Chemical mass closure of atmospheric aerosol collected over Athens, Greece. Paraskevopoulou D. 1, 2, Liakakou E. 1, Theodosi C. 2, Zarmpas P. 2, Gerasopoulos E. 1 1, 2*, Mihalopoulos N. 1 Institute for Environmental Research and Sustainable Development, National Observatory of Athens, I. Metaxa and Vas. Pavlou, 15236, P. Penteli, Athens, Greece 2 Environmental and Analytical Chemical Division, Department of Chemistry, University of Crete, P.O. Box 2208, 71003 Heraklion, Greece *corresponding author e-mail: mihalo@chemistry.uoc.gr Abstract To perform aerosol chemical mass closure in Athens, Greece, particulate matter sampling has been conducted on a 24hour basis, from May 2008 to April 2013. The sampling site is located at the National Observatory of Athens premises in Penteli (38 0 2.94 N, 23 0 51.78 E, 495m a.s.l.) which is considered an urban background station. PM 2.5 and PM 2.5-10 fractions of aerosol are collected on quartz fiber filters, using a dichotomous auto-sampler. In total 1507 samples were collected during the sampling period and after mass quantification, the filters were analyzed for major anions (Cl -, Br -, NO -3, SO 4, PO 4-3, C 2O 4 ), cations (NH4 +, K +, Na +, Mg +2, Ca +2 ), trace elements, organic carbon (OC) and elemental carbon (EC). The concentration of PM 2.5 ranged from 1 to 160 μg m -3 (average of 20 ±9 μg m -3 ). Aerosol chemical mass closure calculations indicated that carbonaceous aerosol constitutes a major component of particulate matter, along with dust and sulfate anions. In the fine mode about 23%, 20% and 15% of the total mass is due to POM, dust and nssso 4, respectively. 1 Introduction Several studies on ambient aerosol have highlighted the significant variability in the concentration and chemical composition of atmospheric aerosol in Europe. In the area of Eastern Mediterranean, the interest on estimating aerosol chemical characteristics has increased, due to long range transported particulate matter, and enhanced local emissions, absence of precipitation during most of the year and favorable conditions for photochemical reactions. In this study, a long term comprehensive composition of PM 2.5 in Athens, Greece, has been established for the first time. The fine aerosol fraction was chemically characterized for inorganic species, such as trace elements and water soluble ions, as well as for organic components, such as organic and elemental carbon. Seasonal trends of PM 2.5 masses and its chemical components were quantified, and the contribution of each to fine aerosol mass was estimated, in order to identify the pollution level and the potential sources of atmospheric aerosol in the city of Athens.
2 Data and Methodology 2 2.1 Data Aerosol sampling was conducted daily at the National Observatory of Athens premises in Penteli (38 0 2.94 N, 23 0 51.78 E, 495m a.s.l.), during the period from May 2008 to April 2013. All chemical analyses were applied on PM 2.5 fraction of aerosol. The aforementioned acquired aerosol samples have been analyzed for organic carbon (OC), elemental carbon (EC), the main water soluble ions and trace elements, in order to perform the chemical mass closure exercise. 2.2 Methodology PM 2.5 and PM 2.5-10 fractions of aerosol were collected using a Dichotomous Partisol sampler and a Partisol FRM Model 2000 air sampler, on 24 hour basis. In total, 1507 samples were collected on prebaked quartz fiber filters which, in continuation, were stored until chemical analysis. All quartz filters were weighed before and after sampling under controlled conditions using a microbalance (1 μg sensitivity). A punch of 1cm 2 from each sample was analyzed by a thermal optical transmission technique, using a Sunset Laboratory carbon analyzer and applying the EUSAAR protocol, as described by Cavalli et al. (2010), in order to measure the OC and EC concentrations. Filter punches of 2 cm 2 were extracted in ultrasonic bath and were then filtered, in order to be analyzed by ion chromatography (IC) for the determination of the main ionic species concentrations (anions: Cl -, Br -, NO -3, SO 4, PO 4-3, C 2O 4 and cations: NH 4 +, K +, Na +, Mg +2, Ca +2 ), as described by Paraskevopoulou et al. (submitted for publication). An acid microwave digestion procedure followed by Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES, Τhermo Electron ICAP 6000 Series) was applied to measure major (Al, Ca, Mn and Fe) and trace metal (V, Cr, Ni, Cu, Zn, Cd and Pb) concentrations, following the procedure described in details by Theodosi et al. (2010). Indium (In, CPI International) was added as an internal standard to the samples before ICP-OES analysis. All reported concentrations were corrected for blanks.
