Kinetics and Products of the Gas-Phase Reaction of SO3with Water

Direct Measurements of Covalently Bonded Sulfuric Anhydrides from Gas-Phase Reactions of SO3 with Acids under Ambient Conditions

Sulfur trioxide (SO3) is an important oxide of sulfur and a key intermediate in the formation of sulfuric acid (H2SO4, SA) in the Earth's atmosphere. This conversion to SA occurs rapidly due to the reaction of SO3 with a water dimer. However, gas-phase SO3 has been measured directly at concentrations that are comparable to that of SA under polluted mega-city conditions, indicating gaps in our current understanding of the sources and fates of SO3. Its reaction with atmospheric acids could be one such fate that can have significant implications for atmospheric chemistry. In the present investigation, laboratory experiments were conducted in a flow reactor to generate a range of previously uncharacterized condensable sulfur-containing reaction products by reacting SO3 with a set of atmospherically relevant inorganic and organic acids at room temperature and atmospheric pressure. Specifically, key inorganic acids known to be responsible for most ambient new particle formation events, iodic acid (HIO3, IA) and SA, are observed to react promptly with SO3 to form iodic sulfuric anhydride (IO3SO3H, ISA) and disulfuric acid (H2S2O7, DSA). Carboxylic sulfuric anhydrides (CSAs) were observed to form by the reaction of SO3 with C2 and C3 monocarboxylic (acetic and propanoic acid) and dicarboxylic (oxalic and malonic acid)-carboxylic acids. The formed products were detected by a nitrate-ion-based chemical ionization atmospheric pressure interface time-of-flight mass spectrometer (NO3--CI-APi-TOF; NO3--CIMS). Quantum chemical methods were used to compute the relevant SO3 reaction rate coefficients, probe the reaction mechanisms, and model the ionization chemistry inherent in the detection of the products by NO3--CIMS. Additionally, we use NO3--CIMS ambient data to report that significant concentrations of SO3 and its acid anhydride reaction products are present under polluted, marine and polar, and volcanic plume conditions. Considering that these regions are rich in the acid precursors studied here, the reported reactions need to be accounted for in the modeling of atmospheric new particle formation.

Deprotonated sulfamic acid and its homodimers: Does sulfamic acid adopt zwitterion during cluster growth?

We present a joint experimental and computational study on the geometric and electronic structures of deprotonated sulfamic acid (SA) clusters [(SA)n-H]- (n = 1, 2) employing negative ion photoelectron spectroscopy and high-level ab initio calculations. The photoelectron spectra provide the vertical/adiabatic detachment energy (VDE/ADE) of the sulfamate anion (SM-) H2N●SO3- at 4.85 ± 0.05 and 4.58 ± 0.08 eV, respectively, and the VDE and ADE of the SM-●SA dimer at 6.41 ± 0.05 and 5.87 ± 0.08 eV, respectively. The significantly increased electron binding energies of the dimer confirm the enhanced electronic stability upon the addition of one SA molecule. The CCSD(T)-predicted VDEs/ADEs agree excellently with the experimental data, confirming the identified structures as the most stable ones. Two types of dimer isomers possessing different hydrogen bonding (HB) motifs are identified, corresponding to SM- binding to a zwitterionic SA (SM-●SAz) and a canonical SA (SM-●SAc), respectively. Two N-H⋯O HBs and one superior O-H⋯O HB are formed in the lowest-lying SM-●SAc, while SM-●SAz has three moderate N-H⋯O HBs, with the former being 4.71 kcal/mol more stable. Further theoretical analyses reveal that the binding strength advantage of SM-●SAc over SM-●SAz arises from its significant contributions of orbital interactions between fragments, illustrating that sulfamate strongly interacts with its parent SA acid and preferably chooses the canonical SA in the subsequent cluster formations. Given the prominent presence of SA, this study provides the first evidence that the canonical dimer model of sulfamic acid should exist as a superior configuration during cluster growth.

