This work details the first direct observation of OH as a product from (R1): HO₂+CH₃C(O)O₂→(products), which has generally been considered an atmospheric radical termination process. The technique of pulsed laser photolysis radical generation, coupled to calibrated laser induced fluorescence detection was used to measure an OH product yield for (R1) of α₁(298 K)=(0.5±0.2). This study of (R1) included the measurement of a rate coefficient k₁(298 K)=(1.4±0.5)×10⁻¹¹cm³ molecule⁻¹ s⁻¹, substantially reducing the uncertainties in modelling this important atmospheric reaction. OH was also detected as a product from the reactions of HO₂ with three other carbonyl-containing peroxy radicals, albeit at smaller yield, e.g. (R2): HO₂+CH₃C(O)CH₂O₂→(products), α₂≈0.15. By contrast, OH was not observed (α<0.06) as a major product from reactions where carbonyl functionality was absent, e.g. HO₂+HOCH₂CH₂O₂ (R8), and HO₂+CH₃CH(OH)CH₂O₂ (R9).

Absolute rate coefficients for the reaction of hydroxyl radicals (HO) with methanol, HO + CH₃OH → products (R1), and with ethanol, HO + C₂H₅OH → products (R2) were measured over a range of temperatures using pulsed laser photolytic generation of HO coupled to its time resolved detection by pulsed laser induced fluorescence. The accuracy of the rate constants obtained was enhanced by on-line optical absorption measurements of the alcohol concentration. The temperature dependence of the rate coefficients is given by: k₁(210–351 K) = 6.67 × 10⁻¹⁸T² exp(140/T) cm³ molecule⁻¹ s⁻¹ with a rate coefficient at room temperature of (9.3 ± 0.7) × 10⁻¹³ cm³ molecule⁻¹ s⁻¹. For k₂ we obtained: k₂(216–368 K) = 4.0 × 10⁻¹² exp(−42/T) and a room temperature rate coefficient of (3.35 ± 0.17) × 10⁻¹² cm³ molecule⁻¹ s⁻¹. The total error (at 95% confidence) associated with the rate coefficients derived from the expressions describing the temperature dependence is estimated as 7% at all temperatures. The present results, which extend the database on these reactions to cover temperatures relevant for the upper troposphere, are compared to previously published measurements, and values of k₁ and k₂ are recommended for atmospheric modelling.

The reaction CH₃C(O)O₂ + HO₂ → CH₃C(O)OOH + O₂ (Reaction R5a), CH₃C(O)OH + O₃ (Reaction R5b), CH₃ + CO₂ + OH + O₂ (Reaction R5c) was studied in a series of experiments conducted at 1000 mbar and (293 ± 2) K in the HIRAC simulation chamber. For the first time, products, (CH₃C(O)OOH, CH₃C(O)OH, O₃ and OH) from all three branching pathways of the reaction have been detected directly and simultaneously. Measurements of radical precursors (CH₃OH, CH₃CHO), HO₂ and some secondary products HCHO and HCOOH further constrained the system. Fitting a comprehensive model to the experimental data, obtained over a range of conditions, determined the branching ratios α_(R5a) = 0.37 ± 0.10, α_(R5b) = 0.12 ± 0.04 and α_(R5c) = 0.51 ± 0.12 (errors at 2σ level). Improved measurement/model agreement was achieved using k_(R5) = (2.4 ± 0.4) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹, which is within the large uncertainty of the current IUPAC and JPL recommended rate coefficients for the title reaction. The rate coefficient and branching ratios are in good agreement with a recent study performed by Groß et al. (2014b); taken together, these two studies show that the rate of OH regeneration through Reaction (R5) is more rapid than previously thought. GEOS-Chem has been used to assess the implications of the revised rate coefficients and branching ratios; the modelling shows an enhancement of up to 5 % in OH concentrations in tropical rainforest areas and increases of up to 10 % at altitudes of 6–8 km above the equator, compared to calculations based on the IUPAC recommended rate coefficient and yield. The enhanced rate of acetylperoxy consumption significantly reduces PAN in remote regions (up to 30 %) with commensurate reductions in background NO_x.