Photochemical cycling of nitrogen oxides (NOx) produces tropospheric ozone (O3), and NOx is traditionally considered to be directly emitted. The inability of current global models to accurately calculate NOx levels, and concurrently, difficulties in performing direct NOx measurements in low-NOx regimes (several pptv or several tens of pptv) globally introduce a large uncertainty in the modeling of O3 formation. Here, we use the near-explicit Master Chemical Mechanism (MCM v3.2) within a 0D box-model framework, to describe the chemistry of NOx and O3 in the remote marine boundary layer at Cape Verde. We explore the impact of a recently discovered NOx recycling route, namely photolysis of particulate nitrate, on the modeling of NOx abundance and O3 formation. The model is constrained to observations of long-lived species, meteorological parameters, and photolysis frequencies. Only a model with this novel NOx recycling route reproduces levels of gaseous nitrous acid, NO, and NO2 within the model and measurement uncertainty. O3 formation from NO oxidation is several times more efficient than previously considered. This study highlights the need for the inclusion of particulate nitrate photolysis in future models for O3 and for the photolysis rate of particulate nitrate to be quantified under variable atmospheric conditions.

A HO2 mass accommodation coefficient of α = 0.23 ± 0.07 was measured onto submicron copper(II)-doped ammonium sulfate aerosols at a relative humidity of 60 ± 3%, at 293 ± 2 K and at an initial HO2 concentration of ∼1 × 109 molecules cm–3 by using an aerosol flow tube coupled to a sensitive fluorescence assay by gas expansion (FAGE) HO2 detection system. The effect upon the HO2 uptake coefficient γ of adding different organic species (malonic acid, citric acid, 1,2-diaminoethane, tartronic acid, ethylenediaminetetraacetic acid (EDTA), and oxalic acid) into the copper(II)-doped aerosols was investigated. The HO2 uptake coefficient decreased steadily from the mass accommodation value to γ = 0.008 ± 0.009 when EDTA was added in a one-to-one molar ratio with the copper(II) ions, and to γ = 0.003 ± 0.004 when oxalic acid was added into the aerosol in a ten-to-one molar ratio with the copper(II). EDTA binds strongly to copper(II) ions, potentially making them unavailable for catalytic destruction of HO2, and could also be acting as a surfactant or changing the viscosity of the aerosol. The addition of oxalic acid to the aerosol potentially forms low-volatility copper–oxalate complexes that reduce the uptake of HO2 either by changing the viscosity of the aerosol or by causing precipitation out of the aerosol forming a coating. It is likely that there is a high enough oxalate to copper(II) ion ratio in many types of atmospheric aerosols to decrease the HO2 uptake coefficient. No observable change in the HO2 uptake coefficient was measured when the other organic species (malonic acid, citric acid, 1,2-diaminoethane, and tartronic acid) were added in a ten-to-one molar ratio with the copper(II) ions.

Measurements of HO2 uptake coefficients (γ) were made onto a variety of organic aerosols derived from glutaric acid, glyoxal, malonic acid, stearic acid, oleic acid, squalene, monoethanol amine sulfate, monomethyl amine sulfate, and two sources of humic acid, for an initial HO2 concentration of 1 × 109 molecules cm–3, room temperature and at atmospheric pressure. Values in the range of γ < 0.004 to γ = 0.008 ± 0.004 were measured for all of the aerosols apart from the aerosols from the two sources of humic acid. For humic acid aerosols, uptake coefficients in the range of γ = 0.007 ± 0.002 to γ = 0.09 ± 0.03 were measured. Elevated concentrations of copper (16 ± 1 and 380 ± 20 ppb) and iron (600 ± 30 and 51 000 ± 3000 ppb) ions were measured in the humic acid atomizer solutions compared to the other organics that can explain the higher uptake values measured. A strong dependence upon relative humidity was also observed for uptake onto humic acid, with larger uptake coefficients seen at higher humidities. Possible hypotheses for the humidity dependence include the changing liquid water content of the aerosol, a change in the mass accommodation coefficient or in the Henry’s law constant.

The low temperature kinetics of the reactions of OH with ethanol and propan-2-ol have been studied using a pulsed Laval nozzle apparatus coupled with pulsed laser photolysis–laser-induced fluorescence (PLP-LIF) spectroscopy. The rate coefficients for both reactions have been found to increase significantly as the temperature is lowered, by approximately a factor of 18 between 293 and 54 K for ethanol, and by ∼10 between 298 and 88 K for OH + propan-2-ol. The pressure dependence of the rate coefficients provides evidence for two reaction channels: a zero pressure bimolecular abstraction channel leading to products and collisional stabilization of a weakly bound OH–alcohol complex. The presence of the abstraction channel at low temperatures is rationalized by a quantum mechanical tunneling mechanism, most likely through the barrier to hydrogen abstraction from the OH moiety on the alcohol.

