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Fourier transform infrared spectroscopy, was used to identify CH 3 OCHO and CH 3 OCH 2 OOH as products of the gas-phase reaction of CH 3 OCH 2 O 2 radicals with HO 2 radicals. At 700 Torr and 295 ± 2 K, branching ratios of k 1a / k 1 = 0.53 ± 0.08 and k 1b / k 1 = 0.40 ± 0.04 were established. Quoted errors are 2 standard deviations together with our estimate of systematic uncertainties. (CH 3 OCH 2 O 2 +HO 2 →CH 3 OCH 2 OOH+O 2 (1a), CH 3 OCH 2 O 2 +HO 2 →CH 3 OCHO+O 2 +H 2 O (1b).) This result is discussed with respect to previous literature data and to computer models of atmospheric chemistry. As part of this work, the rate constants for the reaction of Cl atoms with CH 3 OCH 2 OOH and CH 3 OCHO were determined to be (6.1 ± 0.3) × 10 −11 and (1.4 ± 0.1) × 10 −12 cm 3 molecule −1 s −1 , respectively.

The molecular-modulation—UV/visible-absorption-spectroscopy technique has been used to investigate the simultaneous time-resolved behaviour of IO and HO2 produced by the photolysis of O3 in the presence of CH3OH, I2, O2 and Ar. This has enabled a direct measurement of the rate coefficient for the reaction IO + HO2 → HOI + O2 to be made at 760 Torr and 289 K. The value obtained was k5= (6.4 ± 0.7) × 10−11 cm3 molecule−1 s−1.

Observation of the time-resolved behaviour of OH radicals produced during the modulated photolysis of mixtures of O 3 H 2 O, HONO and Ar at 253.7 nm, has enabled direct measurements of the rate coefficient for the reaction OH+HONO→NO 2 +H 2 O (2) at 11.4 Torr and in the temperature range 278–342.5 K. The value obtained was k 2 = (1.80±0.36) × 10 −11 exp[(−390 ± 80)/ T ] cm 3 molecule −1 s −1 .