ACS Earth and Space Chemistry最新文献

The photochemical stability of organic thin films in contact with iron oxides was investigated under simulated Martian near-surface conditions to elucidate radiation-induced degradation and transformation processes by in situ ultraviolet–visible (UV–Vis) spectroscopy. Highly uniform 20–30 nm films of quercetin, iron tetraphenyl porphyrinato chloride (FeTPPCl), and magnesium tetraphenyl porphyrin (MgTPP) were deposited by thermal evaporation onto UV-transparent magnesium fluoride (MgF₂) substrates, either bare or coated with ultrathin iron oxide layers. Irradiation was performed in a CO₂-dominated atmosphere (≈10 mbar) at −20 °C with a UV flux comparable to present-day Martian surface conditions. The experimental concept and thin-film preparation protocols were motivated by the hardware and sample design of the OREOcube space experiment on the International Space Station (ISS), enabling future comparison between laboratory Mars simulations and space-exposure studies of radiation-driven chemical evolution in organic thin films. UV–Vis measurements reveal molecule-dependent kinetic regimes. Quercetin exhibits apparent first-order decay on all substrate types, with iron oxide coatings altering degradation rates. In contrast, metal porphyrin degradation is more accurately described by kinetic approaches incorporating film-internal optical attenuation effects, indicating that changes in effective light penetration contribute to the observed behavior. Overall, quercetin degradation is largely governed by intrinsic photochemical reactivity, whereas porphyrin kinetics are more sensitive to electronic structure and attenuation effects. Iron oxide interfaces modulate, rather than dominate, degradation pathways, suggesting that mineral association may influence the persistence and spectroscopic detectability of organic compounds on the present-day Martian surface.

Methylglyoxal (MeGX) and glyoxal are commonly occurring α-dicarbonyl compounds that react with nitrogen-containing species to produce aqueous brown carbon (aqBrC), including imidazoles and oligomeric products. The composition and physical properties of aerosol particles that contain these species will influence their effects in the environment via their role in cloud formation, light scattering, and further chemistry. Here, we produce aqBrC from the reaction of MeGX and glycine (Gly) in bulk solution at three possible molar mixing ratios (1:1, 1:2, and 2:1) aged for up to 11 weeks. Individual particles generated from both fresh and aged solutions were levitated in a linear quadrupole electrodynamic balance to measure the hygroscopic growth, phase state, and chemical composition of single particles as a function of bulk aging time and relative humidity (RH). Complementary hygroscopic growth and phase state measurements were conducted using a humidified-tandem differential mobility analyzer and quartz crystal microbalance. Hygroscopic growth measurements reveal a slight decrease in growth with increasing aging time, and a corresponding increase in the refractive index (RI). At low RH, the phase state is shown to be a strong function of the starting composition and drying rate, with samples containing equal quantities of reactants consistently observed to form amorphous phase states, with a corresponding slowing of water diffusion, indicating an increase in viscosity. Compared with bulk solution aging, levitated droplets show much faster product formation. Compounds detected after >3 days in bulk solution appear within <10 min in the droplets. Conversely, other products identified after >3 days of bulk aging were not observed in droplets over the measured time scales. These results show that bulk aqueous aging alone is not sufficient to predict aqBrC aerosol properties. The particle composition, phase state, and water uptake are shaped by the coupled effects of precursor ratio, RH history, drying rate, and accelerated chemistry during and after water loss.

Space weathering causes physical and spectral changes on the surfaces of airless bodies. However, our understanding of how space weathering operates in the presence of volatile ices is in its early stages. Electron irradiation of ice-coated surfaces is expected in astrophysical environments including the early solar system, volatile ice-rich permanently shadowed regions of the Moon and Mercury, and other airless bodies like asteroids. A recent study suggests that anomalous oxygen isotope exchange occurs between water-ice and underlying surfaces when exposed to electron irradiation at extremely low temperatures (10 K). To delve deeper into the physical processes underlying isotopic exchange, we employ nanoscale atomic force microscopy-based infrared (AFM-IR) spectroscopy to identify Si–O bond formation resulting from the electron irradiation of H₂O ice coated silicon targets. Experimental variables include electron energy, amount and timing of water-ice deposition, and surface area exposed to the electron beam. AFM-IR point spectra, surface topography and IR absorption mapping reveal that the degree of surface oxidation is dependent upon experimental conditions. Scanning electron microscopy and (scanning) transmission electron microscope imaging confirm the formation of thicker SiO_x in regions of enhanced interaction between electron irradiation, water-ice, and the silicon substrate. In summary, we find that electron irradiation with energies as low as 1 keV/electron can break the chemical bonds of refractory solids like Si under these simulated cold astrophysical conditions. These results suggest that cosmic rays may play a more significant role than previously thought in the chemical evolution of dust grains in cold astrophysical and protoplanetary environments.

