Search

Iron(III) carboxylate photochemistry plays an important role in aerosol aging, especially in the lower troposphere. These complexes can absorb light over a broad wavelength range, inducing the reduction of iron(III) and the oxidation of carboxylate ligands. In the presence of O2, the ensuing radical chemistry leads to further decarboxylation, and the production of .OH, HO2., peroxides, and oxygenated volatile organic compounds, contributing to particle mass loss. The .OH, HO2., and peroxides in turn reoxidize iron(II) back to iron(III), closing a photocatalytic cycle. This cycle is repeated, resulting in continual mass loss due to the release of CO2 and other volatile compounds. In a cold and/or dry atmosphere, organic aerosol particles tend to attain highly viscous states. While the impact of reduced mobility of aerosol constituents on dark chemical reactions has received substantial attention, studies on the effect of high viscosity on photochemical processes are scarce. Here, we choose iron(III) citrate (FeIII(Cit)) as a model light-absorbing iron carboxylate complex that induces citric acid (CA) degradation to investigate how transport limitations influence photochemical processes. Three complementary experimental approaches were used to investigate kinetic transport limitations. The mass loss of single, levitated particles was measured with an electrodynamic balance, the oxidation state of deposited particles was measured with X-ray spectromicroscopy, and HO2. radical production and release into the gas phase was observed in coated-wall flow-tube experiments. We observed significant photochemical degradation with up to 80 % mass loss within 24 h of light exposure. Interestingly, we also observed that mass loss always accelerated during irradiation, resulting in an increase of the mass loss rate by about a factor of 10. When we increased relative humidity (RH), the observed particle mass loss rate also increased. This is consistent with strong kinetic transport limitations for highly viscous particles. To quantitatively compare these experiments and determine important physical and chemical parameters, a numerical multilayered photochemical reaction and diffusion (PRAD) model was developed that treats chemical reactions and the transport of various species. The PRAD model was tuned to simultaneously reproduce all experimental results as closely as possible and captured the essential chemistry and transport during irradiation. In particular, the photolysis rate of FeIII, the reoxidation rate of FeII, HO2. production, and the diffusivity of O2 in aqueous FeIII(Cit) ∕ CA system as function of RH and FeIII(Cit) ∕ CA molar ratio could be constrained. This led to satisfactory agreement within model uncertainty for most but not all experiments performed. Photochemical degradation under atmospheric conditions predicted by the PRAD model shows that release of CO2 and repartitioning of organic compounds to the gas phase may be very important when attempting to accurately predict organic aerosol aging processes.

The multi-phase oxidation of S(IV) plays a crucial role in the atmosphere, leading to the formation of haze and severe pollution episodes. We here contribute to its understanding on a molecular level by reporting experimentally determined pKa values of the various S(IV) tautomers and reaction barriers for SO2 formation pathways. Complementary state-of-the-art molecular-dynamics simulations reveal a depletion of bisulfite at low pH at the liquid-vapor interface, resulting in a different tautomer ratio at the interface compared to the bulk. On a molecular-scale level, we explain this with the formation of a stable contact ion pair between sulfonate and hydronium ions, and with the higher energetic barrier for the dehydration of sulfonic acid at the liquid-vapor interface. Our findings highlight the contrasting physicochemical behavior of interfacial versus bulk environments, where the pH dependence of the tautomer ratio reported here has a significant impact on both SO2 uptake kinetics and reactions involving NOx and H2O2 at aqueous aerosol interfaces.

