WINTER PRECIPITATION CHEMISTRY IN THE BACKGROUND EMEP STATION IN VÍZNAR (GRANADA, SPAIN) (2002-2006) A. I. Calvo1*, F. J. Olmo2,3, H. Lyamani2,3, L. Alados-Arboledas2,3, A. Castro1, R. Fraile1, M. Fernández-Raga1 1
Department of Physics, University of León, 24071 León, Spain Group for Atmospheric Physics. CEAMA. University of Granada. Av. del Mediterráneo s/n. 18071 Granada, Spain 3 Departament of Applied Physics. University of Granada. 18071 Granada, Spain. *
[email protected]
2
1. INTRODUCTION The removal of gases and aerosols from the atmosphere has an important role in interreservoir transfers. Airborne particulate matter released by natural and anthropogenic sources is transported and scavenged by wet removal or deposited by other mechanisms. The removal of airborne particulate matter through wet deposition, also called scavenging, is achieved through a series of steps that lead eventually to the incorporation of trace particulate into falling raindrops or cloud droplets. The chemical composition of rain and cloud water is a particularly sensitive indicator of pollution emissions. Determining the chemical composition of rainwater provides an understanding of the source types that contribute to rainwater chemistry and enhances the understanding of the local and regional dispersion of pollutants and their potential impacts on ecosystems through deposition processes (Zunckel et al., 2003). The aim of this study is to detect variations in the chemical composition of the rainwater samples collected during five winters (2001/02-2005/06) in a rural EMEP station near Granada. The levels of ions, their variability, and a description of the mean chemistry for the main transport routes reaching this EMEP station are provided. 2. STUDY ZONE The measurements were carried out at the EMEP station (Cooperative Programme for
the Monitoring and Evaluation of Long Range Transmission of Air Pollutants in Europe) of Víznar, in the province of Granada, Spain, (37º 14’ N, 03º 28’ W and 1,260 m above sea level) (Fig.1). Víznar is a rural area located 6 km NE of the city of Granada in one of the surrounding mountains. Because of this location, Víznar is influenced by various factors: North Africa (200 km to the south) is the main source of natural dust; Europe is the main source of anthropogenic pollutants; the Mediterranean Sea lies 50 km to the east; and/or smoke from forest fires may also reach this site. Moreover, the chemical composition of rainwater may vary due to the influence of local sources.
ES07 Víznar ES08 Niembro ES09 Campisábalos ES10 Cabo de Creus ES11 Barcarrota ES12 Zarra ES13 Peñausende ES14 Els Torms ES15 Risco Llano ES16 O Saviñao
Fig.1 Spanish EMEP stations. Only stations in operation (ES07-ES16) are mentioned. 3. MATERIALS AND METHODS The precipitation for the analysis was collected daily in Víznar during five winters (from 2001/02 to 2005/06). Since the station is an EMEP station (ES07), site selection criteria, sampling, analysis and data quality
control protocols are pre-established (EMEP, 2001). Samples were collected daily at 7:00 UTC using an ERNY ARS721 sampler, and were then stored in the refrigerator until they were sent - once a week - to the laboratory at the Carlos III Health Institute (Madrid, Spain). In the samples Cl-, NO3-, SO42- concentrations were measured by means of ion chromatography. Ca2+, Mg2+, Na+, K+ concentrations were measured using an Atomic Absorption or Emission Spectrophotometer and NH4+ concentrations were measured by means of the Spectrophotometric Indophenol method. Back trajectories (120 h, 00 and 12 UTC; FNL) for three different altitudes (500 m, 1,500 m and 3,000 m agl) were calculated with the HYSPLIT model in order to interpret the different source regions of the air masses reaching the study zone. Data relationships and source types for precipitation constituents were explored by means of the Principal Components Factor Analysis (PCA). 4. RESULTS AND DISCUSSION 4.1 Chemical composition A total of 108 samples were collected during the winters 2001/02 - 2005/06. All the events occurring between December 22 and March 30 have been included in this study (2001/02, n=21; 2002/03, n=32; 2003/04, n=20; 2004/05, n=11; 2005/06, n=24). Winter rainfall ranged between 38.4 mm in 2004/05 and 226 mm in 2002/03, and an average of 145 mm has been calculated for the entire studied period. The most important rain events took place in March 2002 and January 2003 when about 110 mm month-1 were registered. The following parameters were determined in the precipitation samples: pH, electrical conductivity and the concentrations of SO42(S), NO3-(N), NH4+ (N), Cl-, Na+, K+, Ca+2, Mg+2 and H+. Statistical information, minimum and maximum concentration (μeq L-1) of the measured anions and cations are summarized in Table 1.