Wintertime Formation of Large Sulfate Particles in China and Implications for Human Health

Outdoor air pollution causes millions of premature deaths annually worldwide. Sulfate is a major component of particulate pollution. Winter sulfate observations in China show both high concentrations and an accumulation mode with a modal size >1 μm. However, we find that this observed size distribution cannot be simulated using classical gaseous and aqueous phase formation (CSF) or proposed aerosol-processing formation (APF) mechanisms. Specifically, the CSF simulation underestimates sulfate concentrations by 76% over megacities in China and predicts particle size distributions with a modal size of ∼0.35 μm, significantly smaller than observations. Although incorporating the APF mechanism in the atmospheric chemical model notably improves sulfate concentration simulation with reasonable parameters, the simulated sulfate particle size distribution remains similar to that using the CSF mechanism. We further conduct theoretical analyses and show that particles with diameters <0.3 μm grow rapidly (2-3 s) to 1 μm through the condensation of sulfuric acid in fresh high-temperature exhaust plumes, referred to as in-source formation (ISF). An ISF sulfate source equivalent to 15% of sulfur emissions from fossil fuel combustion largely explains both observed size distributions and mass concentrations of sulfate particles. The findings imply that ISF is a major source of wintertime micron-sized sulfate in China and underscore the importance of considering the size distribution of aerosols for accurately assessing the impacts of inorganic aerosols on radiative forcing and human health.

Kinetics of Sulfur Trioxide Reaction with Water Vapor to Form Atmospheric Sulfuric Acid

Although experimental methods can be used to obtain the quantitative kinetics of atmospheric reactions, experimental data are often limited to a narrow temperature range. The reaction of SO3 with water vapor is important for elucidating the formation of sulfuric acid in the atmosphere; however, the kinetics is uncertain at low temperatures. Here, we calculate rate constants for reactions of sulfur trioxide with two water molecules. We consider two mechanisms: the SO3···H2O + H2O reaction and the SO3 + (H2O)2 reaction. We find that beyond-CCSD(T) contributions to the barrier heights are very large, and multidimensional tunneling, unusually large anharmonicity of high-frequency modes, and torsional anharmonicity are important for obtaining quantitative kinetics. We find that at lower temperatures, the formation of the termolecular precursor complexes, which is often neglected, is rate-limiting compared to passage through the tight transition states. Our calculations show that the SO3···H2O + H2O mechanism is more important than the SO3 + (H2O)2 mechanism at 5-50 km altitudes. We find that the rate ratio between SO3···H2O + H2O and SO3 + (H2O)2 is greater than 20 at altitudes between 10 and 35 km, where the concentration of SO3 is very high.

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.

Computational chemistry of cluster: Understanding the mechanism of atmospheric new particle formation at the molecular level

New particle formation (NPF), which exerts significant influence over human health and global climate, has been a hot topic and rapidly expands field of research in the environmental and atmospheric chemistry recent years. Generally, NPF contains two processes: formation of critical nucleus and further growth of the nucleus. However, due to the complexity of the atmospheric nucleation, which is a multicomponent process, formation of critical clusters as well as their growth is still connected to large uncertainties. Detection limits of instruments in measuring specific gaseous aerosol precursors and chemical compositions at the molecular level call for computational studies. Computational chemistry could effectively compensate the deficiency of laboratory experiments as well as observations and predict the nucleation mechanisms. We review the present theoretical literatures that discuss nucleation mechanism of atmospheric clusters. Focus of this review is on different nucleation systems involving sulfur-containing species, nitrogen-containing species and iodine-containing species. We hope this review will provide a deep insight for the molecular interaction of nucleation precursors and reveal nucleation mechanism at the molecular level.