The kinetics of the reactions of the hydroxyl radical (OH) with acetone and dimethyl ether (DME) have been studied between 63–148 K and at a range of pressures using laser-flash photolysis coupled with laser induced fluorescence detection of OH in a pulsed Laval nozzle apparatus. For acetone, a large negative temperature dependence was observed, with the rate coefficient increasing from k1 = (1.6 ± 0.8) × 10−12 cm3 molecule−1 s−1 at 148 K to (1.0 ± 0.1) × 10−10 cm3 molecule−1 s−1 at 79 K, and also increasing with pressure. For DME, a similar behaviour was found, with the rate coefficient increasing from k2 = (3.1 ± 0.5) × 10−12 cm3 molecule−1 s−1 at 138 K to (1.7 ± 0.1) × 10−11 cm3 molecule−1 s−1 at 63 K, and also increasing with pressure. The temperature and pressure dependence of the experimental rate coefficients are rationalised for both reactions by the formation and subsequent stabilisation of a hydrogen bonded complex, with a non-zero rate coefficient extrapolated to zero pressure supportive of quantum mechanical tunnelling on the timescale of the experiments leading to products. In the case of DME, experiments performed in the presence of O2 provide additional evidence that the yield of the CH3OCH2 abstraction product, which can recycle OH in the presence of O2, is ≥50%. The experimental data are modelled using the MESMER (Master Equation Solver for Multi Energy Well Reactions) code which includes a treatment of quantum mechanical tunnelling, and uses energies and structures of transition states and complexes calculated by ab initio methods. Good agreement is seen between experiment and theory, with MESMER being able to reproduce for both reactions the temperature behaviour between ∼70–800 K and the pressure dependence observed at ∼80 K. At the limit of zero pressure, the model predicts a rate coefficient of ∼10−11 cm3 molecule−1 s−1 for the reaction of OH with acetone at 20 K, providing evidence that the reaction can proceed quickly in those regions of space where both species have been observed. The results and modelling build considerably on our previous experimental study performed under a much more limited range of conditions (Shannon et al., Phys. Chem. Chem. Phys., 2010, 12, 13511–13514).

Laboratory studies were conducted to investigate the kinetics of HO2 radical uptake onto submicron inorganic salt aerosols. HO2 reactive uptake coefficients were measured at room temperature using an aerosol flow tube and the Fluorescence Assay by Gas Expansion (FAGE) technique that allowed for measurements to be conducted under atmospherically relevant HO2 concentrations ([HO2] = 108 to 109 molecule cm−3). The uptake coefficient for HO2 uptake onto dry inorganic salt aerosols was consistently below the detection limit (γHO2 < 0.004). The mass accommodation coefficient of HO2 radicals onto Cu(II)-doped (NH4)2SO4 aerosols was measured to be αHO2 = 0.4 ± 0.3 representing the kinetic upper limit to γ. For aqueous (NH4)2SO4, NaCl and NH4NO3 aerosols not containing traces of transition metal ions, a range of γHO2 = 0.003–0.02 was measured. These values were much lower than γ values previously measured on aqueous (NH4)2SO4 and NaCl aerosols and also those typically used in atmospheric models (γHO2 = 0.1–1.0). Evidence is presented showing that the HO2 uptake coefficients onto aqueous salt aerosol particles are dependent both on the exposure time to the aerosol and on the HO2 concentration used.

The hydroxyl radical, OH, initiates the removal of the majority of trace gases in the atmosphere, and together with the closely coupled species, the hydroperoxy radical, HO2, is intimately involved in the oxidation chemistry of the atmosphere. This critical review discusses field measurements of local concentrations of OH and HO2 radicals in the troposphere, and in particular the comparisons that have been made with numerical model calculations containing a detailed chemical mechanism. The level of agreement between field measurements of OH and HO2 concentrations and model calculations for a given location provides an indication of the degree of understanding of the underlying oxidation chemistry. We review the measurement-model comparisons for a range of different environments sampled from the ground and from aircraft, including the marine boundary layer, continental low-NOx regions influenced by biogenic emissions, the polluted urban boundary layer, and polar regions. Although good agreement is found for some environments, there are significant discrepancies which remain unexplained, a notable example being unpolluted, forested regions. OH and HO2 radicals are difficult species to measure in the troposphere, and we also review changes in detection methodology, quality assurance procedures such as instrument intercomparisons, and potential interferences.

