Carbonaceous Aerosols in Contrasting Atmospheric Environments in Greek Cities: Evaluation of the EC-tracer Methods for Secondary Organic Carbon Estimation

Abstract

This study examines the carbonaceous-aerosol characteristics at three contrasting urban environments in Greece (Ioannina, Athens, and Heraklion), on the basis of 12 h sampling during winter (January to February 2013), aiming to explore the inter-site differences in atmospheric composition and carbonaceous-aerosol characteristics and sources. The winter-average organic carbon (OC) and elemental carbon (EC) concentrations in Ioannina were found to be 28.50 and 4.33 µg m−3, respectively, much higher than those in Heraklion (3.86 µg m−3 for OC and 2.29 µg m−3 for EC) and Athens (7.63 µg m−3 for OC and 2.44 µg m−3 for EC). The winter OC/EC ratio in Ioannina (6.53) was found to be almost three times that in Heraklion (2.03), indicating a larger impact of wood combustion, especially during the night, whereas in Heraklion, emissions from biomass burning were found to be less intense. Estimations of primary and secondary organic carbon (POC and SOC) using the EC-tracer method, and specifically its minimum R-squared (MRS) variant, revealed large differences between the sites, with a prevalence of POC (67–80%) in Ioannina and Athens and with a larger SOC fraction (53%) in Heraklion. SOC estimates were also obtained using the 5% and 25% percentiles of the OC/EC data to determine the (OC/EC)pri, leading to results contrasting to the MRS approach in Ioannina (70–74% for SOC). Although the MRS method provides generally more robust results, it may significantly underestimate SOC levels in environments highly burdened by biomass burning, as the fast-oxidized semi-volatile OC associated with combustion sources is classified in POC. Further analysis in Athens revealed that the difference in SOC estimates between the 5% percentile and MRS methods coincided with the semi-volatile oxygenated organic aerosol as quantified by aerosol mass spectrometry. Finally, the OC/Kbb+ ratio was used as tracer for decomposition of the POC into fossil-fuel and biomass-burning components, indicating the prevalence of biomass-burning POC, especially in Ioannina (77%).

Assessment of the COVID-19 Lockdown Effects on Spectral Aerosol Scattering and Absorption Properties in Athens, Greece

“…The mean OC/EC ratio in PM 2 . 5 was 6.2 in April 2020, compared to 2.9, averaged for April during 2014-2019 [53,103]. The large increase of the OC/EC ratio can be attributed to the smaller impact of primary combustion emissions and an enhanced presence of secondary organic aerosol [99,104].…”

“…The OC/EC ratio is commonly used to empirically discern between primary and secondary OC, estimating the primary and secondary organic carbon fractions (POC and SOC, respectively), using the EC tracer method in Athens [50,70,102]. The mean OC/EC ratio in PM2.5 was 6.2 in April 2020, compared to 2.9, averaged for April during 2014-2019 [53,103]. The large increase of the OC/EC ratio can be attributed to the smaller impact of primary combustion emissions and an enhanced presence of secondary organic aerosol [99,104].…”

Field Evaluation of Low-Cost PM Sensors (Purple Air PA-II) Under Variable Urban Air Quality Conditions, in Greece

“…The station is located in the city center and is close to a road with moderate traffic. The intercomparison in Ioannina took place from 15 December 2019 until 31 May 2020, covering the spring period but also most of the winter period, when the city is heavily impacted by residential wood burning emissions, leading to persistent wintertime smog episodes [41]. Furthermore, several PA-II devices have been deployed at background locations within the larger urban zones of the two cities, with the objective to gradually form dense monitoring networks.…”

“…Ammonium sulfate is more abundant during summer, while secondary organics (especially the less oxidized fraction) are enhanced in the cold period [43]. Moreover, levels of primary organics and black carbon are largely enhanced during winter, mainly due to residential wood-burning emissions, while they diminish in summer, also due to decreased traffic [38,41,43,62]. The contribution of larger mineral dust particles in PM 2.5 is generally limited in the long-term (typically less than 10%).…”

“…Studies in Ioannina have revealed the important effect of residential wood burning in elevated fine aerosol concentrations during wintertime [41,81]. Three PA-II devices were deployed (Figure 1, Figure S1), specifically at two urban background (VIL, GIR) and one suburban background (ANA) site, in order to monitor the spatial variability of PM 2.5 concentrations in the city.…”