Probing the Electronic Structure and Bond Dissociation of SO3 and SO3- Using High-Resolution Cryogenic Photoelectron Imaging

The SO₃ molecule and its radical anion SO₃^- are important chemical species atmospherically. However, their thermodynamic properties and electronic structures are not well known experimentally. Using cryogenically cooled anions, we have obtained high-resolution photoelectron images of SO₃^- and determined accurately the electron affinity (EA) of SO₃ and the bond dissociation energy of SO₃^- → SO₂ + O^- for the first time. Because of the large geometry changes from the C_3v SO₃^- to the D_3h SO₃, there is a negligible Franck-Condon factor (FCF) for the 0-0 detachment transition, that defines the EA of SO₃. By fitting the high-resolution photoelectron spectra with computed FCFs using structures from high-level ab initio calculations, we have determined the EA of SO₃ to be 2.126(6) eV. By monitoring the appearance of the O^- signal in the photoelectron images at different photon energies, we are able to measure directly the bond dissociation energy of SO₃^-(X²A₁) → SO₂(X¹A₁) + O^-(²P) to be 4.259 ± 0.006 eV, which also allow us to derive the dissociation energy for the spin-forbidden SO₃(X¹A₁') → SO₂(X¹A₁) + O(³P) to be 3.594(6) eV. The excited states of SO₃^- are calculated using high-level ab initio calculations, which are valuable in aiding the interpretation of autodetachment processes observed at various photon energies. The current study provides valuable information about the fundamental molecular properties of SO₃, as well as the radical anion SO₃^-, which is known in redox reactions involving SO₃^2- and may also play a role in the chemistry of SO₂ in the atmosphere.

Reaction of SO3 with HONO2 and Implications for Sulfur Partitioning in the Atmosphere

Sulfur trioxide is a critical intermediate for the sulfur cycle and the formation of sulfuric acid in the atmosphere. The traditional view is that sulfur trioxide is removed by water vapor in the troposphere. However, the concentration of water vapor decreases significantly with increasing altitude, leading to longer atmospheric lifetimes of sulfur trioxide. Here, we utilize a dual-level strategy that combines transition state theory calculated at the W2X//DF-CCSD(T)-F12b/jun'-cc-pVDZ level, with variational transition state theory with small-curvature tunneling from direct dynamics calculations at the M08-HX/MG3S level. We also report the pressure-dependent rate constants calculated using the system-specific quantum Rice-Ramsperger-Kassel (SS-QRRK) theory. The present findings show that falloff effects in the SO₃ + HONO₂ reaction are pronounced below 1 bar. The SO₃ + HONO₂ reaction can be a potential removal reaction for SO₃ in the stratosphere and for HONO₂ in the troposphere, because the reaction can potentially compete well with the SO₃ + 2H₂O reaction between 25 and 35 km, as well as the OH + HONO₂ reaction. The present findings also suggest an unexpected new product from the SO₃ + HONO₂ reaction, which, although very short-lived, would have broad implications for understanding the partitioning of sulfur in the stratosphere and the potential for the SO₃ reaction with organic acids to generate organosulfates without the need for heterogeneous chemistry.

Photochemistry of HOSO2 and SO3 and Implications for the Production of Sulfuric Acid

Sulfur trioxide (SO₃) and the hydroxysulfonyl radical (HOSO₂) are two key intermediates in the production of sulfuric acid (H₂SO₄) on Earth's atmosphere, one of the major components of acid rain. Here, the photochemical properties of these species are determined by means of high-level quantum chemical methodologies, and the potential impact of their light-induced reactivity is assessed within the context of the conventional acid rain generation mechanism. Results reveal that the photodissociation of HOSO₂ occurs primarily in the stratosphere through the ejection of hydroxyl radicals (^•OH) and sulfur dioxide (SO₂). This may decrease the production rate of H₂SO₄ in atmospheric regions with low O₂ concentration. In contrast, the photostability of SO₃ under stratospheric conditions suggests that its removal efficiency, still poorly understood, is key to assess the H₂SO₄ formation in the upper atmosphere.

