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Although particulate organic and elemental carbon (OC and EC) are important constituents of the suspended atmospheric particulate matter (PM), measurements of OC and EC are much less common and more uncertain than measurements of e.g. the ionic components of PM. In the framework of atmospheric research infrastructures supported by the European Union, actions have been undertaken to determine and mitigate sampling artefacts, and assess the comparability of OC and EC data obtained in a network of 10 atmospheric observatories across Europe. Positive sampling artefacts (from 0.4 to 2.8 μg C/m3) and analytical discrepancies (between −50% and +40% for the EC/TC ratio) have been taken into account to generate a robust data set, from which we established the phenomenology of carbonaceous aerosols at regional background sites in Europe. Across the network, TC and EC annual average concentrations range from 0.4 to 9 μg C/m3, and from 0.1 to 2 μg C/m3, respectively. TC/PM10 annual mean ratios range from 0.11 at a Mediterranean site to 0.34 at the most polluted continental site, and TC/PM2.5 ratios are slightly greater at all sites (0.15–0.42). EC/TC annual mean ratios range from 0.10 to 0.22, and do not depend much on PM concentration levels, especially in winter. Seasonal variations in PM and TC concentrations, and in TC/PM and EC/TC ratios, differ across the network, which can be explained by seasonal changes in PM source contributions at some sites. © 2016 The Authors ; EUSAAR (FP6/2001-2006, grant agreement no. RII3-GT-2006-026140), ACTRIS (FP7/2007-2013, grant agreement no. 262254), ACTRIS2 (H2020-INFRAIA-2014-2015, grant agreement no. 654109), EPA Ireland is acknowledged for the research support at Mace Head, the Norwegian Environment Agency for the measurements at Birkenes. Appendix A ; Peer reviewed

Thermal-optical analysis is currently under consideration by the European standardization body (CEN) as the reference method to quantitatively determine organic carbon (OC) and elemental carbon (EC) in ambient air. This paper presents an overview of the critical parameters related to the thermal-optical analysis including thermal protocols, critical factors and interferences of the methods examined, method inter-comparisons, inter-laboratory exercises, biases and artifacts, and reference materials. The most commonly used thermal protocols include NIOSH-like, IMPROVE_A and EUSAAR_2 protocols either with light transmittance or reflectance correction for charring. All thermal evolution protocols are comparable for total carbon (TC) concentrations but the results vary significantly concerning OC and especially EC concentrations. Thermal protocols with a rather low peak temperature in the inert mode like IMPROVE_A and EUSAAR_2 tend to classify more carbon as EC compared to NIOSH-like protocols, while charring correction based on transmittance usually leads to smaller EC values compared to reflectance. The difference between reflectance and transmittance correction tends to be larger than the difference between different thermal protocols. Nevertheless, thermal protocols seem to correlate better when reflectance is used as charring correction method. The difference between EC values as determined by the different protocols is not only dependent on the optical pyrolysis correction method, but also on the chemical properties of the samples due to different contributions from various sources. The overall conclusion from this literature review is that it is not possible to identify the "best" thermal-optical protocol based on literature data only, although differences attributed to the methods have been quantified when possible. ; This work was undertaken under Mandate M/503 "Standardisation mandate to CEN, CENELEC and ETSI in support of the implementation of the Ambient Air Quality Legislation", ENX "Ambient air – Measurement of airborne lemental carbon (EC) and organic carbon (OC) in PM 2.5 deposited on filters". ; EUR 1,920 APC fee funded by the EC FP7 Post-Grant Open Access Pilot ; Peer reviewed

The third intensive measurement period (IMP) organised by the European Monitoring and Evaluation Programme (EMEP) under the UNECE CLTRAP took place in summer 2012 and winter 2013, with PM10 filter samples concurrently collected at 20 (16 EMEP) regional background sites across Europe for subsequent analysis of their mineral dust content. All samples were analysed by the same or a comparable methodology. Higher PM10 mineral dust loadings were observed at most sites in summer (0.5-10 μgm-3) compared to winter (0.2-2 μgm-3), with the most elevated concentrations in the southern-and easternmost countries, accounting for 20-40% of PM10. Saharan dust outbreaks were responsible for the high summer dust loadings at western and central European sites, whereas regional or local sources explained