Long pathlength differential optical absorption spectroscopy (D O A S) measurements of gaseous HONO,NO2 and HCNO in the California South Coast Air Basin

A differential optical absorption spectrometer (DOAS) system was operated at Long Beach, CA during the 1987 SCAQS Fall episodes to measure atmospheric concentrations of nitrous acid (HONO), as well as ambient levels of nitrogen dioxide (NO₂) and formaldehyde (HCHO). The rapid scanning (-3000 spectra per min) spectrometer was interfaced to a 25 m basepath open, multiple reflection system operated routinely at a total optical path of 800 m. During several of the Fall episodes at Long Beach, levels of gaseous HONO were the highest (>15 ppb) reported to date by the DOAS technique. Although approximately half, to all, of the measured nighttime HONO concentrations could be accounted for by proposed heterogeneous formation pathways involving NO₂, HONO concentrations correlated well with primary pollutants such as CO and NO, suggesting that elevated nighttime HONO concentrations in the western end of the Los Angeles basin may be influenced by emissions of HONO from combustion sources. This has significant implications for models which assume HONO arises only from secondary formation, rather than a combination of direct emissions and atmospheric reactions. Estimates of hydroxyl (OH) radical production show that photolysis of HONO shortly after sunrise on these episode days produces a large pulse of OH radicals at a time of the day when OH production from photolysis of O₃ and HCHO is low. In terms of integrated OH radical production, HONO is of comparable importance to HCHO and much more important than O₃ during these Fall periods.     

Formation of nitrous acid and nitric oxide in the heterogeneous dark reaction of nitrogen dioxide and water vapor in a smog chamber

The dark reaction of NO_x and H₂O vapor in 1 atm of air was studied for the purpose of elucidating the recently discussed unknown radical source in smog chambers. Nitrous acid and nitric oxide were found to be formed by the reaction of NO₂ and H₂O in an evacuable and bakable smog chamber. No nitric acid was observed in the gas phase. The reaction is not stoichiometric and is thought to be a heterogeneous wall reaction. The reaction rate is first order with respect to NO₂ and H₂O, and the concentrations of HONO and NO initially increase linearly with time. The same reaction proceeds with a different rate constant in a quartz cell, and the reaction of NO₂ and H₂¹⁸O gave H¹⁸ONO exclusively. Taking into consideration the heterogeneous reaction of NO₂ and H₂O, the upper limit of the rate constant of the third-order reaction NO + NO₂ + H₂O → 2HONO was deduced to be (3.0 ± 1.4) × 10^?10 ppm^?2-min^?1, which is one order of magnitude smaller than the previously reported value. Nitrous acid formed by the heterogeneous dark reaction of NO₂ and H₂O should contribute significantly to both an initially present HONO and a continuous supply of OH radicals by photolysis in smog chamber experiments.     

The heterogeneous formation of N2O in the presence of acidic solutions: Experiments and modeling

We investigated the heterogeneous processes that contribute towards the formation of N₂O in an environment that comes as closely as possible to exhaust conditions containing NO and SO₂ among other constituents. The simultaneous presence of NO, SO₂, O₂, and condensed phase water in the liquid state has been confirmed to be necessary for the production of significant levels of N₂O. The maximum rate of N₂O formation occurred at the beginning of the reaction and scales with the surface area of the condensed phase and is independent of its volume. The replacement of NO by either NO₂ or HONO significantly increases the rate constant for N₂O formation. The measured reaction orders in the rate law change depending upon the choice of the nitrogen reactant used and were fractional in some cases. The rate constants of N₂O formation for the three different nitrogen reactants reveal the following series of increasing reactivity: NO < NO₂ < HONO, indicating the probable sequential involvement of those species in the elementary reactions. Furthermore, we observed a complex dependence of the rate constant on the acidity of the liquid phase where both the initial rate as well as the yield of N₂O are largest at pH=0 of a H₂SO₄/H₂O solution. The results suggest that HONO is the major reacting N(III) species over a wide range of acidities studied. The N₂O formation in synthetic flue gas may be simulated using a relatively simple mechanism based on the model of Lyon and Cole. The first step of the complex overall reaction corresponds to NO oxidation by O₂ to NO₂ mainly in the gas phase, with the presence of both H₂O and active surfaces significantly accelerating NO₂ production. Subsequently, NO₂ reacts with excess NO to obtain HONO which reacts with S(IV) to result in N₂O and H₂SO₄ through a complex reaction sequence probably involving nitroxyl (HON) and its dimer, hyponitrous acid. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet: 29 : 869–891, 1997.     

