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The kinetics of the reactions of the first excited state of methylene, 1CH2, with C2H2, C2H4, and C2H6, has been measured over the temperature range 43–298 K by pulsed laser photolysis, monitoring 1CH2 removal by laser induced fluorescence. Low temperatures were obtained using a pulsed Laval expansion (43–134 K), while a slow flow reaction cell was used for temperatures of 160 K and above. The rate coefficients for the reactions with C2H2, C2H4, and C2H6, all showed a strong negative temperature dependence. In combination with other literature data, the coefficients can be parameterized as: k C 2 H 2 ( 43 ≤ T / K ≤ 298 ) = ( 3.22 ± 0.15 ) × 10 − 10 × ( T / 298 ) ( − 0.394 ± 0.066 ) k C 2 H 4 ( 43 ≤ T / K ≤ 298 ) = ( 2.16 ± 0.14 ) × 10 − 10 × ( T / 298 ) ( − 0.612 ± 0.089 ) k C 2 H 6 ( 43 ≤ T / K ≤ 298 ) = ( 1.78 ± 0.10 ) × 10 − 10 × ( T / 298 ) ( − 0.545 ± 0.078 ) Branching ratios for reactive removal of 1CH2 vs quenching to ground state were also determined for all three colliders and for H2 and CH4, at temperatures between 100 and 298 K. The values measured show that the dominant removal process of 1CH2 by H2, C2H2, and C2H4, changes from reactive removal to quenching to ground state 3CH2 as the temperature decreases from 298 K to 100 K, while for CH4 and C2H6, reactive removal drops from around 85% to around 55%. The impacts of the new measurements for Titan's atmosphere are examined using a 1D chemistry and transport model. A significant increase (∼25%) in the mixing ratio of benzene between 500 and 1550 km is calculated, due to the increased production of C3H3 from the reaction of 1CH2 with C2H2.

Measurements of NO and NO2 were made at a surface site (55.28 degrees N, 77.77 degrees W) near Kuujjuarapik, Canada during February and March 2008. NOx mixing ratios ranged from near zero to 350 pptv with emission from snow believed to be the dominant source. The amount of NOx was observed to be dependent on the terrain over which the airmass has passed before reaching the measurement site. The 24 h average NOx emission rates necessary to reproduce observations were calculated using a zero-dimensional box model giving rates ranging from 6.9 x 10(8) molecule cm(-2) s(-1) to 1.2 x 10(9) molecule cm(-2) s(-1) for trajectories over land and from 3.8 x 10(8) molecule cm(-2) s(-1) to 6.6 x 10(8) molecule cm(-2) s(-1) for trajectories over sea ice. These emissions are higher than those suggested by previous studies and indicate the importance of lower latitude snowpack emissions. The difference in emission rate for the two types of snow cover shows the importance of snow depth and underlying surface type for the emission potential of snow-covered areas. (C) 2010 Elsevier Ltd. All rights reserved.

Alkyl nitrates (C1–C5) were measured at two sites (near urban and rural) in southeast England during the Tropospheric Organic Chemistry Experiment (TORCH). Methyl nitrate was the dominant species during both campaigns accounting for on average about one third of the total measured alkyl nitrates. High mixing ratios (>50 pptv) and variability of methyl nitrate were observed at the near urban site (TORCH1) that were not seen at the rural site (TORCH2) and which could not be explained by local photochemical production or direct emissions. The diurnal variation of methyl nitrate during TORCH1 showed a morning maximum that would be consistent with nighttime chemistry followed by transport to the surface by boundary layer dynamics. Similarly, elevated morning mixing ratios were also observed during TORCH2 although the magnitudes were much smaller. As a result, methyl nitrate could represent a tracer for nighttime chemistry seen at the ground the following day. At both campaigns, the dominant source of short chain alkyl nitrates and carbonyl precursor radicals (≤C4) were from decomposition of larger compounds. The magnitude of the source increased with decreasing carbon number consistent with increasing total precursor abundance. Non-photochemical emissions of acetaldehyde and acetone could not be accounted for by automobile exhaust emissions alone and indicated that other direct sources are likely important in this environment.

Laser flash photolysis of CH3C(O)OH at 248 nm was used to create equal zero time yields of CH3CO and OH. The absolute OH yield from the CH3CO + O2 (+M) reaction was determined by following the OH temporal profile using the zero time OH concentration as an internal standard. The OH yield from CH3CO + O2 (+M) was observed to decrease with increasing pressure with an extrapolated zero pressure yield close to unity (1.1 ± 0.2, quoted uncertainties correspond to 95% confidence limits). The results are in quantitative agreement with those obtained from 248 nm acetone photolysis in the presence of O2.

