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

Abstract

We present an evaluation of the online regional model WRF-Chem over Europe with a focus on ground-level ozone (O-3) 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 O-3 concentrations predicted by the two chemical mechanisms were found to be quite different, with MOZART-4 predicting O-3 concentrations up to 20 mu g m(-3) greater than RADM2 in summer. Compared to observations, MOZART-4 chemistry overpredicted O-3 concentrations for most of Europe in the summer and fall, with a summertime domain-wide mean bias of +10 mu g m(-3) against observations from the AirBase network. In contrast, RADM2 chemistry generally led to an underestimation of O-3 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 O-3 in summer is higher for MOZART-4 than for RADM2 for most of the domain. The largest differences in O-3 production can be seen over Germany, where net O-3 production in MOZART-4 is seen to be higher than in RADM2 by 1.8 ppbh 1 (3.6 mu g 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 mu g m(-3), or 40% of the summertime difference in O-3 predicted by the two mechanisms. Differences in deposition and photolysis schemes explained smaller differences in O-3. Our results highlight the strong dependence of modeled surface O-3 over Europe on the choice of gas-phase chemical mechanism, which we discuss in the context of overall uncertainties in prediction of ground-level O-3 and its associated health impacts (via the health-related metrics MDA8 and SOMO35)

Air quality modelling in the summer over the eastern Mediterranean using WRF-Chem: chemistry and aerosol mechanism intercomparison

“…Knote et al (2014) attributed the differences in pollutants concentrations between these mechanisms to the differences in the treatment of VOCs, since the rate constants for the basic O 3 production and loss reactions are similar. Hourly correlation coefficients for O 3 are low (less than 0.30) for all three mechanisms which is comparable to the findings of Mar et al (2016). In their study, summertime O 3 hourly correlation coefficients at the Ayia Marina station (AQ01) were close to 0.2.…”

“…During the second phase of the Air Quality Model Evaluation International Initiative (AQMEII), the majority of the modelling groups using the RMS and CM mechanisms with the WRF-Chem model also reported O 3 concentrations overestimation over the eastern Mediterranean (Im et al, 2015a). However, Mar et al (2016) reported an underestimation of about 5 ppbV on summertime O 3 concentrations WRF-Chem model using the RMS mechanism. Our sensitivity tests showed that high PM 2.5 concentrations affect the O 3 concentrations through the aerosolradiation feedbacks by altering the radiation budget and as a consequence, the photochemical activity and the concentration of secondary pollutants.…”

“…Mar et al (2016) and Zhang et al (2013) also reported WS 10 m overpredictions by the WRF model over the Mediterranean. The latter study attributed this model behaviour to the poor representation of surface drag exerted by the unresolved topography (mountains, hills and valleys) and other smaller scale terrain features.…”

“…It was noted that the use of a finer grid resolution may improve the overall model performance. Mar et al (2016) evaluated the performance of the Regional Acid Deposition Model (RADM2) and MOZART-4 gas-phase chemistry mechanisms at a horizontal grid resolution of 45 km over Europe. Simulated O 3 consecrations by MOZART-4 were found to be up to 20 µg m −3 higher than RADM2 during the summer due to a higher photochemical O 3 production rate.…”

WRF-Chem simulated surface ozone over south Asia during the pre-monsoon: effects of emission inventories and chemical mechanisms

“…A recent study by Mar et al (2016) highlighted the dependence of WRF-Chem predicted ozone air quality (over Europe) on the chosen chemical mechanism. These results indicate the need for evaluating the effects of emission inventories and chemical mechanisms on the model performance using a network of stations across south Asia, which has not been carried out thus far.…”