A graphical abstract is available for this content

The dispersed fluorescence spectra originating from the v′ = 2 and v′ = 0 levels of the A2Π3/2 state of iodine monoxide (IO) have been recorded for the first time after laser induced fluorescence (LIF) excitation in the A2Π3/2 ← X2Π3/2 electronic transition. The results are used to obtain relative Franck–Condon factors for various v′ → v′′ transitions in the A2Π3/2 → X2Π3/2 system up to v′′ = 12 and compared with theoretical predictions. A fluorescence quenching study of the A2Π3/2 state of IO has also been performed, revealing that collisional quenching and rotational energy transfer (RET) are rapid in the A2Π3/2 state of IO. The J′-dependence to fluorescence quenching of the A2Π3/2 (v′ = 2) state of IO by N2 suggests a collisional predissociation mechanism.

The rate coefficients (k) for reactions of OH with acetone, methyl ethyl ketone (MEK) and dimethyl ether (DME) have been measured in the temperature range 86–112 K using a pulsed Laval nozzle apparatus. Large increases in k at lower temperatures were observed, with k86K/k295K = 334 for acetone, and k93K/k295K = 72 and 3, for MEK and DME respectively. A mechanism involving the formation of a hydrogen bonded complex prior to an overall barrier on the potential energy surface is proposed to explain this behaviour.

The kinetics of reactions of the OH radical with ethene, ethyne (acetylene), propyne (methyl acetylene) and t-butyl-hydroperoxide were studied at temperatures of 69 and 86 K using laser flash-photolysis combined with laser-induced fluorescence spectroscopy. A new pulsed Laval nozzle apparatus is used to provide the low-temperature thermalised environment at a single density of ∼4 × 1016 molecule cm–3 in N2. The density and temperature within the flow are determined using measurements of impact pressure and rotational populations from laser-induced fluorescence spectroscopy of NO and OH. For ethene, rate coefficients were determined to be k2 = (3.22 ± 0.46) × 10–11 and (2.12 ± 0.12) × 10–11 cm3 molecule–1 s–1 at T = 69 and 86 K, respectively, in good agreement with a master-equation calculation utilising an ab initio surface recently calculated for this reaction by Cleary et al. (P. A. Cleary, M. T. Baeza Romero, M. A. Blitz, D. E. Heard, M. J. Pilling, P. W. Seakins and L. Wang, Phys. Chem. Chem. Phys., 2006, 8, 5633–5642) For ethyne, no previous data exist below 210 K and a single measurement at 69 K was only able to provide an approximate upper limit for the rate coefficient of k3 < 1 × 10–12 cm3 molecule–1 s–1, consistent with the presence of a small activation barrier of ∼5 kJ mol–1 between the reagents and the OH–C2H2 adduct. For propyne, there are no previous measurements below 253 K, and rate coefficients of k4 = (5.08 ± 0.65), (5.02 ± 1.11) and (3.11 ± 0.09) × 10–12 cm3 molecule–1 s–1 were obtained at T = 69, 86 and 299 K, indicating a much weaker temperature dependence than for ethene. The rate coefficient k1 = (7.8 ± 2.5) × 10–11 cm3 molecule–1 s–1 was obtained for the reaction of OH with t-butyl-hydroperoxide at T = 86 K. Studies of the reaction of OH with benzene and toluene yielded complex kinetic profiles of OH which did not allow the extraction of rate coefficients. Uncertainties are quoted at the 95% confidence limit and include systematic errors.

The temperature and pressure dependence of the rate coefficient for the reaction of iodine monoxide radicals with dimethyl sulfide (DMS), IO + DMS → I + DMSO (1), was studied using laser induced fluorescence (LIF) to monitor the temporal profile of IO following 351 nm photolysis of RI/DMS/NO2/He (RI = CH3I/CF3I) mixtures. The study was performed over the range T = 296–468 K yielding a positive activation energy and k1 = (9.6 ± 8.8) × 10−12 exp{−(1816 ± 397)/T}. No dependence was observed on total pressure between 5–300 Torr. The rate coefficient at 296 K was determined as (2.0 ± 0.40.6) × 10−14 cm3 molecule−1 s−1, more than an order of magnitude smaller than a recent study but in reasonable agreement with the previous literature.

