Ultrafine particles (UFP, those with diameters ≤ 100 nm), have been reported to potentially penetrate deeply into the respiratory system, translocate through the alveoli, and affect various organs, potentially correlating with increased mortality. The aim of this study is to assess long-term trends (5-11 years) in mostly urban UFP concentrations based on measurements of particle number size distributions (PNSD). Additionally, concentrations of other pollutants and meteorological variables were evaluated to support the interpretations. PNSD datasets from 12 urban background (UB), 5 traffic (TR), 3 suburban background (SUB) and 1 regional background (RB) sites in 15 European cities and 1 in the USA were evaluated. The non-parametric Theil-Sen's method was used to detect monotonic trends. Meta-analyses were carried out to assess the overall trends and those for different environments. The results showed significant decreases in NO, NO2, BC, CO, and particle concentrations in the Aitken (25-100 nm) and the Accumulation (100-800 nm) modes, suggesting a positive impact of the implementation of EURO 5/V and 6/VI vehicle standards on European air quality. The growing use of Diesel Particle Filters (DPFs) might also have clearly reduced exhaust emissions of BC, PM, and the Aitken and Accumulation mode particles. However, as reported by prior studies, there remains an issue of poor control of Nucleation mode particles (smaller than 25 nm), which are not fully reduced with current DPFs, without emission controls for semi-volatile organic compounds, and might have different origins than road traffic. Thus, contrasting trends for Nucleation mode particles were obtained across the cities studied. This mode also affected the UFP and total PNC trends because of the high proportion of Nucleation mode particles in both concentration ranges. It was also found that the urban temperature increasing trends might have also influenced those of PNC, Nucleation and Aitken modes.

Department of Environment CIEMAT Madrid 28040 Spain

Department of Meteorology 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

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 Heinrich Heine University of 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

Laboratory of Aerosols Chemistry and Physics Institute of Chemical Process Fundamentals v v i Academy of Sciences of the Czech Republic Rozvojova 1 Prague Czech Republic

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

Saxon State Office for Environment Agriculture and Geology Dresden German

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