Methyl hydroperoxide, CH 3 OOH, 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 CH 3 OOH was measured and for 3 O photofragments following the photolysis of CH 3 OOH 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 CH 3 O, 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 CH 3 OOH (R1a) to generate CH 3 O 2 + H 2 O products was measured at 295 K to be k (R1a) = (9.0 ± 0.2) × 10 −12 cm 3 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 reaction between C 2 H 5 CO and O 2 (R1a) , (R1b) is observed to directly form OH. This reaction proceeds via the formation of energised C 2 H 5 CO(O 2 ), which can either dissociate to OH or be collisionally thermalized. This reaction mechanism is analogous to CH 3 CO + O 2 (R4a) , (R4b) , which we have previously studied and extended in this study. The kinetics and OH yield of reactions (R1a) , (R1b) , (R4a) , (R4b) are reported as a function of pressure at 295 and 213 K, and are sufficiently different to allow the identification of the corresponding acyl radical and it is demonstrated for 2-butanone photolysis.

The reaction O ( 3 P )+ CF 3 I → k 1 products was monitored by the detection of the product IO using laser-induced fluorescence at 445 nm, with generation of O( 3 P ) via 248 nm photolysis of O 3 or 193 nm photolysis of N 2 O. The temporal evolution of IO as a function of [CF 3 I] was used to determine the rate coefficient, k 1 , in the temperature range 223–523 K at a variety of pressures. At 223 K, a pressure dependence of k 1 was observed, indicative of an association channel, and for N 2 bath gas the third-order rate coefficient was estimated as k assoc =(5.5±2)×10 −31 cm 6 molecule −1 s −1 . For T >223 K, k 1 was found to be independent of pressure and was described by the Arrhenius expression: k 1 (T)=(9.9±1.0)×10 −12 exp − (1930±800) RT cm 3 molecule −1 s −1 The rate coefficient for the reaction IO + NO → k 2 products was measured at T =294 K to be k 2 =(1.82±0.1)×10 −11 cm 3 molecule −1 s −1 , and independent of pressure.

OH formation has been observed directly from the reaction between CH 3 CO and O 2 . This reaction proceeds via the formation of energised CH 3 CO(O 2 ), which can either dissociate to OH or be collisionally stabilized to CH 3 CO(O 2 ). The kinetics of OH formation were measured as a function of pressure at 295 and 213 K, giving rate coefficients in good agreement with the literature. The OH yield was also determined as a function of pressure enabling the reaction to be further quantified.

Low-temperature kinetics of the reactions of C2H radical with ethene (k1), propene (k2), and 1-butene (k3) are studied using a pulsed Laval nozzle supersonic expansion. The C2H decay profiles in the presence of both the alkene reactant and O2 are monitored by the CH(A2Δ) chemiluminescence tracer method. The rate constants k1, k2, and k3 are measured at T=103 K to be (1.4±0.4)×10−10, (2.4±0.6)×10−10, and (2.6±0.6)×10 −10 cm 3 molecule −1 s −1 , respectively. Measurements at room temperature (296 K) and previous data suggest a slight negative temperature dependence for all three reactions.

This paper explores several aspects of the chemistry of a forested region in north-western Greece, from data collected during the AEROBIC97 campaign. An observationally constrained box model has been constructed to enable comparisons between modelled concentrations of OH and HO2 and those determined by the fluorescence assay by gas expansion (FAGE) technique. These results represent the first comparison of measured and modelled OH concentrations in such an environment. The modelled OH concentrations are, on average,∼50% of those measured (range of 16–61%) over 4 days of model and measurement comparison. Possible reasons for the model-measurement discrepancy are discussed. A rate of production analysis illustrates the dominance of isoprene and the monoterpenes on OH loss, as well as the significance of the ozonolysis of biogenic species as an OH source. The measured and modelled [HO2]/[OH] ratio averaged between 11:00 and 15:00 h is much higher than has been found previously for similar NOx concentrations,∼75 and 340, respectively, cf. 10–20. The high ratio reflects the rapid recycling through the OH–HO2 oxidation chain, involving biogenic species. The high biogenic concentrations result in a midday OH lifetime of∼0.15 s. Finally, for the conditions encountered during the campaign, there is high net photochemical ozone production, peaking at∼20 ppbv h−1 around 09:00 h.

