Determination of the High-Pressure Limiting Rate Coefficient and the Enthalpy of Reaction for OH + SO2

Probing the dynamics and bottleneck of the key atmospheric SO2 oxidation reaction by the hydroxyl radical

SO2 (Sulfur dioxide) is the major precursor to the production of sulfuric acid (H2SO4), contributing to acid rain and atmospheric aerosols. Sulfuric acid formed from SO2 generates light-reflecting sulfate aerosol particles in the atmosphere. This property has prompted recent geoengineering proposals to inject sulfuric acid or its precursors into the Earth's atmosphere to increase the planetary albedo to counteract global warming. SO2 oxidation in the atmosphere by the hydroxyl radical HO to form HOSO2 is a key rate-limiting step in the mechanism for forming acid rain. However, the dynamics of the HO + SO2 → HOSO2 reaction and its slow rate in the atmosphere are poorly understood to date. Herein, we use photoelectron spectroscopy of cryogenically cooled HOSO2- anion to access the neutral HOSO2 radical near the transition state of the HO + SO2 reaction. Spectroscopic and dynamic calculations are conducted on the first ab initio-based full-dimensional potential energy surface to interpret the photoelectron spectra of HOSO2- and to probe the dynamics of the HO + SO2 reaction. In addition to the finding of a unique pre-reaction complex (HO⋯SO2) directly connected to the transition state, dynamic calculations reveal that the accessible phase space for the HO + SO2 → HOSO2 reaction is extremely narrow, forming a key reaction bottleneck and slowing the reaction rate in the atmosphere, despite the low reaction barrier. This study underlines the importance of understanding the full multidimensional potential energy surface to elucidate the dynamics of complex bimolecular reactions involving polyatomic reactants.

HO2 + NO2: Kinetics, Thermochemistry, and Evidence for a Bimolecular Product Channel

A master equation (ME) analysis of available experimental data has been carried out on the reaction HO₂ + NO₂ + M ⇋ HO₂NO₂ + M (1a)/(-1a). The analysis, based on the ME code MESMER, uses both the association and dissociation kinetic data from the literature, and provides improved thermochemistry on reaction 1a. Our preferred model assigns two low-frequency vibrations of HO₂NO₂ as hindered rotors and couples these to the external rotations. This model gives Δ_rH°₀(1a) = -93.9 ± 1.0 kJ mol^-1, which implies that Δ_fH°₀ HO₂NO₂ = -42.0 ± 1.0 kJ mol^-1 (uncertainties are 2σ). A significant contributor to the uncertainty derives from modeling the interaction between the internal and external rotors. Using this improved kinetics for reaction 1a/-1a, data at elevated temperatures, 353-423 K, which show no evidence of the expected equilibration, have been reanalyzed, indicating that an additional reaction is occurring that masks the equilibration. Based on a published ab initio study, this additional channel is assigned to the bimolecular reaction HO₂ + NO₂ → H-NO₂ + O₂ (1b); H-NO₂ is nitryl hydride and has not previously been directly observed in experiments. The output of the master equation analysis has been parametrized and Troe expressions are provided for an improved description of k_1a(p,T) and k_-1a(p,T).

Association of Cl with C2H2 by unified variable-reaction-coordinate and reaction-path variational transition-state theory

Barrierless unimolecular association reactions are prominent in atmospheric and combustion mechanisms but are challenging for both experiment and kinetics theory. A key datum for understanding the pressure dependence of association and dissociation reactions is the high-pressure limit, but this is often available experimentally only by extrapolation. Here we calculate the high-pressure limit for the addition of a chlorine atom to acetylene molecule (Cl + C₂H₂→C₂H₂Cl). This reaction has outer and inner transition states in series; the outer transition state is barrierless, and it is necessary to use different theoretical frameworks to treat the two kinds of transition state. Here we study the reaction in the high-pressure limit using multifaceted variable-reaction-coordinate variational transition-state theory (VRC-VTST) at the outer transition state and reaction-path variational transition state theory (RP-VTST) at the inner turning point; then we combine the results with the canonical unified statistical (CUS) theory. The calculations are based on a density functional validated against the W3X-L method, which is based on coupled cluster theory with single, double, and triple excitations and a quasiperturbative treatment of connected quadruple excitations [CCSDT(Q)], and the computed rate constants are in good agreement with some of the experimental results. The chlorovinyl (C₂H₂Cl) adduct has two isomers that are equilibrium structures of a double-well C≡C-H bending potential. Two procedures are used to calculate the vibrational partition function of chlorovinyl; one treats the two isomers separately and the other solves the anharmonic energy levels of the double well. We use these results to calculate the standard-state free energy and equilibrium constant of the reaction.

