Acoustic levitation combined with laboratory-based small-angle X-ray scattering (SAXS) to probe changes in crystallinity and molecular organisation.

Single particle levitation techniques allow us to probe samples in a contactless way, negating the effect that surfaces could have on processes such as crystallisation and phase transitions. Small-angle X-ray scattering (SAXS) is a common method characterising the nanoscale order in aggregates such as colloidal, crystalline and liquid crystalline systems. Here, we present a laboratory-based small-angle X-ray scattering (SAXS) setup combined with acoustic levitation. The capability of this technique is highlighted and compared with synchrotron-based levitation-SAXS and X-ray diffraction. We were able to follow the deliquescence and crystallisation of sucrose, a commonly used compound for the study of viscous atmospheric aerosols. The observed increased rate of the deliquescence-crystallisation transitions on repeated cycling could suggest the formation of a glassy sucrose phase. We also followed a reversible phase transition in an oleic acid-based lyotropic liquid crystal system under controlled humidity changes. Our results demonstrate that the coupling of acoustic levitation with an offline SAXS instrument is feasible, and that the time resolution and data quality are sufficient to draw physically meaningful conclusions. There is a wide range of potential applications including topics such as atmospheric aerosol chemistry, materials science, crystallisation and aerosol spray drying.

Atmospheric Degradation of Ecologically Important Biogenic Volatiles: Investigating the Ozonolysis of (E)-ß-Ocimene, Isomers of α and ß-Farnesene, α-Terpinene and 6-Methyl-5-Hepten-2-One, and Their Gas-Phase Products.

Biogenic volatile organic compounds (bVOCs), synthesised by plants, are important mediators of ecological interactions that can also undergo a series of reactions in the atmosphere. Ground-level ozone is a secondary pollutant generated through sunlight-driven reactions between nitrogen oxides (NOx) and VOCs. Its levels have increased since the industrial revolution and reactions involving ozone drive many chemical processes in the troposphere. While ozone precursors often originate in urban areas, winds may carry these hundreds of kilometres, causing ozone formation to also occur in less populated rural regions. Under elevated ozone conditions, ozonolysis of bVOCs can result in quantitative and qualitative changes in the gas phase, reducing the concentrations of certain bVOCs and resulting in the formation of other compounds. Such changes can result in disruption of bVOC-mediated behavioural or ecological interactions. Through a series of gas-phase experiments using Gas Chromatography Mass Spectrometry (GC-MS) and Proton Transfer Reaction Mass Spectrometry (PTR-MS), we investigated the products and their yields from the ozonolysis of a range of ubiquitous bVOCs, which were selected because of their importance in mediating ecological interactions such as pollinator and natural enemy attraction and plant-to-plant communication, namely: (E)-ß-ocimene, isomers of α and ß-farnesene, α-terpinene and 6-methyl-5-hepten-2-one. New products from the ozonolysis of these compounds were identified, and the formation of these compounds is consistent with terpene-ozone oxidation mechanisms. We present the degradation mechanism of our model bVOCs and identify their reaction products. We discuss the potential ecological implications of the degradation of each bVOC and of the formation of reaction products.

Molecular Self-Organization in Surfactant Atmospheric Aerosol Proxies.

ConspectusAerosols are ubiquitous in the atmosphere. Outdoors, they take part in the climate system via cloud droplet formation, and they contribute to indoor and outdoor air pollution, impacting human health and man-made environmental change. In the indoor environment, aerosols are formed by common activities such as cooking and cleaning. People can spend up to ca. 90% of their time indoors, especially in the western world. Therefore, there is a need to understand how indoor aerosols are processed in addition to outdoor aerosols.Surfactants make significant contributions to aerosol emissions, with sources ranging from cooking to sea spray. These molecules alter the cloud droplet formation potential by changing the surface tension of aqueous droplets and thus increasing their ability to grow. They can also coat solid surfaces such as windows ("window grime") and dust particles. Such surface films are more important indoors due to the higher surface-to-volume ratio compared to the outdoor environment, increasing the likelihood of surface film-pollutant interactions.A common cooking and marine emission, oleic acid, is known to self-organize into a range of 3-D nanostructures. These nanostructures are highly viscous and as such can impact the kinetics of aerosol and film aging (i.e., water uptake and oxidation). There is still a discrepancy between the longer atmospheric lifetime of oleic acid compared with laboratory experiment-based predictions.We have created a body of experimental and modeling work focusing on the novel proposition of surfactant self-organization in the atmosphere. Self-organized proxies were studied as nanometer-to-micrometer films, levitated droplets, and bulk mixtures. This access to a wide range of geometries and scales has resulted in the following main conclusions: (i) an atmospherically abundant surfactant can self-organize into a range of viscous nanostructures in the presence of other compounds commonly encountered in atmospheric aerosols; (ii) surfactant self-organization significantly reduces the reactivity of the organic phase, increasing the chemical lifetime of these surfactant molecules and other particle constituents; (iii) while self-assembly was found over a wide range of conditions and compositions, the specific, observed nanostructure is highly sensitive to mixture composition; and (iv) a "crust" of product material forms on the surface of reacting particles and films, limiting the diffusion of reactive gases to the particle or film bulk and subsequent reactivity. These findings suggest that hazardous, reactive materials may be protected in aerosol matrixes underneath a highly viscous shell, thus extending the atmospheric residence times of otherwise short-lived species.