Atmospheric aerosol particles can contain light-absorbing organic compounds, also referred to as brown carbon (BrC). The ocean surface and sea spray aerosol particles can also contain light-absorbing organic species referred to as chromophoric dissolved organic matter (CDOM). Many BrC and CDOM species can contain carbonyls, dicarbonyls or aromatic carbonyls such as imidazole-2-carboxaldehyde (IC), which may act as photosensitizers because they form triplet excited states upon UV–VIS light absorption. These triplet excited states are strong oxidants and may initiate catalytic radical reaction cycles within and at the surface of atmospheric aerosol particles, thereby increasing the production of condensed-phase reactive oxygen species (ROS). Triplet states or ROS can also react with halides, generating halogen radicals and molecular halogen compounds. In particular, molecular halogens can be released into the gas phase, which is one halogen activation pathway. In this work, we studied the influence of bromide and iodide on the photosensitized production and release of hydroperoxy radicals (HO2) upon UV irradiation of films in a coated wall flow tube (CWFT) containing IC in a matrix of citric acid (CA) irradiated with UV light. In addition, we measured the iodine release upon irradiation of IC ∕ CA films in the CWFT. We developed a kinetic model coupling photosensitized CA oxidation with condensed-phase halogen chemistry to support data analysis and assessment of atmospheric implications in terms of HO2 production and halogen release in sea spray particles. As indicated by the experimental results and confirmed by the model, significant recycling of halogen species occurred via scavenging reactions with HO2. These prevented the full and immediate release of the molecular halogen (bromine and iodine) produced. Recycling was stronger at low relative humidity, attributed to diffusion limitations. Our findings also show that the HO2 production from BrC or CDOM photosensitized reactions can increase due to the presence of halides, leading to high HO2 turnover, in spite of low release due to the scavenging reactions. We estimated the iodine production within sea salt aerosol particles due to iodide oxidation by ozone (O3) at 5.0×10-6 M s−1 assuming O3 was in Henry's law equilibrium with the particle. However, using an O3 diffusion coefficient of 1×10-12 cm2 s−1, iodine activation in an aged, organic-rich sea spray is estimated to be 5.5×10-8 M s−1. The estimated iodine production from BrC photochemistry based on the results reported here amounts to 4.1×10-7 M s−1 and indicates that BrC photochemistry can exceed O3 reactive uptake in controlling the rates of iodine activation from sea spray particles under dry or cold conditions where diffusion is slow within particles.

Abstract The multi-phase oxidation of S(IV) plays a crucial role in the atmosphere, leading to the formation of haze and severe pollution episodes. We here contribute to its understanding on a molecular level by reporting experimentally determined pK a values of the various S(IV) tautomers and reaction barriers for SO2 formation pathways. Complementary state-of-the-art molecular-dynamics simulations reveal a depletion of bisulfite at low pH at the liquid-vapor interface, resulting in a different tautomer ratio at the interface compared to the bulk. On a molecular-scale level, we explain this with the formation of a stable contact ion pair between sulfonate and hydronium ions, and with the higher energetic barrier for the dehydration of sulfonic acid at the liquid-vapor interface. Our findings highlight the contrasting physicochemical behavior of interfacial versus bulk environments, where the pH dependence of the tautomer ratio reported here has a significant impact on both SO2 uptake kinetics and reactions involving NO x and H2O2 at aqueous aerosol interfaces.

The multiphase chemistry of glyoxal is a source of secondary organic aerosol (SOA), including its light-absorbing product imidazole-2-carboxaldehyde (IC). IC is a photosensitizer that can contribute to additional aerosol ageing and growth when its excited triplet state oxidizes hydrocarbons (reactive uptake) via H-transfer chemistry. We have conducted a series of photochemical coated-wall flow tube (CWFT) experiments using films of IC and citric acid (CA), an organic proxy and H donor in the condensed phase. The formation rate of gas-phase HO2 radicals (PHO2) was measured indirectly by converting gas-phase NO into NO2. We report on experiments that relied on measurements of NO2 formation, NO loss and HONO formation. PHO2 was found to be a linear function of (1) the [IC] × [CA] concentration product and (2) the photon actinic flux. Additionally, (3) a more complex function of relative humidity (25 % 2 ∕ N2 ratio (15 % 2 ∕ N2 2 mobility and losses in the film. The maximum PHO2 was observed at 25–55 % RH and at ambient O2 ∕ N2. The HO2 radicals form in the condensed phase when excited IC triplet states are reduced by H transfer from a donor, CA in our system, and subsequently react with O2 to regenerate IC, leading to a catalytic cycle. OH does not appear to be formed as a primary product but is produced from the reaction of NO with HO2 in the gas phase. Further, seed aerosols containing IC and ammonium sulfate were exposed to gas-phase limonene and NOx in aerosol flow tube experiments, confirming significant PHO2 from aerosol surfaces. Our results indicate a potentially relevant contribution of triplet state photochemistry for gas-phase HO2 production, aerosol growth and ageing in the atmosphere.