Chemical characteristics, sources, and formation mechanisms of PM2.5 before and during the Spring Festival in a coastal city in Southeast China

Severe pollution caused by atmospheric particulate matter (PM) has become a global environmental issue. Samples of atmospheric PM were collected before and during the Chinese Spring Festival in Xiamen, a coastal city in Southeast China, to investigate their chemical characteristics, sources, and formation mechanisms. The results indicated that PM_2.5 mass concentrations comprised 53.60% and 56.31% of total suspended particulates before and during the Spring Festival, respectively. Due to the halt of factory production and construction and the reduction of vehicle flow during the Spring Festival, the concentrations of organic carbon, elemental carbon and water soluble ions in PM_2.5 decreased by 78.56%, 84.19% and 27.53%, respectively, compared with those before the Spring Festival. However, the concentrations of K⁺, Mg²⁺, Al, Sr, and Ba increased by 3121.76%, 571.67%, 183.71%, 180.15%, and 137.58%, respectively, resulting from the display of fireworks and firecrackers during the Spring Festival. Analysis of backward air mass trajectory indicated that the concentrations of PM_2.5 and its components were dominated by local pollution sources before and during the Spring Festival. The relationships between meteorological conditions and pollutant concentrations showed that the secondary organic aerosol was generated from the heterogeneous reaction before the Spring Festival, and the secondary inorganic aerosol was formed by the photochemical reaction during the Spring Festival.

Seasonal variation, formation mechanisms and potential sources of PM2.5 in two typical cities in the Central Plains Urban Agglomeration, China

In order to probe the seasonal variation, formation mechanisms as well as geographical origins of fine particles and its chemical components in two cities (Zhengzhou, ZZ and Xinxiang, XX) in Central Plains Urban Agglomeration, daily PM_2.5 aerosol samples were collected for four consecutive seasons during 2017-2018. The annual average concentrations of PM_2.5 (particulate matter with an aerodynamic diameter smaller than 2.5 μm) were calculated at 70.5 ± 50.8 and 69.0 ± 46.3 μg m^-3 at ZZ and XX, respectively. Daily ambient PM_2.5 concentrations ranged from 18.2 to 303.0 μg m^-3, among which >81% of the total sampling days exceeded the National Ambient Air Quality Standard of China (NAAQS, 35 μg m^-3 as an annual average). Additionally, concentrations of PM_2.5 and its major chemical components were seasonally dependent, usually with the highest mass concentration in winter. Compared with previous studies, higher NO₃^-/SO₄^2- were observed in this study depicted that air pollution caused by motor vehicle exhaust cannot be ignored. OC concentration was higher at ZZ than XX during sampling campaign likely partially caused by larger number of motor vehicles, chemical pesticide and solvent used in ZZ. Both homogeneous and heterogeneous reactions played an important role in the formation of nitrate, while heterogeneous reactions dominated the formation of sulfate. We also found a faster increase in nitrate than in sulfate during the evolution of haze. The characteristics of long-range transportation of PM_2.5 and its major chemical components and gaseous precursors were observed at both sites through back trajectories and WPSCF analysis, suggesting the complexity of air pollution and the multi-influence among cities.

Catalytic conversions of atmospheric sulfur dioxide and formation of acid rain over mineral dusts: Molecular oxygen as the oxygen source

Sulfur dioxide (SO₂) ranks as a major air pollutant and is likely to generate acid rain. When molecular oxygen is the oxygen source, the regular surfaces of gibbsite (one of the most abundant mineral dusts) show no reactivity for SO₂ conversions to H₂SO₄, while the partially dehydrated (100) surface with coordination-unsaturated Al sites becomes catalytically effective. Because of the easy availability of molecular oxygen, results manifest that acid rain can form under all atmospheric conditions and may account for the high conversion ratio of atmospheric SO₂. The (100) and (001) surfaces show divergent catalytic effects, and hydrolysis is always the rate-limiting step. Path A (hydrolysis and then oxidation) is preferred for (100) surface, whereas a third path with obviously lower activation barriers is presented for (001) surface, which is non-existent for (100) surface. Atomic oxygen originating from the dissociation of molecular oxygen is catalytically active for (100) surface, while the active site of (001) surface fails to be recovered, suggesting that SO₂ conversions over gibbsite surfaces are facet-controlled. This work also offers an environmentally friendly route for production of H₂SO₄ (one of the essential compounds in chemical industry), directly using molecular oxygen as the oxygen source.

