ACS Earth and Space Chemistry最新文献

Atomic carbon in its ground electronic state, C(³P), is expected to be present at high abundances during the evolution of dense molecular clouds. Consequently, its reactions with other interstellar species could have a strong influence on the chemical composition of these regions. Here, we report the results of an investigation of the reaction between C(³P) and dimethyl ether, CH₃OCH₃, which was recently detected in dark cloud TMC-1. Experiments were performed to study the kinetics of this reaction using a continuous supersonic flow reactor employing pulsed laser photolysis and pulsed laser-induced fluorescence for atomic radical generation and detection, respectively. Rate constants for this process were measured between 50 and 296 K, while additional measurements of the product atomic hydrogen yields were also performed over the 75–296 K range. To better understand the experimental results, statistical rate theory was used to calculate rate constants over the same temperature range and to provide insight on the major product channels. These simulations, based on quantum chemical calculations of the ground triplet state of the C₃H₆O molecule, allowed us to obtain the most important features of the underlying potential energy surface. The measured rate constant increases as the temperature falls, reaching a value of k_C+CH3OCH3 = 7.5 × 10^–11 cm³ s^–1 at 50 K, while the low measured H atom yields support the theoretical prediction that the major reaction products are CH₃ + CH₃ + CO. The effects of this reaction on the abundances of interstellar CH₃OCH₃ and related species were tested using a gas-grain model of dense interstellar clouds, employing an expression for the rate constant, k(T) = α(T/300)^β, with α = 1.27 × 10^–11 and β = −1.01. These simulations predict that the C(³P) + CH₃OCH₃ reaction decreases gas-phase CH₃OCH₃ abundances by more than an order of magnitude at early and intermediate cloud ages.

The interaction of methyl iodine with the surface of amorphous, cubic, and hexagonal ices has been investigated. The CH₃I desorption process has also been evaluated. We have highlighted the difference in CH₃I behavior depending on the trapping on the ice surface. The broadband UV photochemistry of CH₃I trapped at the surface of ices has been studied. Those results have been compared with UV broadband photochemistry of CH₃I bare monomer complexed with water and trapped in argon cryogenic matrices. It appears that if CH₃I interacts with water molecules or water ice by hydrogen bonding, then CH₃I does not fragment under UV irradiation. Thus, energy transfer to the network of hydrogen bonds in the ice or matrix is effective. On the other hand, to a first approximation, if CH₃I interacts by picnogen-type bonding (I···O), then CH₃I fragments, because the electronic relaxation seems to take place mainly at the intramolecular levels of CH₃I. Finally, we demonstrated that water ice does not catalyze the photofragmentation of CH₃I. Rather, it modifies the electronic relaxation paths, some of which lead to the fragmentation of iodomethane. This fundamental work provides an understanding of the molecular processes involved in water/ice–CH₃I interaction and the role of these molecular interactions on CH₃I photochemistry in the atmosphere.

To study the effect of soil erosion on the distribution and migration pattern of radionuclides, the levels of ^239+240Pu and ¹³⁷Cs in alpine meadow soil were measured in the Hongsongwa Nature Reserve, Hebei Province. The measured activities of ^239+240Pu and ¹³⁷Cs in surface soil ranged from 0.028 to 2.781 Bq/kg and from 1.3 to 59.8 Bq/kg, respectively. The distribution of ¹³⁷Cs and ^239+240Pu is uneven and significantly correlated with the organic matter content and altitude variations within the study area. Core samples were collected from both ridge and valley locations to assess erosion rates, revealing that ridge areas experienced approximately 2.5 times higher erosion rates (18.0 t ha^–l a ^–1) compared to valleys (7.3 t ha^–l a ^–1). The vertical migration behavior of ^239+240Pu and ¹³⁷Cs was quantitatively described by a convection-diffusion equation model. Results indicated that core samples taken from the ridge displayed significantly higher apparent diffusion coefficients (D_Cs = 4.57 cm²/y; D_Pu = 2.42 cm²/y) as well as apparent convection coefficient (ν_Cs = 0.31 cm/y; ν_Pu = 0.43 cm/y), which were approximately 10 and 2 times those observed in reference sample (D_Cs = 0.33 cm²/y; D_Pu = 0.32 cm²/y; ν_Cs = 0.16 cm/y; ν_Pu = 0.17 cm/y), respectively. The migration rate of ^239+240Pu is accelerated by 39% compared to that of ¹³⁷Cs due to soil erosion. The diffusion and convection rates of both isotopes in the valley sample are similar to those in the reference sample. In general, soil erosion significantly affects the horizontal and vertical migration of ^239+240Pu and ¹³⁷Cs.

