Introduction

1. DETERMINATION OF ORGANIC AND INORGANIC ANIONS IN ATMOSPHERIC PARTICULATE MATTER AND DUST DEPOSITED ON MONUMENTS N. Papazachou* and C. Samara* * EnvironmentalPollution Control Laboratory, Department of Chemistry, Aristotle University, 541-24 Thessaloniki, Greece Introduction It is well known that the deposition of atmospheric pollutants on the surfaces of buildings and further reaction with building materials is the greatest threat to cultural heritage. Besides gaseous pollutants, equally important is the contribution of PM and dust deposition in both the material (black crusts) and the aesthetic deterioration (blackening) of monuments [Brimblecombe, 2005]. PM plays a larger role in protected or semi-protected sections of a monument because it is not leached by rain and accumulates in large amounts. Specifically, the ionic components of PM are very important because their concentrations are very high and also they are involved in various mechanisms (sulfation, soluble salts). Although the presence of soluble inorganic ions (Cl-, NO3-, SO42-) in PM has been extensively studied, the carboxylic organic anions of low molecular weight (Acetate-Ac, Formate-For and Oxalate-Ox) have been studied to a lesser extent. These anions are found in high concentrations in the atmosphere, they contribute to acidification of precipitation and, concurrently, they are strongly present in weathered layers (black crusts) of natural building materials [Ghedini et al., 2006]. • Ionic components of PM • In table 1 are shown the average concentrations of inorganic and organic anions determined in the two particulate fractions. In general, inorganic ions outweighed organic with both being in the same order of magnitude, with the exception of SO42- that were an order of magnitude higher than all the other ions. Table 1: Concentrations (μg/m3) of inorganic and organic anions From the foregoing it is concluded that a comprehensive study of the atmosphere in such environments of cultural heritage is necessary. In the present study inorganic anions and low molecular weight carboxylic anions were determined in the area of the church Agia Sophia (Fig. 1), a monument of great cultural and historical value for the city of Thessaloniki. PM and dust deposition samples were analyzed in order to examine the ionic content of these particles and their possible effect on weathering of the cultural heritage monument. In the fraction of PM2.5, SO42- were the most abundant anion while the concentrations of NO3- and Cl- were very similar (table 1). These values are characterized in general as normal to low for an urban site [Querol et al., 2008]. On the other hand, in PM2.5-10 fraction, NO3- and Cl- exhibited elevated concentrations compared to PM2.5, accounting for 72% and 62% of total NO3- and Cl- , respectively. SO42- ranged one order of magnitude lower than those of fine particles. Regarding organic anions, Ac and For appeared to contribute equally in PM2.5. Previous measurements in the greater region of Thessaloniki had shown elevated concentrations for Ac (0.67 μg/m3, Mouratidou et al., 2004) while For were very similar to those found in present study. These are expected, as soon as Ac associates strongly with the vehicle exhaust gas [Wang et al., 2007] so is likely to associate with the reduced traffic occurred in the last years in the city while, on the other hand, the main primary sources of For are biogenic which do not appear wide variations over time. Despite Ox are detected mainly in fine particulate fraction (over 90%) [Kerminen et al., 2000] in the present study they did not exceed 0.12 μg/m3. In the fraction of PM2.5-10, Ac were the most abundant anion while mean concentrations of determined For and Ox found to be identical. It is observed that Ac were distributed almost equally in the two particulate fractions. The values of For in coarse particles were much lower than those in fine particles which is in agreement with the literature [Wang et al., 2007]. Ox showed very low concentrations during the sampling period. In Figure 3 the distributions of the determined ions in both PM fractions are depicted. Figure 1: The church of Agia Sophia in the centre of Thessaloniki Methods and materials Sampling PM samplings were performed in central Thessaloniki at a place surrounded by commercial shops and residential houses. A dichotomous sampler (Thermo Andersen, Series 240) was used in order to separate particles in two size fractions: the coarse fraction (PM2.5-10) and the fine fraction (PM2.5). The 48-h measurements were carried out on Teflon filters conditioned for 48 h at 20±1°C and 50±5% RH, at selected dates in the spring (2/4-30/5) and summer (12/7-25/9) of 2011. The church of Agia Sophia, from where the samples of dry deposition were taken, is located approximately 200 m from the