![]() ![]() This was due to an increasing number of detected COVID-19 cases and hospital admissions: Spain detected 9,222 new cases in 24 h on 31 March 2020. After 30 March, the government implemented even more restrictive measures on 9 April, when only some essential services, such as pharmacies, the food sector, and some industries, were allowed. This situation occurred worldwide ( Anjum, 2020 Bao and Zhang, 2020). Where possible, non-essential activities were restricted to teleworking, and travel by car was completely forbidden unless it was indispensable. The viral threat forced the government to implement a state of alarm on 14 March, which restricted the mobility of the citizenry to strictly necessary activities, such as purchasing food and receiving medical assistance. Spain was one of the most affected countries in the first wave of COVID-19. The rapid spread of coronavirus disease 19 (COVID-19) affected the European countries from March 2020 onward. A set of CTM simulations should be done in the future to assess the O3-VOCs-NOx chemistry. The PM 10 was the most difficult case to analyze because of its dependence on external transport phenomena, which were difficult to consider in the models. The O 3 concentration showed a different behavior depending on the type of AQS, pointing to changes in the regime of other pollutants, such as VOCs. The results showed around a 60% reduction in the NO 2 at the three types of AQSs (traffic, suburban, and background) during the most restrictive months of the pandemic. The predictions could then be compared to the real observed AQS data to determine the meteorological normalized reductions. The XGBoost and LightGBM gradient boosting decision tree-based models were applied to the time series recorded in Madrid and used to predict the expected concentrations in 2020 if no restrictions had been made. This study used the approach of modeling a normal business day using both air quality and meteorological data from 2017 to 2019 to train machine-learning models to be able to predict what concentration of the three most concerning pollutants (NO 2, O 3, and PM 10) would be given by the meteorological conditions and the time of the year. Although a direct comparison between the observed measured values given by reference air quality stations (AQSs) and values from before the COVID lockdown provides an idea of the pollution reduction, it cannot be separated from the meteorology, and hence, those studies could be misleading. This constituted a unique and valuable opportunity to study the effect of air quality pollutant concentrations. As a consequence, a low-emission scenario that could only be simulated before, actually occurred. The coronavirus disease (COVID) lockdown was implemented in 2020, which included harsh restrictions on the amount of traffic. ETSII-UPM (Escuela Técnica Superior de Ingenieros Industriales), Madrid, Spain.Jose María Cordero *, Adolfo Narros and Rafael Borge ![]()
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