HOx budgets during HOxComp: A case study of HOx chemistry under NOx-limited conditions
2012
[1] Recent studies have shown that measured OH under
NOx-limited, high-isoprene conditions are many times higher than modeled OH. In this study, a detailed analysis of the HOx radical budgets under low-
NOx, rural conditions was performed employing a
box modelbased on the Master Chemical Mechanism (MCMv3.2). The model results were compared with HOx radical measurements performed during the international HOxComp campaign carried out in Julich, Germany, during summer 2005. Two different air masses influenced the measurement site denoted as high-
NOx(NO, 1–3 ppbv) and low-
NOx(NO, < 1 ppbv) periods. Both modeled OH and HO2 diurnal profiles lay within the measurement range of all HOx 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 HO2 is still in good agreement with the observations at high
NOxbut overpredicts HO2 by a factor of 1.3 to 1.8 at low
NOx. 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-
NOxperiod using the corrected HO2 data. The measured diurnal profile of the HO2/OH ratio, calculated using the corrected HO2, is well reproduced by the MCM at high
NOxbut is significantly overestimated at low
NOx. Thus, the cycling between OH and HO2 is better described by the model at high
NOxthan at low
NOx. Therefore, similar comprehensive field measurements accompanied by model studies are urgently needed to investigate HOx recycling under low-
NOxconditions.
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