measurement station. A total of eight samples of deposited dust were collected from the surfaces of exterior walls of the church. Six of them were from unsheltered points (UN, exposed to wind and rain) and two of them were from sheltered points (S, not exposed to rain and partially exposed in the wind). Examples of these points are shown in Figure 2. Dust samples were collected in plastic sachets of propylene using a paintbrush. Finally, they were stored in a dark and cool place until analysis. Figure 3: Comparison of average concentrations (+SD) of determined anions in PM2.5 and PM2.5-10 • Ionic components of dust • In UN samples, Ac were found to be the dominant carboxylic anion with an average concentration of 52.0 μg/g, while For’s value was significantly lower (9.2 μg/g). On the other hand in S samples both anions exhibited similar concentrations (56.4 μg/g and 48.7 μg/g for For and Ac, respectively) which indicates that For is influenced more by the residence time. It is also notable that elevated concentrations of Ac were detected in the side with higher traffic density confirming vehicular exhausts as the main origin of this anion. Ox were not detected neither in UN nor in S samples. • As far as inorganic anions are concerned, their concentrations were found to be 1 to 2 orders of magnitude higher than the corresponding of organic. Particularly, in UN samples SO42- were the dominant ions (mean: 3.0 mg/g), followed by NO3- and Cl- (mean: 0.4 mg/g and 0.2 mg/g, respectively). These findings indicate the atmospheric deposition of HNO3 and H2SO4 gases and their further heterogeneous reactions with the deposited particles. In S samples all the anions appeared to be elevated compared to the UN samples. Average concentrations of SO42-, NO3- and Cl- determined equal to 4.3 mg/g, 0.7 mg/g and 0.3 mg/g. These values are within the range of values reported in Budapest and Cologne by Török et al. (2010). Figure 2: Examples of unsheltered (UN, left) and sheltered points (S, right) from where samples of dust deposition were collected Analysis In order to determine particle mass (PM) concentrations the filters were weighed before and after sampling using a KERN 870 microbalance (d=0.01mg). Concentrations of the ions Cl-, NO3-, SO42-, Ac, For and Ox were determined using ion chromatography (Shimadzu system). Both filters and dust samples were extracted with 2.5 ml of a mixture of ultra-pure water and isopropanol (9:1) and then treated in an ultrasonic bath for 30 min. All anions, both organic and inorganic, were determined simultaneously using a Slimpack IC-A1 column and a guard column of the same material. The eluent was potassium hydrogen phthalate (C48H6O4) 1.2 mM. • Relative Contributions • In figure 4 is given the relative contribution of individual anions in PM10 fraction (resulted from the sum of determined PM2.5 and PM2.5-10) and in dust samples. It can be observed that the abundance order of ions was SO42- > NO3- > Cl- > Ac > For > Ox and it was similar for both PM10 and dust samples. Results & Discussion Particulate mass The mass concentration of PM2.5 ranged from 7.1 to 18.1 μg/m3(mean: 11.6 μg/m3) while the corresponding values for PM2.5-10ranged from 3.8 to 23.1 μg/m3(mean: 12.0 μg/m3). Both small and coarse particles concentrations were significantly lower than earlier measurements (PM2.5: 21.1 μg/m3, PM2.5-10: 24.0 μg/m3) obtained from the same area [Argyropoulos et al., 2011]. This difference is attributed both in the prevailing weather conditions at the period of sampling (frequent rainfalls), as well as in the reduced traffic observed over the last years in the city of Thessaloniki, mainly due to increased fuel prices and the economic recession in Greece. Figure 4: Relative contribution (%) of individual anions in the total ionic mass of PM10 and dust deposition PM10 Dust References Argyropoulos G, Manoli E, Kouras A, Samara C (2012), Concentrations and source apportionment of PM10 and associated major and trace elements in the Rhodes Island, Greece. Sci. of the Total. 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Greece, Atmospheric Environment, vol. 38, pp.4593-4598 Querol, X., Rodriguez, S., Schneider, J., Spindler, G., Brink, H., Tǿrseth, K., Wiedensohler, A., (2004), A European aerosol phenomenology— 2:chemical characteristics of particulate matter at kerbside, urban, rural and background sites in Europe, Atmospheric Environment, vol. 38, pp.2579–2595 Török, Á., Licha, T., Simon, K., Siegesmund, S., (2010), Urban and rural limestone weathering; the contribution of dust to black crust formation, Environmental Earth Sciences Wang, Y., Zhuang, G., Chen, S., An, Z., Zheng, A., (2007), Characteristics and sources of formic, acetic and oxalic acids in PM2.5 and PM10 aerosols in Beijing, China, Atmospheric Research, vol. 84, pp.169-181 Ac For Ox NO3- SO42- Cl-