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∞.

In environments with high concentrations of biogenic volatile organic compounds and low concentrations of nitrogen oxides (NOx = NO + NO2), significant discrepancies have been found between measured and modeled concentrations of hydroxyl radical (OH). The photolysis of peroxy radicals from isoprene (HO-Iso-O2) in the near ultraviolet represents a potential source of OH in these environments, yet has not been considered in atmospheric models. This paper presents measurements of the absorption cross-sections for OH formation (σRO2,OH) from the photolysis of HO-Iso-O2 at wavelengths from 310–362.5 nm, via direct observation by laser-induced fluorescence of the additional OH produced following laser photolysis of HO-Iso-O2. Values of σRO2,OH for HO-Iso-O2 ranged from (6.0 ± 1.6) × 10−20 cm2 molecule−1 at 310 nm to (0.50 ± 0.15) × 10−20 cm2 molecule−1 at 362.5 nm. OH photodissociation yields from HO-Iso-O2 photolysis, ϕOH,RO2, were determined via comparison of the measured values of σRO2,OH to the total absorption cross-sections for HO-Iso-O2 (σRO2), which were obtained using a newly-constructed spectrometer. ϕOH,RO2 was determined to be 0.13 ± 0.04 at wavelengths from 310–362.5 nm. To determine the impact of HO-Iso-O2 photolysis on atmospheric OH concentrations, a modeling case-study for a high-isoprene, low-NOx environment (namely, the 2008 Oxidant and Particle Photochemical Processes above a South-East Asian Tropical Rainforest (OP-3) field campaign, conducted in Borneo) was undertaken using the detailed Master Chemical Mechanism. The model calculated that the inclusion of HO-Iso-O2 photolysis in the model had increased the OH concentration by only 1% on average from 10:00–16:00 local time. Thus, HO-Iso-O2 photolysis alone is insufficient to resolve the discrepancy seen between measured OH concentrations and those predicted by atmospheric chemistry models in such environments.

A novel technique has been developed for the detection of peroxy radicals in order to study their kinetics with NO2. Peroxy radicals (RO2, where R = H, CH3, and 1,2-C4H9) were produced by laser flash photolysis and were probed by photodissociation of the RO2 and the subsequent detection of either OH or CH3O photofragments by laser-induced fluorescence. Reaction 1, CH3O2 + NO2 + M ⇌ CH3O2NO2 + M (M = N2), was studied between 25 and 400 Torr at 295 K, giving results in excellent agreement with the literature. At temperatures between 333 and 363 K, equilibration was observed and yielded ΔrH⊖298(1) = −93.5 ± 0.3 kJ mol–1. Reaction 2, HO2 + NO2 + M ⇌ HO2NO2 + M (M = N2), was studied at 295 K and showed kinetics in fair agreement with the literature. Equilibration at higher temperatures was obscured by an additional loss of HO2NO2 from the system. In addition, the OH quantum yield from photolysis of HO2NO2 at 248 nm was determined to be 0.15 ± 0.03. Reaction 3, 1,2-C4H9O2 + NO2 + M ⇌ 1,2-C4H9O2NO2 + M (M = He), was studied between 241 and 341 K, and at the higher temperatures equilibration was observed, which yielded ΔrH⊖298(3) = −93.5 ± 0.6 kJ mol–1. The low uncertainties in the enthalpies of formation for both CH3O2 and 1,2-C4H9O2 are a result of using a master equation method that allows global analysis of all the available rate data (present measurements and literature values) for forward and reverse reactions under all conditions of temperature and pressure.

The C1 Criegee intermediate, CH2OO, reaction with water vapour has been studied. The removal rate constant shows a quadratic dependence on [H2O], implying reaction with the water dimer, (H2O)2. The rate constant, kCH2OO+(H2O)2 = (4.0 ± 1.2) × 10−12 cm3 molecule−1 s−1, is such that this is the major atmospheric sink for CH2OO.

Kinetics of CH2OO Criegee intermediate reactions with SO2, NO2, NO, H2O and CH3CHO and CH2I radical reactions with NO2 are reported as a function of pressure at 295 K. Measurements were made under pseudo-first-order conditions using flash photolysis of CH2I2–O2–N2 gas mixtures in the presence of excess co-reagent combined with monitoring of HCHO reaction products by laser-induced fluorescence (LIF) spectroscopy and, for the reaction with SO2, direct detection of CH2OO by photoionisation mass spectrometry (PIMS). Rate coefficients for CH2OO + SO2 and CH2OO + NO2 are independent of pressure in the ranges studied and are (3.42 ± 0.42) × 10−11 cm3 s−1 (measured between 1.5 and 450 Torr) and (1.5 ± 0.5) × 10−12 cm3 s−1 (measured between 25 and 300 Torr), respectively. The rate coefficient for CH2OO + CH3CHO is pressure dependent, with the yield of HCHO decreasing with increasing pressure. Upper limits of 2 × 10−13 cm3 s−1 and 9 × 10−17 cm3 s−1 are placed on the rate coefficients for CH2OO + NO and CH2OO + H2O, respectively. The upper limit for the rate coefficient for CH2OO + H2O is significantly lower than has been reported previously, with consequences for modelling of atmospheric impacts of CH2OO chemistry.

