Chemistry at and near the Surface of Liquid Sulfuric Acid: A Kinetic, Thermodynamic, and Mechanistic Analysis of Heterogeneous Reactions of Acetone

Surface Confinement of Finite-Size Water Droplets for SO3 Hydrolysis Reaction Revealed by Molecular Dynamics Simulations Based on a Machine Learning Force Field

As an important source for sulfuric acid in the atmosphere, hydrolysis of sulfur trioxide (SO₃) takes place with water clusters of sizes from several molecules to several nanometers, resulting in various final products, including neutral (H₂SO₄)-(H₂O) clusters and ionic (HSO₄)^--(H₃O)⁺ clusters. The diverse products may be due to the ability of proton transfer and the formation of hydrated ions for water cluster of finite sizes, especially the sub-micrometer ones. However, the detailed molecular-level mechanism is still unclear due to the lack of available characterization and simulations tools. Here, we developed a quantum chemistry-level machine learning (ML) model to simulate the hydrolysis of SO₃ with water clusters of sizes up to nanometers. The simulation results demonstrate diverse reaction paths taking place between SO₃ and water clusters of different sizes. Generally, neutral (H₂SO₄)-(H₂O) clusters are preferred by water clusters of ultra-small size, and a loop structure-mediated mechanism with SO₃(H₂O)_n≤4 structures and a non-loop structure-mediated mechanism with structure relaxation are observed. As the water cluster size increases to (H₂O)₈, a (HSO₄)^--(H₃O)⁺ ion-pair product emerges; and the Eigen-Zundel ion conversion-like proton transfer mechanism takes place and stabilizes the ion pairs. As the water cluster sizes further increase beyond several nanometers ((H₂O)_n≥32), the (SO₄)^2-[(H₃O)⁺]₂ ion-pair product appears. The reason could be that the surface of these water clusters is large enough to screen Coulomb repulsion between two tri-coordinated ion-pair complexes. These findings would provide new perspectives for understanding SO₃ hydrolysis in the real atmosphere and sulfuric acid chemistry in atmospheric aerosols.

Acidity and the multiphase chemistry of atmospheric aqueous particles and clouds

The acidity of aqueous atmospheric solutions is a key parameter driving both the partitioning of semi-volatile acidic and basic trace gases and their aqueous-phase chemistry. In addition, the acidity of atmospheric aqueous phases, e.g., deliquesced aerosol particles, cloud, and fog droplets, is also dictated by aqueous-phase chemistry. These feedbacks between acidity and chemistry have crucial implications for the tropospheric lifetime of air pollutants, atmospheric composition, deposition to terrestrial and oceanic ecosystems, visibility, climate, and human health. Atmospheric research has made substantial progress in understanding feedbacks between acidity and multiphase chemistry during recent decades. This paper reviews the current state of knowledge on these feedbacks with a focus on aerosol and cloud systems, which involve both inorganic and organic aqueous-phase chemistry. Here, we describe the impacts of acidity on the phase partitioning of acidic and basic gases and buffering phenomena. Next, we review feedbacks of different acidity regimes on key chemical reaction mechanisms and kinetics, as well as uncertainties and chemical subsystems with incomplete information. Finally, we discuss atmospheric implications and highlight the need for future investigations, particularly with respect to reducing emissions of key acid precursors in a changing world, and the need for advancements in field and laboratory measurements and model tools.

Solubility of acetic acid and trifluoroacetic acid in low-temperature (207-245 k) sulfuric acid solutions: implications for the upper troposphere and lower stratosphere

