Confined to an Interface. The Case of Free Radical Oxidation of C30H62 Alkane by OH

We examine in a simple organic aerosol the transition between heterogeneous chemistry under well-mixed conditions to chemistry under interfacial confinement. A single reaction mechanism, shown to reproduce observed OH oxidation chemistry for liquid and semisolid C₃₀H₆₂, is used in reaction-diffusion simulations to explore reactivity over a broad viscosity range. The results show that when internal mixing of the aerosol is fast and the particle interface is enriched in C-H groups, ketone and alcohol products, formed via peroxy radical disproportionation, predominate. As viscosity increases the reactions become confined to a shell at the gas-aerosol interface. The confinement is accompanied by emergence of acyloxy reaction pathways that are particularly active when the shell is 1 nm or less. We quantify this trend using a reaction-diffusion index, allowing the parts of the mechanism that control reactivity as viscosity increases to be identified.

Diffusive confinement of free radical intermediates in the OH radical oxidation of semisolid aerosols

Multiphase chemical reactions (gas + solid/liquid) involve a complex interplay between bulk and interface chemistry, diffusion, evaporation, and condensation. Reactions of atmospheric aerosols are an important example of this type of chemistry: the rich array of particle phase states and multiphase transformation pathways produce diverse but poorly understood interactions between chemistry and transport. Their chemistry is of intrinsic interest because of their role in controlling climate. Their characteristics also make them useful models for the study of principles of reactivity of condensed materials under confined conditions. In previous work, we have reported a computational study of the oxidation chemistry of a liquid aliphatic aerosol. In this study, we extend the calculations to investigate nearly the same reactions at a semisolid gas-aerosol interface. A reaction-diffusion model for heterogeneous oxidation of triacontane by hydroxyl radicals (OH) is described, and its predictions are compared to measurements of aerosol size and composition, which evolve continuously during oxidation. These results are also explicitly compared to those obtained for the corresponding liquid system, squalane, to pinpoint salient elements controlling reactivity. The diffusive confinement of the free radical intermediates at the interface results in enhanced importance of a few specific chemical processes such as the involvement of aldehydes in fragmentation and evaporation, and a significant role of radical-radical reactions in product formation. The simulations show that under typical laboratory conditions semisolid aerosols have highly oxidized nanometer-scale interfaces that encapsulate an unreacted core and may confer distinct optical properties and enhanced hygroscopicity. This highly oxidized layer dynamically evolves with reaction, which we propose to result in plasticization. The validated model is used to predict chemistry under atmospheric conditions, where the OH radical concentration is much lower. The oxidation reactions are more strongly influenced by diffusion in the particle, resulting in a more liquid-like character.

Connecting the Elementary Reaction Pathways of Criegee Intermediates to the Chemical Erosion of Squalene Interfaces during Ozonolysis

Criegee intermediates (CI), formed in alkene ozonolysis, are central for controlling the multiphase chemistry of organic molecules in both indoor and outdoor environments. Here, we examine the heterogeneous ozonolysis of squalene, a key species in indoor air chemistry. Aerosol mass spectrometry is used to investigate how the ozone (O₃) concentration, relative humidity (RH), and particle size control reaction rates and mechanisms. Although the reaction rate is found to be independent of RH, the reaction products and particle size depend upon H₂O. Under dry conditions (RH = 3%) the reaction produces high-molecular-weight secondary ozonides (SOZ), which are known skin irritants, and a modest change in particle size. Increasing the RH reduces the aerosol size by 30%, while producing mainly volatile aldehyde products, increases potential respiratory exposure. Chemical kinetics simulations link the elementary reactions steps of CI to the observed kinetics, product distributions, and changes in particle size. The simulations reveal that ozonolysis occurs near the surface and is O₃-transport limited. The observed secondary ozonides are consistent with the formation of mainly secondary CI, in contrast to gas-phase ozonolysis mechanisms.

Heterogeneous OH Oxidation, Shielding Effects, and Implications for the Atmospheric Fate of Terbuthylazine and Other Pesticides

Terbuthylazine (TBA) is a widely used herbicide, and its heterogeneous reaction with OH radicals is important for assessing its potential to undergo atmospheric long-range transport and to affect the environment and public health. The apparent reaction rate coefficients obtained in different experimental investigations, however, vary by orders of magnitude depending on the applied experimental techniques and conditions. In this study, we used a kinetic multilayer model of aerosol chemistry with reversible surface adsorption and bulk diffusion (KM-SUB) in combination with a Monte Carlo genetic algorithm to simulate the measured decay rates of TBA. Two experimental data sets available from different studies can be described with a consistent set of kinetic parameters resolving the interplay of chemical reaction, mass transport, and shielding effects. Our study suggests that mass transport and shielding effects can substantially extend the atmospheric lifetime of reactive pesticides from a few days to weeks, with strong implications for long-range transport and potential health effects of these substances.

Low energy electron attenuation lengths in core–shell nanoparticles

A velocity map imaging spectrometer is used to measure photoemission from free core–shell nanoparticles, where a salt core is coated with a liquid hydrocarbon shell (i.e. squalane).