% reductions 1 Air quality 1 Atmospheric chemistry 1 Business-as-usual 1 COVID-19 1 Deterioration 1 Diesel 1 Diesel engines 1 Diesel passenger vehicles 1 Diesel vehicles 1 Diesel-powered vehicles 1 Environmental model 1 European cities 1 Learning algorithms 1 Machine learning 1 Machine-learning 1 Nitrogen oxides 1 Urban air quality 1 Vehicles 1 atmospheric chemistry diesel 1 environmental modeling 1 machine learning 1 nitrogen dioxide 1 urban air quality 1