The pH of rain is considered neutral at 5.6. Below this value precipitation is considered acid rain. This station did not have an acid rain problem: the rainwater collected had a typical pH in the 5.8–7.4 range (the average pH was 6.4±0.3). Extensive conductivity range values (between 5 μS cm−1 and 84.0 μS cm−1) were registered. If mean concentration values of the complete study period are observed, wet-only precipitation chemistry is dominated by Ca2+, SO4-2 and Cl-. These elements contributed over 20%, 18% and 17%, respectively, of the total ion concentrations analyzed. NO3-, NH4+ and Na+ constitute about 12% of the total concentration each. The winter 2004/05 can be considered as a relatively dry winter when compared with the rest, with only 38.4 mm of precipitation. Particularly high Ca+2 concentrations were registered in this winter with a mean value of 110 ± 100 μeq L-1. The maximum Ca+2 concentration of the study period was registered on 16 March 2005 with a value of 299.4 μeq L-1 during a Sahara episode. High concentrations of Ca2+ often found in the Mediterranean area are usually attributed to the intrusion of carbonate-rich air masses arriving from the Sahara (Alastuey et al., 1999). In addition to Ca+2, the highest pH and conductivity, Mg+2, K+, NH4+, NO3- and SO42- mean concentrations were also registered during this winter 2004/05. The highest precipitation was registered in the winter 2002/03 with 225.6 μeq L-1 and the winter 2003/04 is characterized by its high Clconcentration. 4.2 Trajectory analysis Many studies have pointed out the fundamental role of meteorological factors in determining the chemical features of precipitation. Using back-trajectory analysis with the aim of studying the influence of different air masses on the chemical winter precipitations in our study zone, the precipitation events were classified into six groups: (1) Mediterranean (2) Tropical Maritime (3) Polar Maritime (4) Local (5) Continental (6) Arctic. Three additional groups were defined to describe the influence
of the frequent Sahara intrusions on the chemical composition of the precipitation in Víznar and in particular to reveal the influence of Sahara air masses at high levels on this chemical composition: (7) Sahara 500 m (8) Sahara 1,500 m and (9) Sahara 3,000 m. To carry out this classification, the Sahara 500 m air masses input days were selected. With the rest of the events, and without taking into account the 500 m classification, the Sahara 1,500 m air masses input days were selected. The same process was carried out with the 3,000 m classification. The rest of the events were classified in the other groups on the basis of 500 m back-trajectories. The geometric mean has been calculated (as a better descriptor of the central tendency in log-normal distributions, as is the case here) for each meteorological group (Fig. 2 and 3). Sahara rain event chemistry is characterized by high pH and Ca2+ concentration levels (Ávila and Alarcón, 1999). They present high
cation and marine ion concentrations. The high cation load and alkalinity of Sahara events have been attributed to the calcareous dust dissolution in the precipitation originated in the North of Africa. According to Ávila and Alarcón (1999), the high SO42-, Na+, and Clconcentrations are probably due to gypsum (CaSO4) and halite (NaCl) dissolution in Sahara dust. Sahara rain events at different heights (500, 1,500 and 3,000 m) present a high mean conductivity (15.9, 15.7 and 14.6 μS cm-1, respectively) and the highest Ca+2 and Mg+2 concentrations of the nine groups studied (Fig. 2 and 3). Sahara 500 m (n=22) and Sahara 1,500 m (n=15) rain events show similar mean concentrations for the different elements (except for Cl-). Sahara 500 m registers the highest Ca+2 (43.9 μeq L-1) concentrations and the highest conductivity means. During Sahara 1,500 m (n=15) rain events, the highest pH (6.6), Mg2+ (15.2 μeq L-1) and K+ (4.1 μeq L-1) mean concentrations were registered.