the elevated concentrations observed at eastern sites. The eastern Mediterranean sites experienced elevated levels due to African dust outbreaks during both summer and winter. The mineral dust composition varied more in winter than in summer, with a higher relative contribution of anthropogenic dust during the former period. A relatively high contribution of K from non-mineral and non-sea-salt sources, such as biomass burning, was evident in winter at some of the central and eastern European sites. The spatial distribution of some components and metals reveals the influence of specific anthropogenic sources on a regional scale: shipping emissions (V, Ni, and SO2- 4 ) in the Mediterranean region, metallurgy (Cr, Ni, and Mn) in central and eastern Europe, high temperature processes (As, Pb, and SO2- 4 ) in eastern countries, and traffic (Cu) at sites affected by emissions from nearby cities. © 2016 Author(s). CC Attribution 3.0 License. ; The present work was supported by the Cooperative Programme for Monitoring and Evaluation of the Longrange Transmission of Air pollutants in Europe (EMEP) under UNECE. We also acknowledge support by the European Union Seventh Framework Programme (FP7/2007-2013) through ACTRIS (grant agreement no. 262254) and by the European Union's Horizon 2020 research and innovation programme under grant agreement No. 654109. The participation of IDAEA-CSIC was supported by the Spanish Ministry of Economy and Competitiveness and FEDER funds under the project PRISMA (CGL2012-39623-C02-1), by the Generalitat de Catalunya (AGAUR 2015 SGR33 and the DGQA). The authors wish to thank the Norwegian Ministry of foreign affairs for support to the measurements in Armenia and Moldova. The French participation to the campaigns was funded by the French Agency of Environment and Energy Management (ADEME, grants 1262C0022 and 1262C0039). Mines Douai acknowledges support from the CaPPA project which is financed by the French National Research Agency (ANR) through the PIA (Programme d'Investissement d'Avenir) under contract ANR-11-LABX-0005- 01, the "Nord-Pas de Calais" Regional Council and the European Regional Development Fund (ERDF). Participation of the ANDRA is also acknowledged for the measurements at OPE-ANDRA. The authors are also indebted to the fieldwork teams at Auchencorth Moss and Harwell (NERC CEH and Riccardo Energy and Environment Staff). The authors would also like to express our gratitude to the Atmospheric Modelling Laboratory from the Barcelona Supercomputing Centre, the Naval Research Laboratory and the SeaWiFS project (NASA) for the provision of the DREAM, NAAPs aerosol maps and the satellite imagery, respectively. The authors wish to thank D. C. Carslaw and K. Ropkins for providing the Openair software used in this paper (Carslaw and Ropkins, 2012; Carslaw, 2012). ; Peer reviewed

This investigation presents the variability of near-surface in-situ aerosol particle light scattering measurements obtained over the past decade at 28 measuring atmospheric observatories which are part of the ACTRIS Research Infrastructure and most of them belong to the GAW network. This manuscript provides a comprehensive picture of the spatial and temporal variability of aerosol particles optical properties in Europe. ; This project has received funding from the European Union's Horizon 2020 research and 11 innovation programme under grant agreement No 654109, ACTRIS (project No. 262254), ACTRIS12 PPP (project No 739530).MAD station is co-financed by the PROACLIM ( CGL2014-52877-R) project. SMR station acknowledges BACCHUS (project No. 603445), CRAICC (project No. 26060) and Academy of Finland (project No. 3073314). UGR station is co-financed by the Spanish Ministry of Economy and Competitiveness through project CGL2016-81092-R. Measurements at Montseny and Montsec stations were supported by the MINECO (Spanish Ministry of Economy and Competitiveness) and FEDER funds under the PRISMA project (CGL2012-39623-C02/00), by the MAGRAMA (Spanish Ministry of Agriculture, Food and Environment) and by the Generalitat de Catalunya (AGAUR 2014 SGR33 and the DGQA). Measurements at Izaña were supported by AEROATLAN project (CGL2015-17 66229-P), co-funded by the Ministry of Economy and Competitiveness of Spain and the European Regional Development Fund. Station Košetice is supported by Ministry of Education, Youth and Sports of the Czech Republic within project for support of national research infrastructure ACTRIS – participation of the Czech Republic (ACTRIS-CZ – LM2015037). Measurements at Puy de Dôme were partly supported by CNRS-INSU, University Clermont- Auvergne, OPGC and the french CLAP program. PAL station acknowledges KONE Foundation, Academy of Finland (project No. 269095 and No. 296302). CHC station received support from Institut de Recherche pour le Développement (IRD) under both Jeune Equipe program attributed ...

