The impact of urban land-surface on extreme air pollution over central Europe

Abstract

Abstract. This paper deals with the urban land-surface impact (i.e., the urban canopy meteorological forcing; UCMF) on extreme air pollution for selected central European cities for present-day climate conditions (2015–2016) using three regional climate-chemistry models: the regional climate models RegCM and WRF-Chem (its meteorological part), the chemistry transport model CAMx coupled to either RegCM and WRF and the “chemical” component of WRF-Chem. Most of the studies dealing with the urban canopy meteorological forcing on air pollution focused on change in average conditions or only on a selected winter and/or summer air pollution episode. Here we extend these studies by focusing on long-term extreme air pollution levels by looking at not only the change in average values, but also their high (and low) percentile values, and we combine the analysis with investigating selected high-pollution episodes too. As extreme air pollution is often linked to extreme values of meteorological variables (e.g., low planetary boundary layer height, low winds, high temperatures), the urbanization-induced extreme meteorological modifications will be analyzed too. The validation of model results show reasonable model performance for regional-scale temperature and precipitation. Ozone is overestimated by about 10–20 µg m−3 (50 %–100 %); on the other hand, extreme summertime ozone values are underestimated by all models. Modeled nitrogen dioxide (NO2) concentrations are well correlated with observations, but results are marked by a systematic underestimation up to 20 µg m−3 (−50 %). PM2.5 (particles with diameter ≤2.5 µm) are systematically underestimated in most of the models by around 5 µg m−3 (50 %–70 %). Our results show that the impact on extreme values of meteorological variables can be substantially different from that of the impact on average ones: low (5th percentile) temperature in winter responds to UCMF much more than average values, while in summer, 95th percentiles increase more than averages. The impact on boundary layer height (PBLH), i.e., its increase is stronger for thicker PBLs and wind speed, is reduced much more for strong winds compared to average ones. The modeled changes in ozone (O3), NO2 and PM2.5 show the expected pattern, i.e., increase in average 8 h O3 up to 2–3 ppbv, decrease in daily average NO2 by around 2–4 ppbv and decrease in daily average PM2.5 by around −2 µg m−3. Regarding the impact on extreme (95th percentile) values of these pollutants, the impact on ozone at the high end of the distribution is rather similar to the impact on average 8 h values. A different picture is obtained however for extreme values of NO2 and PM2.5. The impact on the 95th percentile values is almost 2 times larger than the impact on the daily averages for both pollutants. The simulated impact on extreme values further well corresponds to the UCMF impact simulated for the selected high-pollution episodes. Our results bring light to the principal question: whether extreme air quality is modified by urban land surface with a different magnitude compared to the impact on average air pollution. We showed that this is indeed true for NO2 and PM2.5, while in the case of ozone, our results did not show substantial differences between the impact on mean and extreme values.

The “urban meteorology island”: a multi-model ensemble analysis

“…The remaining RegCM simulations show smaller temperature biases in summer, even less than those of Huszár et al (2018), who used a very similar model setup. Huszar et al (2020b) used the model configuration corresponding to RU simulation (i.e. the Nogherotto MP, Tiedtke convection and UW PBL schemes), but it gives, in 9 km resolution, smaller biases: summer underestimation and winter Figure 12.…”

Advances in Air Quality Research – Current and Emerging Challenges

“…Meteorology is one of the main uncertainties in air quality assessment and forecast in urban areas where meteorological characteristics are very inhomogeneous (Hidalgo et al, 2008;Ching, 2013;Huszar et al, 2018Huszar et al, , 2020. For these reasons, models used at the urban level must achieve greater accuracy in the meteorological fields (wind speed, temperature, turbulence, humidity, cloud water, precipitation).…”

The regional impact of urban emissions on air quality in Europe: the role of the urban canopy effects

“…Two regional climate models (RCMs) as meteorological drivers, RegCM version 4.7 and Weather Research and Forecasting model coupled with chemistry (WRF-Chem) version 4.0.3, and two chemical transport models (CTMs), the Comprehensive Air Quality Model with Extensions (CAMx) version 6.50 and the online coupled chemical module of the WRF-Chem model, were used in the study. As the models and their parameterizations are identical to those in Huszar et al (2020b), here we will provide only the most relevant information.…”

“…Model simulations were performed over the same nested domains and for the same period as in Huszar et al (2020b) with 9, 3 and 1 km resolution centered over Prague, Czechia (50.075 • N, 14.44 • E; Lambert conic conformal projection). The model orography including the placement of the three domains and the cities analyzed are presented in Fig.…”

“…To fulfill the goal of the study, several simulations have been performed with and without including the effects of both the urban canopy meteorological forcing and the chemical footprint of the urban emissions (i.e., the UEI). Part of the simulations that are analyzed in this work had been already analyzed in Huszar et al (2020b): these included experiments with all emissions considered but with or without considering the UCMF. As the focus of this paper is to evaluate the impact of urban emissions, we extend these simulations with those without the inclusion of such emissions (for selected cities).…”