Scavenging Ratios in an Urban Area in the Spanish Basque Country

Abstract

For a period of six years (1995–2000) the scavenging ratio, which is the ratio of a pollutant's concentration in water to its concentration in air, collected at an urban site in the Spanish Basque Country was studied. The aerosol is characterized by SO₄ ²⁻ and NO₃− with 1.79 and 1.61 μg m⁻³, respectively. Greater fractions of SO₄ ²⁻, NO₃−, and NH₄+ ions were present in the fine particle range, while greater fractions of other ions appeared in the coarse range. The most important species found in the precipitation is SO₄ ²⁻ with 3.0 mg l⁻¹. NO₃−, Ca²⁺, and Cl⁻ are the second most important ions. The volume-weighted mean concentration of H⁺ is 4.6 μg l⁻¹ (pH = 5.3). The concentration of all analyzed ions (except H⁺) decreases throughout the rain event, showing the washout phenomenon of the rainwater. The scavenging ratio for the anthropogenic ions NO₃−, SO₄ ²⁻, NH₄+, and K⁺ is lower than the scavenging ratio for the marine-terrigenous ions, Cl⁻, Na⁺, and Ca²⁺.

INTRODUCTION

Atmospheric aerosol is the main source of most of the nonreactive mineral species found in precipitation. The main process for the incorporation of aerosols into precipitation include the incorporation in cloud water (nucleation) and the collection via raindrops in the in-cloud and below-cloud regions (inertial impaction, interception, and diffusion; Tsai et al. 1990).

The concept of scavenging ratios is based on the simplified assumption that the concentration of a component in precipitation is related to the concentration of that compound in the air. Thus, scavenging ratios can be calculated on a mass basis (Kasper-Giebl et al. 1999),

[1]

where W_x is the dimensionless scavenging ratio of X species; (X)_p and (X)_a are the X concentrations in the precipitation (μg g⁻¹) and in the aerosol (μg m⁻³), respectively; and ρ= 1290 g m⁻³ is the air density at 20°C and 1013 hPa.

The scavenging ratio is fundamentally affected by the following factors: (1) particle size and hygroscopicity for aerosols, which is itself dependent upon many factors including chemical speciation and atmospheric life history; and (2) cloud type and precipitation intensity. Moreover, while much of the pollutant incorporation into precipitation occurs at cloud level, concentrations of airborne species are normally measured at the ground. The atmosphere is frequently not well mixed over this vertical distance, especially for components with a strong ground-level source and short lifetime (e.g., ammonia). For these reasons, scavenging ratios vary greatly in time and space (Harrison and Allen 1991).

One important condition for the reliability of the scavenging ratio is to verify simultaneous rainwater and aerosol sampling at the same location and over a long period in order to avoid important deviations in the results.

We are not aware of any other studies on this subject carried out in the north of Spain (the part of the country with the highest levels of annual precipitation). The present study aims at providing data related to this parameter, which is so important for the field.

In this article, the scavenging ratio calculated from rainwater and aerosol ionic content in an urban area over a period of six years (1995–2000) is presented for different chemical species.

EXPERIMENTAL

The sampling point is located at Vitoria, an urban area in the Spanish Basque Country (Figure 1). Vitoria is a town of 200,000 inhabitants, located in the south of the Basque Country (Northern Spain) at an altitude of 580 m above sea level. It is moderately industrialized. However, the northern area of the Basque Country is a highly industrialized region (40–70 km from Vitoria). About half of the Basque Country is forested (Figure 1).

FIG. 1 Location of the sampling point (Vitoria) in the Spanish Basque Country.

A six-stage cascade impactor was used to measure the particle size distribution. Each measurement was carried out once a week over 24 h (from Wednesday at 10:00 a.m. to Thursday at 10:00 a.m.) in order to integrate the dynamic cycle of the atmospheric mixing layer. Table 1 shows the diameter ranges for the particles in each stage of the cascade impactor. The airflow used was 1 m³ h⁻¹.

TABLE 1 Diameter particle range collected by the cascade impactor

Basis Airflow Filter type	Once a week and over 24 h 1 m^3 h^−1 Nuclepore, 0.4 μm
Stage	Diameter range (μm)
1	>9.76
2	9.76–4.21
3	4.21–1.73
4	1.73–0.73
5	0.73–0.45
6	<0.45

The design of the cascade impactor has been described in detail previously (Encinas et al. 1994; Encinas and Casado 1996, 1999). In order to avoid a situation where the particles bounce off the impactor, the impactor was made of electrostatic nylon. The collection surfaces were 0.4 μm Nuclepore filters. The soluble matter was extracted with 5 ml of deionixed H₂O using the ultrasonic method.