Unprecedented Ambient Sulfur Trioxide (SO3) Detection: Possible Formation Mechanism and Atmospheric Implications

Sulfur trioxide (SO₃) is a crucial compound for atmospheric sulfuric acid (H₂SO₄) formation, acid rain formation, and other atmospheric physicochemical processes. During the daytime, SO₃ is mainly produced from the photo-oxidation of SO₂ by OH radicals. However, the sources of SO₃ during the early morning and night, when OH radicals are scarce, are not fully understood. We report results from two field measurements in urban Beijing during winter and summer 2019, using a nitrate-CI-APi-LTOF (chemical ionization-atmospheric pressure interface-long-time-of-flight) mass spectrometer to detect atmospheric SO₃ and H₂SO₄. Our results show the level of SO₃ was higher during the winter than during the summer, with high SO₃ levels observed especially during the early morning (∼05:00 to ∼08:30) and night (∼18:00 to ∼05:00 the next day). On the basis of analysis of SO₂, NO _x , black carbon, traffic flow, and atmospheric ions, we suggest SO₃ could be formed from the catalytic oxidation of SO₂ on the surface of traffic-related black carbon. This previously unidentified SO₃ source results in significant H₂SO₄ formation in the early morning and thus promotes sub-2.5 nm particle formation. These findings will help in understanding urban SO₃ and formulating policies to mitigate secondary particle formation in Chinese megacities.

An Ab Initio Molecular Dynamics Study of the Hydrolysis Reaction of Sulfur Trioxide Catalyzed by a Formic Acid or Water Molecule

Ab initio molecular dynamics (AIMD) calculations have been performed to investigate the role of dynamical and steric effects in formic acid (FA) or H₂O-catalyzed gas phase hydrolysis of SO₃ to form sulfuric acid. This was done by colliding FA or H₂O with the SO₃-H₂O complex and the water dimer with the SO₃ molecule and analyzing the outcomes of 230 AIMD trajectories. Our calculations show that, within simulation times used, sulfuric acid is formed in 5% of FA collisions but is not produced when H₂O collides with the SO₃-H₂O complex or when the water dimer collides with the SO₃ molecule. We also find that FA collisions have about 2 times higher probability to form the prereactive complex than H₂O collisions. Moreover, our simulations show that the SO₃-H₂O-FA prereactive complex is more stable in time than the SO₃-H₂O-H₂O prereactive complex. These findings indicate that the FA-catalyzed mechanism is favored over the H₂O one when looking from the steric and dynamic effect point of view. Additionally, AIMD simulations starting from the optimized structure of the SO₃-H₂O-FA prereactive complex have been computed to qualitatively estimate the rate of the sulfuric acid formation. Collisional energy has been observed to promote sulfuric acid formation more effectively than thermal excitation.

Temperature and isotope effects in the reaction of CH3CHOO with methanol

Carbonyl oxides, also known as Criegee intermediates, are generated from ozonolysis of unsaturated hydrocarbons in the atmosphere. Alcohols are often used as a scavenger of the Criegee intermediates in laboratory studies. In this work, the reaction kinetics of CH₃CHOO with methanol vapor was investigated at various temperatures, pressures, and isotopic substitutions using time-resolved UV absorption spectroscopy. The observed rate coefficients of the reaction of anti-CH₃CHOO with methanol show a linear dependence on [CH₃OH]. The bimolecular rate coefficient was determined to be k_1Ha = (4.8 ± 0.5) × 10^-12 cm³ s^-1 at 298 K and 250 Torr with a negative activation energy E_a = -2.8 ± 0.3 kcal mol^-1 for T = 288-315 K [k(T) = A exp(-E_a/RT)]. For the reaction of syn-CH₃CHOO with methanol vapor, the observed rate coefficients show a quadratic dependence on [CH₃OH], indicating that two methanol molecules participate in the reaction. The termolecular rate coefficient was determined to be k_2Hs = (8.0 ± 1.0) × 10^-32 cm⁶ s^-1 at 298 K and 250 Torr with a strong negative temperature dependence (E_a = -13.2 ± 0.3 kcal mol^-1) at 273-323 K. No significant pressure effect was observed at 250-760 Torr. A kinetic isotope effect, k_2Hs/k_2Ds = 2.5, was observed by changing CH₃OH to CH₃OD. Quantum chemistry and transition state theory calculations suggest that the observed isotope effect is mainly attributed to the changes of the vibrational zero-point energies and partition functions while tunneling plays a very minor role. The reaction of syn-CH₃CHOO with one CH₃OH molecule was not observed in the studied concentration range.