Mapping the effects of ozone pollution and mixing on floral odour plumes and their impact on plant-pollinator interactions.

The critical ecological process of animal-mediated pollination is commonly facilitated by odour cues. These odours consist of volatile organic compounds (VOCs), often with short chemical lifetimes, which form the strong concentration gradients necessary for pollinating insects to locate a flower. Atmospheric oxidants, including ozone pollution, may react with and chemically alter these VOCs, impairing the ability of pollinators to locate a flower, and therefore the pollen and nectar on which they feed. However, there is limited mechanistic empirical evidence to explain these processes within an odour plume at temporal and spatial scales relevant to insect navigation and olfaction. We investigated the impact of ozone pollution and turbulent mixing on the fate of four model floral VOCs within odour plumes using a series of controlled experiments in a large wind tunnel. Average rates of chemical degradation of α-terpinene, ß-caryophyllene and 6-methyl-5-hepten-2-one were slightly faster than predicted by literature rate constants, but mostly within uncertainty bounds. Mixing reduced reaction rates by 8-10% in the first 2 m following release. Reaction rates also varied across the plumes, being fastest at plume edges where VOCs and ozone mixed most efficiently and slowest at plume centres. Honeybees were trained to learn a four VOC blend equivalent to the plume released at the wind tunnel source. When subsequently presented with an odour blend representative of that observed 6 m from the source at the centre of the plume, 52% of honeybees recognised the odour, decreasing to 38% at 12 m. When presented with the more degraded blend from the plume edge, recognition decreased to 32% and 10% at 6 and 12 m respectively. Our findings highlight a mechanism by which anthropogenic pollutants can disrupt the VOC cues used in plant-pollinator interactions, which likely impacts on other critical odour-mediated behaviours such as mate attraction.

Exploring the Nanostructures Accessible to an Organic Surfactant Atmospheric Aerosol Proxy.

The composition of atmospheric aerosols varies with time, season, location, and environment. This affects key aerosol properties such as hygroscopicity and reactivity, influencing the aerosol's impact on the climate and air quality. The organic fraction of atmospheric aerosol emissions often contains surfactant material, such as fatty acids. These molecules are known to form three-dimensional nanostructures in contact with water. Different nanostructures have marked differences in viscosity and diffusivity that are properties whose understanding is essential when considering an aerosol's atmospheric impact. We have explored a range of nanostructures accessible to the organic surfactant oleic acid (commonly found in cooking emissions), simulating variation that is likely to happen in the atmosphere. This was achieved by changing the amount of water, aqueous phase salinity and by addition of other commonly coemitted compounds: sugars and stearic acid (the saturated analogue of oleic acid). The nanostructure was observed by both synchrotron and laboratory small/wide angle X-ray scattering (SAXS/WAXS) and found to be sensitive to the proxy composition. Additionally, the spacing between repeat units in these nanostructures was water content dependent (i.e., an increase from 41 to 54 Šin inverse hexagonal phase d-spacing when increasing the water content from 30 to 50 wt %), suggesting incorporation of water within the nanostructure. A significant decrease in mixture viscosity was also observed with increasing water content from ∼104 to ∼102 Pa s when increasing the water content from 30 to 60 wt %. Time-resolved SAXS experiments on levitated droplets of this proxy confirm the phase changes observed in bulk phase mixtures and demonstrate that coexistent nanostructures can form in droplets. Aerosol compositional and subsequent nanostructural changes could affect aerosol processes, leading to an impact on the climate and urban air pollution.

Indoor Air Pollution and the Health of Vulnerable Groups: A Systematic Review Focused on Particulate Matter (PM), Volatile Organic Compounds (VOCs) and Their Effects on Children and People with Pre-Existing Lung Disease.