Exploring the features on the OH + SO2 potential energy surface using theory and testing its accuracy by comparison to experimental data

Ab initio theory has been used to identify the pre-reaction complex in the atmospherically important reaction between OH + SO2, (R1), where the binding energy of the pre-reaction complex was determined to be 7.2 kJ mol-1. Using reaction rate theory, implemented with the master equation package MESMER, the effects of this complex on the kinetics of R1 at temperatures above 250 K have been investigated. From simulations and fitting to the experimental kinetic data, it is clear that the influence of this pre-reaction complex is negligible and that the kinetics are controlled by the inner transition-state that leads to the product, HOSO2. While the effect of this complex on the thermal kinetics is small it potentially provides an efficient route to remove energy from vibrationally excited OH. The fitting to the past experimental data reveals that this inner transition-state is submerged with a barrier -0.25 kJ mol-1 below the entrance channel, which is outside the range predicted from the best theoretical calculations. The data fitting also yielded ΔR1H0K equal to -(109 ± 5.6) kJ mol-11 and a more precise expression for k∞1(T), (5.95 ± 0.83) × 10-13 × (T/298)-0.11±0.27.

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.

Characteristics and formation of typical winter haze in Handan, one of the most polluted cities in China

Handan, a city within the North China Plain (NCP) region, is a typical city influenced by regional particulate matter (PM) pollution. One-year hourly semi-continuous observation was carried out in 2015 in Handan with the aim of identifying the chemical composition and variations in PM_2.5. Moreover, the concentration of aerosol precursors, meteorological factors, and secondary transformations are considered. The results demonstrate that the annual average PM_2.5 concentration in Handan is 122.35μgm^-3, approximately 3.5 times higher than the Chinese National Ambient Air Quality Standard (NAAQS) (35μgm^-3), and only 12days were below the guideline. As expected, PM concentrations are highest in winter, especially in December. In addition, we measure the concentrations of five species commonly found in PM, nitrate, sulfate, ammonium, inorganic carbon, and organic carbon. Of these, nitrate and sulfate account for the largest proportion of PM_2.5; during periods when the PM_2.5 concentration was below 400μgm^-3, nitrate dominates, while above this concentration, sulfate dominate. This is likely related to the nitrogen and sulfur oxidation ratios, which are in turn, especially the sulfur oxidation ratio, driven by high relative humidity (>60%). In addition, haze events are driven by other meteorological conditions, wind speed and direction, where low wind speeds from the south and southwest enable pollutant accumulation, which are infrequently interspersed with brief periods with high wind speeds that promote pollutant dispersal. Even though Handan is among the ten most polluted cities in China with regard to air pollution, few studies beyond model simulations have analyzed air pollutant concentrations in this city. Therefore, this study makes a significant contribution to understanding air pollution in Handan, which can further be used to improve our understanding of regional pollution in the highly populated North China Plain. These results have implications for the creation of policies and legislation, as well as other pollution control measures.

Photodegradation of methyl thioglycolate particles as a proxy for organosulphur containing droplets

Photochemical generation of elemental sulphur and sulphate at the gas–liquid interface by heterogeneous interaction with gaseous O₂and H₂O.

Formation of a pre-reaction hydrogen-bonded complex and its significance in the potential energy surface of the OH + SO2→ HOSO2 reaction: A computational study

Using computational calculations, we have revisited the potential energy surface (PES) of the reaction between OH and SO2, which is believed as the rate-limiting step in the atmospheric formation of H2SO4. In this work, we report for the first time the presence of a pre-reaction hydrogen-bonded complex between OH and SO2 in the reaction PES. Based on this finding, it has been shown that the reaction can be considered as a two-step process in which the first step is the formation of the pre-reaction complex and the second step is the transformation of this complex to the product. It was observed that due to the presence of this pre-reaction complex as a potential well in the reaction PES, the barrier height got increased by around two-fold for the second step. Based on this observation, it has been proposed that the kinetics of the reaction is going to be affected. Also based on the analysis of the geometries of this pre-reaction complex and the transition state, it has been argued that the step involving the transformation of this pre-reaction complex to the product via the transition state is going to be the slowest step as this transformation involves large structural changes of the stationary points involved.

An Experimental and Master Equation Study of the Kinetics of OH/OD + SO2: The Limiting High-Pressure Rate Coefficients

The kinetics of the reaction OH/OD + SO₂ were studied using a laser flash photolysis/laser-induced fluorescence technique. Evidence for two-photon photolysis of SO₂ at 248 nm is presented and quantified, and which appears to have been evident to some extent in most previous photolysis studies, potentially leading to values for the rate coefficient k₁ that are too large. The kinetics of the reaction OH(v = 0) + SO₂ (T = 295 K, p = 25-300 torr) were measured under conditions where SO₂ photolysis was taken into account. These results, together with literature data, were modeled using a master equation analysis. This analysis highlighted problems with the literature data: the rate coefficients derived from flash photolysis data were generally too high and from the flow tube data too low. Our best estimate of the high-pressure limiting rate coefficient k₁^∞ was obtained from selected data and gives a value of (7.8 ± 2.2) × 10^-13 cm³ molecule^-1 s^-1, which is lower than that recommended in the literature. A parametrized form of k₁([N₂],T) is provided. The OD(v = 0) + SO₂ (T = 295 K, p = 25-300 torr) data are reported for the first time, and master equation analysis reinforces our assignment of k₁^∞.