Aerosol particles are a complex component of the atmospheric system which influence climate directly by interacting with solar radiation, and indirectly by contributing to cloud formation. The variety of their sources, as well as the multiple transformations they may undergo during their transport (including wet and dry deposition), result in significant spatial and temporal variability of their properties. Documenting this variability is essential to provide a proper representation of aerosols and cloud condensation nuclei (CCN) in climate models. Using measurements conducted in 2016 or 2017 at 62 ground-based stations around the world, this study provides the most up-to-date picture of the spatial distribution of particle number concentration (Ntot) and number size distribution (PNSD, from 39 sites). A sensitivity study was first performed to assess the impact of data availability on Ntot's annual and seasonal statistics, as well as on the analysis of its diel cycle. Thresholds of 50g% and 60g% were set at the seasonal and annual scale, respectively, for the study of the corresponding statistics, and a slightly higher coverage (75g%) was required to document the diel cycle. Although some observations are common to a majority of sites, the variety of environments characterizing these stations made it possible to highlight contrasting findings, which, among other factors, seem to be significantly related to the level of anthropogenic influence. The concentrations measured at polar sites are the lowest (g1/4g102gcm-3) and show a clear seasonality, which is also visible in the shape of the PNSD, while diel cycles are in general less evident, due notably to the absence of a regular day-night cycle in some seasons. In contrast, the concentrations characteristic of urban environments are the highest (g1/4g103-104gcm-3) and do not show pronounced seasonal variations, whereas diel cycles tend to be very regular over the year at these stations. The remaining sites, including mountain and non-urban continental and coastal stations, do not exhibit as obvious common behaviour as polar and urban sites and display, on average, intermediate Ntot (g1/4g102-103gcm-3). Particle concentrations measured at mountain sites, however, are generally lower compared to nearby lowland sites, and tend to exhibit somewhat more pronounced seasonal variations as a likely result of the strong impact of the atmospheric boundary layer (ABL) influence in connection with the topography of the sites. ABL dynamics also likely contribute to the diel cycle of Ntot observed at these stations. Based on available PNSD measurements, CCN-sized particles (considered here as either >50gnm or >100gnm) can represent from a few percent to almost all of Ntot, corresponding to seasonal medians on the order of g1/4g10 to 1000gcm-3, with seasonal patterns and a hierarchy of the site types broadly similar to those observed for Ntot. Overall, this work illustrates the importance of in situ measurements, in particular for the study of aerosol physical properties, and thus strongly supports the development of a broad global network of near surface observatories to increase and homogenize the spatial coverage of the measurements, and guarantee as well data availability and quality. The results of this study also provide a valuable, freely available and easy to use support for model comparison and validation, with the ultimate goal of contributing to improvement of the representation of aerosol-cloud interactions in models, and, therefore, of the evaluation of the impact of aerosol particles on climate. ; NOAA base funding supports the observatories BRW, BND, MLO, SMO, SPO and THD, where efforts of the dedicated observatory staff and of programmer Derek Hageman are appreciated. BRW observations are also supported in part by the Atmospheric Radiation Measurement (ARM) user facility, a US Department of Energy (DOE) Office of Science user facility managed by the Biological and Environmental Research programme. Measurements at Welgegund are supported by North-West University, the University of Helsinki and the Finnish Meteorological Institute. This publication also forms part of the output of the Biogeochemistry Research Infrastructure Platform (BIOGRIP) of the Department of Science and Innovation of South Africa. Pallas and SMEAR II are grateful for the support of the Academy of Finland Centre of Excellence programme (project no. 272041), the Academy of Finland project Greenhouse gas, aerosol and albedo variations in the changing Arctic (project no. 269095), and the