Air pollution affects health, but much of the focus to this point has been on outdoor air. Higher indoor pollution is anticipated due to increasingly energy-efficient and less leaky buildings together with more indoor activities. Studies of indoor air pollution focusing on children and people with respiratory disease from the database Web of Science (1991-2021) were systemically reviewed according to the PRISMA guidelines, with 69 studies included in the final selection. Emissions from building materials affected indoor air quality, and ventilation also had an influence. The main indoor air pollutants are Volatile Organic Compounds (VOCs) and Particulate Matter (PM). PM sources included smoking, cooking, heating, candles, and insecticides, whereas sources of coarse particles were pets, housework and human movements. VOC sources included household products, cleaning agents, glue, personal care products, building materials and vehicle emissions. Formaldehyde levels were particularly high in new houses. Personal exposure related to both indoor and outdoor pollutant levels, highlighting home characteristics and air exchange rates as important factors. Temperature, humidity, educational level, air purifiers and time near sources were also related to personal exposure. There was an association between PM and Fractional exhaled Nitric Oxide (FeNO), lung function, oxygen saturation, childhood asthma and symptoms of chronic obstructive pulmonary disease (COPD) patients. High VOCs were associated with upper airways and asthma symptoms and cancer. Effective interventional studies for PM in the future might focus on human behavior together with air purifiers and increased ventilation, whereas VOC interventions might center more on building materials and household products, alongside purification and ventilation.

Anthropogenic air pollutants reduce insect-mediated pollination services.

Common air pollutants, such as nitrogen oxides (NOx), emitted in diesel exhaust, and ozone (O3), have been implicated in the decline of pollinating insects. Reductionist laboratory assays, focused upon interactions between a narrow range of flowering plant and pollinator species, in combination with atmospheric chemistry models, indicate that such pollutants can chemically alter floral odors, disrupting the cues that foraging insects use to find and pollinate flowers. However, odor environments in nature are highly complex and pollination services are commonly provided by suites of insect species, each exhibiting different sensitivities to different floral odors. Therefore, the potential impacts of pollution-induced foraging disruption on both insect ecology, and the pollination services that insects provide, are currently unknown. We conducted in-situ field studies to investigate whether such pollutants could reduce pollinator foraging and as a result the pollination ecosystem service that those insects provide. Using free-air fumigation, we show that elevating diesel exhaust and O3, individually and in combination, to levels lower than is considered safe under current air quality standards, significantly reduced counts of locally-occurring wild and managed insect pollinators by 62-70% and their flower visits by 83-90%. These reductions were driven by changes in specific pollinator groups, including bees, flies, moths and butterflies, and coincided with significant reductions (14-31%) in three different metrics of pollination and yield of a self-fertile test plant. Quantifying such effects provides new insights into the impacts of human-induced air pollution on the natural ecosystem services upon which we depend.

The persistence of a proxy for cooking emissions in megacities: a kinetic study of the ozonolysis of self-assembled films by simultaneous small and wide angle X-ray scattering (SAXS/WAXS) and Raman microscopy.

Cooking emissions account for a significant proportion of the organic aerosols emitted into the urban environment and high pollution events have been linked to an increased organic content on urban particulate matter surfaces. We present a kinetic study on surface coatings of self-assembled (semi-solid) oleic acid-sodium oleate cooking aerosol proxies undergoing ozonolysis. We found clear film thickness-dependent kinetic behaviour and measured the effect of the organic phase on the kinetics for this system. In addition to the thickness-dependent kinetics, we show that significant fractions of unreacted proxy remain after extensive ozone exposure and that this effect scales approximately linearly with film thickness, suggesting that a late-stage inert reaction product may form and inhibit reaction progress - effectively building up an inert crust. We determine this by using a range of simultaneous analytical techniques; most notably Small-Angle X-ray Scattering (SAXS) has been used for the first time to measure the reaction kinetics of films of a wide range of thicknesses from ca. 0.59 to 73 µm with films <10 µm thick being of potential atmospheric relevance. These observations have implications for the evolution of particulate matter in the urban environment, potentially extending the atmospheric lifetimes of harmful aerosol components and affecting the local urban air quality and climate.

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.

Control of Nanomaterial Self-Assembly in Ultrasonically Levitated Droplets.

