Atmospheric photochemistry of aromatic hydrocarbons: OH budgets during SAPHIR chamber experiments

Abstract. Current photochemical models developed to simulate the atmospheric degradation of aromatic hydrocarbons tend to underestimate OH radical concentrations. In order to analyse OH budgets, we performed experiments with benzene, toluene, p-xylene and 1,3,5-trimethylbenzene in the atmosphere simulation chamber SAPHIR. Experiments were conducted under low-NO conditions (typically 0.1–0.2 ppb) and high-NO conditions (typically 7–8 ppb), and starting concentrations of 6–250 ppb of aromatics, dependent on OH rate constants. For the OH budget analysis a steady-state approach was applied in which OH production and destruction rates (P_OH and D_OH) have to be equal. The P_OH were determined from measurements of HO₂, NO, HONO, and O₃ concentrations, considering OH formation by photolysis and recycling from HO₂. The D_OH were calculated from measurements of the OH concentrations and total OH reactivities. The OH budgets were determined from D_OH/P_OH ratios. The accuracy and reproducibility of the approach were assessed in several experiments using CO as a reference compound where an average ratio D_OH/P_OH = 1.13 ± 0.19 was obtained. In experiments with aromatics, these ratios ranged within 1.1–1.6 under low-NO conditions and 0.9–1.2 under high-NO conditions. The results indicate that OH budgets during photo-oxidation experiments with aromatics are balanced within experimental accuracies. Inclusion of a further, recently proposed OH production via HO₂ + RO₂ reactions led to improvements under low-NO conditions but the differences were small and insignificant within the experimental errors.