Table 1. Minimum, maximum and mean concentrations for ions, conductivity and pH in rain water samples collected at Víznar during the winter study campaign (December 2001-Mars 2006) with the standard deviation. Number of events (N) and precipitation (P) in mm have been included. Concentrations are reported in μeq L-1 units except pH and Conductivity (μS cm-1) Winter
N
P
2001/02
21
192.0
2002/03
32
225.6
2003/04
20
112.6
2004/05
11
38.4
2005/06
24
155.6
109
144.8
2001/06
min max mean σ min max mean σ min max mean σ min max mean σ min max mean σ mean σ
Conduc.
pH
H+
Cl-
NO3-
SO42-
Na+
NH4+
K+
Mg2+
Ca2+
5.1 45.4 15.4 9.2 5.2 39.3 16.2 8.9 5.2 50.1 19.6 11.2 11.0 84.1 32.1 22.6 5.0 33.9 13.6 7.3 17.8 12.2
6.1 7.0 6.6 0.2 5.9 7.0 6.2 0.2 5.9 6.7 6.4 0.2 6.4 7.0 6.7 0.2 5.8 6.7 6.5 0.2 6.4 0.3
0.2 0.6 0.3 0.1 0.1 1.2 0.7 0.3 0.2 1.2 0.5 0.2 0.1 0.4 0.2 0.1 0.2 1.5 0.4 0.3 0.5 0.3
16.1 39.8 26.0 7.1 13.3 135.4 40.8 31.7 15.5 209.0 58.6 59.3 5.1 28.8 15.3 8.2 10.2 65.4 22.9 14.5 35.3 33.9
10.7 128.5 30.4 26.5 10.0 57.8 20.3 10.2 6.4 102.1 33.3 22.0 9.3 82.1 37.2 24.4 5.7 57.1 16.2 11.3 25.2 19.4
17.5 116.0 39.5 23.7 19.3 69.2 35.9 12.7 11.9 111.7 50.5 26.6 15.6 117.3 49.7 34.8 8.1 85.5 23.8 17.6 37.7 22.7
4.8 30.0 17.8 7.0 4.3 87.0 30.0 21.5 10.0 40.4 26.5 8.9 10.4 47.0 27.8 14.2 5.2 63.9 22.0 15.7 24.6 15.5
5.7 73.5 18.3 18.5 5.7 64.2 18.1 14.1 5.7 74.2 28.7 16.1 8.6 82.8 45.7 52.5 5.0 77.1 29.3 17.8 24.1 18.4
1.5 7.4 3.5 1.4 1.5 5.1 3.2 1.0 2.8 8.7 5.6 1.6 3.6 9.0 5.8 2.1 1.5 7.2 3.2 1.6 3.9 1.8
5.8 57.6 16.5 12.3 4.1 23.0 12.4 6.3 6.6 38.7 17.8 9.4 10.7 65.8 31.2 21.5 5.8 19.8 10.1 4.0 15.1 11.1
18.5 168.7 51.3 34.2 7.5 90.8 28.4 21.4 24.0 116.3 46.1 23.3 33.4 299.4 111.3 99.4 14.0 49.4 25.2 9.8 42.1 41.4
Sahara 3,000 m (n=3) rain events present different characteristics. The highest load of pollutant species (SO42-, NO3- and NH4+) was registered in this group. The pH mean value is the lowest observed in the nine groups studied with a value of 6.1. Although the minimum pH value in this group was 6.0, this is not a low value for Sahara events, as minimum 4.2 pH values have been registered for this type of events by Ávila and Alarcón (1999). In order to explain this fact we have to consider that, on the one hand, anthropogenic species (NO3- and nss-SO42-) are usually transported with mineral dust (Savoie et al., 1992, Prospero et al, 1995). Concentrations of Particulate Matter (PM) of non-sea-salt sulphate between 2 and 6 μg m-3 have been registered in the Canary Islands during Sahara events (Prospero et al., 1995). On the other hand, it is necessary to consider that we have a Sahara air mass at high levels, but the characteristics of air masses at low levels (500 m and 1,500 m) influence
rain chemistry too. The events included in the Sahara 3,000 m group have mainly a continental and tropical maritime origin at low levels (500 m and 1,500 m). If Sahara 3,000 m events are compared with the continental or tropical maritime group, no real similarities are founded. Ca+2 concentrations at 3,000 m are similar to the mean Sahara 500 m and 1,500 m concentrations for this element. It is remarkable that the third highest Ca+2 concentration of the study period was registered in one of the three events included in this class (168.7 μeq L-1), only preceded by two Sahara 500 m events. Thus, the Sahara 3,000 m group seems to have an important influence on chemical rain composition. Tropical and Polar Maritime groups present similar mean values (except for NH4+ and Ca+2). These marine groups present high concentrations for ions of marine origin (Na+ and Cl-).