Ab initio kinetics of the HOSO2 + 3O2 → SO3 + HO2 reaction

The detailed kinetic mechanism of the HOSO₂ + ³O₂ reaction, which plays a pivotal role in the atmospheric oxidation of SO₂, was investigated using accurate electronic structure calculations and novel statistical thermodynamic/kinetic models. Explored using the accurate composite method W1U, the detailed potential energy surface (PES) revealed that the addition of O₂ to a HOSO₂ radical to form the adduct (HOSO₄) proceeds via a transition state with a slightly positive barrier (i.e., 0.7 kcal mol^-1 at 0 K). Such a finding compromises a long-term hypothesis about this channel of being a barrierless process. Moreover, the overall reaction was found to be slightly exothermic by 1.7 kcal mol^-1 at 0 K, which is in good agreement with recent studies. On the newly-constructed PES, the temperature- and pressure-dependent behaviors of the title reaction were characterized in a wide range of conditions (T = 200-1000 K & P = 10-760 Torr) using the integrated deterministic and stochastic master equation/Rice-Ramsperger-Kassel-Marcus (ME/RRKM) rate model in which corrections for hindered internal rotation (HIR) and tunneling treatments were included. The calculated numbers were found to be in excellent agreement with literature data. The sensitivity analyses on the derived rate coefficients with respect to the ab initio input parameters (i.e., barrier height and energy transfer) were also performed to further understand the kinetic behaviors of the title reaction. The detailed kinetic mechanism, consisting of thermodynamic and kinetic data (in NASA polynomial and modified Arrhenius formats, respectively), was also provided at different T & P for further use in the modeling/simulation of any related systems.

Mechanistic investigation of the atmospheric reaction of NH2 with NO2 and study of the catalytic effects of water molecule on kinetic path

The gas-phase reaction mechanism of NO2 with NH2 on the singlet potential energy surface is investigated at the B3LYP, MP2, CCSD(T) and G3B3 levels of calculation in connection with the 6-311[Formula: see text]G(3df, 3pd) basis sets. Seven different kinds of products have been obtained. The results reveal that P7 [N2[Formula: see text]2O] is the main product in thermodynamic viewpoint with the Gibbs free energy of [Formula: see text][Formula: see text]kcal/mol. The product P2 due to less number of transition states and low level barrier height of transition states is introduced as the kinetic product. The calculation of rate constants in different temperatures confirms our predication for P7 adduct in comparison with other products. In the first step, the reactions without catalytic effects are considered. All of these reactions begin with pre-reactive collision complex CR [NO2–NH2]. This complex has been produced from barrier-less process via collision of reactants. From variation of bond lengths and angles or dissociation of this complex, other species are produced. In the second step, the catalytic effect of water and water dimer molecules on the reaction rate of kinetic path, P2, have been attended. Calculation of the rate constant was done in the temperature range from 500[Formula: see text]K to 3000[Formula: see text]K. Our results showed that the rate constant of path P2 increases with temperature and the number of water molecules. So, we can conclude that the number of water molecules corresponding to relative humidity of atmosphere has catalytic effect on the mentioned reaction rate.

