Effects of near-surface atmospheric stability and turbulence on air pollution levels in Madrid 

<p>Air pollution is one of the main hazards for human health worldwide, especially in cities. Surface emissions are the main responsible for the presence of pollutants in the near atmosphere, but meteorological conditions typically play a fundamental role in their accumulation or dispersion. In this work, we focus on the near-surface atmospheric stability and turbulence, for which data from four field campaigns with meteorological and pollutant measurements belonging to the AIRTEC-CM project^(*) have been analysed. Available data correspond to the winter and summer seasons of 2020 and 2021 at two public sites located in the city centre of Madrid: a University (ETSII) and a Hospital (HCSC). The evolution of turbulence and stability and their relationships with pollutants such as NO₂, PM_2,5 and PM₁₀ are investigated. To study turbulence, an analysis of friction velocity and turbulent kinetic velocity is carried out, while the stability parameter (Obukhov length) and the Richardson number obtained by means of the simplified universal similarity functions have been considered for the analysis of the stability. Through the analysis of the mean daily cycle of pollutants, it has been observed how its evolution shows two maxima centered on the morning and evening transitions. The results obtained indicate that pollutant concentrations are strongly influenced by the daily cycle of stability and turbulence, marked by the presence of a maximum of turbulence in the central hours of the day (when the sensible heat flux is greater) and a strong decay of this in the day-night transitions, which in the case of the afternoon transition coincides with the shift from the convective to the stable boundary layer. In addition, the time when this turbulence decay occurs modulates the pollutant concentration values reached, added to the intensity of the stability itself.</p> <p>&#160;</p> <p>(*) AIRTEC-CM research project (S2018/EMT-4329) is funded by The Regional Government of Madrid (Spain) and the European Union.</p>