Acetaldehyde represents a key carbonyl compound in star-forming regions of the Sagittarius B2 molecular cloud, where photolysis generates reactive intermediates central to astrochemical complex molecule formation. Yet, its dominant photodissociation mechanisms remain poorly understood. Here, we use extreme ultraviolet (21.5 eV) time-resolved photoelectron spectroscopy, supported by high level ab initio calculations, to investigate the wavelength-dependent photodissociation dynamics of acetaldehyde following excitation at 295 and 308 nm. At 295 nm, population of the S₁ $(n_O\to {\pi }_{C=O}^{*})$ state decays on a 60 fs time scale via an energetically accessible S₁/S₀ conical intersection seam, leading to both direct dissociation via conventional transition state dynamics and the formation of longer-lived (4.0 ps) CH₃ roaming intermediates on S₀. These roaming intermediates subsequently yield radical and molecular products over a hundreds-of-picoseconds time scale. In contrast, excitation at 308 nm produces low intensity structure between 7.0–8.5 eV at intermediate and late delay times, which, together with EOM-IP-CCSD calculations of representative geometries, suggest that additional pathways, such as H-atom roaming, may be active, leading to acetyl radical formation on a tens-of-picoseconds time scale. These findings demonstrate that access to competing S₁ pathways in acetaldehyde may be strongly wavelength- and mode-dependent, controlling the balance between conventional dissociation and distinct roaming mechanisms.

Although energetic and nonenergetic processing of interstellar dust grain surfaces and ice is thought to be one of the dominant mechanisms for the extraterrestrial synthesis of prebiotic molecules, the role that “ion–ice” chemistry (bulk ion–neutral and ion–ion reactions within the dust-grain ice mantle) plays is still a largely unexplored topic. In this work, we present the results of our study to investigate the astrochemical importance of ions produced within dust grain ice mantles via irradiation by VUV photons, though the methods are applicable to other kinds of ionizing radiation. To this end, we present new models of the chemical evolution of a pure oxygen ice irradiated by UV photons, using a chemical network that explicitly includes bulk reactions involving secondary ions. A comparison of our calculations with previous data suggests that ion–neutral and ion–ion reactions may play a critical role in understanding the chemistry of interstellar ice.

This work presents a quantum chemical investigation of the atmospheric oxidation mechanism of 1,3,3,4,4-pentafluorocyclobutene (cyc-CF₂CF₂CF=CH–, HFO-1325yz) initiated by Cl atoms. The potential energy surface (PES) of the title reaction was explored at the M11/6-311++G(d,p) and CCSD(T)//M11/6-311++G(d,p) methods. Thermodynamic and energetic evaluations clearly indicate that Cl-atom addition to the C═C bond is substantially more favorable than the competing H-abstraction pathway. Further, rate constant calculations using variational transition state theory indicate that the addition pathways exhibit significantly higher reaction rates than the H-abstraction pathway and unambiguously dominate the overall oxidation mechanism. The overall rate constant at 298.15 K is estimated to be 4.52 × 10^–11 cm³molecule^–1s^–1, corresponding to an atmospheric lifetime of approximately 25.6 days, suggesting a relatively short persistence and a low global warming potential (GWP) over various time horizons. Secondary degradation processes of the primary product radicals in the presence of O₂ and HO₂^•were also examined to elucidate subsequent oxidation routes. The major stable products identified include CF₂=CHCl, CF₂=CFCl, and COF₂, alongside several key radicals such as F(O)CCF₂(•), H(O)CCF₂(•), F(O)CCH(Cl)(•), and H(O)CCF(Cl)(•). Ecotoxicity assessments of HFO-c1325yz and its primary oxidation products toward three representative aquatic organisms-fish, daphnia, and green algae- are classified as “Harmful”. On the other hand, the secondary degradation products are largely nontoxic. Further, TD-DFT analysis indicates that HFO-c1325yz and some stable products are photochemically stable under tropospheric conditions, whereas selected oxidation products cyc-CF₂CF₂CF(Cl)^•CH- (P2) and cyc-CF₂CF₂CF=C^•- (P3) exhibit absorption in the near-UV/visible region and may undergo photolytic degradation. In general, this work sheds comprehensive insight into the reaction mechanisms, kinetics, and environmental implications of Cl-initiated oxidation of HFO-c1325yz, contributing to a deeper understanding of its atmospheric fate and potential ecological impact.