An Experimental Study of the Kinetics of OH/OD(v = 1,2,3) + SO2: The Limiting High-Pressure Rate Coefficients as a Function of Temperature

The kinetics of the reaction OH/OD(v = 1,2,3) + SO₂ were studied using a photolysis/laser-induced fluorescence technique. The rate coefficients OH/OD(v = 1,2,3) + SO₂, k₁, over the temperature range of 295-810 K were used to determine the limiting high-pressure limit k₁^∞. This method is usually applicable if the reaction samples the potential well of the adduct HOSO₂ and if intramolecular vibrational relaxation is fast. In the present case, however, the rate coefficients showed an additional fast removal contribution as evidenced by the increase in k₁ with vibrational level; this behavior together with its temperature dependence is consistent with the existence of a weakly bound complex on the potential energy surface prior to adduct formation. The data were analyzed using a composite mechanism that incoporates energy-transfer mechanisms via both the adduct and the complex, and yielded a value of k₁^∞(295 K) equal to (7.2 ± 3.3) × 10^-13 cm³ molecule^-1 s^-1 (errors at 1σ), a factor of between 2 and 3 smaller than the current recommended IUPAC and JPL values of (2.0_-1.0^+2.0) and (1.6 ± 0.4) × 10^-12 cm³ molecule^-1 s^-1 at 298 K, respectively, although the error bars do overlap. k₁^∞ was observed to only depend weakly on temperature. Further evidence for a smaller k₁^∞ is presented in the companion paper.

Ammonia Catalyzed Formation of Sulfuric Acid in Troposphere: The Curious Case of a Base Promoting Acid Rain

Electronic structure calculations have been performed to investigate the role of ammonia in catalyzing the formation of sulfuric acid through hydrolysis of SO₃ in Earth's atmosphere. The uncatalyzed process involves a high activation barrier and, until date, is mainly known to occur in Earth's atmosphere only when catalyzed by water and acids. Here we show that hydrolysis of SO₃ can be very efficiently catalyzed by ammonia, the most abundant basic component in Earth's atmosphere. It was found, based on magnitude of relative potential energies as well as rate coefficients, that ammonia is the best among all the catalysts studied until now (water and acids) and could be a considerable factor in formation of sulfuric acid in troposphere. The calculated rate coefficient (at 298 K) of ammonia catalyzed reaction has been found to be ∼10⁵-10⁷ times greater than that for water catalyzed ones. It was found, based on relative rates of ammonia and water catalyzed processes, that in troposphere ammonia, together with water, could be the key factor in determining the rate of formation of sulfuric acid. In fact, ammonia could surpass water in catalyzing sulfuric acid formation via hydrolysis of SO₃ at various altitudes in troposphere depending upon their relative concentrations.

Reaction of SO2with OH in the atmosphere

We theoretically investigate the rate constantk(T,p) of the OH + SO₂reaction with experimental accuracy.

Gas-phase hydrolysis of triplet SO2: A possible direct route to atmospheric acid formation

Sulfur chemistry is of great interest to the atmospheric chemistry of several planets. In the presence of water, oxidized sulfur can lead to new particle formation, influencing climate in significant ways. Observations of sulfur compounds in planetary atmospheres when compared with model results suggest that there are missing chemical mechanisms. Here we propose a novel mechanism for the formation of sulfurous acid, which may act as a seed for new particle formation. In this proposed mechanism, the lowest triplet state of SO₂ (³B₁), which may be accessed by near-UV solar excitation of SO₂ to its excited ¹B₁ state followed by rapid intersystem crossing, reacts directly with water to form H₂SO₃ in the gas phase. For ground state SO₂, this reaction is endothermic and has a very high activation barrier; our quantum chemical calculations point to a facile reaction being possible in the triplet state of SO₂. This hygroscopic H₂SO₃ molecule may act as a condensation nucleus for water, giving rise to facile new particle formation (NPF).