The rainwater samples were collected using an sequential “wet-only” collector (giving 0.13 mm fractions of precipitation). The collector features a plate equipped with a resistance that maintains the plate's temperature at 120°C. When there is precipitation, the first raindrops that touch the plate will evaporate, and a servomechanism opens the cupola. The date and time of opening are registered, and the first 10 ml of rain (0.13 mm of precipitation) are sent through a system of valves to the conductivity cell, where the temperature is measured. When these two parameters have been registered, the 10 ml of rainfall are injected into the first test tube of the tray. When this is done, the tray automatically turns until the second tube is placed below the injector. The process is repeated until the end of the rain event, when the water on the plate evaporates and the cupola closes.

Once the samples were collected, they were analyzed within 24 h or, if that was not possible, kept in a refrigerator at a temperature below 4°C.

The Cl⁻, NO₃−, SO₄ ²⁻, Na⁺, NH₄+, K⁺, Ca²⁺, and Mg²⁺ ions were the species determined in both the soluble extracts from the filter and the rainwater samples. The analytical technique was the ionic chromatography with conductimetric detection (Casado et al. 1989, 1995, 1996; Encinas and Casado 1999; Ezcurra et al. 1998). Moreover, the pH was measured in the rainwater samples.

The ionic concentrations from the filters have been corrected for blanks. The mean concentrations of the different ions analyzed at each event, weighed by volume, have been calculated using the fraction samples.

RESULTS

The study of the scavenging ratio comprises the period between January 1995 and December 2000. During this period, a total of 127 samples (762 filters) were collected in the cascade impactor.

For the rainwater, 4380 mm of precipitation were registered at Vitoria. The precipitation collected in the wet-only collector was 4220 mm, distributed among 637 events, which correspond to 96% of the total registered precipitation.

Size Distribution of Atmospheric Particulate Fraction

Fine particle range, integrated both in the Aitken nucleus mode and in the accumulation mode (D< 2.5 μm), is of a distinct anthropogenic nature—that is, the particles are either the outcome of nucleation and coagulation processes, or are emitted directly by anthropogenic sources (such as combustion processes, industrial activity, etc.). Coarse or sedimentable particle range (D>2.5 μm) are produced mainly by natural processes. It includes sea-salt aerosols, which are produced by direct dispersal of ocean water, and crustal aerosols, which originate from the solid surface of the earth (Morawska et al. 1999; Parmar et al. 2001).

Figure 2 shows the average ionic concentrations measured in each stage of the cascade impactor.

FIG. 2 Average ionic concentrations of atmospheric particulate fraction collected at Vitoria. Period: January 1995 to December 2000. Units: μg m⁻³.

As can be seen from Figure 2, the Mg²⁺ ion was not included in the study because its concentration is below the detection limit in the majority of the samples collected.

The maximum concentrations of anthropogenic ions, NO₃−, SO₄ ²⁻, and NH₄+, are logically in the fine particles mode (D< 1.73 μm). Of the total mass of SO₄ ²⁻ and NH₄+ ions, 41% and 55% respectively, are in the 0.45–0.73 μm range. For NO₃−, 55% of the total mass are in the 0.45–1.73 μm range. This result is probably due to the absorption of the HNO₃ gas by the coarse particles carrying the Ca²⁺ ion, which have a basic character. Similar results have been reported in other sites in the Spanish Basque Country (Casado et al. 1996; Encinas et al. 1994; Encinas and Casado 1996, 1999) and by other authors (Pratt and Krupa 1985; Ohta and Okita 1990).

Both marine ions (Cl⁻ and Na⁺) and a crustal ion (Ca²⁺) are present in higher concentration levels in the coarse particles range, as can be expected from their mechanical origin. Of the total mass of Cl⁻ and Na⁺ ions, 60% and 70%, respectively, are in the 0.73–4.21 μm range. For the Ca²⁺ ion, the percentage of the total mass in the 1.73–9.76 μm range is 48%. There is not a distinct size distribution for the K⁺ ion.

Total Atmospheric Particulate Fraction

The total atmospheric particulate fraction was calculated as the addition of the average ionic concentrations in each stage of the cascade impactor. Figure 3 shows the total atmospheric particulate fraction collected at Vitoria during the study period.

FIG. 3 Total atmospheric particulate fraction calculated as the addition of the average ionic concentrations in each stage of the CRA-88 impactor. Period: January 1995 to December 2000. Units: μg m⁻³.