Methyl hydroperoxide, CH3OOH, has been synthesised with >99.5% purity, confirmed using UV absorption spectroscopy and high-pressure liquid chromatography (HPLC) followed by post-column derivatisation. The UV absorption cross-section for CH3OOH was measured and for <325 nm was in good agreement with the literature. Laser-flash photolysis combined with laser-induced fluorescence (LIF) spectroscopy has been used to measure both OH and CH3O photofragments following the photolysis of CH3OOH in the wavelength range 223–355 nm. Using the previously measured unity quantum yield for OH at 248 nm as a reference, the LIF signals immediately following photolysis were used to measure wavelength dependent quantum yields for OH and CH3O, taking into account changes in laser pulse energy and absorption cross-section. The quantum yields for both species were unity within experimental error. The rate coefficient for the reaction of OH with CH3OOH (R1a) to generate CH3O2 + H2O products was measured at 295 K to be k(R1a) = (9.0 ± 0.2) × 10−12 cm3 molecule−1 s−1, considerably higher (by about a factor of two) than previous values measured by Vaghjiani and Ravishankara [G.L. Vaghjiani, A.R. Ravishankara, J. Phys. Chem. 93 (1989) 1948–1959] and Niki et al. [H. Niki, P.D. Maker, C.M. Savage, L.P. Breitenbach, J. Phys. Chem. 87 (1983) 2190–2193].

The kinetics of reactions of the OH radical with ethene, ethyne (acetylene), propyne (methyl acetylene) and t-butyl-hydroperoxide were studied at temperatures of 69 and 86 K using laser flash-photolysis combined with laser-induced fluorescence spectroscopy. A new pulsed Laval nozzle apparatus is used to provide the low-temperature thermalised environment at a single density of ∼4 × 1016 molecule cm–3 in N2. The density and temperature within the flow are determined using measurements of impact pressure and rotational populations from laser-induced fluorescence spectroscopy of NO and OH. For ethene, rate coefficients were determined to be k2 = (3.22 ± 0.46) × 10–11 and (2.12 ± 0.12) × 10–11 cm3 molecule–1 s–1 at T = 69 and 86 K, respectively, in good agreement with a master-equation calculation utilising an ab initio surface recently calculated for this reaction by Cleary et al. (P. A. Cleary, M. T. Baeza Romero, M. A. Blitz, D. E. Heard, M. J. Pilling, P. W. Seakins and L. Wang, Phys. Chem. Chem. Phys., 2006, 8, 5633–5642) For ethyne, no previous data exist below 210 K and a single measurement at 69 K was only able to provide an approximate upper limit for the rate coefficient of k3 < 1 × 10–12 cm3 molecule–1 s–1, consistent with the presence of a small activation barrier of ∼5 kJ mol–1 between the reagents and the OH–C2H2 adduct. For propyne, there are no previous measurements below 253 K, and rate coefficients of k4 = (5.08 ± 0.65), (5.02 ± 1.11) and (3.11 ± 0.09) × 10–12 cm3 molecule–1 s–1 were obtained at T = 69, 86 and 299 K, indicating a much weaker temperature dependence than for ethene. The rate coefficient k1 = (7.8 ± 2.5) × 10–11 cm3 molecule–1 s–1 was obtained for the reaction of OH with t-butyl-hydroperoxide at T = 86 K. Studies of the reaction of OH with benzene and toluene yielded complex kinetic profiles of OH which did not allow the extraction of rate coefficients. Uncertainties are quoted at the 95% confidence limit and include systematic errors.

Laboratory studies were conducted to investigate the kinetics of HO2 radical uptake onto submicron inorganic salt aerosols. HO2 reactive uptake coefficients were measured at room temperature using an aerosol flow tube and the Fluorescence Assay by Gas Expansion (FAGE) technique that allowed for measurements to be conducted under atmospherically relevant HO2 concentrations ([HO2] = 108 to 109 molecule cm−3). The uptake coefficient for HO2 uptake onto dry inorganic salt aerosols was consistently below the detection limit (γHO2 < 0.004). The mass accommodation coefficient of HO2 radicals onto Cu(II)-doped (NH4)2SO4 aerosols was measured to be αHO2 = 0.4 ± 0.3 representing the kinetic upper limit to γ. For aqueous (NH4)2SO4, NaCl and NH4NO3 aerosols not containing traces of transition metal ions, a range of γHO2 = 0.003–0.02 was measured. These values were much lower than γ values previously measured on aqueous (NH4)2SO4 and NaCl aerosols and also those typically used in atmospheric models (γHO2 = 0.1–1.0). Evidence is presented showing that the HO2 uptake coefficients onto aqueous salt aerosol particles are dependent both on the exposure time to the aerosol and on the HO2 concentration used.

