There is a body of evidence that ultrafine particles (UFP, those with diameters ≤ 100 nm) might have significant impacts on health. Accordingly, identifying sources of UFP is essential to develop abatement policies. This study focuses on urban Europe, and aims at identifying sources and quantifying their contributions to particle number size distribution (PNSD) using receptor modelling (Positive Matrix Factorization, PMF), and evaluating long-term trends of these source contributions using the non-parametric Theil-Sen's method. Datasets evaluated include 14 urban background (UB), 5 traffic (TR), 4 suburban background (SUB), and 1 regional background (RB) sites, covering 18 European and 1 USA cities, over the period, when available, from 2009 to 2019. Ten factors were identified (4 road traffic factors, photonucleation, urban background, domestic heating, 2 regional factors and long-distance transport), with road traffic being the primary contributor at all UB and TR sites (56-95 %), and photonucleation being also significant in many cities. The trends analyses showed a notable decrease in traffic-related UFP ambient concentrations, with statistically significant decreasing trends for the total traffic-related factors of -5.40 and -2.15 % yr-1 for the TR and UB sites, respectively. This abatement is most probably due to the implementation of European emissions standards, particularly after the introduction of diesel particle filters (DPFs) in 2011. However, DPFs do not retain nucleated particles generated during the dilution of diesel exhaust semi-volatile organic compounds (SVOCs). Trends in photonucleation were more diverse, influenced by a reduction in the condensation sink potential facilitating new particle formation (NPF) or by a decrease in the emissions of UFP precursors. The decrease of primary PM emissions and precursors of UFP also contributed to the reduction of urban and regional background sources.

Andalusian Institute for Earth System Research University of Granada Granada Spain

Department of Environment CIEMAT Madrid 28040 Spain

Department of Meteorology Institute of Geography and Earth Sciences Eötvös Loránd University Budapest Hungary

Department of Public Health Sciences University of Rochester School of Medicine and Dentistry Rochester NY 14642 USA

Division of Environmental Health and Risk Management School of Geography Earth and Environmental Sciences University of Birmingham Edgbaston Birmingham B15 2TT United Kingdom

Division of Environmental Health and Risk Management School of Geography Earth and Environmental Sciences University of Birmingham Edgbaston Birmingham B15 2TT United Kingdom; Department of Environmental Sciences Faculty of Meteorology Environment and Arid Land Agriculture King Abdulaziz University Jeddah Saudi Arabia

ENRACT Institute of Nuclear and Radiological Science and Technology Energy and Safety NCSR Demokritos 15310 Ag Paraskevi Athens Greece

Environment and Health Administration SLB analys Box 8136 104 20 Stockholm Sweden

Environmental Chemical Processes Laboratory Department of Chemistry University of Crete 71003 Heraklion Greece; Institute for Environmental Research and Sustainable Development National Observatory of Athens 11810 Athens Greece

European Commission Joint Research Centre 21027 Ispra Italy

Finnish Meteorological Institute Atmospheric Composition Research Helsinki Finland

Flanders Environment Agency Dokter De Moorstraat 24 26 9300 Aalst Belgium

Flemish Institute for Technological Research Boeretang 200 2400 Mol Belgium

Helsinki Region Environmental Services Authority 00240 Helsinki Finland

Hevesy György Ph D School of Chemistry Eötvös Loránd University Budapest Hungary

Institut National de l'Environnement Industriel et des Risques Parc Technologique Alata BP2 60550 Verneuil en Halatte France

Institute for Atmospheric and Earth System Research Faculty of Science University of Helsinki Finland

Institute for Environmental Research and Sustainable Development National Observatory of Athens 11810 Athens Greece

Institute for Environmental Research and Sustainable Development National Observatory of Athens 11810 Athens Greece; Department of Environment University of the Aegean 81100 Mytilene Greece

Institute for Occupational Social and Environmental Medicine Medical Faculty and University Hospital Düsseldorf Heinrich Heine University Düsseldorf Germany

Institute of Atmospheric Sciences and Climate of National Research Council ISAC CNR 73100 Lecce Italy

Institute of Chemistry Eötvös Loránd University Budapest Hungary

Institute of Environmental Assessment and Water Research 08034 Barcelona Spain

Institute of Environmental Assessment and Water Research 08034 Barcelona Spain; Department of Applied Physics Meteorology University of Barcelona Barcelona 08028 Spain

Institute of Environmental Assessment and Water Research Manresa 08242 Spain

Laboratoire des Sciences du Climat et de l'Environnement CEA Orme des Merisiers 91191 Gif sur Yvette France

Laboratory for Air Pollution and Environmental Technology Swiss Federal Laboratories for Materials Science and Technology 8600 Duebendorf Switzerland

Leibniz Institute for Tropospheric Research Leipzig Germany

MRC Centre for Environment and Health Environmental Research Group Imperial College London United Kingdom

MRC Centre for Environment and Health Environmental Research Group Imperial College London United Kingdom; NIHR HPRU in Environmental Exposures and Health Imperial College London United Kingdom

Research Group of Aerosol Chemistry and Physics Institute of Chemical Process Fundamentals v v i Academy of Sciences of the Czech Republic Rozvojova 1 Prague Czech Republic

Saxon State Office for Environment Agriculture and Geology Dresden German

University Lille CNRS UMR 8518 Laboratoire d'Optique Atmosphérique Lille France

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