Extreme PM and ozone pollution over central Europe: interactions of the urban canopy meteorological forcing and radiative effects of urban emissions

Extreme air pollution in European cities, especially those in central and eastern Europe is, regardless of strict pollution control measures, still present, representing a large health burden on their inhabitants. Understanding the processes that control or modulate such events over urban areas is therefore crucial. In general, the climate-chemistry interactions over urban areas are complex with multiple feedbacks. In this study, based on two air pollution events with i) high winter PM concentrations and stagnant conditions (14 days in January 2017), ii) elevated ozone levels during a dry sunny summer period (14 days in August 2015), we will examine the mutual role of urban emissions (and secondary pollutants formed from them) and the urban canopy meteorological forcing (UCMF) over central Europe. We performed a series of WRF-Chem simulations with/without urban land-surface (effect of rural-urban transition) and with/without urban emissions, while six large central European cities were considered. Impact on both meteorological conditions and chemical species is examined.

Regarding the impact on meteorological conditions (temperature, windspeed, boundary layer height), we showed that the direct effect of UCMF (1-2K for temperature) is much larger than the secondary effects of the radiative impacts of urban emissions (driven mainly by aerosol effects; 0.1 K for temperature in average). It was also shown that these radiative impacts depend whether UCMF is included or not, with differences up to 2 K in hourly values. The impact on chemical concentrations is driven especially by UCMF causing decrease of PM and increase of ozone while the indirect effects of urban emissions induced meteorological changes are substantially smaller.