Abstract Aircraft soot has a significant impact on global and local air pollution and is of particular concern for the population working at airports and living nearby. The morphology and chemistry of soot are related to its reactivity and depend mainly on engine operating conditions and fuel-type. We investigated the morphology (by transmission electron microscopy) and chemistry (by X-ray micro-spectroscopy) of soot from the exhaust of a CFM 56-7B26 turbofan engine, currently the most common engine in aviation fleet, operated in the test cell of SR Technics, Zurich airport. Standard kerosene (Jet A-1) and a biofuel blend (Jet A-1 with 32% HEFA) were used at ground idle and climb-out engine thrust, as these conditions highly influence air quality at airport areas. The results indicate that soot reactivity decreases from ground idle to climb-out conditions for both fuel types. Nearly one third of the primary soot particles generated by the blended fuel at climb-out engine thrust bear an outer amorphous shell implying higher reactivity. This characteristic referring to soot reactivity needs to be taken into account when evaluating the advantage of HEFA blending at high engine thrust. The soot type that is most prone to react with its surrounding is generated by Jet A-1 fuel at ground idle. Biofuel blending slightly lowers soot reactivity at ground idle but does the opposite at climb-out conditions. As far as soot reactivity is concerned, biofuels can prove beneficial for airports where ground idle is a common situation; the benefit of biofuels for climb-out conditions is uncertain. Graphical abstract Image 1 Highlights • Soot produced by conventional kerosene Jet A-1 at ground idle is most prone for reaction. • Biofuel leads to soot reactivity reduction at ground idle but not at climb-out conditions. • Soot reactivity decreases with increasing engine thrust. • Considering soot reactivity, biofuel may prove beneficial for ground idle; the benefit for climb-out is uncertain. Conventional kerosene (Jet A-1) at ground idle produces the most reactive soot; biofuel can prove beneficial for ground idle; its advantage for climb-out is uncertain. [ABSTRACT FROM AUTHOR]

Sunlight can change the composition of atmospheric aerosol particles, but the mechanisms through which this happens are not well known. Here, the authors show that fast radical reaction and slow diffusion near viscous organic particle surfaces can cause oxygen depletion, radical trapping and humidity dependent oxidation.

Heterogeneous oxidation of bromide in atmospheric aqueous environments has long been suspected to be accelerated at the interface between aqueous solution and air. Here, the authors provide spectroscopic, kinetic and theoretical evidence for a rate limiting, surface active ozonide formed at the interface.