The solubility of gas-phase acetic acid (CH(3)COOH, HAc) and trifluoroacetic acid (CF(3)COOH, TFA) in aqueous sulfuric acid solutions was measured in a Knudsen cell reactor over ranges of temperature (207-245 K) and acid composition (40-75 wt %, H(2)SO(4)). For both HAc and TFA, the effective Henry's law coefficient, H*, is inversely dependent on temperature. Measured values of H* for TFA range from 1.7 × 10(3) M atm(-1) in 75.0 wt % H(2)SO(4) at 242.5 K to 3.6 × 10(8) M atm(-1) in 40.7 wt % H(2)SO(4) at 207.8 K. Measured values of H* for HAc range from 2.2 × 10(5) M atm(-1) in 57.8 wt % H(2)SO(4) at 245.0 K to 3.8 × 10(8) M atm(-1) in 74.4 wt % H(2)SO(4) at 219.6 K. The solubility of HAc increases with increasing H(2)SO(4) concentration and is higher in strong sulfuric acid than in water. In contrast, the solubility of TFA decreases with increasing sulfuric acid concentration. The equilibrium concentration of HAc in UT/LS aerosol particles is estimated from our measurements and is found to be up to several orders of magnitude higher than those determined for common alcohols and small carbonyl compounds. On the basis of our measured solubility, we determine that HAc in the upper troposphere undergoes aerosol partitioning, though the role of H(2)SO(4) aerosol particles as a sink for HAc in the upper troposphere and lower stratosphere will only be discernible under high atmospheric sulfate perturbations.

Accretion reactions of octanal catalyzed by sulfuric acid: product identification, reaction pathways, and atmospheric implications

Atmospheric accretion reactions of octanal with sulfuric acid as a catalyst were investigated in bulk liquid-liquid experiments and gas-particle experiments. In bulk studies, trioxane, alpha,beta-unsaturated aldehyde, and trialkyl benzene were identified by gas chromatography-mass spectrometry as major reaction products with increasing sulfuric acid concentrations (0-86 wt%). Cyclotrimerization and one or multiple steps of aldol condensation are proposed as possible accretion reaction pathways. High molecular weight (up to 700 Da) oligomers were also observed by electrospray ionization-mass spectrometry in reactions under extremely high acid concentration conditions (86 wt%). Gas-particle experiments using a reaction cell were carried out using both high (approximately 20 ppmv) and low (approximately 900 ppbv) gas-phase octanal concentrations under a wide range of relative humidity (RH, from < 1% to 50%, corresponding to > 80 wt% to 43 wt% H2SO4) and long reaction durations (24 h). One or multiple steps of aldol condensation occurred under low RH (< 1% and 10%, > 80 wt% and 64 wt% H2SO4, respectively) and high octanal concentration (approximately 20 ppmv) conditions. No cyclotrimerization was observed in the gas-particle experiments even under RH conditions corresponding to similar sulfuric acid concentration conditions that favor cyclotrimerization in bulk studies. No accretion reaction product was found in the low octanal concentration (approximately 900 ppbv) experiments, which indicates that the accretion reactions are not significant as expected when the gas-phase octanal concentration is low. A kinetic analysis of the first-step aldol condensation product was performed to understand the discrepancies between the bulk and gas-particle experiments and between the high and low octanal concentrations in the gas-particle experiments. The comparisons between experimental results and kinetic estimations suggest that caution should be exercised in the extrapolation of laboratory experiment results to ambient conditions.

Uptake and Dissolution of Gaseous Ethanol in Sulfuric Acid

The solubility of gas-phase ethanol (ethyl alcohol, CH3CH2OH, EtOH) in aqueous sulfuric acid solutions was measured in a Knudsen cell reactor over ranges of temperature (209-237 K) and acid composition (39-76 wt % H2SO4). Ethanol is very soluble under these conditions: effective Henry's law coefficients, H, range from 4 x 10(4) M atm(-1) in the 227 K, 39 wt % acid to greater than 10(7) M atm(-1) in the 76 wt % acid. In 76 wt % sulfuric acid, ethanol solubility exceeds that which can be precisely determined using the Knudsen cell technique but falls in the range of 10(7)-10(10) M atm(-1). The equilibrium concentration of ethanol in upper tropospheric/lower stratospheric (UT/LS) sulfate particles is calculated from these measurements and compared to other small oxygenated organic compounds. Even if ethanol is a minor component in the gas phase, it may be a major constituent of the organic fraction in the particle phase. No evidence for the formation of ethyl hydrogen sulfate was found under our experimental conditions. While the protonation of ethanol does augment solubility at higher acidity, the primary reason H increases with acidity is an increase in the solubility of molecular (i.e., neutral) ethanol.