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.

Reaction of SO2with OH in the atmosphere

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

Reactive nitrogen chemistry in aerosol water as a source of sulfate during haze events in China

Fine-particle pollution associated with winter haze threatens the health of more than 400 million people in the North China Plain. Sulfate is a major component of fine haze particles. Record sulfate concentrations of up to ~300 μg m^-3 were observed during the January 2013 winter haze event in Beijing. State-of-the-art air quality models that rely on sulfate production mechanisms requiring photochemical oxidants cannot predict these high levels because of the weak photochemistry activity during haze events. We find that the missing source of sulfate and particulate matter can be explained by reactive nitrogen chemistry in aerosol water. The aerosol water serves as a reactor, where the alkaline aerosol components trap SO₂, which is oxidized by NO₂ to form sulfate, whereby high reaction rates are sustained by the high neutralizing capacity of the atmosphere in northern China. This mechanism is self-amplifying because higher aerosol mass concentration corresponds to higher aerosol water content, leading to faster sulfate production and more severe haze pollution.

Aircraft engine exhaust emissions and other airport-related contributions to ambient air pollution: A review

Civil aviation is fast-growing (about +5% every year), mainly driven by the developing economies and globalisation. Its impact on the environment is heavily debated, particularly in relation to climate forcing attributed to emissions at cruising altitudes and the noise and the deterioration of air quality at ground-level due to airport operations. This latter environmental issue is of particular interest to the scientific community and policymakers, especially in relation to the breach of limit and target values for many air pollutants, mainly nitrogen oxides and particulate matter, near the busiest airports and the resulting consequences for public health. Despite the increased attention given to aircraft emissions at ground-level and air pollution in the vicinity of airports, many research gaps remain. Sources relevant to air quality include not only engine exhaust and non-exhaust emissions from aircraft, but also emissions from the units providing power to the aircraft on the ground, the traffic due to the airport ground service, maintenance work, heating facilities, fugitive vapours from refuelling operations, kitchens and restaurants for passengers and operators, intermodal transportation systems, and road traffic for transporting people and goods in and out to the airport. Many of these sources have received inadequate attention, despite their high potential for impact on air quality. This review aims to summarise the state-of-the-art research on aircraft and airport emissions and attempts to synthesise the results of studies that have addressed this issue. It also aims to describe the key characteristics of pollution, the impacts upon global and local air quality and to address the future potential of research by highlighting research needs.

Yield of HO2 Radicals in the OH-Initiated Oxidation of SO2

The HO2 radical yield in the OH initiated oxidation of SO2 has been determined by direct observation of HO2 concentration time profiles following the 248 nm photolysis of H2O2/SO2/O2 mixtures. Initial OH radical concentrations have been deduced from a fit of the absolute HO2 concentration time profiles after 248 nm photolysis of H2O2 in the absence of SO2, an increase in the HO2 concentration upon addition of SO2 to this reaction mixture is observed and can be explained by a decrease of HO x -radical losses due to a faster decay of OH radicals in the presence of SO2. Simulations of theses profiles using recommended rate constants in a simple model are in agreement with an HO2-yield of 1.0 ± 0.1 from the OH initiated oxidation of SO2.

Transport impacts on atmosphere and climate: Aviation

Aviation alters the composition of the atmosphere globally and can thus drive climate change and ozone depletion. The last major international assessment of these impacts was made by the Intergovernmental Panel on Climate Change (IPCC) in 1999. Here, a comprehensive updated assessment of aviation is provided. Scientific advances since the 1999 assessment have reduced key uncertainties, sharpening the quantitative evaluation, yet the basic conclusions remain the same. The climate impact of aviation is driven by long-term impacts from CO₂ emissions and shorter-term impacts from non-CO₂ emissions and effects, which include the emissions of water vapour, particles and nitrogen oxides (NO _x ). The present-day radiative forcing from aviation (2005) is estimated to be 55 mW m^-2 (excluding cirrus cloud enhancement), which represents some 3.5% (range 1.3-10%, 90% likelihood range) of current anthropogenic forcing, or 78 mW m^-2 including cirrus cloud enhancement, representing 4.9% of current forcing (range 2-14%, 90% likelihood range). According to two SRES-compatible scenarios, future forcings may increase by factors of 3-4 over 2000 levels, in 2050. The effects of aviation emissions of CO₂ on global mean surface temperature last for many hundreds of years (in common with other sources), whilst its non-CO₂ effects on temperature last for decades. Much progress has been made in the last ten years on characterizing emissions, although major uncertainties remain over the nature of particles. Emissions of NO _x result in production of ozone, a climate warming gas, and the reduction of ambient methane (a cooling effect) although the overall balance is warming, based upon current understanding. These NO _x emissions from current subsonic aviation do not appear to deplete stratospheric ozone. Despite the progress made on modelling aviation's impacts on tropospheric chemistry, there remains a significant spread in model results. The knowledge of aviation's impacts on cloudiness has also improved: a limited number of studies have demonstrated an increase in cirrus cloud attributable to aviation although the magnitude varies: however, these trend analyses may be impacted by satellite artefacts. The effect of aviation particles on clouds (with and without contrails) may give rise to either a positive forcing or a negative forcing: the modelling and the underlying processes are highly uncertain, although the overall effect of contrails and enhanced cloudiness is considered to be a positive forcing and could be substantial, compared with other effects. The debate over quantification of aviation impacts has also progressed towards studying potential mitigation and the technological and atmospheric tradeoffs. Current studies are still relatively immature and more work is required to determine optimal technological development paths, which is an aspect that atmospheric science has much to contribute. In terms of alternative fuels, liquid hydrogen represents a possibility and may reduce some of aviation's impacts on climate if the fuel is produced in a carbon-neutral way: such fuel is unlikely to be utilized until a 'hydrogen economy' develops. The introduction of biofuels as a means of reducing CO₂ impacts represents a future possibility. However, even over and above land-use concerns and greenhouse gas budget issues, aviation fuels require strict adherence to safety standards and thus require extra processing compared with biofuels destined for other sectors, where the uptake of such fuel may be more beneficial in the first instance.

