Submicron aerosol chemical composition was studied during a year-long period (26 July 2016–31 July 2017) and two wintertime intensive campaigns (18 December 2013–21 February 2014 and 23 December 2015–17 February 2016), at a central site in Athens, Greece, using an Aerosol Chemical Speciation Monitor (ACSM). Concurrent measurements included a particle-into-liquid sampler (PILS-IC), a scanning mobility particle sizer (SMPS), an AE-33 Aethalometer, and ion chromatography analysis on 24 or 12 h filter samples. The aim of the study was to characterize the seasonal variability of the main submicron aerosol constituents and decipher the sources of organic aerosol (OA). Organics were found to contribute almost half of the submicron mass, with 30 min resolution concentrations during wintertime reaching up to 200 µg m⁻³. During winter (all three campaigns combined), primary sources contributed about 33 % of the organic fraction, and comprised biomass burning (10 %), fossil fuel combustion (13 %), and cooking (10 %), while the remaining 67 % was attributed to secondary aerosol. The semi-volatile component of the oxidized organic aerosol (SV-OOA; 22 %) was found to be clearly linked to combustion sources, in particular biomass burning; part of the very oxidized, low-volatility component (LV-OOA; 44 %) could also be attributed to the oxidation of emissions from these primary combustion sources. These results, based on the combined contribution of biomass burning organic aerosol (BBOA) and SV-OOA, indicate the importance of increased biomass burning in the urban environment of Athens as a result of the economic recession. During summer, when concentrations of fine aerosols are considerably lower, more than 80 % of the organic fraction is attributed to secondary aerosol (SV-OOA 31 % and LV-OOA 53 %). In contrast to winter, SV-OOA appears to result from a well-mixed type of aerosol that is linked to fast photochemical processes and the oxidation of primary traffic and biogenic emissions. Finally, LV-OOA presents a more regional character in summer, owing to the oxidation of OA over the period of a few days.

Organic carbon (OC), elemental carbon (EC), water-soluble organic carbon (WSOC) and main ions were measured in a total of 1510 PM_2.5 daily aerosol samples collected from May 2008 to April 2013 in Athens, Greece. OC and EC concentrations were 2.1 ± 1.3 μg m⁻³ and 0.54 ± 0.39 μg m⁻³, accounting for 11 ± 3% and 3 ± 1% of PM_2.5 mass, respectively, with an average OC/EC ratio of 4.7 ± 3.1. Significant correlation was found between OC and EC during the whole period, indicating emissions by common primary sources on a regional scale. WSOC concentration ranged from 0.03 to 10.6 μg m⁻³, with an average of 1.5 ± 0.9 μg m⁻³. By considering the Finokalia (Crete) station as a reference, it was estimated that, during the warm season in Athens, 67 ± 7% of emitted OC and 53 ± 12% of emitted EC is regional, while, during cold months, the regional contribution of OC is only 33 ± 7% and of EC 29 ± 8%. Furthermore, secondary organic carbon (SOC) was calculated for the warm period of the year (April to October). The estimated SOC constituted about 75 ± 6% of PM_2.5 organic carbon in Athens, highlighting significant aging processes on a regional scale. In the period 2011–2013 and during wintertime, an increase in OC and EC levels was observed, attributed to an increase in wood burning for domestic heating due to the economic crisis.

In an attempt to take effective action towards mitigating pollution episodes in Athens, precise knowledge of PM_2.5 composition and its sources is a prerequisite. Thus, a 2-year chemical composition dataset from aerosol samples collected in an urban background site in central Athens from December 2013 to March 2016 has been obtained and a positive matrix factorization (PMF) was applied in order to identify and apportion fine aerosols to their sources. A total of 850 aerosol samples were collected on a 12 to 24 h basis and analyzed for major ions, trace elements, and organic and elemental carbon, allowing us to further assess the impact of residential heating as a source of air pollution over Athens. The ionic and carbonaceous components were found to constitute the major fraction of the PM_2.5 aerosol mass. The annual contribution of the ion mass (IM), particulate organic mass (POM), dust, elemental carbon (EC), and sea salt (SS) was calculated at 31 %, 38 %, 18 %, 8 %, and 3 %, respectively, and exhibited considerable seasonal variation. In winter, 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 the three intensive winter campaigns indicated the impact of heating on the levels of a series of compounds. Indeed, PM_2.5, EC, POM, NO₃⁻, C₂O₄²⁻, non sea salt (nss) K⁺ and selected trace metals including Cd and Pb were increased by up to a factor of 4 in the night compared to the day, highlighting the importance of heating on air quality in Athens. Furthermore, in order to better characterize wintertime aerosol sources and quantify the impact of biomass burning on PM_2.5 levels, source apportionment was performed. The data can be interpreted on the basis of six sources, namely biomass burning (31 %), vehicular emissions (19 %), heavy oil combustion (7 %), regional secondary (21 %), marine aerosols (9 %), and dust particles (8 %). Regarding night-to-day patterns their contributions shifted from 19 %, 19 %, 8 %, 31 %, 12 %, and 10 % of the PM_2.5mass during day to 39 %, 19 %, 6 %, 14 %, 7 %, and 7 % during the night, underlining the significance of biomass burning as the main contributor to fine particle levels during nighttime in winter.