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. In an attempt to take effective action towards mitigating pollution episodesin Athens, precise knowledge of PM 2.5 composition and its sources is aprerequisite. Thus, a 2-year chemical composition dataset from aerosolsamples collected in an urban background site in central Athens fromDecember 2013 to March 2016 has been obtained and a positive matrixfactorization (PMF) was applied in order to identify and apportion fineaerosols to their sources. A total of 850 aerosol samples were collected ona 12 to 24 h basis and analyzed for major ions, trace elements, and organic andelemental carbon, allowing us to further assess the impact of residentialheating as a source of air pollution over Athens. The ionic and carbonaceous components were found to constitute the majorfraction of the PM 2.5 aerosol mass. The annual contribution of the ionmass (IM), particulate organic mass (POM), dust, elemental carbon (EC), andsea salt (SS) was calculated at 31 %, 38 %, 18 %, 8 %, and3 %, respectively, and exhibited considerable seasonal variation. Inwinter, the share of IM was estimated down to 23 %, with POM  +  EC being the dominant component accounting for 52 % of the PM 2.5 mass,while in summer, IM (42 %) and carbonaceous aerosols (41 %)contributed almost equally. Results from samples collected on a 12 h basis (day and night) during thethree intensive winter campaigns indicated the impact of heating on thelevels of a series of compounds. Indeed, PM 2.5 , EC, POM, NO 3 - , C 2 O 4 2 - , non sea salt (nss) K+ and selected trace metalsincluding Cd and Pb were increased by up to a factor of 4 in the nightcompared to the day, highlighting the importance of heating on air quality inAthens. Furthermore, in order to better characterize wintertime aerosolsources and quantify the impact of biomass burning on PM 2.5 levels,source apportionment was performed. The data can be interpreted on the basisof six sources, namely biomass burning (31 %), vehicular emissions(19 %), heavy oil combustion (7 %), regional secondary (21 %),marine aerosols (9 %), and dust particles (8 %). Regardingnight-to-day patterns their contributions shifted from 19 %, 19 %,8 %, 31 %, 12 %, and 10 % of the PM 2.5  mass during dayto 39 %, 19 %, 6 %, 14 %, 7 %, and 7 % during thenight, underlining the significance of biomass burning as the maincontributor to fine particle levels during nighttime in winter.

Multi-year chemical composition of the fine-aerosol fraction in Athens, Greece, with emphasis on the contribution of residential heating in wintertime