H Atom Yields from the Reactions of CN Radicals with C2H2, C2H4, C3H6, trans-2-C4H8, and iso-C4H8

The kinetics and H atom channel yield at both 298 and 195 K have been determined for reactions of CN radicals with C2H2 (1.00+/-0.21, 0.97+/-0.20), C2H4 (0.96+/-0.032, 1.04+/-0.042), C3H6 (pressure dependent), iso-C4H8 (pressure dependent), and trans-2-C4H8 (0.039+/-0.019, 0.029+/-0.047) where the first figure in each bracket is the H atom yield at 298 K and the second is that at 195 K. The kinetics of all reactions were studied by monitoring both CN decay and H atom growth by laser-induced fluorescence at 357.7 and 121.6 nm, respectively. The results are in good agreement with previous studies where available. The rate coefficients for the reaction of CN with trans-2-butene and iso-butene have been measured at 298 and 195 K for the first time, and the rate coefficients are as follows: k298K=(2.93+/-0.23)x10(-10) cm3 molecule(-1) s(-1), k195K=(3.58+/-0.43)x10(-10) cm3 molecule(-1) s(-1) and k298K=(3.17+/-0.10)x10(-10) cm3 molecule(-1) s(-1), k195K=(4.32+/-0.35)x10(-10) cm3 molecule(-1) s(-1), respectively, where the errors represent a combination of statistical uncertainty (2sigma) and an estimate of possible systematic errors. A potential energy surface for the CN+C3H6 reaction has been constructed using G3X//UB3LYP electronic structure calculations identifying a number of reaction channels leading to either H, CH3, or HCN elimination following the formation of initial addition complexes. Results from the potential energy surface calculations have been used to run master equation calculations with the ratio of primary:secondary addition, the average amount of downward energy transferred in a collision DeltaEd, and the difference in barrier heights between H atom elimination and an H atom 1, 2 migration as variable parameters. Excellent agreement is obtained with the experimental 298 K H atom yields with the following parameter values: secondary addition complex formation equal to 80%, DeltaEd=145 cm(-1), and the barrier height for H atom elimination set 5 kJ mol(-1) lower than the barrier for migration. Finally, very low temperature master equation simulations using the best fit parameters have been carried out in an increased precision environment utilizing quad-double and double-double arithmetic to predict H and CH3 yields for the CN+C3H6 reaction at temperatures and pressures relevant to Titan. The H and CH3 yields predicted by the master equation have been parametrized in a simple equation for use in modeling.

Experimental and Master Equation Study of the Kinetics of OH + C2H2: Temperature Dependence of the Limiting High Pressure and Pressure Dependent Rate Coefficients