Brown carbon (BrC) directly impacts the global radiative balance due to its unique light-absorbing capacity and plays a key role in the atmospheric environment, climate change, and human health. Iron (Fe) has been found to drive the secondary BrC formation in atmospheric aqueous water. Because of the strong photochemical activity, iron-based BrC particles can occur complex physicochemical processes via photoaging reactions, especially at the surface of particles and in the aqueous water. However, the evolution of chemical and optical properties during the photoaging of iron-based BrC is still insufficient. Here, we investigated the photoaging behavior of iron-based BrC, formed by the reaction between fumaric acid (FA) and Fe(III), in both dispersed aqueous systems and particulate phases. Iron-based BrC particles displayed enhanced absorptivity between 200 nm and 800 nm during the photoaging process. Under the atmospheric aqueous water condition, Fe(III) in the BrC particles converted to dissolved Fe(II) and therefore migrated into the aqueous phase during the photoaging process. Reactive oxygen species (ROS) were generated during the photoaging process, potentially enhancing atmospheric oxidative capacity. The photogenerated Fe(II) displayed strong stability in atmospheric aqueous water and can be reoxidized to regenerate iron-based BrC upon addition of FA and H₂O₂ to simulate nighttime atmospheric oxidation. This work elucidates the photochemical evolution of BrC at multiphase interfaces and simulates its natural cycle of formation, aging, and regeneration in atmospheric aqueous water under daytime photolysis and nighttime chemical replenishment conditions. These results highlight the critical role of atmospheric aqueous water during the photoaging and cyclic generation of iron-based BrC in the real atmosphere.

Vernadite, a ubiquitous layered manganese oxide, exerts strong control on metal geochemical cycles in diverse environments. Its nanocrystalline nature imparts high reactivity toward metal adsorption but poses challenges for detailed structural characterization, particularly of the layer curvature. Here, we present the first atomistic models of vernadite with particle dimensions comparable to those observed experimentally, obtained using molecular dynamics (MD) simulations. Analysis of mean and Gaussian curvature computed from the MD trajectories shows that vernadite monolayers adopt anticlastic curvatures, with the degree of curvature modulated by the nature of charge-compensating cations and the spatial distribution of Mn vacancies. We further find that a minimum Mn vacancy concentration of 5% is required for layered structural stability, indicating the essential role of vacancies in maintaining layer integrity. These atomistic structural models of vernadite provide new insights into the structural controls of vernadite’s chemical reactivity, advancing our understanding of Mn oxide behavior with broad implications.

A significant fraction of atmospheric mercury enters the aquatic and terrestrial environments via the deposition of gaseous oxidized mercury (GOM). The photochemically produced GOM comprises many molecules, but most experimental work employs HgCl₂ and HgBr₂ as surrogates. This calls for a more realistic representation of GOM for the development of detection and sampling methods. Here, several mixed mercuric compounds XHgY (X and Y are Cl^–, Br^–, I^–, NO₂^–, and NO₃^–) were synthesized by exchange reactions involving mercuric and nonmercuric compounds, isolated in crystalline form, and characterized by Raman spectroscopy. Vapor above these solids was analyzed by ion drift–chemical ionization mass spectrometry (ID-CIMS), using the SF₆^–, HNO₃·NO₃^–, and I^– reagent ions. When X and Y are both halogens, XHgY readily volatilizes and can be detected by ID-CIMS, based on unique ion products XHgY·F^–, XHgY·NO₃^–, and XHgY·I^–. On the contrary, mixed mercuric salts containing at least one nonhalide ligand cannot be volatilized and detected. Furthermore, solid ClHgNO₂ and BrHgNO₂ rapidly oxidize upon exposure to ambient air, and the oxidation is accompanied by hydrolysis to form red HgO. We conclude that ClHgNO₂ and BrHgNO₂ are insufficiently redox-stable to survive multiday preconcentration from ambient air on sorbents and insufficiently volatile to be thermally desorbed for analysis in their original form.