| Abstract |
Liquid and solid particles with different diameters and chemical compositions suspended in the air may pose a threat to human health and life expectancy. Particulate matter (PM) also affects the climate and various ecosystems directly and indirectly. Understanding and obtaining a detailed characterization of their sources, chemical processes and atmospheric transportation mechanisms governing their highly variable spatial and temporal variability is of pivotal importance to further facilitate regulative strategies and abatement legislation.
This dissertation focuses on the chemical characterization of submicron (PM1) aerosol using high temporal resolution measurements via aerosol mass spectrometry, using the Aerosol Chemical Speciation Monitor (ACSM) in the remote and urban environments in the Eastern Mediterranean. Collected data allow for the determination of organic aerosol (OA) sources via the application of appropriate receptor modeling techniques (Positive Matrix Factorization). Results from measurements conducted for approximately 3 years in Finokalia, a one-year period at the urban background Thissio station in Athens plus two preceding intensive campaigns, followed by the analysis of source specific case studies in the long-term, and finally two season specific intensive campaigns in Piraeus, are herein presented. Aiming at the spatial characterization of delineated sources, the calibration and implementation of a dense low-cost PM monitoring network was also pursued in the context of this thesis.
In Finokalia, average PM1 concentrations were 6.4 ± 4.6 μg m-3 with sulfate being the dominant component throughout the period, contributing on average 42.6%. Organic aerosol followed, contributing 36.5%, while ammonium contributed 12.6%. Nitrate and BC contributed 4.4% and 3.8% respectively. Concentrations in general, were higher during the warmer months of the year. A contrast was recorded among the two periods in terms of species contributions. For the cold period, OA and sulfate contributed equally at 39% each, while during the warm months sulfate clearly dominated ambient PM1 aerosol contributing 44% as opposed to a 36% contribution from organics. All species exhibited limited diurnal variability, in the absence of important local sources, varying by as much as ± 12% around their respective seasonal averages. Long range transport was mainly driving measured concentrations with main sources including industrial activity, shipping and biomass burning in areas of South Western Europe, the Black Sea region, Turkey and the Aegean Sea. A noteworthy contribution from the heavily polluted Nile river Delta and the Middle East was also documented.
In Athens, on an annual basis, the most abundant component was OA (44.5%), followed by sulfate (27.8%), BC (15.1 %), ammonium (7.9 %), and nitrate (4.3 %). During the cold season, three primary sources were identified, namely a hydrocarbon-like organic aerosol factor (HOA; 13%) mainly representing liquid fuel combustion (traffic and residential heating), a cooking organic aerosol factor (COA; 10%) and a biomass burning factor (BBOA; 10). Two secondary factors were additionally modeled, a semi volatile oxygenated organic aerosol factor (SV-OOA; 22%), linked to the fast atmospheric processing of biomass burning plumes and a low volatility OOA factor (LV-OOA; 45%), linked to long range transporting mechanisms. Several severe air quality deterioration events were recorded, pointing to the adverse effects of residential wood burning to the atmosphere over the city. During the warm period, in the absence of an active biomass burning source, four factors were resolved, namely an HOA (5%) and a COA (11%) factor with the same characteristics found for the cold period, an SV-OOA (33%) factor related to both primary aerosol processing and secondary formation, and an LV-OOA (51%) factor representing transboundary pollution.
In Piraeus, the combined traffic, shipping and, especially, residential emissions led to considerably elevated submicron aerosol levels (22.8 μg m-3 ) in winter, which frequently became episodic late at night under stagnant conditions. Carbonaceous compounds comprised the major portion of this submicron aerosol in winter, with mean OA and BC contributions of 61% (13.9 μg m-3 ) and 16% (3.7 μg m-3 ), respectively. The contribution of BB to BC concentrations was considerable and spatially uniform. OA related to BB emissions (fresh and processed) and hydrocarbon-like OA (from vehicular traffic and port-related fossil fuel emissions including shipping) accounted for 37% and 30% of OA, respectively. In summer, the average PM1 concentration was significantly lower (14.8 μg m-3 ) and less variable, especially for the components associated with secondary aerosols (such as OA and sulfate). The effect of the port sector was evident in summer and maintained BC concentrations at high levels (2.8 μg m-3 ), despite the absence of BB and improved atmospheric dispersion. Oxygenated components yielded over 70% of OA in summer, with the more oxidized secondary component of regional origin being dominant (41%) despite the intensity of local sources, in the Piraeus environment. For the field evaluation of Purple Air PA-II devices (low-cost PM sensors), measurements were conducted in Athens for five months and in Ioannina, a medium-sized city in northwestern Greece for a six-month period. The sensor output correlates strongly with reference measurements (R2 = 0.87 against a beta attenuation monitor and R2 = 0.98 against an optical reference-grade monitor). Deviations in the sensor-reference agreement are identified as mainly related to elevated coarse particle concentrations and high ambient relative humidity. Simple and multiple regression models are tested to compensate for these biases, drastically improving the sensor’s response. Large decreases in sensor error are observed after implementation of models, leading to mean absolute percentage errors of 0.18 and 0.12 for the Athens and Ioannina datasets, respectively. Overall, a quality-controlled and robustly evaluated low-cost network can be an integral component for air quality monitoring in a smart city. Case studies are presented along this line, where a network of PA-II devices is used to monitor the air quality deterioration during a peri-urban forest fire event affecting the area of Athens and during extreme wintertime smog events in Ioannina, related to wood burning for residential heating.
Focusing on the cooking source in the urban environment, festive celebrations involving meat grilling i.e., Fat Thursday and Easter Sunday in Athens and the Old Town Festival in the town of Xanthi in northern Greece were examined via a combination of aerosol mass spectrometry and spatially dense PM2.5 data coming from a low-cost sensor network. In Xanthi, during festivities PM2.5 concentrations recorded a 170% increment when compared to the rural background in the area. In Athens, during Fat Thursday PM2.5 concentrations rose significantly in the evening throughout the basin recording a clear pattern of elevated PM2.5 concentrations, being in most cases close to double and in specific sites up to more than triple during Fat Thursday, with respect to a 2-week long control period around the event. A similar trend spatially and concentration-wise was established when examining the Easter Sunday events. COA night-time maxima were up to 23.3 μg m-3 at Thissio during fat Thursday, being more than 5 times higher than the maxima during the control period. On Easter Sunday, the peaks were characteristically shifted to the early afternoon, and COA concentrations exhibited an extreme average value of 27.4 μg m-3 at 15:00 LST signifying a staggering 50-fold increase compared to the average COA concentration of 0.5 μg m-3 for the same hour in the control period. A COA driven organic aerosol dominance was recorded for both cases with OA contributions to PM1 increasing to about 70% while being close to 50% during the control period. Moving away from the event-based analysis, revisiting the full-year dataset, a pattern linked to leisure activities was established, with the weekly variability of COA pointing to increased concentrations and contributions towards the weekend.
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