The kinetics of the reaction OH + C2H2 have been studied using laser flash photolysis at 248 nm to generate OH radicals and laser-induced fluorescence to monitor OH removal. An attempt was made to use the rate coefficients OH (v = 1,2) + C2H2 to determine the limiting high-pressure rate coefficient, k(1a)(infinity), over the temperature range of 195-823 K. This method is usually applicable if the reaction samples the potential energy well of the adduct, HOC2H2, and if intramolecular vibrational relaxation is fast. In the present case, however, the rate coefficients for loss of the vibrationally excited states by reaction with C2H2 also contain a substantial contribution from nonreactive vibrational relaxation, which occurs via a mechanism that does not sample the adduct potential energy well but involves, at least at low temperatures, collisions that access a shallower, longer range van der Waals well. The data were analyzed using a composite mechanism that incorporates both reactive and nonreactive energy transfer mechanisms, which allows the determination of k(1a)(infinity)(T) for OH + C2H2 with satisfactory accuracy over the temperature range 195-823 K. The kinetics of the reaction OH (v = 0) + C2H2 were also studied in He over the range of conditions: 210-373 K and 5-760 Torr. A one-dimensional master equation (ME) analysis of the experimental data provided a further determination of k(1a)(infinity)(T) and also <DeltaE>(down) for He. Combining the two sets of results gives a consistent dataset for k(1a)(infinity) and the Arrhenius parameters A1ainfinity = 7.3 x 10(-12) cm(3) molecule(-1) s(-1) and E(1a)(infinity) = 5.3 kJ mol(-1), with <DeltaE>(down) = 150(T/300 K) cm(-1). Additional experiments were conducted at room temperature in N(2) and SF(6) by laser flash photolysis with cavity ring down spectroscopy, and ME calculations were then optimized for the pressure falloff in N(2) by varying the average downward energy transfer parameter (<DeltaE>(down)). The output from the best fit ME was parametrized using a modified Troe expression to provide rate data for use in atmospheric modeling.

Theoretical Studies of the Reaction Channels on the SO2/OH/NO Singlet Potential Energy Surface

Ab initio MP2/6-311++G(2d,2p) investigation of the SO2/OH/NO singlet potential energy surface (PES) has been performed with the aim to localize and describe the existing minima and transition states linking them. The systematic studies have revealed seven minima, with the trans-HONO-SO2 complex (1t) being the global minimum. Eight transition states between minima or between minima and the relevant reactant species have been described. Several available izomerization and dissociation routes have been identified and discussed. The most favorable association of HOSO2 and NO was found to be a barrierless process forming nitrososulfonic acids. Isomerizations between trans-, cis-, and gauche- nitrososulfonic acids (2t, 2c, and 2g) are possible with low-energy barriers. The HOSO2 and NO species can also react via another channels involving high-energy transition states to produce the HOSO-NO2 (3) and HNO-SO3 (4) complexes.

Reactions of SO3 with the O/H radical pool under combustion conditions

The reactions of SO3 with H, O, and OH radicals have been investigated by ab initio calculations. For the SO3 + H reaction (1), the lowest energy pathway involves initial formation of HSO3 and rearrangement to HOSO2, followed by dissociation to OH + SO2. The reaction is fast, with k(1) = 8.4 x 10(9)T(1.22) exp(-13.9 kJ mol(-1)/RT) cm(3) mol(-1) s(-1) (700-2000 K). The SO3 + O --> SO2 + O2 reaction (2) may proceed on both the triplet and singlet surfaces, but due to a high barrier the reaction is predicted to be slow. The rate constant can be described as k(2) = 2.8 x 10(4)T(2.57) exp(-122.3 kJ mol(-1)/RT) cm(3) mol(-1) s(-1) for T > 1000 K. The SO3 + OH reaction to form SO2 + HO2 (3) proceeds by direct abstraction but is comparatively slow, with k(3) = 4.8 x 10(4)T(2.46) exp(-114.1 kJ mol(-) 1/RT) cm(3) mol(-1) s(-1) (800-2000 K). The revised rate constants and detailed reaction mechanism are consistent with experimental data from batch reactors, flow reactors, and laminar flames on oxidation of SO2 to SO3. The SO3 + O reaction is found to be insignificant during most conditions of interest; even in lean flames, SO3 + H is the major consumption reaction for SO3.

The relaxation of OH (v=1) and OD (v=1) by H2O and D2O at temperatures from 251 to 390 K

We report rate coefficients for the relaxation of OH(v=1) and OD(v=1) by H2O and D2O as a function of temperature between 251 and 390 K. All four rate coefficients exhibit a negative dependence on temperature. In Arrhenius form, the rate coefficients for relaxation (in units of 10(-12) cm3 molecule-1 s-1) can be expressed as: for OH(v=1)+H2O between 263 and 390 K: k=(2.4+/-0.9) exp((460+/-115)/T); for OH(v=1)+D2O between 256 and 371 K: k=(0.49+/-0.16) exp((610+/-90)/T); for OD(v=1)+H2O between 251 and 371 K: k=(0.92+/-0.16) exp((485+/-48)/T); for OD(v=1)+D2O between 253 and 366 K: k=(2.57+/-0.09) exp((342+/-10)/T). Rate coefficients at (297+/-1 K) are also reported for the relaxation of OH(v=2) by D2O and the relaxation of OD(v=2) by H2O and D2O. The results are discussed in terms of a mechanism involving the formation of hydrogen-bonded complexes in which intramolecular vibrational energy redistribution can occur at rates competitive with re-dissociation to the initial collision partners in their original vibrational states. New ab initio calculations on the H2O-HO system have been performed which, inter alia, yield vibrational frequencies for all four complexes: H2O-HO, D2O-HO, H2O-DO and D2O-DO. These data are then employed, adapting a formalism due to Troe (J. Troe, J. Chem. Phys., 1977, 66, 4758), in order to estimate the rates of intramolecular energy transfer from the OH (OD) vibration to other modes in the complexes in order to explain the measured relaxation rates-assuming that relaxation proceeds via the hydrogen-bonded complexes.