Fig. 2. Geometric ion concentrations for the different air masses.
Fig. 3. Conductivity (μS cm-1) and pH geometric mean for air masses. Mediterranean rain events (n=4) can be considered as one of the lowest pollution rains. They have the lower mean concentration of SO42-, Mg+2, Na+ Ca+2, Cland K+ of the nine groups studied. As for continental rains (n=17), they present intermediate characteristics when compared with the rest of air masses. There are studies that have demonstrated the influence of European pollutants in the precipitation chemistry in north-eastern Spain (Carratalá and Bellot, 1998; Ávila and Alarcón, 1999). In these studies, very high SO42-, NO3- and NH4+ concentrations have been registered. In our study the concentrations of these elements are moderate. The Sahara groups (at the three levels) show higher concentrations of these three elements than the continental group. Escudero et al. (2007) study the origin of the exceedances of European daily PM limit in regional background areas of Spain (Víznar is included). These authors question the fact that these exceedances are caused by European transport only, and argue that the common scenario giving rise to European PM transport also favors the formation of typical local and regional winter pollution episodes at urban and industrial sites due to the anticyclone over the Bay of Biscay. It is therefore difficult to say whether the proportion of high PM levels measured under this scenario should be attributed to long-range transport from Europe or to local/regional emissions (Viana et al., 2003). On the other hand, these high PM level episodes only refer to regional background
sites, which should not be strongly influenced by anthropogenic sources. PM exceedances associated with European episodes were only registered in the northern Iberian flank. The pollutants in the European air masses suffer less dilution/dispersion reaching the northern Iberian Peninsula than the center or the south owing to the shorter transect from central Europe (Escudero et al., 2007). This fact could support the influence of Sahara air masses on SO42-, NO3- and NH4+ concentrations. Local rain events (n=3) register the lowest NO3-, NH4+, H+ concentrations and conductivity. They present a pH of 6.5. Nevertheless, they register the highest Clmean concentration (with a concentration similar to Polar and Tropical Maritime groups). Only one arctic event was registered, so no sufficient data are available to draw any conclusion. 4.3 Statistical approach Multivariate techniques have generally been employed to determine the sources of data variability. These techniques are particularly useful for the identification of pollution sources in studies of air quality and will be applied here for a comparison with the meteorological classification. The principal component analysis extracted three factors explaining 80.0 % of the total variance (Table 3). Component one, with the highest variance of 48.0%, was positively correlated to most ions SO42-, NO3-, NH4+, K+, Mg+2, Ca+2) and conductivity. This ion grouping may be interpreted as a size factor. For the second component, which explains 18.1 % of the total variance of the investigated data set, high loadings negatively correlated –as expected- for pH and H+ were obtained. The third component represents 13.9% of the total variance and has high loadings of Cl- and Na+, therefore representing the marine contribution. Similar results in the interpretation of the principal components have been reported for the precipitation chemistry in the Iberian
