This study was partially supported by the European Union's projects ACTRIS (EU FP7-262254) and ACTRIS-2 (EU Horizon 2020–654109). COST Action CA16109 COLOSSAL, Chemical On-Line cOmpoSition and Source Apportionment of fine aerosoL, is acknowledged. The ACSM observations at Birkenes was funded by the Norwegian Ministry of Climate and Environment Strategic Institute Program. IDAEA-CSIC (3 datasets: BCN, MSA, MSY) was partially supported by the Spanish Ministry of Economy and Competitiveness and FEDER funds under the PRISMA project (CGL 2012-39623-C02-1). The London measurements were supported by the UK National Research Council through the ClearfLo project and a PhD studentship (grant refs. NE/H008136/1 and NE/I528142/1) and the Department of Environment, Food and Rural Affairs (DEFRA). ECPL personel, namely Nikolaos Mihalopoulos, Aikaterini Bougiatioti and Iasonas Stavroulas acknowledge support by the project “Panhellenic infrastructure for atmospheric composition and climate change, PANACEA” (MIS 5021516) which is implemented under the Action “Reinforcement of the Research and Innovation Infrastructure”, funded by the Operational Programme” Competitiveness, Entrepreneurship and Innovation” (NSRF 2014–2020) and co-financed by Greece and the European Union (European Regional Development Fund). IMT Lille Douai acknowledges financial support from the CaPPA (Chemical and Physical Properties of the Atmosphere) project funded by the French National Research Agency (ANR) through the PIA (Programme d'Investissement d'Avenir) under contract ANR-11-LABX-0005-01, and two CPER projects funded by the French Ministry of Higher Education and Research, the CNRS, the Regional Council “Hauts-de-France” and the European Regional Development Fund (ERDF): Climibio, and IRENI (additionally financed by the Communauté Urbaine de Dunkerque). S. Zhang thanks IMT Lille Douai and the Regional Council “Hauts-de-France” for her PhD grant. Prague co-authors would like to acknowledge a Czech MEYS's project under INTER-EXCELENCE INTERCOST program under grant agreement LTC18068 and from European Regional Development Fund-Project under the grant ACTRIS-CZ RI (CZ.02.1.01/0.0/0.0/16_013/0001315). EPA Ireland, Department of Communications, Climate Action and Environment (DCCAE) and the European Union's Seventh Framework Programme (FP7/2007–2013) project BACCHUS under grant agreement n_603445 are acknowledged for research support at Mace Head. The physical measurements were also funded by the German Ultrafine Aerosol Network GUAN, which was jointly established with help of the German Federal Environment Ministry (BMU) grants F&E 370343200 (German title: “Erfassung der Zahl feiner und ultrafeiner Partikel in der Auβenluft”), 2008–2010, and F&E 371143232 (German title: “Trendanalysen gesundheitsgefährdender Fein-und Ultrafeinstaubfraktionen unter Nutzung der im German Ultrafine Aerosol Network (GUAN) ermittelten Immissionsdaten durch Fortführung und Interpretation der Messreihen”) 2012–2014. We also acknowledge the WCCAP (World Calibration Center for Aerosol Physics) as part of the WMO-GAW program. The WCCAP is base-funded by the German Federal Environmental Agency (Umweltbundesamt), Germany. Support by the European Regional Development Funds (EFRE – Europe funds Saxony) is gratefully acknowledged. Atmospheric measurements performed in Corsica is part of the ChArMEx project supported by CNRS-INSU, ADEME, Météo-France and CEA in the framework of the multidisciplinary programme MISTRALS (Mediterranean Integrated Studies aT Regional And Local Scales; http://mistrals-home.org/, last access: June 10, 2020). Final data processing of these measurements has been supported by the EMME-CARE (Eastern Mediterranean and Middle East Climate and Atmosphere Research Center) which has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No. 856612 and the Cyprus Government. The measurements in Switzerland were supported by the Federal Office for the Environment. We thank the International Foundation High Altitude Research Stations Jungfraujoch and Gornergrat (HFSJG) for the opportunity to perform experiments on the Jungfraujoch.

Similarities and differences in the submicron atmospheric aerosol chemical composition are analyzed from a unique set of measurements performed at 21 sites across Europe for at least one year. These sites are located between 35 and 62°N and 10° W – 26°E, and represent various types of settings (remote, coastal, rural, industrial, urban). Measurements were all carried out on-line with a 30-min time resolution using mass spectroscopy based instruments known as Aerosol Chemical Speciation Monitors (ACSM) and Aerosol Mass Spectrometers (AMS) and following common measurement guidelines. Data regarding organics, sulfate, nitrate and ammonium concentrations, as well as the sum of them called non-refractory submicron aerosol mass concentration ([NR-PM1]) are discussed. NR-PM1 concentrations generally increase from remote to urban sites. They are mostly larger in the mid-latitude band than in southern and northern Europe. On average, organics account for the major part (36–64%) of NR-PM1 followed by sulfate (12–44%) and nitrate (6–35%). The annual mean chemical composition of NR-PM1 at rural (or regional background) sites and urban background sites are very similar. Considering rural and regional background sites only, nitrate contribution is higher and sulfate contribution is lower in mid-latitude Europe compared to northern and southern Europe. Large seasonal variations in concentrations (μg/m³) of one or more components of NR-PM1 can be observed at all sites, as well as in the chemical composition of NR-PM1 (%) at most sites. Significant diel cycles in the contribution to [NR-PM1] of organics, sulfate, and nitrate can be observed at a majority of sites both in winter and summer. Early morning minima in organics in concomitance with maxima in nitrate are common features at regional and urban background sites. Daily variations are much smaller at a number of coastal and rural sites. Looking at NR-PM1 chemical composition as a function of NR-PM1 mass concentration reveals that although organics account for the major fraction of NR-PM1 at all concentration levels at most sites, nitrate contribution generally increases with NR-PM1 mass concentration and predominates when NR-PM1 mass concentrations exceed 40 μg/m³ at half of the sites.

Peer reviewed