Thermal Dissociation of SO3 at 1000−1400 K

The thermal dissociation of SO3 has been studied for the first time in the 1000-1400 K range. The experiments were conducted in a laminar flow reactor at atmospheric pressure, with nitrogen as the bath gas. On the basis of the flow reactor data, a rate constant for SO3 + N2 --> SO2 + O + N2 (R1b) of 5.7 x 10(17) exp(-40000/T) cm3/(mol s) is derived for the temperature range 1273-1348 K. The estimated uncertainty is a factor of 2. The rate constant corresponds to a value of the reverse reaction of k1 approximately 1.8 x 10(15) cm6 mol(-2) s(-1). The reaction is in the fall-off region under the investigated conditions. The temperature and pressure dependence of SO2 + O (+N2) was estimated from the extrapolation of low temperature results for the reaction, together with an estimated broadening parameter and the high-pressure limit determined recently by Naidoo, Goumri, and Marshall (Proc. Combust. Inst. 2005, 30, 1219-1225). The theoretical rate constant is in good agreement with the experimental results. The improved accuracy in k(1) allows a reassessment of the rate constant for SO3 + O --> SO2 + O2 (R2) based on the data of Smith, Tseregounis, and Wang (Int. J. Chem. Kinet. 1982, 14, 679-697), who conducted experiments on a low-pressure CO/O2/Ar flame doped with SO2. At the location in the flame where the net SO3 formation rate is zero, k2 = k1[SO2][M]/[SO3]. A value of 6.9 x 10(10) cm3 mol(-1) s(-1) is obtained for k2 at 1269 K with an uncertainty a factor of 3. A recommended rate constant k2 = 7.8 x 10(11) exp(-3065/T) cm3 mol(-1) s(-1) is consistent with other flame results as well as the present flow reactor data.

Combined Experimental and Master Equation Investigation of the Multiwell Reaction H + SO2

The temperature and pressure dependence of the rate coefficient for the reaction H + SO2 has been measured using a laser flash photolysis/laser-induced fluorescence technique, for 295 <or= T/K <or= 423 and for 3 <or= [He]/10(18) molecules cm(-3) <or= 23. Under these conditions, the reaction occurs exclusively to form HSO2. These data have been fitted to, and used to refine, a master equation (ME) model of the H + SO2 reaction system that includes the isomers HSO2 and HOSO and the high-temperature products OH + SO. The potential energy for the system is based on literature ab initio calculations. The three eigenvalues of lowest magnitude, obtained from solution of the ME, are the chemically significant eigenvalues that are related to the pressure and temperature dependent rate coefficients for the chemical system. The eigenvalues are generally well separated from each other and from the eigenvalues relating to collisional relaxation, except at high temperatures. The rate coefficients were extracted from the eigenpairs of the ME solution, using an analysis developed by Klippenstein and Miller (J. Phys. Chem. A 2002, 106, 9267) and an adaptation of an earlier analysis by Bartis and Widom (J. Chem. Phys. 1974, 60, 3474). The relationship between the temperature dependence of the chemically significant eigenvalues and combinations of rate coefficients was elucidated via an approximate analysis of the chemical rate equations and it was demonstrated that each eigenvalue relates primarily to passage over a single transition state. Some problems were encountered in equating ratios of rate coefficients for forward and reverse reactions to the equilibrium constant. This problem arises from the nonconservative nature of the reaction system; it was resolved by imposing orthogonality on the coupled rate equations of the chemical system, giving good agreement between rate coefficient ratios and equilibrium constants. The temperature and pressure dependences of the rate coefficients were parametrized according to Troe and modified Troe expressions. The key processes, from a combustion perspective, are the formation and dissociation of HOSO and the formation of OH + SO. Except at pressures >10(3) atm, the latter proceeds directly from H + SO2, via the energized states of HOSO. The derived rate coefficients rely heavily on measurements of the reverse reaction, OH + SO, which has only been determined at temperatures up to 700 K.

