HO_x budgets during HOxComp: A case study of HO_x chemistry under NO_x-limited conditions

Recent studies have shown that measured OH under NO_x-limited, high-isoprene conditions are many times higher than modeled OH. In this study, a detailed analysis of the HO_x radical budgets under low-NO _x, rural conditions was performed employing a box model based on the Master Chemical Mechanism (MCMv3.2). The model results were compared with HO_x radical measurements performed during the international HOxComp campaign carried out in Jülich, Germany, during summer 2005. Two different air masses influenced the measurement site denoted as high-NO_x (NO, 1-3 ppbv) and low-NO_x (NO, < 1 ppbv) periods. Both modeled OH and HO₂ diurnal profiles lay within the measurement range of all HO _x measurement techniques, with correlation slopes between measured and modeled OH and HO₂ around unity. Recently discovered interference in HO₂ measurements caused by RO₂ cross sensitivity was found to cause a 30% increase in measured HO₂ during daytime on average. After correction of the measured HO₂ data, the model HO ₂ is still in good agreement with the observations at high NO _x but overpredicts HO₂ by a factor of 1.3 to 1.8 at low NO_x. In addition, for two different set of measurements, a missing OH source of 3.6 ± 1.6 and 4.9 ± 2.2ppbh^-1 was estimated from the experimental OH budget during the low-NO_x period using the corrected HO₂ data. The measured diurnal profile of the HO ₂/OH ratio, calculated using the corrected HO₂, is well reproduced by the MCM at high NO_x but is significantly overestimated at low NO_x. Thus, the cycling between OH and HO₂ is better described by the model at high NO_x than at low NO_x. Therefore, similar comprehensive field measurements accompanied by model studies are urgently needed to investigate HO_x recycling under low-NO _x conditions.