Furanoids are a class of reactive volatile organic compounds that are major products from the pyrolysis and combustion of biomass polymers, including cellulose, hemicellulose, and lignin. Biomass burning is an atmospheric source of furanoids that is increasing in frequency and intensity throughout regions of the world. Once emitted to the atmosphere, furanoids may react with the major atmospheric oxidants to form secondary pollutants that are hazardous to human health, including ozone (O₃) and secondary organic aerosol (SOA). This review is a comprehensive assessment of the literature between 1977 and the present describing the emissions and atmospheric fate of furanoids emitted from wild, prescribed, and domestic fires. The review is organized by presenting the physical properties of key furanoids first, followed by a summary of the biopolymer pyrolysis and combustion reactions that lead to furanoid formation. Next, furanoid emissions factors are compiled across the typical fuels consumed by biomass burning to highlight the key species emitted in smoke. We next review the available kinetic and atmospheric degradation mechanism data that characterize the reaction rates, gas-phase products, and SOA formed as a result of furanoid reactions with OH, NO₃, O₃, and Cl radicals. We then describe studies that have focused on evaluating furanoid atmospheric chemistry and their impacts on air quality using a combination of field observations and model simulations. We conclude with a perspective that identifies future research directions that would address key data gaps and improve the understanding of furanoid atmospheric processes.

The distribution and chemical form of carbon in the universe are of great interest to astrobiology; however, the measured abundance of carbon in interstellar environments is significantly less than that predicted by astrochemical modeling. Polycyclic aromatic hydrocarbons (PAHs) have been postulated to account for some of this discrepancy, given that carbon in this molecular form is difficult to measure by astronomical techniques due to their large partition functions, resulting in weak spectroscopic signatures. However, with new advances in observation in recent years, a small number of PAH-derivatives have been detected in molecular clouds, leading to the important question regarding their formation routes under a diverse range of cloud environment conditions. Several bimolecular gas-phase routes have been proposed for cold regions, but surface-mediated formation routes, where polymerization of small hydrocarbons takes place on exposed interstellar dust grains, may also play a role at warmer temperatures. In the present work, we have applied computational techniques to investigate possible reaction mechanisms, from acetylene to the smallest aromatic benzene, on model forsterite surfaces. A large (>2 eV) barrier to initial C–C bond formation between adsorbed C₂H₂ is found, suggesting gas-phase routes to form C₄ species is likely imperative. However, significantly lower barriers (∼0.5 eV) are observed for subsequent C₄–C₂ bond formation and cyclization to form benzene on forsterite. While these barriers likely preclude purely surface-based polymerization in cold cloud environments where grains are coated in ice mantles anyway, this study offers support for recent laboratory studies that identified reaction bottlenecks for PAH formation, which other mechanisms may surmount.

Emissions from biomass burning (BB) occurring at midlatitudes can reach the Arctic, where they influence the remote aerosol population. By using measurements of levoglucosan and black carbon, we identify seven BB events reaching Svalbard in 2020. We find that most of the BB events are significantly different to the rest of the year (nonevents) for most of the chemical and physical properties. Aerosol mass and number concentrations are enhanced by up to 1 order of magnitude during the BB events. During BB events, the submicrometer aerosol bulk composition changes from an organic- and sulfate-dominated regime to a clearly organic-dominated regime. This results in a significantly lower hygroscopicity parameter κ for BB aerosol (0.4 ± 0.2) compared to nonevents (0.5 ± 0.2), calculated from the nonrefractory aerosol composition. The organic fraction in the BB aerosol showed no significant difference for the O:C ratios (0.9 ± 0.3) compared to the year (0.9 ± 0.6). Accumulation mode particles were present during all BB events, while in the summer an additional Aitken mode was observed, indicating a mixture of the advected air mass with locally produced particles. BB tracers (vanillic, homovanillic, and hydroxybenzoic acid, nitrophenol, methylnitrophenol, and nitrocatechol) were significantly higher when air mass back trajectories passed over active fire regions in Eastern Europe, indicating agricultural and wildfires as sources. Our results suggest that the impact of BB on the Arctic aerosol depends on the season in which they occur, and agricultural and wildfires from Eastern Europe have the potential to disturb the background conditions the most.