3 Results 3 Table 1 presents the average and the standard deviation values of PM 2.5 mass, OC, EC, dust, POM and main ions concentrations determined in this study. Daily concentrations of PM 2.5 mass and nssso 4 are plotted in Figure 1 as a function of time, demonstrating the decrease in nssso 4 concentration during the last years and the concurrent reduction of PM 2.5 masses, which is statistically not significant (p=0.05). PM 2.5 and PM 2.5-10 masses range from 1 to 160 μg m -3 (average of 20 ±9 μg m -3 ) and from 0.2 to 125 μg m -3 (average of 16 ±12 μg m -3 ), respectively, for the period from May 2008 to April 2013. Compared to respective PM concentrations reported for urban sites in Athens, this study reveals lower levels (e.g.chaloulakou et al., 2005). The average seasonal cycle of PM 2.5 concentrations is displayed in Figure 2. Maximum contribution of dust transport from Africa is observed in spring while during winter intense anthropogenic activities, such as heating, can be associated, as well, with the observed increase in the fine aerosol masses. The daily concentration levels of OC range from 0.1 to 8.5 μg m -3 (average: 2.1 ±1.3 μg m -3 ), contributing on average about 11% to the total PM 2.5 mass. Accordingly, the mass concentration of EC ranges from 0.01 to 3.3 μg m -3 (average: 0.5 ±0.4 μg m -3 ), contributing 3% to the total PM 2.5. The average seasonal cycle of OC and EC is presented in Figure 3, showing maximum values during winter, as expected, since during this period there is increased fuel combustion mainly for domestic heating purposes. The average concentrations of the main anions and cations are included in Table 1. According to the acquired results, sulfate and nitrate are the main ion contributors to fine aerosol mass. Concentrations of PM elements have been measured by ICP-OES in 100 randomly selected samples and the mean obtained concentrations are plotted in Figure 4. For the purpose of mass closure the concentrations of dust, particulate organic matter (POM), ion mass and water content have been also taken into account. The amount of dust was calculated through the concentration of nssca +2 as described by Sciare et al. (2005). For the estimation of POM, a conversion factor of 2.1 was applied (Turpin and Lim. 2001, Sciare et al. 2005), since measurements are conducted at an urban background location affected by long range transport. Water content was calculated using the method described by Ohta and Okita (1990). Figure 2 presents the chemical mass closure on a monthly basis for PM 2.5 fraction, in Penteli, from May 2008 to April 2013. The chemical mass closure exercise can explain about 87% of the recorded fine aerosol mass. In total, the ion mass, POM, dust account and water account for about 32%, 23%, 18%, and 27%, respectively, of PM 2.5 mass. The mean estimated contributions to PM 2.5 mass and ion mass are presented in Figure 5a and b. Aerosol chemical mass closure calculations indicate that carbonaceous aerosol constitutes a major component of particulate matter, along with dust and sulfate anions. The estimated aerosol water concentration (27%) is in accordance with reported findings (e.g. 25-35% of PM 2.5 samples in Tsyro, 2005). In the fine mode about 55% and 10% of the ion mass is
due to SO 4 and NH 4 +, respectively, highlighting the secondary origin of ions in fine aerosol. 4 Table 1. Mean measured concentrations for PM2.5 samples, collected from May 2008 to April 2013 at Penteli. PM 2.5 OC EC POM Dust Na + K + (μg m -3 ) (μg m -3 ) (μg m -3 ) (μg m -3 ) (μg m -3 ) (μg m -3 ) (μg m -3 ) Average 20 2.1 0.5 4.5 3.4 0.16 0.18 StdDev 9 1.3 0.4 2.7 6.3 0.18 0.20 + NH 4 Mg +2 Ca +2 Cl - - NO 3 SO 4 C 2 O 4 (μg m -3 ) (μg m -3 ) (μg m -3 ) (μg m -3 ) (μg m -3 ) (μg m -3 ) (μg m -3 ) Average 0.68 0.03 0.37 0.16 0.45 3.1 0.18 StdDev 0.60 0.05 0.58 0.29 0.43 1.9 0.13 Fig. 1. Time evolution of daily PM2.5 mass and nssso4 concentrations from May 2008 to April 2013 at Penteli. Fig. 2. Mass closure and standard deviation (Ion Mass, Dust, POM, Water and EC) versus mean PM2.5 mass, on a seasonal basis, from May 2008 to April 2013 at Penteli.