Photoenhancement of the nitrous acid formation in the surface reaction of nitrogen dioxide and water vapor: Extra radical source in smog chamber experiments

The surface reaction of NO₂ and H₂O vapor to emit HONO into the gas phase was studied in the evacuable and bakeable photochemical chamber under the irradiation of UV-visible light (? 290 nm). Kinetic analysis of the NO, NO₂, and HONO with the aid of computer modeling strongly suggested that the formation of HONO by the surface reaction is photoenhanced. When a linear regression was assumed, the photoenhancement factor defined by {(k′₂₁/k₂₁) ? 1} was expressed as (6.8 ± 2.5)k₁ under our experimental conditions, where k₁ is the primary photolysis rate of NO₂, and k₂₁, k′₂₁ are the second-order-equivalent rate constants of the HONO formation reaction in dark and under irradiation, respectively. The discussion was made that this photocatalitic enhancement of HONO formation would explain the nature of the extra OH radical flux in the smog chamber experiments, which has been discussed as “unknown radical source” and has still been unexplained by the surface dark reaction of NO₂ and H₂O to emit HONO.     

An investigation of the dark formation of nitrous acid in environmental chambers

The formation of nitrous acid (HONO) in the dark from initial concentrations of NO₂ of 0.1–20 ppm in air, and the concurrent disappearance of NO₂, were monitored quantitatively by UV differential optical absorption spectroscopy in two different environmental chambers of ca.4300- and 5800-L volume (both with surface/volume ratios of 3.4 m^?1). In these environmental chambers the initial HONO formation rate was first order in the NO₂ concentration and increased with the water vapor concentration. However, the HONO formation rate was independent of the NO concentration and relatively insensitive to temperature. The initial pseudo-first-order consumption rate of NO₂ was (2.8 ± 1.2) × 10^?4 min^?1 in the 5800-L Teflon-coated evacuable chamber and (1.6 ± 0.5) × 10^?4 min^?1 in a 4300-L all-Teflon reaction chamber at ca.300 K and ca.50% RH. The initial HONO yields were ca.40–50% of the NO₂ reacted in the evacuable chamber and ca.10–30% in the all-Teflon chamber. Nitric oxide formation was observed during the later stages of the reaction in the evacuable chamber, but ca.50% of the nitrogen could not be accounted for, and gas phase HNO₃ was not detected. The implications of these data concerning radical sources in environmental chamber irradiations of NO_x? organic-air mixtures, and of HONO formation in polluted atmospheres, are discussed.     

Kinetic and mechanistic aspects of the NO oxidation by O2 in aqueous phase

The oxidation kinetics of NO by O₂ in aqueous solution was observed using a stopped flow apparatus. The kinetics follows a third order rate law of the form k · [NO]² · [O₂] in analogy to gas-phase results. The rate constant at 296 K was measured as (6.4 ± 0.8) · 10⁶ M^?2 s^?1 with an activation energy of 2.3 kcal/mol and a preexponential factor of (4.0 ± 0.5) · 10⁸ M^?2 s^?1. The rate constant displays a very slight pH dependence corresponding to less than a factor of three over the range 0 to 12. The system NO/O₂ in aqueous solution is an efficient nitrosating agent which has been tested using phenol as a substrate over the pH range 0 to 12. The rate limiting step leading to formation of 4-nitrosophenol is the formation of the reactive intermediate whose competitive hydrolysis yields HONO or NO₂^?. The absence of NO₃^? in the autoxidation of NO, the exclusive presence of NO₂^? as a product of the nitrosation reaction of phenol, and the kinetic results of the N₃^? trapping experiments point towards N₂O₃ as the reactive intermediate. © 1994 John Wiley & Sons, Inc.     

