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

Abstract

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 HO 2 diurnal profiles lay within the measurement range of all HO x measurement techniques, with correlation slopes between measured and modeled OH and HO 2 around unity. Recently discovered interference in HO 2 measurements caused by RO 2 cross sensitivity was found to cause a 30% increase in measured HO 2 during daytime on average. After correction of the measured HO 2 data, the model HO 2 is still in good agreement with the observations at high NO x but overpredicts HO 2 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 ± 2.2 ppb h −1 was estimated from the experimental OH budget during the low‐NO x period using the corrected HO 2 data. The measured diurnal profile of the HO 2 /OH ratio, calculated using the corrected HO 2 , is well reproduced by the MCM at high NO x but is significantly overestimated at low NO x . Thus, the cycling between OH and HO 2 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.