ACS Earth and Space Chemistry最新文献

Hydroxyacetone (HA) is an atmospheric oxidation product of isoprene and other organic precursors that can form brown carbon (BrC). Measured bulk aqueous-phase reaction rates of HA with ammonium sulfate, methylamine, and glycine suggest that these reactions cannot compete with aqueous-phase hydroxyl radical oxidation. In cloud chamber photooxidation experiments with either gaseous or particulate HA in the presence of the same N-containing species, BrC formation was minor, with similar mass absorption coefficients at 365 nm (<0.05 m² g^–1). However, rapid changes observed in aerosol volume and gas-phase species concentrations suggest that the lack of BrC was not due to slow reactivity. Filter-based UHPLC/(+)ESI-HR-QTOFMS analysis revealed that the SOA became heavily oligomerized, with average molecular masses of ∼400 amu in all cases. Oligomers contained, on average, 3.9 HA, 1.5 ammonia, and 1.6 other small aldehydes, including, in descending order of abundance, acetaldehyde, glycolaldehyde, glyoxal, and methylglyoxal. PTR-ToF-MS confirmed the production of these aldehydes. We identify C₁₇H₂₆O₅, C₁₀H₂₂O₉, C₁₅H₂₇NO₇, C₁₇H₂₃NO₅, and C₁₈H₃₂N₂O₉ as potential tracer ions for HA oligomers. We hypothesize that efficient oligomerization without substantial BrC production is due to negligible N-heterocycle (e.g., imidazoles/pyrazines) formation. While HA photooxidation is unlikely a significant atmospheric BrC source, it may contribute significantly to aqueous SOA formation.

Hydroxyacetone (HA) is an atmospheric oxidation product of isoprene and other organic precursors that can form brown carbon (BrC). Measured bulk aqueous-phase reaction rates of HA with ammonium sulfate, methylamine, and glycine suggest that these reactions cannot compete with aqueous-phase hydroxyl radical oxidation. In cloud chamber photooxidation experiments with either gaseous or particulate HA in the presence of the same N-containing species, BrC formation was minor, with similar mass absorption coefficients at 365 nm (<0.05 m² g^-1). However, rapid changes observed in aerosol volume and gas-phase species concentrations suggest that the lack of BrC was not due to slow reactivity. Filter-based UHPLC/(+)ESI-HR-QTOFMS analysis revealed that the SOA became heavily oligomerized, with average molecular masses of ∼400 amu in all cases. Oligomers contained, on average, 3.9 HA, 1.5 ammonia, and 1.6 other small aldehydes, including, in descending order of abundance, acetaldehyde, glycolaldehyde, glyoxal, and methylglyoxal. PTR-ToF-MS confirmed the production of these aldehydes. We identify C₁₇H₂₆O₅, C₁₀H₂₂O₉, C₁₅H₂₇NO₇, C₁₇H₂₃NO₅, and C₁₈H₃₂N₂O₉ as potential tracer ions for HA oligomers. We hypothesize that efficient oligomerization without substantial BrC production is due to negligible N-heterocycle (e.g., imidazoles/pyrazines) formation. While HA photooxidation is unlikely a significant atmospheric BrC source, it may contribute significantly to aqueous SOA formation.

Laboratory investigations of photochemical reactions in simulated Titan-like atmospheric systems provide insight into the formation of gas and aerosol products and the influence of different environmental parameters on the types of organic molecules generated. Studying the gas-phase products as a function of reaction time provides further insight into the reaction pathways that lead to organic production. The stable isotopes in the reactants and products serve as tracers and help to disentangle these reaction pathways. We report a time study on the chemical composition and relative abundance of the evolved gas-phase products formed by far-ultraviolet reactions between 5% CH₄ and N₂ in a closed system. Two experimental setups are used, where one fully removes hydrogen from the experimental system using a palladium membrane (hydrogen-poor experiments) and the other does not remove hydrogen during the experiment (hydrogen-rich experiments). Carbon isotope values (δ¹³C) of CH₄, C₂H₆, and C₃H₈ are also reported and are used, along with the gas-phase composition and relative abundance measurements, to constrain the chemical reactions occurring during our experiments. The gas-phase products C₂H₆, C₃H₈, n-C₄H₁₀, iso-C₄H₁₀, n-C₅H₁₂, iso-C₅H₁₂, C₂H₂, C₂H₄, HCN, and CH₃CN were detected, with some variations between both sets of experiments. The hydrogen-poor experiments highlight the importance of hydrogen in the formation of HCN, n-C₅H₁₂, iso-C₅H₁₂, and CH₃CN. By monitoring the chemical composition and the carbon isotopic ratios of the gas phase during CH₄/N₂ photochemistry, especially under a hydrogen-poor and hydrogen-rich environment, the photochemical reaction pathways and the influence of hydrogen on these pathways in a Titan-like atmosphere can be better understood.