A mechanism for the photochemical transformation of nitrate in snow

Photochemical reactions of trace compounds in snow have important implications for the composition of the atmospheric boundary layer in snow-covered regions and for the interpretation of concentration profiles in snow and ice regarding the composition of the past atmosphere. One of the prominent reactions is the photolysis of nitrate, which leads to the formation of OH radicals in the snow and to the release of reactive nitrogen compounds, like nitrogen oxides (NO and NO₂) and nitrous acid (HONO) to the atmosphere. We performed photolysis experiments using artificial snow, containing variable initial concentrations of nitrate and nitrite, to investigate the reaction mechanism responsible for the formation of the reactive nitrogen compounds. Increasing the initial nitrite concentrations resulted in the formation of significant amounts of nitrate in the snow. A possible precursor of nitrate is NO₂, which can be transformed into nitrate either by the attack of a hydroxy radical or the hydrolysis of the dimer (N₂O₄). A mechanism for the transformation of the nitrogen-containing compounds in snow was developed, assuming that all reactions took place in a quasi-liquid layer (QLL) at the surface of the ice crystals. The unknown photolysis rates of nitrate and nitrite and the rates of NO and NO₂ transfer from the snow to the gas phase, respectively, were adjusted to give an optimum fit of the calculated time series of nitrate, nitrite, and gas phase NO_x with respect to the experimental data. Best agreement was obtained with a ∼25 times faster photolysis rate of nitrite compared to nitrate. The formation of NO₂ is probably the dominant channel for the nitrate photolysis. We used the reaction mechanism further to investigate the release of NO_x and HONO under natural conditions. We found that NO_x emissions are by far dominated by the release of NO₂. The release of HONO to the gas phase depends on the pH of the snow and the HONO transfer rate to the gas phase. However, due to the small amounts of nitrite produced under natural conditions, the formation of HONO in the QLL is probably negligible. We suggest that observed emissions of HONO from the surface snow are dominated by the heterogeneous formation of HONO in the firn air. The reaction of NO₂ on the surfaces of the ice crystals is the most likely HONO source to the gas phase.     

Flow reactor studies and kinetic modeling of the H2/O2/NOX and CO/H2O/O2/NOX reactions

Flow reactor experiments were performed over wide ranges of pressure (0.5–14.0 atm) and temperature (750–1100 K) to study H₂/O₂ and CO/H₂O/O₂ kinetics in the presence of trace quantities of NO and NO₂. The promoting and inhibiting effects of NO reported previously at near atmospheric pressures extend throughout the range of pressures explored in the present study. At conditions where the recombination reaction H + O₂ (+M) = HO₂ (+M) is favored over the competing branching reaction, low concentrations of NO promote H₂ and CO oxidation by converting HO₂ to OH. In high concentrations, NO can also inhibit oxidative processes by catalyzing the recombination of radicals. The experimental data show that the overall effects of NO addition on fuel consumption and conversion of NO to NO₂ depend strongly on pressure and stoichiometry. The addition of NO₂ was also found to promote H₂ and CO oxidation but only at conditions where the reacting mixture first promoted the conversion of NO₂ to NO. Experimentally measured profiles of H₂, CO, CO₂, NO, NO₂, O₂, H₂O, and temperature were used to constrain the development of a detailed kinetic mechanism consistent with the previously studied H₂/O₂, CO/H₂O/O₂, H₂/NO₂, and CO/H₂O/N₂O systems. Model predictions generated using the reaction mechanism presented here are in good agreement with the experimental data over the entire range of conditions explored. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 705–724, 1999     

Modeling Chemical Composition for an Atmospheric Pressure DC Discharge in Air with Water Cathode by 0-D model

This paper reports the results of the chemical composition modeling for an atmospheric pressure DC air discharge with water cathode. The modeling was based on the combined solution of Boltzmann equation for electrons, equations of vibrational kinetics for ground states of N₂, O₂, H₂O and NO molecules, equations of chemical kinetics and plasma conductivity equation. Calculations were carried out using experimental values of E/N and gas temperatures for the discharge currents range of 20–50 mA. The effect of H₂O concentration on the plasma composition was studied. The main particles of plasma were shown to be O₂(a¹Δ, b¹Σ), O(³P), NO, NO₂, HNO₃, H₂O₂ and OH. Effective vibrational temperatures of molecules were higher than gas temperature and they did not depend on the discharge current. Distribution functions on vibrational levels for N₂, O₂, H₂O and NO ground states were non-equilibrium ones.     

