Carbonaceous aerosols (CA), composed of black carbon (BC) and organic matter (OM), significantly impact the climate. Light absorption properties of CA, particularly of BC and brown carbon (BrC), are crucial due to their contribution to global and regional warming. We present the absorption properties of BC (bAbs,BC) and BrC (bAbs,BrC) inferred using Aethalometer data from 44 European sites covering different environments (traffic (TR), urban (UB), suburban (SUB), regional background (RB) and mountain (M)). Absorption coefficients showed a clear relationship with station setting decreasing as follows: TR > UB > SUB > RB > M, with exceptions. The contribution of bAbs,BrC to total absorption (bAbs), i.e. %AbsBrC, was lower at traffic sites (11-20 %), exceeding 30 % at some SUB and RB sites. Low AAE values were observed at TR sites, due to the dominance of internal combustion emissions, and at some remote RB/M sites, likely due to the lack of proximity to BrC sources, insufficient secondary processes generating BrC or the effect of photobleaching during transport. Higher bAbs and AAE were observed in Central/Eastern Europe compared to Western/Northern Europe, due to higher coal and biomass burning emissions in the east. Seasonal analysis showed increased bAbs, bAbs,BC, bAbs,BrC in winter, with stronger %AbsBrC, leading to higher AAE. Diel cycles of bAbs,BC peaked during morning and evening rush hours, whereas bAbs,BrC, %AbsBrC, AAE, and AAEBrC peaked at night when emissions from household activities accumulated. Decade-long trends analyses demonstrated a decrease in bAbs, due to reduction of BC emissions, while bAbs,BrC and AAE increased, suggesting a shift in CA composition, with a relative increase in BrC over BC. This study provides a unique dataset to assess the BrC effects on climate and confirms that BrC can contribute significantly to UV-VIS radiation presenting highly variable absorption properties in Europe.

Aerosol d o o Kamniška 39A 1000 Ljubljana Slovenia

Aerosol Physics Laboratory Faculty of Engineering and Natural Sciences Tampere University Tampere University P O Box 692 FI 33014 Finland; Univ Grenoble CNRS IRD IGE 38000 Grenoble France

Aix Marseille Univ CNRS LCE Marseille France

Andalusian Institute for Earth System Research University of Granada Granada Spain

ANDRA DRD GES Observatoire Pérenne de l'Environnement 55290 Bure France

ARPA Lazio Regional Environmental Protection Agency Rome Italy

Arpa Lombardia Settore Monitoraggi Ambientali Unità Operativa Qualità dell'Aria Milano Italy

Atmospheric Composition Research Finnish Meteorological Institute 00560 Helsinki Finland

Atmospheric Composition Research Finnish Meteorological Institute 00560 Helsinki Finland; Aerosol Physics Laboratory Faculty of Engineering and Natural Sciences Tampere University Tampere University P O Box 692 FI 33014 Finland

Atmospheric Composition Research Finnish Meteorological Institute 00560 Helsinki Finland; Institute for Atmospheric and Earth System Research Physics Faculty of Science University of Helsinki Helsinki Finland

Atmospheric Pollution Laboratory Department of Applied Physics Miguel Hernández University Elche 03202 Spain

Center for Atmospheric Research University of Nova Gorica Ajdovščina 5270 Slovenia

Center for Atmospheric Research University of Nova Gorica Ajdovščina 5270 Slovenia; Aerosol d o o Kamniška 39A 1000 Ljubljana Slovenia

Center for Atmospheric Research University of Nova Gorica Ajdovščina 5270 Slovenia; Haze Instruments d o o Ljubljana 1000 Slovenia; Department of Condensed Matter Physics Jozef Stefan Institute Ljubljana 1000 Slovenia

Division of Environmental Health and Risk Management School of Geography Earth and Environmental Sciences University of Birmingham Edgbaston Birmingham UK

Division of Environmental Health and Risk Management School of Geography Earth and Environmental Sciences University of Birmingham Edgbaston Birmingham UK; 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 analysis Stockholm Sweden

Environment Department CIEMAT Madrid Spain

Environmental Research Group MRC Centre for Environment and Health Imperial College London London W12 0BZ UK

Estonian Environmental Research Centre Air Quality Management Department Tallinn Estonia

European Commission Joint Research Centre Ispra Italy

Federal Office of Meteorology and Climatology MeteoSwiss Payerne Switzerland

German Meteorological Service Observatory Hohenpeissenberg Germany

Global Change Research Institute Czech Academy of Sciences 603 00 Brno Czech Republic

Helsinki Region Environmental Services Authority Helsinki Finland

IMT Nord Europe Institut Mines Télécom Univ Lille Centre for Energy and Environment Lille France

Institut National de l'Environnement Industriel et des Risques Verneuil en Halatte France

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

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

Institute of Atmospheric Sciences and Climate 00133 Rome Italy

Institute of Atmospheric Sciences and Climate 40129 Bologna Italy

Institute of Chemical Process Fundamentals of the CAS Rozvojová 135 1 16502 Prague Czech Republic

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 Universitat de Barcelona Barcelona 08028 Spain

Institute of Environmental Assessment and Water Research Manresa 08242 Spain

Laboratoire de Météorologie Physique UMR6016 Université Clermont Auvergne CNRS Aubière France

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

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

National Institute of Research and Development for Optoelectronics INOE 2000 Magurele Romania

NILU 2027 Kjeller Norway

PSI Center for Energy and Environmental Sciences Paul Scherrer Institute 5232 Villigen PSI Switzerland

PSI Center for Energy and Environmental Sciences Paul Scherrer Institute 5232 Villigen PSI Switzerland; Aix Marseille Univ CNRS LCE Marseille France

PSI Center for Energy and Environmental Sciences Paul Scherrer Institute 5232 Villigen PSI Switzerland; Datalystica Ltd Parkstrasse 1 5234 Villigen Switzerland

School of Natural Sciences Physics Centre for Climate and Air Pollution Studies Ryan Institute University of Galway University Road Galway H91 CF50 Ireland

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