Background: The incorporation of multi-energy capabilities into radiotherapy flat-panel detectors offers advantages including enhanced soft tissue visualization by reduction of signal from overlapping anatomy such as bone in 2D image projections; creation of virtual monoenergetic images for 3D contrast enhancement, metal artefact reduction and direct acquisition of relative electron density. A novel dual-layer on-board imager offering dual energy processing capabilities is being designed. As opposed to other dual-energy implementation techniques which require separate acquisition with two different x-ray spectra, the dual-layer detector design enables simultaneous acquisition of high and low energy images with a single exposure. A computational framework is required to optimize the design parameters and evaluate detector performance for specific clinical applications., Purpose: In this study, we report on the development of a Monte Carlo (MC) model of the imager including model validation., Methods: The stack-up of the dual-layer imager (DLI) was implemented in GEANT4 Application for Tomographic Emission (GATE). The DLI model has an active area of 43×43 cm 2 , with top and bottom Cesium Iodide (CsI) scintillators of 600 and 800 μm thickness, respectively. Measurement of spatial resolution and imaging of dedicated multi-material dual-energy (DE) phantoms were used to validate the model. The modulation transfer function (MTF) of the detector was calculated for a 120 kVp x-ray spectrum using a 0.5 mm thick tantalum edge rotated by 2.5 o . For imaging validation, the DE phantom was imaged using a 140 kVp x-ray spectrum. For both validation simulations, corresponding measurements were done using an initial prototype of the imager. Agreement between simulations and measurement was assessed using normalized root mean square error (NRMSE) and 1D profile difference for the MTF and phantom images respectively. Further comparison between measurement and simulation was made using virtual monoenergetic images (VMIs) generated from basis material images derived using precomputed look-up tables., Results: The MTF of the bottom layer of the dual-layer model shows values decreasing more quickly with spatial frequency, compared to the top layer, due to the thicker bottom scintillator thickness and scatter from the top layer. A comparison with measurement shows NRMSE of 0.013 and 0.015 as well as identical MTF 50 of 0.8 mm 1 and 1.0 mm 1 for the top and bottom layer respectively. For the DE imaging of the DE-phantom, although a maximum deviation of 3.3% is observed for the 10 mm aluminum and Teflon inserts at the top layer, the agreement for all other inserts is less than 2.2% of the measured value at both layers. Material decomposition of simulated scatter-free DE images gives an average accuracy in PMMA and aluminum composition of 4.9% and 10.3% for 11-30 mm PMMA and 1-10 mm aluminum objects respectively. A comparison of decomposed values using scatter containing measured and simulated DE images shows good agreement within statistical uncertainty., Conclusion: Validation using both MTF and phantom imaging shows good agreement between simulation and measurements. With the present configuration of the digital prototype, the model can generate material decomposed images and virtual monoenergetic images., (© 2023 American Association of Physicists in Medicine.)

Objective . Megavoltage cone-beam computed tomography (MV-CBCT) imaging offers several advantages including reduced metal artifacts and accurate electron density mapping for adaptive or emergent situations. However, MV-CBCT imaging is limited by the poor efficiency of current detectors. Here we examine a new MV imager and compare CBCT reconstructions under clinically relevant scenarios. Approach . A multilayer imager (MLI), consisting of four vertically stacked standard flat-panel imagers, was mounted to a clinical linear accelerator. A custom anthropomorphic pelvis phantom with replaceable femoral heads was imaged using MV-CBCT and kilovoltage CBCT (kV-CBCT). Bone, aluminum, and titanium were used as femoral head inserts. 8 MU 2.5 MV scans were acquired for all four layers and (as reference) the top layer. Prostate and bladder were contoured on a reference CT and transferred to the other scans after rigid registration, from which the structural similarity index measure (SSIM) was calculated. Prostate and bladder were also contoured on CBCT scans without guidance, and Dice coefficients were compared to CT contours. Main results . kV-CBCT demonstrated the highest SSIMs with bone inserts (prostate: 0.86, bladder: 0.94) and lowest with titanium inserts (0.32, 0.37). Four-layer MV-CBCT SSIMs were preserved with bone (0.75, 0.80) as compared to titanium (0.67, 0.74), outperforming kV-CBCT when metal is present. One-layer MV-CBCT consistently underperformed four-layer results across all phantom configurations. Unilateral titanium inserts and bilateral aluminum insert results fell between the bone and bilateral titanium results. Dice coefficients trended similarly, with four-layer MV-CBCT reducing metal artifact impact relative to KV-CBCT to provide better soft-tissue identification. Significance . MV-CBCT with a four-layer MLI showed improvement over single-layer MV scans, approaching kV-CBCT quality for soft-tissue contrast. In the presence of artifact-producing metal implants, four-layer MV-CBCT scans outperformed kV-CBCT by eliminating artifacts and single-layer MV-CBCT by reducing noise. MV-CBCT with a novel multi-layer imager may be a valuable alternative to kV-CBCT, particularly in the presence of metal., (© 2023 Institute of Physics and Engineering in Medicine.)