We demonstrate that acoustic trapping can be used to levitate and manipulate droplets of soft matter, in particular, lyotropic mesophases formed from self-assembly of different surfactants and lipids, which can be analyzed in a contact-less manner by X-ray scattering in a controlled gas-phase environment. On the macroscopic length scale, the dimensions and the orientation of the particle are shaped by the ultrasonic field, while on the microscopic length scale the nanostructure can be controlled by varying the humidity of the atmosphere around the droplet. We demonstrate levitation and in situ phase transitions of micellar, hexagonal, bicontinuous cubic, and lamellar phases. The technique opens up a wide range of new experimental approaches of fundamental importance for environmental, biological, and chemical research.

Time-Resolved Gas-Phase Kinetic, Quantum Chemical, and RRKM Studies of the Reaction of Silylene with 2,5-Dihydrofuran.

Time-resolved kinetics studies of silylene, SiH2, generated by laser flash photolysis of phenylsilane, were performed to obtain rate coefficients for its bimolecular reaction with 2,5-dihydrofuran (2,5-DHF). The reaction was studied in the gas phase over the pressure range of 1-100 Torr in SF6 bath gas, at five temperatures in the range of 296-598 K. The reaction showed pressure dependences characteristic of a third body assisted association. The second-order rate coefficients obtained by Rice-Ramsperger-Kassel-Marcus (RRKM)-assisted extrapolation to the high-pressure limit at each temperature fitted the following Arrhenius equation where the error limits are single standard deviations: log(k/cm(3) molecule(-1) s(-1)) = (-9.96 ± 0.08) + (3.38 ± 0.62 kJ mol(-1))/RT ln 10. End-product analysis revealed no GC-identifiable product. Quantum chemical (ab initio) calculations indicate that reaction of SiH2 with 2,5-DHF can occur at both the double bond (to form a silirane) and the O atom (to form a donor-acceptor, zwitterionic complex) via barrierless processes. Further possible reaction steps were explored, of which the only viable one appears to be decomposition of the O-complex to give 1,3-butadiene + silanone, although isomerization of the silirane cannot be completely ruled out. The potential energy surface for SiH2 + 2,5-DHF is consistent with that of SiH2 with Me2O, and with that of SiH2 with cis-but-2-ene, the simplest reference reactions. RRKM calculations incorporating reaction at both π- and O atom sites, can be made to fit the experimental rate coefficient pressure dependence curves at 296-476 K, giving values for k(∞)(π) and k(∞)(O) that indicate the latter is larger in magnitude at all temperatures, in contrast to values from individual model reactions. This unexpected result suggests that, in 2,5-DHF with its two different reaction sites, the O atom exerts the more pronounced electrophilic attraction on the approaching silylene. Arrhenius parameters for the individual pathways were obtained. The lack of a fit at 598 K is consistent with decomposition of the O-complex to give 1,3-butadiene + silanone.

Ozonolysis of methyl oleate monolayers at the air-water interface: oxidation kinetics, reaction products and atmospheric implications.

Ozonolysis of methyl oleate monolayers at the air-water interface results in surprisingly rapid loss of material through cleavage of the C=C bond and evaporation/dissolution of reaction products. We determine using neutron reflectometry a rate coefficient of (5.7 ± 0.9) × 10(-10) cm(2) molecule(-1) s(-1) and an uptake coefficient of ∼3 × 10(-5) for the oxidation of a methyl ester monolayer: the atmospheric lifetime is ∼10 min. We obtained direct experimental evidence that <2% of organic material remains at the surface on atmospheric timescales. Therefore known long atmospheric residence times of unsaturated fatty acids suggest that these molecules cannot be present at the interface throughout their ageing cycle, i.e. the reported atmospheric longevity is likely to be attributed to presence in the bulk and viscosity-limited reactive loss. Possible reaction products were characterized by ellipsometry and uncertainties in the atmospheric fate of organic surfactants such as oleic acid and its methyl ester are discussed. Our results suggest that a minor change to the structure of the molecule (fatty acid vs. its methyl ester) considerably impacts on reactivity and fate of the organic film.

Reaction of a phospholipid monolayer with gas-phase ozone at the air-water interface: measurement of surface excess and surface pressure in real time.

The reaction between gas-phase ozone and monolayers of the unsaturated lipid 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine, POPC, on aqueous solutions has been studied in real time using neutron reflection and surface pressure measurements. The reaction between ozone and lung surfactant, which contains POPC, leads to decreased pulmonary function, but little is known about the changes that occur to the interfacial material as a result of oxidation. The results reveal that the initial reaction of ozone with POPC leads to a rapid increase in surface pressure followed by a slow decrease to very low values. The neutron reflection measurements, performed on an isotopologue of POPC with a selectively deuterated palmitoyl strand, reveal that the reaction leads to loss of this strand from the air-water interface, suggesting either solubilization of the product lipid or degradation of the palmitoyl strand by a reactive species. Reactions of (1)H-POPC on D(2)O reveal that the headgroup region of the lipids in aqueous solution is not dramatically perturbed by the reaction of POPC monolayers with ozone supporting degradation of the palmitoyl strand rather than solubilization. The results are consistent with the reaction of ozone with the oleoyl strand of POPC at the air-water interface leading to the formation of OH radicals. The highly reactive OH radicals produced can then go on to react with the saturated palmitoyl strands leading to the formation of oxidized lipids with shorter alkyl tails.

