Observational analysis of the wind speed and turbulence relationship with NO2 concentration

<p>Nitrogen dioxide (NO₂) is one of the main concerns regarding air quality due to its impacts on human health in cities, where the main emissions come from the combustion of the vehicle&#8217;s engines. However, it is also well known that, apart from variations in emissions, meteorology plays a very important role on the evolution of ground-level concentrations in urban areas.</p><p>In this work, we compute correlations between NO₂ concentration with different meteorological and turbulent variables during two contrasting field campaigns in winter 2020 and summer 2021 in the city of Madrid, within the framework of the AIRTEC-CM project (*). Specifically, we compare the correlation of NO₂ with wind speed and with turbulent variables which include the vertical velocity component of the wind: the turbulent kinetic energy and the friction velocity, computed from two sonic anemometers installed at two different heights, the first one close to the street and the second one on the terrace of a tall building. Hence, the overall objective is to determine the variable and the emplacement that shows better correlation under specific conditions (summer, winter, and a stable period in winter). To this aim, we also investigate the usefulness of using some threshold values of these variables to be associated with high levels of pollutant concentration.</p><p>In our study, we highlight the importance of the different atmospheric processes observed during the diurnal cycle, which affect the NO₂levels reached. The highest NO₂ concentrations are observed during the evening and the initial part of the night with fair-weather conditions in winter due to the incipient atmospheric-boundary-layer (ABL) stabilization. Nonetheless, the levels reached are very sensitive to small variations in the afternoon and evening wind speed and turbulence. Besides, these days are also characterized by the arrival of nocturnal thermally-driven flows, which are formed only a few hours after the aforementioned stabilization. These wind values are found to play a crucial role on the rapid reduction of the concentrations, becoming very important phenomena for the pollutant reduction in areas of complex terrain and heterogeneous surfaces, as Madrid region is. This result contrasts with the pollutant evolution observed in cities located also in complex terrain, but with different features, like cities in cold pools (i.e., valleys). Finally, we show the importance of some short-lived -and difficult to predict- wind events that generate intermittent turbulence and affect the evolution of the pollutant concentration during the first hours of the stable ABL.</p><p>(*) AIRTEC-CM project (S2018/EMT-4329), funded by The Regional Government of Madrid.</p>