Strong Negative Temperature Dependence of the Simplest Criegee Intermediate CH2OO Reaction with Water Dimer

The kinetics of the reaction of CH2OO with water vapor was measured directly with UV absorption at temperatures from 283 to 324 K. The observed CH2OO decay rate is second order with respect to the H2O concentration, indicating water dimer participates in the reaction. The rate coefficient of the CH2OO reaction with water dimer can be described by an Arrhenius expression k(T) = A exp(-Ea/RT) with an activation energy of -8.1 ± 0.6 kcal mol(-1) and k(298 K) = (7.4 ± 0.6) × 10(-12) cm(3) s(-1). Theoretical calculations yield a large negative temperature dependence consistent with the experimental results. The temperature dependence increases the effective loss rate for CH2OO by a factor of ~2.5 at 278 K and decreases by a factor of ~2 at 313 K relative to 298 K, suggesting that temperature is important for determining the impact of Criegee intermediate reactions with water in the atmosphere.

The gas-phase reaction of methane sulfonic acid with the hydroxyl radical without and with water vapor

The gas phase reaction between methane sulfonic acid (CH3SO3H; MSA) and the hydroxyl radical (HO), without and with a water molecule, was investigated with DFT-B3LYP and CCSD(T)-F12 methods. For the bare reaction we have found two reaction mechanisms, involving proton coupled electron transfer and hydrogen atom transfer processes that produce CH3SO3 and H2O. We also found a third reaction mechanism involving the double proton transfer process, where the products and reactants are identical. The computed rate constant for the oxidation process is 8.3 × 10(-15) cm(3) s(-1) molecule(-1). CH3SO3H forms two very stable complexes with water with computed binding energies of about 10 kcal mol(-1). The presence of a single water molecule makes the reaction between CH3SO3H and HO much more complex, introducing a new reaction that consists in the interchange of H2O between HO and CH3SO3H. Our kinetic calculations show that 99.5% of the reaction involves this interchange of the water molecule and, consequently, water vapor does not play any role in the oxidation reaction of methane sulfonic acid by the hydroxyl radical.

Calibration of a chemical ionization mass spectrometer for the measurement of gaseous sulfuric acid

The accurate measurement of the gaseous sulfuric acid concentration is crucial within many fields of atmospheric science. Instruments utilizing chemical ionization mass spectrometry (CIMS) measuring H(2)SO(4), therefore, require a careful calibration. We have set up a calibration source that can provide a stable and adjustable concentration of H(2)SO(4). The calibration system initiates the production of sulfuric acid through the oxidation of SO(2) by OH. The hydroxyl radical is produced by UV photolysis of water vapor. A numerical model calculates the H(2)SO(4) concentration provided at the outlet of the calibration source. From comparison of this concentration and the signals measured by CIMS, a calibration factor is derived. This factor is evaluated to be 1.1 × 10(10) cm(-3), which is in good agreement with values found in the literature for other CIMS instruments measuring H(2)SO(4). The calibration system is described in detail and the results are discussed. Because the setup is external to the CIMS instrument, it offers the possibility for future CIMS intercomparison measurements by providing defined and stable concentrations of sulfuric acid.

Impact of fuel quality regulation and speed reductions on shipping emissions: implications for climate and air quality

Atmospheric emissions of gas and particulate matter from a large ocean-going container vessel were sampled as it slowed and switched from high-sulfur to low-sulfur fuel as it transited into regulated coastal waters of California. Reduction in emission factors (EFs) of sulfur dioxide (SO₂), particulate matter, particulate sulfate and cloud condensation nuclei were substantial (≥ 90%). EFs for particulate organic matter decreased by 70%. Black carbon (BC) EFs were reduced by 41%. When the measured emission reductions, brought about by compliance with the California fuel quality regulation and participation in the vessel speed reduction (VSR) program, are placed in a broader context, warming from reductions in the indirect effect of SO₄ would dominate any radiative changes due to the emissions changes. Within regulated waters absolute emission reductions exceed 88% for almost all measured gas and particle phase species. The analysis presented provides direct estimations of the emissions reductions that can be realized by California fuel quality regulation and VSR program, in addition to providing new information relevant to potential health and climate impact of reduced fuel sulfur content, fuel quality and vessel speed reductions.