5 Fig. 3. Seasonal variability of OC and EC concentrations from May 2008 to April 2013 at Penteli. Fig. 4. Mean concentrations of fine fraction elements in 100 randomly selected samples from May 2008 to April 2013 at Penteli. Fig. 5. Mean contribution of a) ion mass, dust, POM and EC to PM2.5 mass, and of b) main ionic species to ion mass, from May 2008 to April 2013, in Penteli, Athens.
4 Conclusions 6 This study presents the first time long-term chemical mass closure measurements in the area of Athens. The determination of the composition of PM 2.5 daily samples in an urban background site for a period of five years has indicated that the most abundant constituents of fine aerosol are carbonaceous aerosol, dust and sulfates while; simultaneously, water contributes significantly to fine aerosol mass that has been collected on quartz fiber filters. In the fine mode about 23%, 18% and 16% of the total mass is due to POM, dust and SO 4, respectively. In particular, the chemical mass closure exercise can explain about 87% of the recorded fine aerosol mass. Acknowledgments This research has been co-financed by the European Union (European Social Fund ESF) and Greek national funds through the Operational Program "Education and Lifelong Learning" of the National Strategic Reference Framework (NSRF) - Research Funding Program: Heracleitus II. Investing in knowledge society through the European Social Fund. References Cavalli, F., Viana, M., Yttri, K. E., Genberg, J., and Putaud, J. P.: Toward a standardised thermaloptical protocol for measuring atmospheric organic and elemental carbon: the EUSAAR protocol, Atmos. Meas. Tech., 3, 79-89, 10.5194/amt-3-79010, 2010. Chaloulakou, A., Kassomenos, P., Grivas, G., Spyrellis, N., 2005. Particulate matter and black smoke concentration levels in central Athens, Greece. Environment International 31, 651-659. Ohta, S., Okita, T., 1990. A chemical characterization of atmospheric aerosol in Sapporo. Atmospheric Environment. Part A. General Topics 24, 815-822. Paraskevopoulou D., Liakakou E., Gerasopoulos E., Theodosi C., and Mihalopoulos N., Long term characterization of organic and elemental carbon in the PM2.5 fraction: The case of Athens, Greece, submitted for publication. Sciare, J., Oikonomou, K., Cachier, H., Mihalopoulos, N., Andreae, M. O., Maenhaut, W., and Sarda-Esteve, R.: Aerosol mass closure and reconstruction of the light scattering coefficient over the Eastern Mediterranean Sea during the MINOS campaign, Atmos. Chem. Phys., 5, 2253265, 10.5194/acp-5253005, 2005. Theodosi, C., Markaki, Z., Tselepides, A., and Mihalopoulos, N.: The significance of atmospheric inputs of soluble and particulate major and trace metals to the eastern Mediterranean seawater, Marine Chemistry, 120, 154-163, http://dx.doi.org/10.1016/j.marchem.2010.02.003, 2010. Tsyro, S.G., 2005. To what extent can aerosol water explain the discrepancy between model calculated and gravimetric PM10 and PM2.5? Atmos. Chem. Phys. 5, 515-532. Turpin, B. J., and Lim, H.-J.: Species Contributions to PM2.5 Mass Concentrations: Revisiting Common Assumptions for Estimating Organic Mass, Aerosol Science and Technology, 35, 602-610, 10.1080/02786820119445, 2001.