ACS Earth and Space Chemistry最新文献

Astrochemical models of interstellar clouds, the sites of stars, and planet formation require information about spin-state chemistry to allow quantitative comparison with spectroscopic observations. In particular, it is important to know if full scrambling or H abstraction (also known as proton hopping) takes place in ion-neutral reactions. The reaction of Cl⁺ and HCl⁺ with H₂ and isotopologues has been studied at cryogenic temperatures between 20 and 180 K using a 22 pole radio frequency ion trap. Isotopic exchange processes are used to probe the reaction mechanism of the HCl⁺ + H₂ reaction. The results are compared with previous measurements and theoretical predictions. The rate coefficients for the Cl⁺ + H₂ and HCl⁺ + H₂ reactions are found to be constant in the range of temperatures studied, except for the DCl⁺ + D₂ reaction, where a weak negative temperature dependence is observed, and reactions with D₂ are found to be significantly slower than the Langevin rate. No isotopic exchange reactions are observed to occur for the H₂Cl⁺ ion. The analysis of the products of the HCl⁺ + H₂ isotopic system clearly indicates that the reaction proceeds via simple hydrogen atom abstraction.

Halogen radicals, such as atomic chlorine (Cl·), can contribute to secondary wintertime fine particulate matter in the Salt Lake Valley. One source of Cl· is the photolysis of nitryl chloride (ClNO₂), formed from the reaction of dinitrogen pentoxide (N₂O₅) with chloride-containing aerosol. However, sources of chloride-containing aerosols in the Salt Lake Valley, and their subsequent reaction kinetics, remain poorly constrained. We analyzed playa (i.e., dried saline lakebed) samples collected from dust-emitting regions along the northern and southern areas of the shrinking Great Salt Lake to investigate their mineralogy, reactivity, and ClNO₂ forming potential. The reactive uptake coefficients (γN₂O₅) for all samples ranged from 0.005 to 0.064, with the average γN₂O₅ of the northern area samples approximately double the average γN₂O₅ of the southern area samples. We attribute the increased γN₂O₅ of northern playas to increased particulate chloride and silicate, while the reduced γN₂O₅ in southern playas is due to particulate organics and high quantities of gypsum, a nonreactive mineral. The yield of ClNO₂ is > 50% for all playas tested, with one exception. Using our kinetic data during an ambient wintertime case study, we estimate playa dust contributes up to 5% of observed ClNO₂, a lower estimate which likely increases during the spring when dust emissions are higher. Our work highlights the importance of including playa dust in current air quality models, especially as reductions of anthropogenic halogen sources are implemented in the United States, and ephemeral lakes continue to shrink globally.

The chemical evolution of biomass burning aerosol occurs through reactive and nonreactive pathways, with both involving the partitioning of semivolatile organic compounds (SVOCs) between the gas and particle phase. Here, we explore the vapor pressure of imidazoles, a class of compounds characterized by an aromatic N-containing five-membered ring and commonly found in atmospheric particles. We estimate liquid phase vapor pressures of these compounds to be greater than 0.2 Pa, indicating that these compounds are highly volatile SVOCs. Despite this, ambient measurements identified imidazoles in the particle phase. In this work, we show that when imidazoles are internally mixed with certain inorganic salts, they are stabilized in the particle phase. In these mixed particles, we measure two distinct phases of evaporation, characterized by fast and slow changes. We analyze these regions separately, allowing the evolving composition of the particle to be determined from an evaporation model and identifying the characteristic composition at which stabilization occurs. Based on these observations, further supported by water uptake behavior and optical properties, we determine that the stabilization is driven by ammonium depletion due to protonation of the imidazole and evaporation of ammonia. This work highlights the importance of considering cosolutes and their stabilizing effect on SVOCs, with important implications for understanding and predicting the composition of biomass burning aerosol particles.

Excited triplet states of organic molecules (³C*) present in aerosols have been investigated for their ability as oxidants in the aqueous phase. Aerosols were collected in Grenoble (France), in winter and summer times during the period December 2021 through June 2022, and steady-state concentrations and quantum yields of ³C*, ¹O₂, and ^•OH were determined under solar-simulated conditions using various specific chemical probes. For comparison purposes, the steady-state concentrations of these three oxidants in these aerosols were determined at sample concentrations of 10 mg C L^–1. The resulting steady-state concentrations of all oxidants were larger in the winter samples than in the summer ones and ranked as follows [¹O₂]_ss > [³C*]_ss > [OH]_ss in agreement with previous reports. However, those oxidants do exhibit different reactivities with different classes of organic compounds, which can be ranked in a generic way as k_OH,ORG > k_3C*,ORG > k_1O2,ORG. If we combine the steady-state concentrations and the reactivity of these three oxidants, it appears that triplet states of organic matter are the main oxidants for most classes of organic compounds. This study emphasizes the relevance of excited triplet states compared to singlet oxygen and OH radicals in aqueous aerosols and the need for additional studies under various conditions.

Volatile chemical products (VCPs) and their transformation mechanisms are increasingly important in assessing air quality, as regulations on the atmospheric volatile organic compounds emitted by industries and fossil fuel-powered vehicles have become more stringent. 2-Hydroxy-benzothiazole (2-OH-BTH) is an important class of VCPs, also known as a high production volume chemical, and is employed in numerous industrial and domestic products. Therefore, understanding the 2-OH-BTH’s atmospheric fate is crucial for assessing its environmental risk. In the present work, the ^•OH-initiated transformation mechanism and kinetics of 2-OH-BTH were explored using density functional theory calculations. The results suggest that for the reaction of 2-OH-BTH with ^•OH, H-abstraction is the dominant pathway. The most favorable intermediate formed from the H-abstraction pathway further goes for unimolecular C–S bond rupture to form an S-centered radical intermediate. The S-centered radical intermediate reacts with O₂ to produce alkyl peroxy radicals, that mainly react with NO to form organonitrates/alkoxy radical-related products. The final formed product has a higher toxicity compared to its parent corresponding compound. The current study provides a comprehensive investigation of ^•OH-initiated atmospheric oxidation of 2-OH-BTH, which is valuable for understanding the transformation mechanisms and assessing its risk in the atmospheric environment.