The photolysis of glyoxal has been investigated in the 355–414 nm region by dye laser photolysis coupled with cavity ring-down spectroscopy. Absolute quantum yields of HCO, ΦHCO, were determined using the reaction of chlorine atoms with formaldehyde as an actinometer. The dependence of the quantum yield on pressure was investigated in 3–400 Torr of nitrogen buffer gas and at four temperatures: 233 K, 268 K, 298 K and 323 K. For 355 nm ≤ λ < 395 nm the HCO quantum yield is pressure dependent with linear Stern–Volmer (SV) plots (1/ΦHCOvs. pressure). The zero pressure quantum yield, obtained by extrapolation of the SV plots, rises from 1.6 to 2 between 355 and 382 nm and remains at 2 up to 395 nm. For λ ≥ 395 nm ΦHCO shows a stronger pressure dependence and non-linear SV plots, compatible with formation of HCO by dissociation from two electronic states of glyoxal with significantly different lifetimes. These observations are used to develop a mechanism for the photolysis of glyoxal over the wavelength range studied.

Yields of CH2OO and CH2IO2 from the reaction of CH2I radicals with O2 are reported as a function of total pressure, [N2] and [O2] at T = 295 K using three complementary methods. Results from the three methods are similar, with no observed additional dependence on [O2]. The CH2I + O2 reaction has a yield of ∼18% CH2OO at atmospheric pressure.

The formation of HCO and of H in the photolysis of glyoxal have been investigated over the wavelength ranges 310–335 nm for HCO and 193–340 nm for H. Dye laser photolysis was coupled with cavity ring-down spectroscopy for HCO, and with laser induced fluorescence spectroscopy for H. Absolute quantum yields were determined using actinometers based on (a) Cl2 photolysis and the Cl + HCHO reaction for HCO and (b) N2O photolysis (and O1D + H2) and CH2CO photolysis (and CH2 + O2) for H. The quantum yields were found to be pressure independent in this wavelength region. Quantum yields for all product channels under atmospheric conditions were calculated and compared with literature values. Differences between this work and previously published work and their atmospheric implications are discussed.

Abstract

Li et al. (Reports, 21 March 2008, p. 1657) suggested that the reaction between electronically excited nitrogen dioxide and water vapor is an important atmospheric source of the hydroxyl radical. However, under conditions that better approximate the solar flux, we find no evidence for OH production from this reaction.

Photoexcitation of glyoxal at wavelengths over the range of 395−414 nm was observed to initiate a chemical reaction that produces the HCO radical in addition to the photolytic production of HCO. The technique of dye laser flash photolysis coupled to cavity ring-down spectroscopy was used to determine the time dependence of the HCO radical signal, analysis of which suggests that the chemical source of HCO is the self-reaction of triplet glyoxal As the photoexcitation wavelength increases, the production from the triplet glyoxal reaction increases relative to that of HCO from direct photolysis, and at 414 nm, the dominant source of HCO in the system is from the self-reaction of the triplet. The formation of HCO via this process complicates the assignment of the photolysis quantum yield at longer wavelengths and may have been overlooked in some previous glyoxal photolysis studies.

Kinetics of CH2OO Criegee intermediate reactions with SO2, NO2, NO, H2O and CH3CHO and CH2I radical reactions with NO2 are reported as a function of pressure at 295 K. Measurements were made under pseudo-first-order conditions using flash photolysis of CH2I2–O2–N2 gas mixtures in the presence of excess co-reagent combined with monitoring of HCHO reaction products by laser-induced fluorescence (LIF) spectroscopy and, for the reaction with SO2, direct detection of CH2OO by photoionisation mass spectrometry (PIMS). Rate coefficients for CH2OO + SO2 and CH2OO + NO2 are independent of pressure in the ranges studied and are (3.42 ± 0.42) × 10−11 cm3 s−1 (measured between 1.5 and 450 Torr) and (1.5 ± 0.5) × 10−12 cm3 s−1 (measured between 25 and 300 Torr), respectively. The rate coefficient for CH2OO + CH3CHO is pressure dependent, with the yield of HCHO decreasing with increasing pressure. Upper limits of 2 × 10−13 cm3 s−1 and 9 × 10−17 cm3 s−1 are placed on the rate coefficients for CH2OO + NO and CH2OO + H2O, respectively. The upper limit for the rate coefficient for CH2OO + H2O is significantly lower than has been reported previously, with consequences for modelling of atmospheric impacts of CH2OO chemistry.