The anthropogenic ions, NO₃− and SO₄ ²⁻, are the most important species found in the atmospheric particulate fraction, with 1.61 and 1.79 μg m⁻³, respectively. This was expected due to the urban character of the sampling point. Ca²⁺, Cl⁻, and NH₄+ are the second-most important species, with 0.92, 0.91, and 0.85 μg m⁻³, respectively. These five ions represent on the whole 87% of the water-soluble aerosol mass collected at Vitoria during the study period.

In order to determine the marine influence on the composition of aerosols, sea-salt ratios were calculated using Na⁺ as reference, assuming all Na⁺ to be of marine origin (Parmar et al. 2001). These sea-salt ratios are given in Table 2.

TABLE 2 Sea salt ratio values of atmospheric particulate fraction components and the corresponding values for sea water

	Cl^−/Na^+	SO4 ^2−/Na^+	K^+/Na^+	Ca^2+/Na^+
Sea water	1.8	0.25	0.037	0.038
This work	1.64	3.24	0.67	1.66

It is evident from Table 2 that the sea-salt ratios for SO₄ ²⁻, K⁺, and Ca²⁺ are higher than the seawater ratios, which indicates incorporation of nonmarine constituents in aerosols. The Cl⁻/Na⁺ ratio is similar to the marine aerosol ratio, denoting the marine source as the only one for the Cl⁻ ion in the atmospheric particulate fraction. Encinas and Casado (1999) stated that the Cl⁻/Na⁺ ratio at Salvatierra (Figure 1) was 2.0 due to the presence of a waste incinerator that affects Cl⁻ concentration in the atmospheric particulate fraction collected at this site.

Precipitation

The volume-weighted mean concentrations of the different ions analyzed in the precipitation samples collected at Vitoria are represented in Figure 4.

FIG. 4 Volume-weighted mean concentrations of precipitation collected at Vitoria. Period: January 1995 to December 2000. Units: mg l⁻¹. H⁺ concentration is expressed in μg l⁻¹.

The precipitation at Vitoria is characterized by SO₄ ²⁻ with 3.0 mg l⁻¹. The second-most important species are NO₃−, Ca²⁺, and Cl⁻ with 2.0, 1.8, and 1.7 mg l⁻¹, respectively. On the whole these four ions represent 77% of the ionic content of rainwater.

The volume-weighted mean concentration of H⁺ is 4.6 μg l⁻¹, which corresponds to a mean pH value of 5.3. Consequently, the precipitation at Vitoria has neutral characteristics. The pH value of rain events varies between 4.0 and 7.9, with 17% of the samples showing a pH value less to 5.3. Rain events with strong acidity (pH ≤ 4.5) represent only 3% of the total number of samples, and they proceeded mainly from the European continent (Encinas and Casado 1995).

Figure 5 shows the concentration percentage of the analyzed ions throughout the ten first fractions of the rain events. The rest of the collected fractions were omitted in order to reduce the complexity of the figure.

FIG. 5 Average concentration percentage of ionic species found in the ten first fraction of precipitation.

The concentration of all analyzed ions (except H⁺) decreases throughout the rain event, as can be seen from Figure 5, which implies that there is an important washout process in the first millimeter of precipitation. Percentage values between 25% (for Mg²⁺) and 16% (for NH₄+) are collected in the first 0.13 mm of precipitation. The concentration percentage drops gradually to an approximate value of 6%.

Finally, the concentration percentage of H⁺ initially decreases in the three first samples and then increases progressively, probably due to a neutralization effect by alkaline substances (Casado et al. 1992).

Scavenging Ratio

The scavenging ratio for each analyzed species from the precipitation and aerosol data set was calculated as in Equation (1) and represented in Figure 6. In order to avoid errors that may derive from the variability of conditions throughout the collecting period, only the impactor measurements that preceded a rain event have been considered.

FIG. 6 Scavenging ratios (W) calculated as the relationship between the ionic concentrations in the precipitation and aerosol collected at Vitoria.

As stated by Jaffrezo and Colin (1988), aerosol particle size seems to be one of the main factors that determine scavenging efficiency; depending on the diameter of the particle, the collection efficiency between a raindrop and a particle is predicted to vary over several orders of magnitude.

The anthropogenic NO₃−, SO₄ ²⁻, NH₄+, and K⁺ ions present the lower scavenging ratios, as can be expected because of their size distributions. The higher W(SO₄ ²⁻) compared with W(NO₃−) and W(NH₄+) was probably due to the particulate SO₄ ²⁻ from the marine source.

