| Abstract |
The key objectives of the present study consisted of: I) the study of the occurrence of PM2.5, PM10 and PM2.5-associated Polycyclic Aromatic Hydrocarbons (PAH) in the urban atmosphere of Heraklion Crete, by following a specific protocol implemented in the framework of the ESCAPE project (http://www.escapeproject.eu) during 2009-2010; and II) the study of the occurrence of toxic organic compounds, such as PAHs, Polychlorinated Biphenyls (PCBs) and Organochlorine Pesticides (OCPs), transported by air masses during the presence or absence of African dust incursions in the downwind area of Eastern Mediterranean (island of Crete). To achieve the aforementioned objective, in contrast to previous studies that have been focused to a small amount of dust episodes, a meticulous 3-year sampling campaign was conducted (2012-2015) in the framework of the CHEMISAND project (http://chemisand.chemistry.uoc.gr/).
For that purpose, an efficient analytical methodology was developed that allowed the qualitative and quantitative determination of a relatively large number of toxic organic compounds (37 PAH members, 50 PCB congeners and 24 OCP members) by using a high resolution gas chromatographer-mass spectrometer (HRGC-MS) in Electron Ionization (EI) and Electron Capture Negative Ionization (ECNI) mode, respectively. The methodology was characterized by high quality assurance and control levels (enhanced repeatability, low uncertainty, reliable compound separation, high recovery levels).
The median adjusted PM2.5 concentration in the urban area of Heraklion was 14.3 and 13.2 μg m-3, for the urban traffic (N=20) and background (N=20) sites, respectively, while the corresponding median adjusted PM10 concentration was 37.7 and 35.7 μg m-3 , respectively. We did not observe a statistically significant differentiation in PM2.5 and PM10 levels between urban traffic and background sites. We measured lower PM2.5 concentrations than the annual arithmetic mean benchmark level, established by EU (25 μg m-3), which is currently in effect from January 1st 2015. On the other hand, relatively higher PM10 concentrations were reported in a few urban sites, having regard to the corresponding EU PM10 annual limit value (40 μg m-3).
We measured the highest daily PM concentrations (PM2.5, PM10, TSP) during intensive African dust episodes, while relatively lower PM concentrations were measured during dust episodes mixed with aerosol transport from other areas. Conversely, we measured the lowest daily PM concentrations during the majority of periods with absence of African dust episodes into the downwind area (Eastern Mediterranean). We also observed a significant statistical differentiation in PM levels between intensive African dust and mixed episodes and days with absence of African dust episodes. The aforementioned differentiation was also observed regarding the PM2.5/PM10 ratio (0.32 and 0.54 median values, during African dust and non-dust events, respectively). A statistically significant positive correlation was observed between PM2.5 and PM10 levels measured at the downwind area, demonstrating common emission PM sources, the burden of which is controlled by air masses, transporting African mineral dust. The median ΣPAH concentration measured in PM2.5 urban samples (sum of 10 traffic
and background sites) was 0.63 ng m -3, while
the
arithmetic mean concentration
was 0.64 (±0.25) ng m-3.
We measured relatively higher ΣPAH concentrations in the urban
traffic, compared to the background sites, without any significant statistical difference between
them.
Accordingly, the mean seasonal ΣPAH levels did not significantly differ between urban
traffic and background sites, suggesting common PAH sources.
The arithmetic mean ΣPAH particulate concentration during pure African dust events was 0.86 (±0.98) ng m-3. Among that group of samples, there has been a statistically insignificant variation in ΣPAH levels, relative to the origin of the air masses (72-h back trajectories) reaching the downwind area. BaP arithmetic mean concentration was 0.05 (±0.06) ng m-3, much lower than the corresponding annual limit value (1.0 ng m-3), established by EU. The arithmetic mean ΣPAH particulate concentration during mixed episodes was 1.10 ng m-3, while the corresponding BaP concentration was 0.07 (±0.07) ng m-3, also lower than the EU limit value (1.00 ng m-3). During the absence of African dust episodes, the mean ΣPAH particulate concentration was 1.60 (±2.40) ng m-3. The relative arithmetic (but statistically insignificant) differentiation in ΣPAH levels between African dust episodes (pure and mixed) and non-dust intervals is ascribed to the enhanced influence of air masses originated from more polluted areas (Northern Europe) which more likely affect the downwind area, compared to the corresponding air parcels originated from more arid areas, such as the Sahara-Sahel region.
The mean total (gas and particulate phase) ΣPCB concentration measured during the presence of pure African dust events was 77.1 (±41.1) pg m-3, while the corresponding mean total ΣPCB concentration during mixed dust episodes was 92.5 (±44.9) pg m-3. We did not observe a statistically significant variation in ΣPCB levels, relative to the origin of air masses. The mean total (gas and particle) ΣPCB concentration during periods of non-African dust conditions was 81.9 (±38.7) pg m-3. We observed low and statistically insignificant correlation between ambient temperature and ΣPCB concentrations, illustrating the lack of seasonality in PCB atmospheric levels in the downwind study area. ΣPCB concentration differences between dust (pure and mixed) and non-dust conditions were also statistically insignificant. Given the lack of seasonality in PCB (gas & particulate phase) ambient levels in the downwind area, the low and insignificant correlation between partial vapour pressures of gas phase PCBs and temperature and the shallow slopes obtained from Clausius-Clapeyron plots, we assume that PCB atmospheric levels in the Eastern Mediterranean are more likely controlled by a combination of physicochemical processes and long-range transport (LRT) phenomena.
Mean total (gas and particulate phase) concentrations of 6DDX (2,4´-DDD, 4,4´-DDD, 2,4´-DDT, 2,4´-DDE, 4,4´-DDE and 4,4´-DDT), 3Chlordanes (cis-Chlordane, trans-Chlordane and oxy-Chlordane), 3Endosulfans (α-Endosulfan, β-Endosulfan and Endosulfan sulfate) and 4HCHs (α-HCH, β-HCH, γ-HCH and δ-HCH), measured during periods of pure African dust incursions, were 17.1 (±5.7), 1.1 (±0.3), 3.8 (±2.2) and 3.6 (±1.4) pg m-3, respectively. The corresponding mean total (gas and particle) concentrations during mixed African dust events were 22.8 (±10.5), 1.6 (±1.0), 5.4 (±4.8) and 4.4 (±2.2) pg m-3, respectively. Most OCPs targeted in this study were found to be predominant in the gas phase
, while Endosulfan isomers (β-Endosulfan, Endosulfan sulfate, α-Endosulfan) were mostly distributed in the particulate phase of the atmosphere.
Mean total (gas & particle) concentrations of 6DDX, 3Chlordanes, 3Endosulfans and 4HCHs
during periods of non-dust conditions were measured relatively similar to those detected during pure and mixed dust conditions [32.7 (±13.6), 2.1 (±1.3), 7.3 (±4.9) and 11.3 (±7.5) pg
m-3, respectively], with statistically insignificant concentration differences between them.
The aforementioned observation illustrates the lack of any kind of OCP emission source in the vicinity of the downwind study area (Eastern Mediterranean).
Finally, long-range transport (LRT) is considered as the main factor controlling the tmospheric levels of OCPs in this area, owing to the low and statistically insignificant correlations between partial vapor pressures of gas phase OCPs and ambient temperature, as well as the shallow slope values obtained from Clausius-Clapeyron plots.
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