Methylmercury (MeHg) is highly toxic and is mainly produced in anoxic environments by certain microorganisms. Net MeHg production involves a series of separate cellular processes: the uptake of inorganic divalent Hg (Hg(II)) by the cell, intracellular enzymatic Hg(II) methylation, and the release of MeHg into the extracellular medium, as well as MeHg demethylation. As a biological process, saturation at the cellular level can be anticipated at all stages of the Hg transformation. The aim of this study was to investigate the kinetics of Hg(II) methylation and MeHg demethylation over a 24-h period in the model sulfate-reducing strain Pseudodesulfovibrio hydrargyri BerOc1, across a range of Hg(II) concentrations from 0.03 to 3.15 μM. The distribution of Hg(II) and MeHg over 24 h within three cellular fractions (extracellular, adsorbed to the cells, and intracellular) was determined to estimate Hg uptake and export. With increasing Hg(II) concentrations, we observed (i) an increase in the accumulated intracellular Hg(II), (ii) a reduction in the methylation rate, and (iii) an increase in MeHg associated with the cells after a short Hg(II) exposure time (<1 h). Our study suggests that the saturation of MeHg production is likely not driven by Hg(II) uptake but rather by Hg(II) intracellular speciation, Hg(II) methylation by HgcAB proteins, and/or MeHg export. These results are essential to better predict and understand the parameters influencing MeHg production within more complex environments, such as anoxic sediments and soils.

The search for the molecular origins of life likely must go through polyimines, and this quantum chemical work shows that the anti-(E,Z)-1,2-diiminoethane molecule would be the next natural step as a detection target. This conformer exhibits a 2.78 D dipole moment and is the second-lowest-energy conformer after the nonpolar anti-(E,E) form. Additionally, the previously assigned N–H antisymmetric stretch of the lowest-energy anti-(E,E) conformer should likely be decreased in frequency to closer to 2930 cm^–1. The full set of anharmonic, fundamental vibrational frequencies and spectroscopic constants of all six conformers are produced in this work and will aid in any possible, future characterization of 1,2-diiminoethane and its possible role in the buildup of prebiotic molecules where multiple C = N bonds are required.

Modeling atmospheric reactions that lead to the formation of secondary organic aerosol (SOA) is an important tool for understanding the current and future impacts of human activity on the environment. Vapor pressure is a key parameter in modeling these reactions, as it largely determines the gas-particle partitioning of atmospheric oxidation products. However, the vapor pressures of many atmospherically relevant molecules are still poorly constrained. To aid modeling efforts, several structure–activity relationships (SARs) based on group contribution methods have been developed for estimating compound vapor pressures. The current study evaluates how four of these SARs: SIMPOL, EVAPORATION, SPARC, and Nannoolal impact the modeled predictions of SOA yields for reactions of C₈–C₁₄ 1-alkenes and C₉–C₁₅ 2-methyl-1-alkenes with OH radicals in the presence of NO_x. The models include well-constrained, quantitative reaction mechanisms developed by our research group from several previous environmental chamber studies of product yields, gas-particle and gas-wall partitioning, and secondary reactions with OH radicals. Based on our previous product studies and the results of this study, there was no need to account for particle-phase oligomer formation. Comparison of modeled and measured SOA yields provide insight into the major products responsible for SOA formation over the large range of carbon numbers, and the sources of discrepancies between model predictions and measurements. The generally moderate to poor model-measurement agreement exemplifies the need for further development of vapor pressure estimation methods, which have a major impact on atmospheric SOA modeling. The systems studied here could be useful to others interested in developing and evaluating models for simulating SOA formation.

Efflux of carbon dioxide from soil is a central component of the carbon cycle and climate models, but one potentially large source is rarely indicated, namely, photochemical decomposition of soil organic matter. A survey from diverse environments around the world shows that this process is almost ubiquitous and comparable in magnitude to respiration. Moreover, it can persist when respiration subsides, such as in dry and cold soils.

The phase state of secondary organic aerosols (SOA) can range from liquid through amorphous semisolid to glassy solid, which is important to consider as it influences various multiphase processes including SOA formation and partitioning, multiphase chemistry, and cloud activation. In this study, we simulate the glass transition temperature and viscosity of SOA over the globe using the global chemical transport model, GEOS-Chem. The simulated spatial distributions show that SOA at the surface exist as liquid over equatorial regions and oceans, semisolid in the midlatitude continental regions, and glassy solid over lands with low relative humidity. The predicted SOA viscosities are mostly consistent with the available measurements. In the free troposphere, SOA particles are mostly predicted to be semisolid at 850 hPa and glassy solid at 500 hPa, except over tropical regions including Amazonia, where SOA are predicted to be low viscous. Phase state also exhibits seasonal variation with a higher frequency of semisolid and solid particles in winter compared to warmer seasons. We calculate equilibration time scales of SOA partitioning (τ_eq) and effective mass accommodation coefficient (α_eff), indicating that τ_eq is shorter than the chemical time step of GEOS-Chem of 20 min and α_eff is close to unity for most locations at the surface level, supporting the application of equilibrium SOA partitioning. However, τ_eq is prolonged and α_eff is lowered over drylands and most regions in the upper troposphere, suggesting that kinetically limited growth would need to be considered for these regions in future large-scale model studies.