Articles | Volume 9, issue 10
Development and technical paper
19 Oct 2016
Development and technical paper |  | 19 Oct 2016

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

Kathleen A. Mar, Narendra Ojha, Andrea Pozzer, and Tim M. Butler

Abstract. We present an evaluation of the online regional model WRF-Chem over Europe with a focus on ground-level ozone (O3) and nitrogen oxides (NOx). 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 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 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 NOx, with both MOZART-4 and RADM2 resulting in a slight underestimation of NOx 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 h−1 (3.6 µg m−3 h−1) or more. We also show that while the two mechanisms exhibit similar NOx 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 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).

Short summary
Ground-level ozone is an air pollutant with adverse effects on human and ecosystem health and is also a climate forcer with a significant warming effect. This paper presents the setup and evaluation of a model for ozone air quality over Europe. Within the model evaluation, we compare the use of two commonly used photochemical schemes, and we conclude that uncertainties in the representation of chemistry are important to consider when using air quality models for policy applications.