Laboratory investigations of photochemical reactions in simulated Titan-like atmospheric systems provide insight into the formation of gas and aerosol products and the influence of different environmental parameters on the types of organic molecules generated. Studying the gas-phase products as a function of reaction time provides further insight into the reaction pathways that lead to organic production. The stable isotopes in the reactants and products serve as tracers and help to disentangle these reaction pathways. We report a time study on the chemical composition and relative abundance of the evolved gas-phase products formed by far-ultraviolet reactions between 5% CH₄ and N₂ in a closed system. Two experimental setups are used, where one fully removes hydrogen from the experimental system using a palladium membrane (hydrogen-poor experiments) and the other does not remove hydrogen during the experiment (hydrogen-rich experiments). Carbon isotope values (δ¹³C) of CH₄, C₂H₆, and C₃H₈ are also reported and are used, along with the gas-phase composition and relative abundance measurements, to constrain the chemical reactions occurring during our experiments. The gas-phase products C₂H₆, C₃H₈, n-C₄H₁₀, iso-C₄H₁₀, n-C₅H₁₂, iso-C₅H₁₂, C₂H₂, C₂H₄, HCN, and CH₃CN were detected, with some variations between both sets of experiments. The hydrogen-poor experiments highlight the importance of hydrogen in the formation of HCN, n-C₅H₁₂, iso-C₅H₁₂, and CH₃CN. By monitoring the chemical composition and the carbon isotopic ratios of the gas phase during CH₄/N₂ photochemistry, especially under a hydrogen-poor and hydrogen-rich environment, the photochemical reaction pathways and the influence of hydrogen on these pathways in a Titan-like atmosphere can be better understood.

We report an extensive ¹H to ²⁹Si cross-polarization (CP) nuclear magnetic resonance (NMR) investigation of a wide range of opal-AG, opal-AN and opal-CT samples, including both spectra and contact time dependent kinetics. After an extensive study of Hartmann–Hahn optimization, spin rates and power levels we are forced to conclude that the kinetics of the system is only amenable to comparative analysis rather than determination of absolute values. Q₃ peaks showed both signal growth (T_IS) and decay (T_1ρ^I) while Q₄ centers only showed the T_IS component for all opals studied, consistent with isolated proton sources in the latter. Q₂ centers are only a minor factor in most cases. Initial ¹H–²⁹Si 2D-HETCOR spectral evidence suggests that multiple Q₃ and Q₄ sites, with differing chemical shifts, are involved in the CP process. Active silicate centers and water sites may differ for single pulse (SP) and CP modes. Both SP and CP techniques are best used for comparative studies within each and between opal classes. Differing geometries are implied for all three types of opal.

Photoionization by high-energy photons creates nonthermal electrons with a broad range of energies that heat and chemically transform the atmospheres of planets. The specifics of the interactions are notably different when the gas is atomic or molecular. Motivated by the idea that molecules survive to high altitudes in some exoplanets, we built a model for the energy transfer from nonthermal electrons to the H₂O, H₂, and O₂ molecules. Our calculations show that the primary electrons of energy above about a hundred eV, a likely outcome from X-ray photoionization at moderately high atmospheric densities, expend most of their energy in ionization, dissociation, and electronic excitation collisions. In contrast, the primary electrons of less than about ten eV, such as those produced by extreme-ultraviolet photons at low densities, expend most of their energy in momentum transfer (heating), rotational, and vibrational excitation collisions. The partitioning between channels with weak thresholds is particularly sensitive to local fractional ionization. The transition between these two situations introduces a parallel transition in the way that the stellar energy is deposited in the atmosphere. Our calculations show that the nonthermal electrons enhance the ionization rate by a factor of a few or more with respect to photoionization alone but may not greatly contribute to the direct dissociation of molecules unless the local flux of far-ultraviolet photons is relatively weak. These findings highlight the importance of tracking the energy from the incident photons to the nonthermal electrons and onto the gas for problems concerned with the remote sensing and energy balance of exoplanet atmospheres.

In this article, we examine the reactions between methane molecules as a starting point for hydrocarbon growth in space and assess the effectiveness of the ion–ion reaction between CH₄⁺ and CH₄⁺ using quantum mechanical and molecular dynamics methods. We modeled the reaction starting from the dicationically ionized [CH₄···CH₄]²⁺ cluster. Initially, attractive interactions occur between the facing C–H bonds of the tetrahedral structures, which are electron-deficient. As the structure transitions to a trigonal pyramid, a bond begins to form between two carbon atoms with unpaired electrons, resulting in a metastable configuration due to the balance between Coulombic repulsion and attractive forces. The stabilization energy for C–C bond formation was 176.8 kcal/mol, with a bond formation efficiency of 32.6%, and the corresponding rate coefficient was 1.394 × 10^–2 fs^–1. This stabilization by C–C bond formation generates kinetic energy, and if sufficient energy is redistributed to the vibrational mode of the reaction, the reaction can proceed. Reactions involving C–C bond formation produced precursors of ethane, ethylene, and acetylene, such as C₂H₆²⁺, C₂H₅⁺, C₂H₄⁺, and C₂H₃⁺, as well as CH₃⁺, a key species in ion–molecule reactions in space. Even without C–C bond formation, a significant amount of CH₃⁺ was produced. Our findings underscore the importance of exploring novel ion–ion reactions to deepen our understanding of molecular growth in space.