Determination of the temperature and pressure dependence of the reaction OH + C2H4from 200–400 K using experimental and master equation analyses

The pressure and temperature dependence for the reaction of OH + C(2)H(4) was studied over the range of conditions: 200-400 K and 5-600 Torr by laser flash photolysis, laser-induced fluorescence (FP-LIF). Additional experiments were conducted at room temperature by laser flash photolysis, cavity ring-down spectroscopy to facilitate determination of the high pressure limit. One-dimensional master equation calculations were conducted to test the temperature and pressure dependence of the reaction in He and in N(2). The energetics of the reaction and geometries of intermediate species were calculated by ab initio calculations (DFT-BH&HLYP/6-311+G(3df,2p) and CBS-APNO level along DFT-IRC, respectively. An investigation into the importance of a pre-reaction van der Waals complex on the kinetics over the pressure range of the troposphere is discussed. The high pressure rate coefficient was extracted by fitting the master equation calculations to the data and yields k(infinity) = 5.01 x 10(-12) exp(148/T) cm(3) molecule(-1) s(-1). The master equation calculations were then optimized for the pressure fall-off in He and N(2) by varying the average downward energy transfer parameter (DeltaE(down)) for the different collision partners and finally fitted to a Troe expression to determine k(o) and F(cent) for use in atmospheric modeling.

Hydrogen Transfer between Sulfuric Acid and Hydroxyl Radical in the Gas Phase: Competition among Hydrogen Atom Transfer, Proton-Coupled Electron-Transfer, and Double Proton Transfer

In an attempt to assess the potential role of the hydroxyl radical in the atmospheric degradation of sulfuric acid, the hydrogen transfer between H2SO4 and HO* in the gas phase has been investigated by means of DFT and quantum-mechanical electronic-structure calculations, as well as classical transition state theory computations. The first step of the H2SO4 + HO* reaction is the barrierless formation of a prereactive hydrogen-bonded complex (Cr1) lying 8.1 kcal mol(-1) below the sum of the (298 K) enthalpies of the reactants. After forming Cr1, a single hydrogen transfer from H2SO4 to HO* and a degenerate double hydrogen-exchange between H2SO4 and HO* may occur. The single hydrogen transfer, yielding HSO4* and H2O, can take place through three different transition structures, the two lowest energy ones (TS1 and TS2) corresponding to a proton-coupled electron-transfer mechanism, whereas the higher energy one (TS3) is associated with a hydrogen atom transfer mechanism. The double hydrogen-exchange, affording products identical to reactants, takes place through a transition structure (TS4) involving a double proton-transfer mechanism and is predicted to be the dominant pathway. A rate constant of 1.50 x 10(-14) cm(3) molecule(-1) s(-1) at 298 K is obtained for the overall reaction H2SO4 + HO*. The single hydrogen transfer through TS1, TS2, and TS3 contributes to the overall rate constant at 298 K with a 43.4%. It is concluded that the single hydrogen transfer from H2SO4 to HO* yielding HSO4* and H2O might well be a significant sink for gaseous sulfuric acid in the atmosphere.

Oxidation of CO by SO2: A Theoretical Study

The elementary reaction SO(2) + CO --> CO(2) + SO((3)Sigma) (1) and the subsequent reaction SO((3)Sigma) + CO --> CO(2) + S((3)P) (2) have been studied by the application of the Gaussian-3//B3LYP quantum chemical approach to characterize the potential energy surfaces and transition state kinetic analysis to derive rate coefficients. Reaction 1 is found to take place via two transition states (TS), a cis-OSOCO TS and a trans-OSOCO TS. Reaction via the cis-TS is concerted and takes place on a singlet surface. Intersystem crossing to the final products occurs after passage through the barrier on the singlet surface. The trans-TS leads to a very weakly bound singlet OSOCO intermediate that then passes through a second TS (on the triplet surface) to form the products. Reaction 2 takes place on triplet surfaces. There is a concerted reaction through a cis-SOCO TS and a weakly bound trans-SOCO has also been identified. Reaction 2 is analogous to the reaction CO + O(2)((3)Sigma) --> CO(2) + O((3)P) (3), and this reaction has been reinvestigated at a similar level of theory and the rate coefficient derived by quantum chemistry is compared with experiment. The sensitive effects of trace impurities such as H(2), H(2)O, and hydrocarbons on the accurate experimental determination of the rate coefficient of reaction 3 is discussed. Using rate coefficients for reactions 1 and 2 obtained via quantum chemical calculations, we have been unable to model the extent of decomposition of SO(2) measured in a shock tube study of reaction between SO(2) and CO [Bauer, S. H.; Jeffers, P.; Lifshitz, A.; Yadava, B. P. Proc. Combust. Inst. 1971, 13, 417]. In light of the known sensitivity of reaction 3 to trace impurities, we have incorporated trace amounts of H(2), CH(4), or H(2)O, together with our rate coefficients for (1) and (2), in a kinetic model of Alzueta et al. [Combust. Flame 2001, 127, 2234], which is then shown to be able to substantially model the SO(2) data of Bauer et al. In the course of this modeling study we also computed heats of formation for a number of sulfur-containing small molecules: HS, HSO, HSOH, HOSO, HS(2), HSO(2), HOSO(2), HOSOH, and HOSHO.