Microwave plasma removal of sulphur hexafluoride

Sulphur hexafluoride (SF(6)) gas is a common pollutant emitted during the plasma etching of thin films and plasma cleaning chemical vapor deposition (CVD) production processes used in the semiconductor industry. In this paper a method using microwave (2.45GHz frequency) plasmas sustained at atmospheric pressure for the abatement of SF(6) is investigated experimentally for various gas mixture constituents and operating conditions, with respect to its ability to decompose SF(6) to less harmful molecules. The destruction and removal efficiencies (DRE) of plasma abatement of SF(6) at concentrations between 1.7 and 5% in nitrogen in the presence of water vapor were studied as a function of the total gas flow rate and microwave power. Water vapor proved to be an effective source of free radical species that reacts with the radicals and ions resulting from SF(6) fragmentation in the plasma and also, it proved to reduce the process by-products. It was measured that approximately 25% of the initial SF(6) is converted to SO(2). Destruction and removal efficiencies of SF(6) up to 99.9% have been achieved.

Accurate computational determination of the binding energy of the SO3∙H2O complex

Reliable thermochemical data for the reaction SO3 + H2O<-->SO3 x H2O (1a) are of crucial importance for an adequate modeling of the homogeneous H2SO4 formation in the atmosphere. We report on high-level quantum chemical calculations to predict the binding energy of the SO3 x H2O complex. The electronic binding energy is accurately computed to De = 40.9+/-1.0 kJ/mol = 9.8+/-0.2 kcal/mol. By using harmonic frequencies from density functional theory calculations (B3LYP/cc-pVTZ and TPSS/def2-TZVP), zero-point and thermal energies were calculated. From these data, we estimate D0 = -Delta H(1a)0(0 K) = 7.7+/-0.5 kcal/mol and Delta H(1a)0(298 K) = -8.3+/-1.0 kcal/mol.

Mass Accommodation of H2SO4 and CH3SO3H on Water−Sulfuric Acid Solutions from 6% to 97% RH

The uptake of H2SO4 and CH3SO3H onto particles composed of water and sulfuric acid was studied in a laminar flow reactor at atmospheric pressure. Their first-order gas-phase loss rate coefficients were determined using a chemical ionization mass spectrometer. Relative humidity was varied from 6% to 97% at 295-297.5 K. The mass accommodation coefficient, alpha, was found to be close to unity for both species. These findings show that alpha does not limit particle growth rates resulting from H2SO4 and CH3SO3H uptake. Diffusion coefficients in N2 for these two species are also reported and a significant dependence upon relative humidity was seen for H2SO4 but not for CH3SO3H. Last, production of small particles was observed due to the presence of SO2 in particle chargers. Formation of these particles can be significantly reduced by adding an OH scavenger such as propane.

Rapid Formation of Sulfuric Acid Particles at Near-Atmospheric Conditions

We investigated the formation of new particles in a laboratory study, starting from H2SO4 produced in situ through the reaction of OH radicals with SO2. Newly formed particles were observed for H2SO4 concentrations above 7 x 10(6) per cubic centimeter. At 293 kelvin, a rough estimate yielded a nucleation rate of 0.3 to 0.4 particles per cubic centimeter per second for approximately 10(7) particles per cubic centimeter of H2SO4 (particle size >/= 3 nanometers). These findings are in agreement with observations from the atmosphere. The results demonstrate that under laboratory conditions similar to the atmosphere, particle formation occurs at atmospheric H2SO4 concentration levels.