A graphical abstract is available for this content

The dispersed fluorescence spectra originating from the v′ = 2 and v′ = 0 levels of the A2Π3/2 state of iodine monoxide (IO) have been recorded for the first time after laser induced fluorescence (LIF) excitation in the A2Π3/2 ← X2Π3/2 electronic transition. The results are used to obtain relative Franck–Condon factors for various v′ → v′′ transitions in the A2Π3/2 → X2Π3/2 system up to v′′ = 12 and compared with theoretical predictions. A fluorescence quenching study of the A2Π3/2 state of IO has also been performed, revealing that collisional quenching and rotational energy transfer (RET) are rapid in the A2Π3/2 state of IO. The J′-dependence to fluorescence quenching of the A2Π3/2 (v′ = 2) state of IO by N2 suggests a collisional predissociation mechanism.

The hydroxyl radical, OH, initiates the removal of the majority of trace gases in the atmosphere, and together with the closely coupled species, the hydroperoxy radical, HO2, is intimately involved in the oxidation chemistry of the atmosphere. This critical review discusses field measurements of local concentrations of OH and HO2 radicals in the troposphere, and in particular the comparisons that have been made with numerical model calculations containing a detailed chemical mechanism. The level of agreement between field measurements of OH and HO2 concentrations and model calculations for a given location provides an indication of the degree of understanding of the underlying oxidation chemistry. We review the measurement-model comparisons for a range of different environments sampled from the ground and from aircraft, including the marine boundary layer, continental low-NOx regions influenced by biogenic emissions, the polluted urban boundary layer, and polar regions. Although good agreement is found for some environments, there are significant discrepancies which remain unexplained, a notable example being unpolluted, forested regions. OH and HO2 radicals are difficult species to measure in the troposphere, and we also review changes in detection methodology, quality assurance procedures such as instrument intercomparisons, and potential interferences.

The rate coefficients (k) for reactions of OH with acetone, methyl ethyl ketone (MEK) and dimethyl ether (DME) have been measured in the temperature range 86–112 K using a pulsed Laval nozzle apparatus. Large increases in k at lower temperatures were observed, with k86K/k295K = 334 for acetone, and k93K/k295K = 72 and 3, for MEK and DME respectively. A mechanism involving the formation of a hydrogen bonded complex prior to an overall barrier on the potential energy surface is proposed to explain this behaviour.

The kinetics of the reactions of the hydroxyl radical (OH) with acetone and dimethyl ether (DME) have been studied between 63–148 K and at a range of pressures using laser-flash photolysis coupled with laser induced fluorescence detection of OH in a pulsed Laval nozzle apparatus. For acetone, a large negative temperature dependence was observed, with the rate coefficient increasing from k1 = (1.6 ± 0.8) × 10−12 cm3 molecule−1 s−1 at 148 K to (1.0 ± 0.1) × 10−10 cm3 molecule−1 s−1 at 79 K, and also increasing with pressure. For DME, a similar behaviour was found, with the rate coefficient increasing from k2 = (3.1 ± 0.5) × 10−12 cm3 molecule−1 s−1 at 138 K to (1.7 ± 0.1) × 10−11 cm3 molecule−1 s−1 at 63 K, and also increasing with pressure. The temperature and pressure dependence of the experimental rate coefficients are rationalised for both reactions by the formation and subsequent stabilisation of a hydrogen bonded complex, with a non-zero rate coefficient extrapolated to zero pressure supportive of quantum mechanical tunnelling on the timescale of the experiments leading to products. In the case of DME, experiments performed in the presence of O2 provide additional evidence that the yield of the CH3OCH2 abstraction product, which can recycle OH in the presence of O2, is ≥50%. The experimental data are modelled using the MESMER (Master Equation Solver for Multi Energy Well Reactions) code which includes a treatment of quantum mechanical tunnelling, and uses energies and structures of transition states and complexes calculated by ab initio methods. Good agreement is seen between experiment and theory, with MESMER being able to reproduce for both reactions the temperature behaviour between ∼70–800 K and the pressure dependence observed at ∼80 K. At the limit of zero pressure, the model predicts a rate coefficient of ∼10−11 cm3 molecule−1 s−1 for the reaction of OH with acetone at 20 K, providing evidence that the reaction can proceed quickly in those regions of space where both species have been observed. The results and modelling build considerably on our previous experimental study performed under a much more limited range of conditions (Shannon et al., Phys. Chem. Chem. Phys., 2010, 12, 13511–13514).