Moreover, the NO₃−, SO₄ ²⁻, and NH₄+ scavenging ratios are influenced by the presence of gaseous compounds such as HNO₃, SO₂, and NH₃. Many studies have been made regarding the contribution of gases to rainwater concentrations (Chaumerliac et al. 2000; Crutzen and Lawrence 2000; Graedel 1984; Voisin et al. 2000). Thus, Kasper-Giebl et al. (1999) stated that precipitation sulfate originates predominately from particulate sulfate (89–96%). Reactive gas-phase scavenging of SO₂ is only of minor importance. Precipitation NO₃−, on the other hand, is predominantly formed by gas-phase scavenging of HNO₃ (88–96%), while particulate NO₃− contributes to a much lesser extent (4–12%). Precipitation NH₄+ is formed by comparable amount of particulate NH₄+ (49–79%) and NH₃ (51–21%). Therefore, W(NO₃−) and W(NH₄+) based on particulate matter only have to be regarded as an upper limit of the actual value.

On the other hand, the marine (Cl⁻ and Na⁺) and crustal (Ca²⁺) ions are in the coarse particle range; therefore, they present a high value of the scavenging ratio.

Table 3 compares our results with those of the reference. In this chart we also include the scavenging ratios obtained a Olaeta and Salvatierra, two municipalities located near Vitoria (see Figure 1). The results obtained at Vitoria are similar to the ones obtained for other areas. However, the scavenging coefficient of the Cl⁻ ion at Olaeta and Salvatierra is higher due to the gaseous HCl from a waste incinerator located near the stations (Casado et al. 1996; Encinas and Casado 1999). This waste incinerator does not affect the scavenging ratios of the Vitoria station.

TABLE 3 Scavenging ratios in other areas

Reference	Site	Type	Cl^−	NO3−	SO4 ^2−	Na^+	NH4+	K^+	Ca^2+
This work	Vitoria	Urban	3030	2303	2830	3625	1739	1151	3983
Casado et al. 1996	Olaeta	Rural	21936	2319	843	4965	433	1736	4721
Encinas and Casado 1999	Salvatierra	Rural	8729	2788	963	7106	1080	1392	1834
Jaffrezo and Colin 1988	Paris-Francia	Urban	7710		940	744		1325	1759
Dasch and Cadle 1985	Lapeer-USA	Rural	5129	1252	1808	1260		1193	1433
Harrison and Pio 1983	Hazlerigg-UK	Rural	600	370	700	560	350	620	1890
Forti et al. 1990	S. Paulo-Brasil	Urban	3888			468		684	1404
Kane et al. 1994	Mannington-UK	Coastal		204	540	800	1320		420
Guerzoni et al. 1995	Sardinia-Italia	Coastal	2419	754	1728	1708		1630	3459
Pratt and Krupa 1985	Alma-USA	Forestal	886	2927	708		691		706
	Ely-USA	Forestal	949	4913	889		687		2358
	Grand Rapids-USA	Forestal	1392	3833	866		1306		1844
	Sherburne-USA	Rural	902	2118	616		1095		500
	Wright-USA	Rural	1534	2381	597		668		667
	Lamberton-USA	Rural	1464	1799	575		863		540

CONCLUSIONS

The ratio of the pollutant's concentration in water to its concentration in air (named scavenging ratio) was studied at Vitoria, an urban site in the Spanish Basque Country, over a period of six years (1995–2000).

The aerosol is characterized by SO₄ ²⁻ and NO₃− with 1.79 and 1.61 μg m⁻³, respectively. The aerosol size distribution presents SO₄ ²⁻ and NH₄+ in the fine particle range (D< 0.73 μm) and the rest of the ions in the coarse range (D > 0.73 μm), as can be expected from their respective origins. The NO₃− ion presents a high concentration in the 0.45–1.73 μm range, probably due to the absorption of the HNO₃ gas by the coarse particles that carry the Ca²⁺ ion, which have a basic character.

The most important species found in the precipitation is SO₄ ²⁻ with 3.0 mg l⁻¹. NO₃−, Ca²⁺, and Cl⁻ are the second-most important species. The volume-weighted mean concentration of H⁺ is 4.6 μg l⁻¹ (pH= 5.3). The concentration of all analyzed ions (except H⁺) decreases throughout the precipitation event, showing the washout phenomenon of the rainwater.

The scavenging ratio for the anthropogenic ions, NO₃−, SO₄ ²⁻, NH₄+, and K⁺, is lower than the one for the marine-terrigenous ions, Cl⁻, Na⁺, and Ca²⁺. Therefore, the higher the particle diameter, the higher the scavenging ratio.

Acknowledgments

This work is a part of the “Cuantificación y estudio del depósito contaminante atmosférico en la CAPV y sus posibles efectos sobre las poblaciones de Pinus radiataD. Don” programme. The Basque Government sponsors this program.