The C1 Criegee intermediate, CH2OO, reaction with water vapour has been studied. The removal rate constant shows a quadratic dependence on [H2O], implying reaction with the water dimer, (H2O)2. The rate constant, kCH2OO+(H2O)2 = (4.0 ± 1.2) × 10−12 cm3 molecule−1 s−1, is such that this is the major atmospheric sink for CH2OO.

Yields of CH2OO and CH2IO2 from the reaction of CH2I radicals with O2 are reported as a function of total pressure, [N2] and [O2] at T = 295 K using three complementary methods. Results from the three methods are similar, with no observed additional dependence on [O2]. The CH2I + O2 reaction has a yield of ∼18% CH2OO at atmospheric pressure.

In environments with high concentrations of biogenic volatile organic compounds and low concentrations of nitrogen oxides (NOx = NO + NO2), significant discrepancies have been found between measured and modeled concentrations of hydroxyl radical (OH). The photolysis of peroxy radicals from isoprene (HO-Iso-O2) in the near ultraviolet represents a potential source of OH in these environments, yet has not been considered in atmospheric models. This paper presents measurements of the absorption cross-sections for OH formation (σRO2,OH) from the photolysis of HO-Iso-O2 at wavelengths from 310–362.5 nm, via direct observation by laser-induced fluorescence of the additional OH produced following laser photolysis of HO-Iso-O2. Values of σRO2,OH for HO-Iso-O2 ranged from (6.0 ± 1.6) × 10−20 cm2 molecule−1 at 310 nm to (0.50 ± 0.15) × 10−20 cm2 molecule−1 at 362.5 nm. OH photodissociation yields from HO-Iso-O2 photolysis, ϕOH,RO2, were determined via comparison of the measured values of σRO2,OH to the total absorption cross-sections for HO-Iso-O2 (σRO2), which were obtained using a newly-constructed spectrometer. ϕOH,RO2 was determined to be 0.13 ± 0.04 at wavelengths from 310–362.5 nm. To determine the impact of HO-Iso-O2 photolysis on atmospheric OH concentrations, a modeling case-study for a high-isoprene, low-NOx environment (namely, the 2008 Oxidant and Particle Photochemical Processes above a South-East Asian Tropical Rainforest (OP-3) field campaign, conducted in Borneo) was undertaken using the detailed Master Chemical Mechanism. The model calculated that the inclusion of HO-Iso-O2 photolysis in the model had increased the OH concentration by only 1% on average from 10:00–16:00 local time. Thus, HO-Iso-O2 photolysis alone is insufficient to resolve the discrepancy seen between measured OH concentrations and those predicted by atmospheric chemistry models in such environments.

The photolysis of glyoxal has been investigated in the 355–414 nm region by dye laser photolysis coupled with cavity ring-down spectroscopy. Absolute quantum yields of HCO, ΦHCO, were determined using the reaction of chlorine atoms with formaldehyde as an actinometer. The dependence of the quantum yield on pressure was investigated in 3–400 Torr of nitrogen buffer gas and at four temperatures: 233 K, 268 K, 298 K and 323 K. For 355 nm ≤ λ < 395 nm the HCO quantum yield is pressure dependent with linear Stern–Volmer (SV) plots (1/ΦHCOvs. pressure). The zero pressure quantum yield, obtained by extrapolation of the SV plots, rises from 1.6 to 2 between 355 and 382 nm and remains at 2 up to 395 nm. For λ ≥ 395 nm ΦHCO shows a stronger pressure dependence and non-linear SV plots, compatible with formation of HCO by dissociation from two electronic states of glyoxal with significantly different lifetimes. These observations are used to develop a mechanism for the photolysis of glyoxal over the wavelength range studied.

The formation of HCO and of H in the photolysis of glyoxal have been investigated over the wavelength ranges 310–335 nm for HCO and 193–340 nm for H. Dye laser photolysis was coupled with cavity ring-down spectroscopy for HCO, and with laser induced fluorescence spectroscopy for H. Absolute quantum yields were determined using actinometers based on (a) Cl2 photolysis and the Cl + HCHO reaction for HCO and (b) N2O photolysis (and O1D + H2) and CH2CO photolysis (and CH2 + O2) for H. The quantum yields were found to be pressure independent in this wavelength region. Quantum yields for all product channels under atmospheric conditions were calculated and compared with literature values. Differences between this work and previously published work and their atmospheric implications are discussed.