Novel Assessment of Black Carbon in the Eurasian Arctic: From Historical Concentrations and Sources to Future Climate Impacts (NABCEA, project no. 296302). Aerosol measurements at Anmyeon-do were supported by the Korea Meteorological Administration Research and Development Program "Development of Monitoring and Analysis Techniques for Atmospheric Composition in Korea" under grant KMA2018-00522. Measurements at Gosan were supported by the National Research Foundation of Korea (2017R1D1A1B06032548) and the Korea Meteorological Administration Research and Development Program under grant KMI2018-01111. The Lulin station is operated under the grants funded by the Taiwan Environmental Protection Administration. WLG is supported by the China Meteorological Administration, where efforts of the dedicated observatory staff are appreciated. Sites PDM, PUY, GIF, CHC and RUN are partially operated with the support of CNRS-INSU under the long-term observation programme and the French Ministry for Research under the ACTRIS-FR national research infrastructure. PDM and GIF received specific support from the French Ministry of the Environment. ATMO Occitanie is mentioned for sampling operations at PDM. Measurements at SIRTA are hosted by CNRS and by the alternative energies and atomic energy commission (CEA) with additional contributions from the French Ministry of the Environment through its funding to the reference laboratory for air quality monitoring (LCSQA). PUY is grateful for support from ATMO Auvergne Rhône Alpes for sampling operations and the support from the personnel of the Observatoire de Physique du Globe de Clermont-Ferrand (OPGC). The specific support of the Institut de Recherche et Développement (IRD) in France and the Universidad Mayor de San Andrés in Bolivia support operations at CHC operations. The Steamboat Ski Resort provided logistical support and in-kind donations for SPL. The Desert Research Institute is a permittee of the Medicine Bow–Routt National Forests and an equal opportunity service provider and employer. SPL appreciates the extensive assistance of the NOAA/ESRL Federated Aerosol Network, of Ian McCubbin, site manager of SPL, and of Ty Atkins, Joe Messina, Dan Gilchrist and Maria Garcia, who provided technical assistance with the maintenance and data quality control for the aerosol instruments. SGP measurements/mentorship were supported by DOE-7F-30118 and staff on site. The Cape Grim Baseline Air Pollution Monitoring Station is grateful to the Australian Bureau of Meteorology for their long-term and continued support and all the staff from the Bureau of Meteorology and CSIRO, who have contributed to the generation of records reported here. The aerosol measurements at the Jungfraujoch were conducted with financial support from MeteoSwiss (GAW-CH aerosol monitoring programme) and from the European Union as well as the Swiss State Secretariat for Education, Research and Innovation (SERI) for the European Research Infrastructure for the observation of Aerosol, Clouds and Trace Gases (ACTRIS). The International Foundation High Altitude Research Station Jungfraujoch and Gornergrat (HFSJG) is mentioned for providing the research platform at the Jungfraujoch. The aerosol measurements at Kosetice received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement no. 654109 and from the project for support of the national research infrastructure ACTRIS – participation of the Czech Republic (ACTRIS-CZ – LM2015037) supported by the Ministry of Education, Youth and Sports of CR within National Sustainability Program I (NPU I), grant agreement no. LO1415. The measurements were also supported by ERDF "ACTRIS-CZ RI" (no. CZ.02.1.01/0.0/0.0/16_013/0001315). Measurements at the Madrid site were funded by the following projects: CRISOL (CGL2017–85344-R MINECO/AEI/FEDER, UE), TIGAS-CM (Madrid Regional Government Y2018/EMT5177), AIRTEC-CM (Madrid Regional Government P2018/EMT4329), REDMAAS2020 (RED2018-102594-T CIENCIA) and Red de Excelencia ACTRIS-ESPAÑA (CGL2017-90884-REDT). Measurements at Montsec and Montseny were supported by the Spanish Ministry of Economy, Industry and Competitiveness and FEDER funds under project HOUSE (CGL2016-78594-R) and by the Generalitat de Catalunya (AGAUR 2017 SGR41 and the DGQA). Aerosol measurements at El Arenosillo Observatory are supported by the National Institute for Aerospace Technology and by different R&D projects of the Ministerio Español de Economía, Industria y Competitividad (MINECO). Aerosol measurements at UGR are supported by the Spanish Ministry of Economy and