The Uptake of Methyl Vinyl Ketone, Methacrolein, and 2-Methyl-3-butene-2-ol onto Sulfuric Acid Solutions

To investigate the link between molecular structure, reactivity, and partitioning of oxygenated organic compounds in acidic aerosols, the uptake of three compounds found in the atmosphere, methyl vinyl ketone (MVK), methacrolein (MACR), and 2-methyl-3-butene-2-ol (MBO), by sulfuric acid solutions has been measured using a rotated wetted-wall reactor (RWW) coupled to a chemical ionization mass spectrometer (CIMS). MVK was found to partition reversibly into 20-75 wt % H(2)SO(4) solutions, and we report Henry's law coefficients between 20 and 7000 M atm(-1) over this range. A chemical reaction for MVK was likely responsible for the uptake observed for 80-96 wt % H(2)SO(4) solutions. We derive an upper limit to the aldol self-reaction rate coefficient for MVK in 80 wt % solution of approximately 3 M(-1) s(-1). MACR partitioned reversibly over most of the acidity range, and in contrast to that for MVK, the Henry's law coefficient was relatively independent of H(2)SO(4) content. These differences indicate that the increase of the coefficient with acidity is likely due to the ability of the carbonyl molecule to form an enol. These results indicate that aldol condensation can be facile in concentrated sulfuric acid solutions, but it should be negligibly slow in dilute acid solutions such as tropospheric aerosols. MBO uptake could be explained by a Henry's law coefficient that decreases slightly as acid content varies from 20 to 55 wt % H(2)SO(4); we also measured the value in water, 70 M atm(-1) at 298 K. A steady-state uptake of MBO was observed onto 40-80 wt % H(2)SO(4) solutions, a reaction product was observed, and the reaction was tentatively identified as Pinacol rearrangement. Similar rearrangements could be at the origin of some substituted oxygenated species found in atmospheric aerosols.

Uptake and Reaction Kinetics of Acetone, 2-Butanone, 2,4-Pentanedione, and Acetaldehyde in Sulfuric Acid Solutions

This work presents a study of the uptake of acetone, 2-butanone (methyl ethyl ketone), 2,4-pentanedione, and acetaldehyde by sulfuric acid solutions with an aim at understanding the reactivity of carbonyl compounds present in the atmosphere toward acidic aerosols. Experiments were performed in a rotating wetted-wall reactor coupled to a mass spectrometer at room temperature (298 +/- 3 K) with 0-96 wt % H(2)SO(4) solutions. For all compounds, a reactive uptake was observed at high acidity (>or=64 wt % H(2)SO(4)). The corresponding reactions were found to follow a second-order kinetics, and their rate constants, k (M(-1) s(-1)) were found to increase exponentially with acidity. These rate constants and their variations with acid concentration were in good agreement with the kinetic behavior of acid-catalyzed aldol condensation reported in the organic chemical literature, except for 2,4-pentanedione. The results of this work suggest that aldol condensation should be too slow to account for the enhanced organic aerosol mass observed in smog chamber studies and should have an even smaller contribution under atmospheric conditions. The rate constants of other compounds, such as large aldehydes, remain however to be measured. However, in order to contribute significantly to organic aerosol formation, a liquid phase reaction would have to result in an uptake coefficient of the order of 10(-2).

Theoretical Study of the First Acid Dissociation of H2SO4 at a Model Aqueous Surface

Electronic structure calculations on the H2SO4.(H2O)(4,6) model system embedded at the surface of an aqueous layer have been performed to examine the feasibility of the first acid dissociation of H2SO4 to an H2SO4-.H3O+ contact ion pair over a wide temperature range, with a special focus on the 190-250 K range relevant for atmospheric sulfate aerosols. The results indicate that the acid dissociation can be either thermodynamically favored or disfavored depending on the degree of solvation of the acid and the produced ions, as well as on the temperature.