Boundary layer concentrations of hydroxyl (OH) and hydroperoxyl (HO2) radicals were measured at 1180 m elevation in a mountainous, forested region of north-western Greece during the AEROsols formation from BIogenic organic Carbon (AEROBIC) field campaign held in July–August 1997. In situ measurements of OH radicals were made by laser-induced fluorescence (LIF) at low pressure, exciting in the (0, 0) band of the A–X system at 308 nm. HO2 radicals were monitored by chemical titration to OH upon the addition of NO, with subsequent detection by LIF. The instrument was calibrated regularly during the field campaign, and demonstrated a sensitivity towards OH and HO2 of 5.2×105 and 2.4×106 molecule cm−3, respectively, for a signal integration period of 2.5 min and a signal-to-noise ratio of 1. Diurnal cycles of OH and HO2 were measured on 10 days within a small clearing of a forest of Greek Fir (Abies Borisi-Regis). In total 4165 OH data points and 1501 HO2 data points were collected at 30 s intervals. Noon-time OH and HO2 concentrations were between 4–12×106 and 0.4–9×108 molecule cm−3, respectively. The performance of the instrument is evaluated, and the data are interpreted in terms of correlations with controlling variables. A significant correlation (r2=0.66) is observed between the OH concentration and the rate of photolysis of ozone, J(O1D). However, OH persisted into the early evening when J(O1D) had fallen to very low values, consistent with the modelling study presented in the following paper (Carslaw et al., 2001, OH and HO2 radical chemistry in a forest region of north-western Greece. Atmospheric Environment 35, 4725–4737) that predicts a significant radical source from the ozonolysis of biogenic alkenes. Normalisation of the OH concentrations for variations in J(O1D) revealed a bell-shaped dependence of OH upon NOx (NO+NO2), which peaked at [NOx] ∼1.75 ppbv. The diurnal variation of HO2 was found to be less correlated with J(O1D) compared to OH.

The rate constants of the reactions of C2H radical with HC  CH (k 1 ) , O 2 (k 2 ) , CH 3 C  CH (k 3 ) , and H 2 C  C  CH 2 (k 4 ) are measured at 103 K using a pulsed Laval nozzle supersonic expansion and the CH(A2Δ) chemiluminescence monitor method: k1=(1.3±0.3)×10−10, k2=(3.8±0.9)×10−11, k3=(2.7±0.6)×10−10, and k 4 =(2.5±0.6)×10 −10 cm 3 molecule −1 s −1 . These values are compared to previous results and measurements in a continuous Laval nozzle apparatus (accompanying paper by Carty et al.). The non-Arrhenius behavior and possible implications of the results for atmospheric chemistry of Titan are discussed.

The cross-section for electronic quenching of OH( A 2 Σ + , v′=0) by H2O was found to increase from 82±8 A2 at 294 K to 119±11 A2 at 211 K. Both OH( A 2 Σ + ) and H2O display relatively large dipole moments, and a `harpooned' mechanism occurring on a Lennard-Jones potential energy surface modified to include dipole moment effects was found to reproduce the observed temperature dependence of the cross-sections in the range 211–2300 K. The model, which is general, will enable corrections for the effects of quenching on laser-induced fluorescence signals to be made with a high degree of confidence in combustion and atmospheric media.

A rovibrational spectrum of the acetylene-argon van der Waals (vdW) complex, C2H2Ar, has been measured in the 1.525 μm ‘2νCH’ overtone region by a time-resolved infrared-ultraviolet double resonance method. Infrared absorption by the vdW complex is recorded as a depletion of laser-induced rovibronic fluorescence intensity, providing a novel way to measure rovibrational spectra of simple vdW complexes and to study their predissociation dynamics.

Laser-induced fluorescence measurements have been made of the OH, CH, and NO radicals in slighly rich methane/air flames burning at 30, 70, and 120 torr. Absolute NO and OH concentrations were determined using separate calibration experiments. Temperature profiles were deduced from OH rotational excitation scans, and fluorescence quenching rates for NO and CH were determined. Predicted profiles of the concentrations of these radical species as a function of height above the burner were obtained from a computer model of the flame, and comparison made with experiment. Good agreement is achieved for the relative concentration profiles, the absolute NO concentration, and concentration ratios between 30 and 70 torr, although slight disagreement with the CH profile indicates there remain some unknown aspects of the flame chemistry.

Quenching of electronically excited CH and NO radicals in methane/air flames between 30 and 70 Torr has been studied using the time decay of laser-induced fluorescence. Rates of 0.3 and 1 μs−1 Torr−1 were determined for A 2Δ CH and A 2Σ+ NO respectively at a temperature near 1700 K. These are compared with rates calculated using previously determined bimolecular quenching cross sections. The agreement is good, within 20 to 30%. The temperature dependence of H2O quenching of CH and of N2 and CO2 quenching of NO need further investigation.

A time-resolved interferometer has been used to study the product vibrational infrared chemiluminescence from atom-radical reactions. The entire temporal evolution of the emission spectrum is obtained from a single interferometric scan. Results are given for the O( 3 P) + CF 2 (X 1 A 1 ) reaction, which exhibits strong emission from highly vibrationally excited CO 2 , formed by reaction of O atoms with FCO, the major primary product of the title reaction.