Competitiveness through projects no. CGL2016-81092-R, CGL2017-90884-REDT, RTI2018-097864-B-I00 and PGC2018-098770-B-I00 and by the Andalusia Regional Government through project no. P18-RT-3820. FKL, HAC and DEM are grateful for funding by project PANhellenic infrastructure for Atmospheric Composition and climate change (MIS 5021516), which is implemented under action Reinforcement of the Research and Innovation Infrastructure, funded by operational programme Competitiveness, Entrepreneurship and Innovation (NSRF 2014-2020) and co-financed by Greece and the European Union (European Regional Development Fund). CPC measurements at Sonnblick are supported by the Climate and Air Quality Commission of the Austrian Academy of Sciences and the office of the provincial government Salzburg, Unit 5/02. At CMN, aerosol measurements were partially supported by the Italian Ministry of Research and Education. Measurements at Birkenes II are financed by the Norwegian Environment Agency. VAV is grateful for various Swedish FORMAS, Swedish Research Council (VR) grants and the Magnus Bergvall and Märta och Erik Holmberg foundations and the Swedish EPA for making the research possible at the VAV site. NMY wishes to thank the many technicians and scientists of the Neumayer overwintering crews, whose outstanding commitment enabled continuous, high-quality aerosol records over many years. Gunter Löschau is acknowledged for his contribution to the data acquisition at ANB, DTC and DRN. Financial support This research was supported by the European Commission's Horizon 2020 Framework Programme (ACTRIS2 (grant agreement no. 654109)), the University of Helsinki, the Finnish Meteorological Institute, the Department of Science and Innovation of South Africa, the Academy of Finland Centre of Excellence programme (project no. 272041), the Academy of Finland project Greenhouse gas, aerosol and albedo variations in the changing Arctic (project no. 269095), the Novel Assessment of Black Carbon in the Eurasian Arctic: From Historical Concentrations and Sources to Future Climate Impacts (NABCEA, project no. 296302), the Korea Meteorological Administration Research and Development Program "Development of Monitoring and Analysis Techniques for Atmospheric Composition in Korea" (grant no. KMA2018-00522), the National Research Foundation of Korea (grant no. 2017R1D1A1B06032548), the Korea Meteorological Administration Research and Development Program (grant no. KMI2018-01111), the Taiwan Environmental Protection Administration, the China Meteorological Administration, the National Scientific Foundation of China (41675129, 41875147), the National Key R&D Program of the Ministry of Science and Technology of the People's Republic of China (grant no. 2016YFC0203305 and 2018YFC0213204), the Chinese Academy of Meteorological Sciences (2020KJ001), the Innovation Team for Haze-fog Observation and Forecasts of MOST and CMA, CNRS-INSU, the French Ministry for Research under the ACTRIS-FR national research infrastructure, the French Ministry of the Environment, MeteoSwiss (GAW-CH aerosol monitoring programme), the Swiss State Secretariat for Education, Research and Innovation (SERI), the Ministry of Education, Youth and Sports of CR within National Sustainability Program I (NPU I, grant no. LO1415), ERDF "ACTRISCZ RI" (grant no. CZ.02.1.01/0.0/0.0/16_013/0001315), CRISOL (CGL2017-85344-R MINECO/AEI/FEDER, UE), TIGAS-CM (Madrid Regional Government Y2018/EMT-5177), AIRTEC-CM (Madrid Regional Government P2018/EMT4329), REDMAAS2020 (RED2018-102594-T CIENCIA), Red de Excelencia ACTRIS-ESPAÑA (CGL2017-90884-REDT), the Spanish Ministry of Economy, Industry and Competitiveness, FEDER funds (project HOUSE, grant no. CGL2016-78594-R), the Generalitat de Catalunya (AGAUR 2017 SGR41 and the DGQA), the National Institute for Aerospace Technology, the Ministerio Español de Economía, Industria y Competitividad (MINECO), the Spanish Ministry of Economy and Competitiveness (projects no. CGL2016-81092-R, CGL2017-90884-REDT, RTI2018-097864-B-I00 and PGC2018-098770-B-I00), the Andalusia Regional Government (project no. P18-RT-3820), the PANhellenic infrastructure for Atmospheric Composition and climate change (MIS 5021516), Research and Innovation Infrastructure, Competitiveness, Entrepreneurship and Innovation (grant no. NSRF 2014-2020), the Italian Ministry of Research and Education, the Norwegian Environment Agency, Swedish FORMAS, the Swedish Research Council (VR), the Magnus Bergvall foundation, the Märta och Erik Holmberg foundation, and the Swedish EPA. ; Peer reviewed