The atmospheric degradation of methyl dichloroacetate can be initiated by ^•OH and Cl^• radicals through H atom abstraction from the alkyl groups (Cl₂HC– or –CH₃) of the chloroester. Product yields for the gas-phase reaction with ^•OH were determined experimentally in a 480 L Pyrex glass atmospheric-simulation reactor coupled to an in situ Fourier transform infrared (FTIR) spectrometer. In addition to those results, we present in this paper a complete degradation mechanism based on thermodynamic data obtained by identifying all critical points on the potential-energy surface for these reactions, employing density functional calculations with the M06-2X and MN15 hybrid exchange–correlation functionals and the aug-cc-pVTZ basis sets. A conformational search for reactants and transition states was performed. The energies of these conformers were later corrected at the CCSD(T,Full)-F12/complete basis set level by using the SVECV-f12 composite method. The corrected energies were then used to obtain the theoretical rate coefficients in a multiconformer approach. The global rate coefficient calculated for the reaction of methyl dichloroacetate with ^•Cl atoms is (7.34 × 10^–12 cm³ molecule^–1·s^–1), and the global rate coefficient calculated for the reaction with ^•OH radicals is (1.07 × 10^–12 cm³ molecule^–1·s^–1). The identified products and their respective yield percentages for the reaction of MDCA with ^•OH were Cl₂CHCOOH (44 ± 3%), COCl₂ (43 ± 3%), and CO (41 ± 6%). The analysis of the mechanism suggests that formation of P1 (Cl₂CO, phosgene) occurs mainly by abstraction from the Cl₂HC– group since the formation of P4 (Cl₂CHC(O)OH, dichloroacetic acid) and P5 (CO, carbon monoxide) is more favorable in the path for abstraction from the –OCH₃ group. The multiconformer calculated rate constant values were compared with the values obtained employing only the low-lying TSs and with our own previous experimental studies. Branching ratios for the reaction with ^•Cl were compared to the experimental product yields.

Iodine in the atmosphere destroys ozone and can nucleate particles by formation of iodic acid, HIO₃. Recent field observations suggest iodate recycles from particles sustaining significant gas-phase IO radical concentrations (0.06 pptv) in aged stratospheric air, and in elevated dust plumes. However, laboratory evidence for iodine activation from aerosols is currently missing. Here, a series of coated-wall flow tube (CWFT) experiments test for iodine release from thin aqueous films containing iodate. Photocatalyzed reactions were studied using iron(III) citrate (Fe–Cit), Arizona Test Dust (ATD), and Fe₂O₃, along with the dark reaction of iodate with H₂O₂ at 90% RH and 293 K. Fresh films were separately irradiated with visible and UV-A light, and the efficient release of molecular iodine, I₂, was observed from all irradiated films containing photocatalysts. For films with Fe–Cit, visible light reduced larger amounts of iodate than UV-A light, activating ∼40% of iodate as I₂. The formation of oxygenated volatile organic compounds (OVOC) and iodinated OVOC was also observed. Dark exposure of films to H₂O₂ led to I₂ release in smaller amounts than suggested by Bray–Liebhafsky kinetics, consistent with H₂O₂ salting-out in the films, or possibly other reasons. Photochemical activation is enhanced by dust proxies in the film, and by aging the film with H₂O₂ in the dark prior to irradiation. These findings help explain recent field observations of elevated IO radical concentrations in lofted dust layers, and warrant the inclusion of photocatalyzed iodate reduction in atmospheric models.

Iodine in the atmosphere destroys ozone and can nucleate particles by formation of iodic acid, HIO₃. Recent field observations suggest iodate recycles from particles sustaining significant gas-phase IO radical concentrations (0.06 pptv) in aged stratospheric air, and in elevated dust plumes. However, laboratory evidence for iodine activation from aerosols is currently missing. Here, a series of coated-wall flow tube (CWFT) experiments test for iodine release from thin aqueous films containing iodate. Photocatalyzed reactions were studied using iron(III) citrate (Fe-Cit), Arizona Test Dust (ATD), and Fe₂O₃, along with the dark reaction of iodate with H₂O₂ at 90% RH and 293 K. Fresh films were separately irradiated with visible and UV-A light, and the efficient release of molecular iodine, I₂, was observed from all irradiated films containing photocatalysts. For films with Fe-Cit, visible light reduced larger amounts of iodate than UV-A light, activating ∼40% of iodate as I₂. The formation of oxygenated volatile organic compounds (OVOC) and iodinated OVOC was also observed. Dark exposure of films to H₂O₂ led to I₂ release in smaller amounts than suggested by Bray-Liebhafsky kinetics, consistent with H₂O₂ salting-out in the films, or possibly other reasons. Photochemical activation is enhanced by dust proxies in the film, and by aging the film with H₂O₂ in the dark prior to irradiation. These findings help explain recent field observations of elevated IO radical concentrations in lofted dust layers, and warrant the inclusion of photocatalyzed iodate reduction in atmospheric models.