Ozone air quality simulations with WRF-Chem (v3.5.1) over Europe: model evaluation and chemical mechanism comparison

We present an evaluation of the online regional model WRF-Chem over Europe with a focus on ground-level ozone (O₃) and nitrogen oxides (NO_<i>x</i>). The model performance is evaluated for two chemical mechanisms, MOZART-4 and RADM2, for year-long simulations. Model-predicted surface meteorological variables (e.g., temperature, wind speed and direction) compared well overall with surface-based observations, consistent with other WRF studies. WRF-Chem simulations employing MOZART-4 as well as RADM2 chemistry were found to reproduce the observed spatial variability in surface ozone over Europe. However, the absolute O₃ concentrations predicted by the two chemical mechanisms were found to be quite different, with MOZART-4 predicting O₃ concentrations up to 20 µg&thinsp;m⁻³ greater than RADM2 in summer. Compared to observations, MOZART-4 chemistry overpredicted O₃ concentrations for most of Europe in the summer and fall, with a summertime domain-wide mean bias of +10 µg&thinsp;m⁻³ against observations from the AirBase network. In contrast, RADM2 chemistry generally led to an underestimation of O₃ over the European domain in all seasons. We found that the use of the MOZART-4 mechanism, evaluated here for the first time for a European domain, led to lower absolute biases than RADM2 when compared to ground-based observations. The two mechanisms show relatively similar behavior for NO_<i>x</i>, with both MOZART-4 and RADM2 resulting in a slight underestimation of NO_<i>x</i> compared to surface observations. Further investigation of the differences between the two mechanisms revealed that the net midday photochemical production rate of O₃ in summer is higher for MOZART-4 than for RADM2 for most of the domain. The largest differences in O₃ production can be seen over Germany, where net O₃ production in MOZART-4 is seen to be higher than in RADM2 by 1.8 ppb&thinsp;h⁻¹ (3.6 µg&thinsp;m⁻³&thinsp;h⁻¹) or more. We also show that while the two mechanisms exhibit similar NO_<i>x</i> sensitivity, RADM2 is approximately twice as sensitive to increases in anthropogenic VOC emissions as MOZART-4. Additionally, we found that differences in reaction rate coefficients for inorganic gas-phase chemistry in MOZART-4 vs. RADM2 accounted for a difference of 8 µg&thinsp;m⁻³, or 40 % of the summertime difference in O₃ predicted by the two mechanisms. Differences in deposition and photolysis schemes explained smaller differences in O₃. Our results highlight the strong dependence of modeled surface O₃ over Europe on the choice of gas-phase chemical mechanism, which we discuss in the context of overall uncertainties in prediction of ground-level O₃ and its associated health impacts (via the health-related metrics MDA8 and SOMO35)