Transient Spark Discharge Generated in Various N2/O2 Gas Mixtures: Reactive Species in the Gas and Water and Their Antibacterial Effects

Reactive species generated in the gas and in water by cold air plasma of the transient spark discharge in various N₂/O₂ gas mixtures (including pure N₂ and pure O₂) have been examined. The discharge was operated without/with circulated water driven down the inclined grounded electrode. Without water, NO and NO₂ are typically produced with maximum concentrations at 50% O₂. N₂O was also present for low O₂ contents (up to 20%), while O₃ was generated only in pure O₂. With water, gaseous NO and NO₂ concentrations were lower, N₂O was completely suppressed and HNO₂ increased; and O₃ was lowered in O₂ gas. All species production decreased with the gas flow rate increasing from 0.5 to 2.2 L/min. Liquid phase species (H₂O₂, NO₂￣, NO₃￣, ^·OH) were detected in plasma treated water. H₂O₂ reached the highest concentrations in pure N₂ and O₂. On the other hand, nitrites NO₂￣ and nitrates NO₃￣ peaked between 20 and 80% O₂ and were associated with pH reduction. The concentrations of all species increased with the plasma treatment time. Aqueous ^·OH radicals were analyzed by terephthalic acid fluorescence and their concentration correlated with H₂O₂. The antibacterial efficacy of the transient spark on bacteria in water increased with water treatment time and was found the strongest in the air-like mixture thanks to the peroxynitrite formation. Yet, significant antibacterial effects were found even in pure N₂ and in pure O₂ most likely due to high ^·OH radical concentrations. Controlling the N₂/O₂ ratio in the gas mixture, gas flow rate, and water treatment time enables tuning the antibacterial efficacy.

     

Smog chamber study of H2O2 formation in ethene?NOx and propene?NOx mixtures

Hydrogen peroxide formation in the photooxidation of CO? NO_x, ethene? NO_x, and propene? NO_x mixtures has been determined in the TVA 31 cubic meter smog chamber under the following conditions: [NO_x] ca. 22–46 ppb; ethene = 0.22–1.1 ppm, [propene] = 0.12–0.97 ppm; [H₂O] ca. 8 × 10^?3 ppm. Ethene, propene, NO, NO_x, PAN, HCHO, and CH₃CHO were also monitored. Computer modeling was performed using the gas phase ethene and propene mechanism of the Regional Acid Deposition Model. There is good agreement between the model predicted and observed H₂O₂ concentrations. However, to successfully model all the propene? NO_x experimental results, organic nitrate formation from the reaction of peroxy radicals with NO must be included in the mechanism.     

Computational study on the kinetics and mechanisms for the reactions of HCO with HONO and HNOH

The kinetics and mechanisms of the HCO reactions with HONO and HNOH have been studied at the G2M level of theory based on the geometric parameters optimized at BH&HLYP/6‐311G(d,p). The rate constants in the temperature range 200–3000 K at different pressures have been predicted by microcanonical RRKM and/or variational transition state theory calculations with Eckart tunneling corrections. For the HCO + HONO reaction, hydrogen abstraction from trans‐HONO and cis‐HONO by HCO produces H₂CO + NO₂, with the latter being dominant. Two other channels involving cis‐HONO by the association/decomposition mechanism via the HC(O)N(O)OH intermediate, which could fragment to give H₂O + CO + NO at high temperatures, were also found to be important. For the HCO + HNOH reaction, three reaction channels were identified: one association reaction giving a stable intermediate, HC(O)N(H)OH (LM2), and two hydrogen abstraction channels producing H₂CO and H₂NOH. The dominant products were predicted to be the formation of LM2 at low temperatures and H₂NOH + CO at middle and high temperatures. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 178–187 2004     

Daytime HONO formation in the suburban area of the megacity Beijing,China

Nitrous acid(HONO),as a primary precursor of OH radicals,has been considered one of the most important nitrogencontaining species in the atmosphere.Up to 30%of primary OH radical production is attributed to the photolysis of HONO.However,the major HONO formation mechanisms are still under discussion.During the Campaigns of Air Quality Research in Beijing and Surrounding Region(CAREBeijing2006)campaign,comprehensive measurements were carried out in the megacity Beijing,where the chemical budget of HONO was fully constrained.The average diurnal HONO concentration varied from 0.33 to 1.2 ppbv.The net OH production rate from HONO,POH(HONO)net,was on average(from 05:00 to 19:00)7.1×106 molecule/(cm3 s),2.7 times higher than from O3 photolysis.This production rate demonstrates the important role of HONO in the atmospheric chemistry of megacity Beijing.An unknown HONO source(Punknown)with an average of 7.3×106molecule/(cm3 s)was derived from the budget analysis during daytime.Punknown provided four times more HONO than the reaction of NO with OH did.The diurnal variation of Punknown showed an apparent photo-enhanced feature with a maximum around 12:00,which was consistent with previous studies at forest and rural sites.Laboratory studies proposed new mechanisms to recruit NO2 and J(NO2)in order to explain a photo-enhancement of of Punknown.In this study,these mechanisms were validated against the observation-constraint Punknown.The reaction of exited NO2 accounted for only 6%of Punknown,and Punknown poorly correlated with[NO2](R=0.26)and J(NO2)[NO2](R=0.35).These results challenged the role of NO2 as a major precursor of the missing HONO source.     

