J. Alistair Kerr

Photolysis of Formaldehyde as a Hydrogen Atom Source in the Lower Atmosphere

The rates of formaldehyde photodecomposition into hydrogen and formyl radicals and hydrogen and carbon monoxide molecules in sunlightirradiated atmospheres have been estimated from extinction data and photochemical results. These data should prove useful in the development of models for the chemical changes that take place in the polluted atmosphere.

Hydroxyl-radical-initiated oxidation of isobutyl isopropyl ether under laboratory conditions related to the troposphere Product studies and proposed mechanism

The products formed by the hydroxyl-radical-initiated oxidation of the model ether, isobutyl isopropyl ether [(CH 3 ) 2 CHCH 2 OCH(CH 3 ) 2 ], have been investigated by irradiating synthetic air mixtures containing the substrate, methyl nitrite, and nitric oxide at ppm levels in a Teflon bag reactor at room temperature. The decay of reactant and formation of products were monitored by gas chromatography, mass spectrometry and by HPLC. The molar yields of the major products (mol of product formed/mol of isobutyl isopropyl ether consumed) were as follows: acetone, 0.56 ± 0.04; isopropyl formate, 0.48 ± 0.03; isobutyl acetate, 0.28 ± 0.02; 2-hydroxy-2-methylpropyl acetate [CH 3 C(O)OCH 2 C(OH)(CH 3 ) 2 ], 0.25 ± 0.1. The molar yields of the minor products were as follows: isobutyraldehyde, 0.06 ± 0.05; isopropyl nitrate, 0.09 ± 0.06; 1,1,4-trimethyl-3-oxapentyl nitrate [(CH 3 ) 2 CHOCH 2 C(CH 3 ) 2 (ONO 2 )], 0.07 ± 0.02; isopropyl isobutyrate [(CH 3 ) 2 CHC(O)OCH(CH 3 ) 2 ] ca. 0.01; and isobutyl formate, ca. 0.01. The major products are explained by a mechanism involving initial OH attack at the –CH– and –CH 2 – groups in the alkyl side chains of the ether followed by the subsequent reactions of the resulting carbon-centred, organic peroxy, and organic oxy radicals. The observed products, in conjunction with the proposed reaction pathways, account for a total yield of about 1.15, indicating that all the main routes are accounted for in the degradation of this ether. The major reaction pathways of the three principal organic oxy radicals are summarised as follows (percentage of overall reaction in brackets):(CH3)2C(O˙)OCH2CH(CH3)2 → CH3C(O)OCH2CH(CH3)2]C˙H3 (28%)(CH3)2CHOCH(O˙)CH(CH3)2 → (CH3)2CHOC(O)H]C˙H(CH3)2 (⩽48%)(CH3)2CHOCH2C(O˙)(CH3)2 → (CH3)2C˙OCH2C(OH)(CH3)2 (25%)This study supports the finding that organic oxy radicals generated from ethers and containing the structure RCH(O˙)OR undergo mainly decomposition by C–C bond cleavage, whereas those oxy radicals with the structure RCH(O˙)CH 2 OR undergo preferential 1,5-H-atom transfer isomerisation reactions. The following rate coefficients (10 −12 cm 3 molecule −1 s −1 ) at room temperature for the reactions of OH radicals with the reactant and products have been determined by the relative rate technique: isobutyl isopropyl ether, 19.5 ± 0.4; isobutyl acetate, 6.0 ± 0.5; isobutyraldehyde, 25.8 ± 0.7; isopropyl formate, 2.1 ± 0.1; isopropyl isobutyrate, 6.5 ± 0.4; 1,1,4-trimethyl-3-oxapentyl nitrate, 16.5 ± 0.7; and 2-hydroxy-2-methylpropyl acetate, 9.5 ± 1.6.

Computer Simulation of the Chemistry of a Simple Analogue to the Sunlight-Irradiated Auto-Exhaust Polluted Atmosphere

Abstract Computer simulations have been made of the chemistry of a simple analogue to the sunlight-irradiated, auto-exhaust polluted atmosphere: a mixture of 0.075 ppm NO, 0.025 ppm NO2, 1.5 ppm CH4, 10 ppm CO, 0.10 ppm trans-2-butene, 0.10 ppm CH2O and 0.06 ppm CH3CHO in air with a relative humidity of 50% (25°C). The relative importance towards attack on trans-2-butene of the various possible reactive intermediates has been calculated as a function of time. O(3P), O3, HO2 and HO all seem to be important during some stage of the reaction with trans-2-butene. The question of which reactions trigger the NO to NO2 conversion is considered by examining the relative importance of the several possible sources of the HO (or HO2) radicals which initiate the olefin oxidation. In the simulated system in which the rate of HONO forming reaction is assumed to be zero, the major initial push to oxidize NO and the olefin comes from aldehyde photolyses. While in the system in which HONO is formed at an appreciable rate,...

The Predicted Effect of Carbon Monoxide on the Ozone Levels in Photochemical Smog Systems

Abstract Computer simulations have been made of the rates of chemical changes expected to occur in certain polluted atmospheres. The results point to an unexpected conclusion: Even in the absence of reactive hydrocarbons, the presence of small levels of carbon monoxide in an NO-containing atmosphere may enhance the photooxidation of NO to NO2 and may lead ultimately to significant levels of ozone. In view of the technological difficulties associated with the complete removal of NOX and CO from auto-exhaust and NOX from stack gases, significant ambient levels of ozone may be anticipated to continue to form in many urban areas even though a near total removal of the reactive hydrocarbons might be accomplished.

The effect of carbon monoxide on the chemistry of photochemical smog systems.

Recent computer simulations of the rates of chemical changes expected to occur in NO-NO2-CO-containing atmospheres have been repeated in the light of new kinetic data for some of the key reactions. At the same time the computer simulations of the chemical changes which occur in smog chamber experiments involving NO-NO2-CO-air mixtures have been shown to compare reasonably well with the experimental analytical data which is now available. The previous conclusion, that the presence of small levels of CO in an NO-containing atmosphere may enhance the photooxidation of NO to NO2 and may lead ultimately to significant levels of ozone, is further confirmed by these studies.

The relative importance of the various intermediate species in olefin removal reactions in photochemical smog.

Abstract Computer simulations have been made of the chemical changes which occur in smog chamber experiments employing NO-NO2-trans-2-butene-air mixtures. The results indicate that significant differences are expected between experiments carried out with steady illumination and with periods of illumination punctuated with relatively long dark periods. In particular the ozone, olefin, NOX concentration-time profiles are altered appreciably by using the alternative irradiation techniques. When proper account is taken of the effect, an excellent match is had between the present computer simulation and the experimental studies of Tuesday in which a light-dark period cycle was employed. The computer simulation of this system with steady illumination provides a basis for judging the relative importance toward olefin attack of the various possible reactive intermediates in this system. O(3P), O3, and OH species all seem to be important during some stage of the olefin-NO-photo-oxidation sequence. HO2 and CH3O rad...