Photochemistry and budget of ozone during the Mauna Loa Observatory Photochemistry Experiment (MLOPEX 2)

During the Mauna Loa Observatory Photochemistry Experiment (MLOPEX 2), simultaneous measurements of a large number of photochemical species were measured during different seasons at Mauna Loa Observatory (MLO), Hawaii. In this study, these measurements are used to constrain a detailed photochemical box model and evaluate our understanding of the tropospheric photochemistry in this region of the Pacific. The simulations generally reproduce satisfactorily the NO/NO2 photostationary state, which controls the ozone production rate. However, the model fails in simulating the concentration of peroxy radicals (PO2) during all seasons and of hydroxyl radical (OH) during summer. Several hypotheses are considered to assess this discrepancy, including the removal of radicals by unidentified mechanisms and the potential impact of biogenic organic compounds. None of the tested hypotheses give satisfactorily results in terms of OH, PO2 and NO/NO2 simultaneously. Although experimental uncertainties are large for radicals, this issue constitutes a major inconsistency between measurements and model results during MLOPEX. Another disagreement arises from the simulation of peroxides for free tropospheric conditions. The model tends to overestimate H2O2 and CH3OOH by a factor of 1.5–2.5. On the other hand, a fair agreement is achieved in simulating formaldehyde when CH3OOH is constrained in the model. Finally, we find that the gross ozone production and destruction rates are nearly in balance in this region of the Pacific troposphere. The net production is slightly negative, ranging from nearly 0 in winter to about −1.4 ppbv/d during summer. In contrast, the NOx budget shows a severe imbalance. Our results indicate that an additional source of NOx ranging from 18 to 48 pptv/d (in winter and summer, respectively) would be required to sustain the 30 pptv of NOx measured on average at the site during free tropospheric conditions. Acetone has little effect on the budget of HOx at the altitude of MLO (3.4 km). However, including this species in the model induces an even larger imbalance in the NOx budget through the production of peroxyacetylnitrate.