Effects of nitric oxide on the thermal decomposition of methyl nitrite: Overall kinetics and rate constants for the HNO + HNO and HNO + 2NO reactions

The effects of NO on the decomposition of CH₃ONO have been investigated in the temperature range 450–520 K at a constant pressure of 710 torr using He as buffer gas. The measured time-dependent concentration profiles of CH₃ONO, NO, N₂O, and CH₂O can be quantitatively accounted for with a general mechanism consisting of various reactions of CH₃O, HNO, and (HNO)₂. The results of kinetic modeling with sensitivity analyses indicate that the disappearance rate of CH₃ONO is weakly affected by NO addition, whereas that of the HNO intermediate strongly altered by the added NO. In the presence of low NO concentrations, the modeling of N₂O yields leads to the rate constant for the bimolecular reaction, HNO + HNO → N₂O + H₂O (25): In the presence of high NO concentrations (P_NO > 50 torr), the modeling of CH₂O yields gives the rate constant for the termolecular radical formation channel, HNO + 2NO → HN₂O + NO₂ (35): Discussion on the mechanisms for reactions (25) and (35), and the alkyl homolog of (35), RNO + 2NO, is presented herein. © John Wiley & Sons, Inc.     

Rate coefficients for the reaction of OH with HONO between 298 and 373 K

Rate coefficients, k₁, for the reaction OH + HONO → H₂O + NO₂, have been measured over the temperature range 298 to 373 K. The OH radicals were produced by 266 nm laser photolysis of O₃ in the presence of a large excess of H₂O vapor. The temporal profiles of OH were measured under pseudo-first-order conditions, in an excess of HONO, using time resolved laser induced fluorescence. The measured rate coefficient exhibits a slight negative temperature dependence, with k₁ = (2.8 ± 1.3) × 10^?12 exp((260 ± 140)/T) cm³ molecule^?1 s^?1. The measured values of k₁ are compared with previous determinations and the atmospheric implications of our findings are discussed.     

Plasma conductivity as a probe for ambient air admixture in an atmospheric pressure plasma jet

By utilizing a fully floating double electrical probe system, the conductivity of a linear atmospheric pressure plasma jet, utilizing nitrogen as process gas, was measured. The floating probe makes it possible to measure currents in the nanoamp range, in an environment where capacitive coupling of the probes to the powered electrodes is on the order of several kilovolts. Using a chemical kinetic model, the production of reactive nitrogen oxide and hydrogen-containing species through admixture of ambient humid air is determined and compared to the measured gas conductivity. The chemical kinetic model predicts an enhanced diffusion coefficient for admixture of O₂ and H₂O from ambient air of 2.7 cm² s^?1, compared to a literature value of 0.21 cm² s^?1, which is attributed to rapid mixing between the plasma jets and the surrounding air. The dominant charge carriers contributing to the conductivity, aside from electrons, are NO⁺, NO₂ ^? and NO₃ ^?. Upon admixture of O₂ and H₂O, the dominant neutral products formed in the N₂ plasma jet are O, NO and N₂O, while O₂(¹Δ_g) singlet oxygen is the only dominant excited species.     

Spin non-conservation in low-energy molecular charge-transfer reactions

The reaction H₂O⁺(²B)+NO₂(²A) → H₂O(¹A) + NO₂⁺(¹Σ) occurs at near the collision rate constant 1.2 × 10^?9 cm³ s^?1, in spite of the fact that the reactants produce both a singlet and a triplet state and the products correlate only with the singlet state. This would be expected to yield a statistical weight factor of $\text{1}\text{4}$ to be multiplied by the collision rate constant to obtain the maximum charge-tranfer rate constant. The triplet products of the charge transfer are clearly endothermic. The singlet—triplet intersection has not been identified but the available information about the singlet and triplet states of the intermediate protonated nitric acid molecule is discussed. Four other examples of apparent “spin violation” charge-transfer reactions have been noted H₂O⁺ + NO, N₂O⁺ + NO.CO⁺ + NO and CH₄⁺ + O₂.     

