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The reaction of OH with dimethyl ether (CH3OCH3) has been studied from 195 to 850 K using laser flash photolysis coupled to laser induced fluorescence detection of OH radicals. The rate coefficient from this work can be parametrized by the modified Arrhenius expression k = (1.23 ± 0.46) × 10(-12) (T/298)(2.05±0.23) exp((257 ± 107)/T) cm(3) molecule(-1) s(-1). Including other recent literature data (923-1423 K) gives a modified Arrhenius expression of k1 = (1.54 ± 0.48) × 10(-12) (T/298 K)(1.89±0.16) exp((184 ± 112)/T) cm(3) molecule(-1) s(-1) over the range 195-1423 K. Various isotopomeric combinations of the reaction have also been investigated with deuteration of dimethyl ether leading to a normal isotope effect. Deuteration of the hydroxyl group leads to a small inverse isotope effect. To gain insight into the reaction mechanisms and to support the experimental work, theoretical studies have also been undertaken calculating the energies and structures of the transition states and complexes using high level ab initio methods. The calculations also identify pre- and post-reaction complexes. The calculations show that the pre-reaction complex has a binding energy of ~22 kJ mol(-1). Stabilization into the complex could influence the kinetics of the reaction, especially at low temperatures (300 K), but there is no direct evidence of this occurring under the experimental conditions of this study. The experimental data have been modeled using the recently developed MESMER (master equation solver for multi energy well reactions) code; the calculated rate coefficients lie within 16% of the experimental values over the temperature range 200-1400 K with a model based on a single transition state. This model also qualitatively reproduces the observed isotope effects, agreeing closely above ~600 K but overestimating them at low temperatures. The low temperature differences may derive from an inadequate treatment of tunnelling and/or from an enhanced role of an outer transition state leading to the pre-reaction complex.

An apparatus has been developed for the direct determination of ambient, atmospheric formaldehyde using gas chromatography. A loop sample of 1–6 ml is transferred onto the column and the formaldehyde is focused with a cold trap. Following separation, formaldehyde is detected with an argon doped, pulsed discharge helium ionization detector. Absolute values are obtained via calibration with a permeation source. A detection limit of 42 pptv is calculated. The relatively high duty cycle (∼11 h−1) allows measurements to be taken on timescales that are short compared to the atmospheric lifetime of formaldehyde. The reliability of the apparatus has been investigated via a 4-week field campaign at an urban site in Leeds, UK. Diurnal profiles are observed with evidence for both primary emissions from traffic and secondary, photochemical production. A decrease in formaldehyde concentrations was observed during rain events.

International audience; Peroxy radical (HO2+?RO2) measurements, using the PEroxy Radical Chemical Amplification (PERCA) technique at the North Atlantic Marine Boundary Layer EXperiment (NAMBLEX) at Mace Head in summer 2002, are presented and put into the context of marine, boundary-layer chemistry. A suite of other chemical parameters (NO, NO2, NO3, CO, CH4, O3, VOCs, peroxides), photolysis frequencies and meteorological measurements, are used to present a detailed analysis of the role of peroxy radicals in tropospheric oxidation cycles and ozone formation. Under the range of conditions encountered the peroxy radical daily maxima varied from 10 to 40 pptv. The diurnal cycles showed an asymmetric shape typically shifted to the afternoon. Using a box model based on the master chemical mechanism the average model measurement agreement was 2.5 across the campaign. The addition of halogen oxides to the model increases the level of model/measurement agreement, apparently by respeciation of HOx. A good correlation exists between j(HCHO).[HCHO] and the peroxy radicals indicative of the importance of HCHO in the remote atmosphere as a HOx source, particularly in the afternoon. The peroxy radicals showed a strong dependence on [NO2] with a break point at 0.1 ppbv, where the radicals increased concomitantly with the reactive VOC loading, this is a lower value than seen at representative urban campaigns. The HO2/(HO2+?RO2) ratios are dependent on [NOx] ranging between 0.2 and 0.6, with the ratio increasing linearly with NOx. Significant night-time levels of peroxy radicals were measured up to 25 pptv. The contribution of ozone-alkenes and NO3-alkene chemistry to night-time peroxy radical production was shown to be on average 59 and 41%. The campaign mean net ozone production rate was 0.11±0.3 ppbv h-1. The ozone production rate was strongly dependent on [NO] having linear sensitivity (dln(P(O3))/dln(NO)=1.0). The results imply that the N(O3) (the in-situ net photochemical rate of ozone production/destruction) will be strongly sensitive in the marine boundary layer to small changes in [NO] which has ramifications for changing NOx loadings in the European continental boundary layer.

Ambient formaldehyde concentrations are reported from the North Atlantic Marine Boundary Layer Experiment (NAMBLEX) campaign at Mace Head on the west coast of Eire during August 2002. The results from two techniques, using direct determination via gas chromatography and the Hantzsch technique, show similar trends but a significant off set in concentrations. For westerly air flows characteristic of the marine boundary layer, formaldehyde concentrations from the gas chromatographic and Hantzsch technique ranged from 0.78–1.15 ppb and 0.13–0.43 ppb, respectively. Possible reasons for the discrepancy have been investigated and are discussed, however, no satisfactory explanation has yet been found. In a subsequent laboratory intercomparison the two techniques were in good agreement. The observed concentrations have been compared with previous formaldehyde measurements in the North Atlantic marine boundary layer and with other measurements from the NAMBLEX campaign. The measurements from the Hantzsch technique and the GC results lie at the lower and upper ends respectively of previous measurements. In contrast to some previous measurements, both techniques show distinct diurnal profiles with day maxima and with an amplitude of approximately 0.15 ppb. Strong correlations were observed with ethanal concentrations measured during NAMBLEX and the ratio of ethanal to formaldehyde determined by the gas chromatographic technique is in good agreement with previous measurements. Some simple box modelling has been undertaken to investigate possible sources of formaldehyde. Such models are not able to predict absolute formaldehyde concentrations as they do not include transport processes, but the results show that oxygenated VOCs such as ethanal and methanol are very significant sources of formaldehyde in the air masses reaching Mace Head.