Oxidation of oleic acid at the air-water interface and its potential effects on cloud critical supersaturations.

The oxidation of organic films on cloud condensation nuclei has the potential to affect climate and precipitation events. In this work we present a study of the oxidation of a monolayer of deuterated oleic acid (cis-9-octadecenoic acid) at the air-water interface by ozone to determine if oxidation removes the organic film or replaces it with a product film. A range of different aqueous sub-phases were studied. The surface excess of deuterated material was followed by neutron reflection whilst the surface pressure was followed using a Wilhelmy plate. The neutron reflection data reveal that approximately half the organic material remains at the air-water interface following the oxidation of oleic acid by ozone, thus cleavage of the double bond by ozone creates one surface active species and one species that partitions to the bulk (or gas) phase. The most probable products, produced with a yield of approximately (87 +/- 14)%, are nonanoic acid, which remains at the interface, and azelaic acid (nonanedioic acid), which dissolves into the bulk solution. We also report a surface bimolecular rate constant for the reaction between ozone and oleic acid of (7.3 +/- 0.9) x 10(-11) cm2 molecule s(-1). The rate constant and product yield are not affected by the solution sub-phase. An uptake coefficient of ozone on the oleic acid monolayer of approximately 4 x 10(-6) is estimated from our results. A simple Kohler analysis demonstrates that the oxidation of oleic acid by ozone on an atmospheric aerosol will lower the critical supersaturation needed for cloud droplet formation. We calculate an atmospheric chemical lifetime of oleic acid of 1.3 hours, significantly longer than laboratory studies on pure oleic acid particles suggest, but more consistent with field studies reporting oleic acid present in aged atmospheric aerosol.

Oxidation of biogenic and water-soluble compounds in aqueous and organic aerosol droplets by ozone: a kinetic and product analysis approach using laser Raman tweezers.

The results of an experimental study into the oxidative degradation of proxies for atmospheric aerosol are presented. We demonstrate that the laser Raman tweezers method can be used successfully to obtain uptake coefficients for gaseous oxidants on individual aqueous and organic droplets, whilst the size and composition of the droplets is simultaneously followed. A laser tweezers system was used to trap individual droplets containing an unsaturated organic compound in either an aqueous or organic (alkane) solvent. The droplet was exposed to gas-phase ozone and the reaction kinetics and products followed using Raman spectroscopy. The reactions of three different organic compounds with ozone were studied: fumarate anions, benzoate anions and alpha-pinene. The fumarate and benzoate anions in aqueous solution were used to represent components of humic-like substances, HULIS; alpha-pinene in an alkane solvent was studied as a proxy for biogenic aerosol. The kinetic analysis shows that for these systems the diffusive transport and mass accommodation of ozone is relatively fast, and that liquid-phase diffusion and reaction are the rate determining steps. Uptake coefficients, gamma, were found to be (1.1 +/- 0.7) x 10(-5), (1.5 +/- 0.7) x 10(-5) and (3.0-7.5) x 10(-3) for the reactions of ozone with the fumarate, benzoate and alpha-pinene containing droplets, respectively. Liquid-phase bimolecular rate coefficients for reactions of dissolved ozone molecules with fumarate, benzoate and alpha-pinene were also obtained: kfumarate = (2.7 +/- 2) x 10(5), kbenzoate = (3.5 +/- 3) x 10(5) and kalpha-pinene = (1-3) x 10(7) dm3 mol(-1) s(-1). The droplet size was found to remain stable over the course of the oxidation process for the HULIS-proxies and for the oxidation of alpha-pinene in pentadecane. The study of the alpha-pinene/ozone system is the first using organic seed particles to show that the hygroscopicity of the particle does not increase dramatically over the course of the oxidation. No products were detected by Raman spectroscopy for the reaction of benzoate ions with ozone. One product peak, consistent with aqueous carbonate anions, was observed when following the oxidation of fumarate ions by ozone. Product peaks observed in the reaction of ozone with alpha-pinene suggest the formation of new species containing carbonyl groups.