A shock tube study of the CH2O + NO2 reaction at high temperatures

The reaction of CH₂O with NO₂ has been studied with a shock tube equipped with two stabilized ew CO lasers. The production of CO, NO, and H₂O has been monitored with the CO lasers in the temperature range of 1140–1650 K using three different Ar-diluted CH₂O-NO₂ mixtures. Kinetic modeling and sensitivity analysis of the observed CO, NO, and H₂O production profiles over the entire range of reaction conditions employed indicate that the bimolecular metathetical reaction, NO₂ + CH₂O → HONO + CHO (1) affects most strongly the yields of these products. Combination of the kinetically modeled values of ??₁ with those obtained recently from a low temperature pyrolytic study, ref. [8], leads to for the broad temperature range of 300–2000 K.     

Theoretical Investigation of the Photoexcited NO2+H2O reaction at the Air–Water Interface and Its Atmospheric Implications

The atmospheric role of photochemical processes involving NO₂ beyond its dissociation limit (398 nm) is controversial. Recent experiments have confirmed that excited NO₂^* beyond 420 nm reacts with water according to NO₂^*+H₂O→HONO+OH. However, the estimated kinetic constant for this process in the gas phase is quite small (k≈10⁻¹⁵–3.4×10⁻¹⁴ cm³ molecule⁻¹ s⁻¹) suggesting minor atmospheric implications of the formed radicals. In this work, ab initio molecular dynamics simulations of NO₂ adsorbed at the air–water interface reveal that the OH production rate increases by about 2 orders of magnitude with respect to gas phase, attaining ozone reference values for NO₂ concentrations corresponding to slightly polluted rural areas. This finding substantiates the argument that chemistry on clouds can be an additional source of OH radicals in the troposphere and suggests directions for future laboratory experimental studies.     

Experimental and kinetic modeling study of the effect of sulfur dioxide on the mutual sensitization of the oxidation of nitric oxide and methane

New experimental results were obtained for the mutual sensitization of the oxidation of NO and methane in a fused silica jet‐stirred reactor operating at 10⁵ Pa, over the temperature range 800–1150 K. The effect of the addition of sulfur dioxide was studied. Probe sampling followed by online FTIR analyses and off‐line GC‐TCD/FID analyses allowed the measurement of concentration profiles for the reactants, stable intermediates, and final products. A detailed chemical kinetic modeling of the present experiments was performed. An overall reasonable agreement between the present data and modeling was obtained. According to the present modeling, the mutual sensitization of the oxidation of methane and NO proceeds via the NO to NO₂ conversion by HO₂ and CH₃O₂. The conversion of NO to NO₂ by CH₃O₂ is more important at low temperatures (800 K) than at higher temperatures (850–900 K) where the production of NO₂ is mostly due to the reaction of NO with HO₂. The NO to NO₂ conversion is favored by the production of the HO₂ and CH₃O₂ radicals yielded from the oxidation of the fuel. The production of OH resulting from the oxidation of NO accelerates the oxidation of the fuel: NO + HO₂ → OH+ NO₂ followed by OH + CH₄→ CH₃. In the lower temperature range of this study, the reaction further proceeds via CH₃ + O₂→ CH₃O₂; CH₃O₂+ NO → CH₃O + NO₂. At higher temperatures, the production of CH₃O involves NO₂: CH₃+ NO₂→ CH₃O. This sequence of reactions is followed by CH₃O → CH₂O + H; CH₂O +OH → HCO; HCO + O₂ → HO₂ and H + O₂ → HO₂ → CH₂O + H; CH₂O +OH → HCO; HCO + O₂ → HO₂ and H + O₂ → HO₂. The data and the modeling show that unexpectedly, SO₂ has no measurable effect on the kinetics of the mutual sensitization of the oxidation of NO and methane in the present conditions, whereas it frequently acts as an inhibitor in combustion. This result was rationalized via a detailed kinetic analysis indicating that the inhibiting effect of SO₂ via the sequence of reactions SO₂+H → HOSO, HOSO+O₂ → SO₂+HO₂, equivalent to H+O₂?HO₂, is balanced by the reaction promoting step NO+HO₂ → NO₂+OH. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 406–413, 2005