Seasonal and diurnal variability of fine aerosol sources derived from receptor modeling of multi-instrument online composition measurements in Athens, Greece

To characterize the diurnal variability of fine aerosol sources on a seasonal basis, a monitoring campaign was conducted at the NOA-Thissio urban background site in central Athens. Online measurements were performed during warm (June-September 2019), cold (February-March 2020), and COVID-19 lockdown (March-May 2020) periods. Concentrations of 15 elements in PM2.5 were measured at 3-h resolution with a Horiba PX-375 EDXRF analyzer. Collocated data were obtained for organic aerosol and ions (Q-ACSM), black carbon and its components (AE33 aethalometer), and selected cations (PILS-IC). Metals related to non-exhaust emissions (Cu, Fe, Zn) exhibited bimodal cycles and substantial lockdown reductions (39-44%). Levels of crustal elements (Si, Al, Ti, Mn) were elevated at midday in summer due to enhanced dust erosion and resuspension. Episodic elemental concentrations were observed during Saharan dust outbreaks, wildfires (through emission and pyroconvection), and firework events. The speciated data were combined in PMF receptor modeling, identifying eight PM2.5 factors related to traffic emissions (exhaust and non-exhaust), biomass burning (BB), oil combustion in shipping, natural particles (long-range transported and local), and secondary aerosol (regional and locally processed). Substantial lockdown decreases in traffic sources were observed (−28% combined), but only the evening peak in diurnal cycles was fully suppressed by the traffic ban. For the shipping source, the diel variation of passenger ship emissions appeared to build upon a strong background from commercial shipping. The results corroborate ongoing research in Athens, which indicates that achieving the new PM2.5 standard will require further control of traffic emissions, proactive regulation of residential BB, and reevaluation of the urban impacts of heavy-oil combustion in shipping, considering the declaration of the Mediterranean as an emissions control zone.