Background Despite improvements in air quality, levels of particulate matter (PM) remain a concern across the European Union. Evidence shows that fine particulates (PM2.5) are the most detrimental to human health, but coarser fractions (PM of 2.5 to 10 μm diameter) are also problematic. An EU directive proposing comprehensive monitoring of ambient levels in urban areas is a necessary first step to reducing exposure of citizens to the high risks from PM2.5. A reduction of 20% relative to average concentrations is the target set for all Member States, to be attained between 2010 and 2020. High ambient PM concentrations have been recorded in Greece. Although national air monitoring programmes record PM2.5 and PM10 levels at fixed operating stations, specifically nine stations in the project study areas, more information on source apportionment is needed to enable concrete measures targeting emission sources to be implemented. Objectives The ACEPT-AIR project aimed to distinguish sources of particulate matter that lead to concentrations exceeding limits set by EU legislation. In order to achieve this objective, the project sought to determine quantitative weighed contributions caused by human versus natural sources in areas above limit values. A key objective was to develop a database to detail the anthropogenic emission source strength and the naturally emitted primary and secondary materials, so that authorities could adopt and evaluate control strategies. Results The ACEPT-AIR project recorded detailed airborne PM levels at three urban locations in Greece: Athens, Thessaloniki and Volos. This information was processed through a software tool for pollution source distribution assessment, and was made publically available on a database. Six emission inventories for the three urban areas, with two types of source identification (anthropogenic and natural) were produced, along with guidelines for effective action plans to reduce PM emissions. The project compiled time-series (historical data) for PM10 and PM2.5 to observe trends in air pollution. Historical data was obtained from air monitoring stations of the Ministry of Environment and the Municipality of Thessaloniki, scientific reports and published articles, and data previously collected by the beneficiaries. Coordinating beneficiary NCSR also developed and verified analytical techniques for a range of trace metals in urban air. During the last decade, mean concentrations of PM10 for summer were found to be below the limit value for all three urban areas, although exceedances were recorded at traffic intersections. However, winter measurements were significantly above the target values. In Athens, PM10 levels were above the 50 ug/m3 limit value for one-third of the measurement days, while in Volos the levels exceeded the limit value during 70% of the measurement days. This was attributed to the increase in residential wood burning. A comprehensive chemical characterisation and mass determination of the two particulate matter fractions, PM10 and PM2.5, was conducted in the three cities, during cold and warm seasons in 2011 and 2012, with emission sources distinguished as either anthropogenic or natural in origin. Mitigation measures were elaborated which can be used for the formulation of a national Action Plan on PM reduction. The application of the tool and the information generated by the project is expected to improve the capacity of decision-makers to meet the requirements of EC Air Quality Directive (2008/50/EC), as well as WHO guidelines for the protection of human health. Emission inventories were compiled by associated beneficiary AXON, while emissions from natural sources were assessed by the Technical University of Crete (TUC). Despite significant African dust loads contributing to PM10 concentrations, it was found that most exceedances are still attributed to anthropogenic sources; although severe fire events in certain years (2000 and 2007) were responsible for many of the exceedances. The project developed a user-friendly software tool, the Operational Platform Policy Tool, to estimate levels of PM reduction based on different emission scenarios and the application of control measures. End-users can respond to environmental issues, specific to particulate matter concentrations and target their response to specific PM sources. The tool therefore provides the means to take informed decisions based on quantified PM reductions and to guide policy decisions aiming to reduce PM levels. The ACEPT-AIR Operational Platform Policy Tool is based on innovative software relating PM concentrations with emissions. The project team produced a guide and a manual for the tool, which are available to the public free of charge. The tool can be easily updated with data from future years and can be applied for other regions and cities besides the three used in the project. Ten measures were proposed to reduce PM pollution from road traffic, including low emission zones, street cleaning, promoting new technology for low emission vehicles, expanding public transport and reducing fares. Key stakeholders were consulted throughout the project's duration, and a scientific advisory body periodically reviewed outputs and provided suggestions. The project trained 110 educators in the three cities. Dissemination actions comprised a website, brochures and leaflets, a DVD, public awareness-raising at events, technical seminars, an ACEPT-AIR international conference, and a series of scientific publications. Further information on the project can be found in the project's layman report (see "Read more" section).