Table 3. Principal component analysis of elemental composition pattern in wet-only winter precipitation in Víznar. Components Conductivity -
NO3
2-
SO4
2+
Mg
2+
Ca +
K
+
NH4 pH +
H
-
Cl
+
Na
1 0,97
2 0,09
3 0,04
0,91
0,18
-0,11
0,91
0,19
-0,04
0,90
0,02
-0,05
0,86
-0,16
-0,12
0,72
0,05
0,26
0,71
0,01
-0,18
0,22
-0,97
0,03
-0,17
0,96
-0,11
0,02
-0,03
0,87
0,18
0,19
0,79
Peninsula (Escarré et al., 1998; Ávila and Alarcon, 1999). 5. CONCLUSIONS - The winters 2001/2006 present different characteristics as regards wet-only precipitation chemistry and height precipitation according to the data registered at the Viznar EMEP station. - In the total study, wet-only precipitation chemistry was dominated by Ca2+, SO4-2 and Cl-. - Rain events were classified into nine groups on the basis of back-trajectories. They present different characteristics. - The Sahara 1,500 m and 3,000 m classes seem to have an important influence in chemical rain composition. - ACP extracted three components that explained 80.0 % of the total variance, interpreted as size, acidity and marine factors. 6. BIBLIOGRAPHY Alastuey, A., Querol, X., Chaves, A., Ruiz, C.R., Carratalá, A., and López-Soler, A. 1999. Bulk deposition in a rural area located around a large coal-fired power
station, northeast Spain. Environmental. Pollution. 106, 359-367. Avila, A., Alarcón M. 1999. Relationship between precipitation chemistry and meteorological situations at a rural site in northeastern Spain. Atmospheric Environment 33, 1663-1677. Carratalá, A., Bellot, J., 1998. Neutralization of nitrate and sulphate in precipitation on the eastern Mediterranean coast of Spain. Implications for acidification risk. Water, Air and Soil Pollution 104, 237– 257. EMEP, 2001. EMEP Manual for sampling and chemical analysis. EMEP/CCCReport 1/95, Revision 1/2001, NILU, Norge. Escarré, A., Carratalá, A., Avila, A., Bellot, J., Piñol, J., Millán, M. 1998. Precipitation chemistry anda ir pollution. In: Rodà, F., Retana, J., Gracia, C., Bellot, J. (Eds.), Ecology of Mediterranean Evergreen Oak Forest. Berlin. Ed. Springer-Verlag, Series Ecological Studies, pp. 195-206. Escudero, M., Querol, X., Ávila, A., Cuevas, E. 2007. Origin of the exceedances of the European daily PM limit value in regional background areas of Spain. Atmospheric Environment, 41, 4, 730744. Prospero, J.M., Schmitt, R., Cuevas, E., Savoie, D.L., Graustein, W.C., Turekian, K.K., Volz-Thomas, A., Díaz, A., Oltmans, S.J., Levy II, H., 1995. Temporal variability of summer-time ozone and aerosols in the free troposphere over the eastern North Atlantic. Geophysical Research Letters 22, pp. 2925–2928. Savoie, D.L., Prospero, J.M., Oltmans, S.J., Graustein, W.C., Turekian, K.K., Merril, J.T. and Levy II, H., 1992. Sources of nitrate and ozone in the marine boundary layer of the Tropical North Atlantic. Journal of Geophysical Research 97 11, pp. 575–589. Viana, M., Querol, X., Alastuey, A., Gangoiti, G., Menendez, M., 2003. PM levels in the Basque Country (Northern Spain): analysis of a 5 yr data record and interpretation of seasonal variations.
Atmospheric Environment 37 (21), 2879– 2891. Zunckel, M., Saizar, C. and Zarauz, J., 2003. Rainwater composition in northeast Uruguay, Atmospheric Environment, 37, 1601–1611. Acknowledgements The authors would like to thank Alberto Gonzalez Ortíz (MMA) and Jaume Almera
Institut (CSIC) for the information provided about Viznar station. Borja Ruíz Reverter is gratefully acknowledged for the backtrajectories classification. This work was supported by the Spanish Ministry of Education CGL2004-05984-C07-03 and by Andalusian Regional Government P06RNM-01503 .