Optical confinement (OC) structures the optical field and amplifies light intensity inside atmospheric aerosol particles, with major consequences for sunlight-driven aerosol chemistry. Although theorized, the OC-induced spatial structuring has so far defied experimental observation. Here, x-ray spectromicroscopic imaging complemented by modeling provides direct evidence for OC-induced patterning inside photoactive particles. Single iron(III)-citrate particles were probed using the iron oxidation state as a photochemical marker. Based on these results, we predict an overall acceleration of photochemical reactions by a factor of two to three for most classes of atmospheric aerosol particles. Rotation of free aerosol particles and intraparticle molecular transport generally accelerate the photochemistry. Given the prevalence of OC effects, their influence on aerosol particle photochemistry should be considered by atmospheric models.

The reaction of ozone with sea-salt derived bromide is relevant for marine boundary layer atmospheric chemistry. The oxidation of bromide by ozone is enhanced at aqueous interfaces. Ocean surface water and sea spray aerosol are enriched in organic compounds, which may also have a significant effect on this reaction at the interface. Here, we assess the surface propensity of cationic tetrabutylammonium at the aqueous liquid-vapor interface by liquid microjet X-ray photoelectron spectroscopy (XPS) and the effect of this surfactant on ozone uptake to aqueous bromide solutions. The results clearly indicate that the positively charged nitrogen group in tetrabutylammonium (TBA), along with its surface activity, leads to an enhanced interfacial concentration of both bromide and the bromide ozonide reaction intermediate. In parallel, off-line kinetic experiments for the same system demonstrate a strongly enhanced ozone loss rate in the presence of TBA, which is attributed to an enhanced surface reaction rate. We used liquid jet XPS to obtain detailed chemical composition information from the aqueous-solution-vapor interface of mixed aqueous solutions containing bromide or bromide and chloride with and without TBA surfactant. Core level spectra of Br 3d, C 1s, Cl 2p, N 1s, and O 1s were used for this comparison. A model was developed to account for the attenuation of photoelectrons by the carbon-rich layer established by the TBA surfactant. We observed that the interfacial density of bromide is increased by an order of magnitude in solutions with TBA. The salting-out of TBA in the presence of 0.55 M sodium chloride is apparent. The increased interfacial bromide density can be rationalized by the association constants for bromide and chloride to form ion-pairs with TBA. Still, the interfacial reactivity is not increasing simply proportionally with the increasing interfacial bromide concentration in response to the presence of TBA. The steady state concentration of the bromide ozonide intermediate increases by a smaller degree, and the lifetime of the intermediate is 1 order of magnitude longer in the presence of TBA. Thus, the influence of cationic surfactants on the reactivity of bromide depends on the details of the complex environment at the interface., Competing Interests: The authors declare no competing financial interest., (© 2021 The Authors. Published by American Chemical Society.)

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 FeCl 2 and observed the in situ chemical reaction that oxidized Fe 2+ to Fe 3+ 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 Fe 2+ accelerated with increasing ozone concentration and relative humidity, RH. Concentric 2-D column integrated profiles of the Fe 2+ 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 Fe 2+ 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 O 3 , and diffusion coefficients for ozone and iron, D O 3 and D Fe , respectively. We found that H O 3 is higher in our xanthan gum/FeCl 2 particles than for water and increases when RH decreased from about 80% to dry conditions. This coincided with a decrease in both D O 3 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.

Oxidation of bromide in aqueous environments initiates the formation of molecular halogen compounds, which is important for the global tropospheric ozone budget. In the aqueous bulk, oxidation of bromide by ozone involves a [Br•OOO - ] complex as intermediate. Here we report liquid jet X-ray photoelectron spectroscopy measurements that provide direct experimental evidence for the ozonide and establish its propensity for the solution-vapour interface. Theoretical calculations support these findings, showing that water stabilizes the ozonide and lowers the energy of the transition state at neutral pH. Kinetic experiments confirm the dominance of the heterogeneous oxidation route established by this precursor at low, atmospherically relevant ozone concentrations. Taken together, our results provide a strong case of different reaction kinetics and mechanisms of reactions occurring at the aqueous phase-vapour interface compared with the bulk aqueous phase.Heterogeneous oxidation of bromide in atmospheric aqueous environments has long been suspected to be accelerated at the interface between aqueous solution and air. Here, the authors provide spectroscopic, kinetic and theoretical evidence for a rate limiting, surface active ozonide formed at the interface.