~wrpkrir Ewirwmt Vol. 21. No. 6. pp. 1275-1283. 1987. OMI4-6981/87 13.00+0.00

Primed in Great Britain. 0 1987 Perpmon Joumalr Ltd.

SOURCE CHARACTERIZATION FOR ATMOSPHERIC TRACE METALS OVER KIEL BIGHT

BERND SCHNEIDER*

Institut fiir Meereskunde an der Universitiit Kiel, Diisternbrooker Weg 20.2300 Kiel. Federal Republic of Germany

(Pirsr received 14 February 1986 and infinuljorm 25 Nowmber 1986)

Abstract-Atmospheric concentrations of Na. Al, K, Ca, Ti. V. Cr. Mn, Fe, Ni. Cu, Zn. As. Se, Rb, Sr, Ba and Pb are reported for 59 weekly air filter samples collected over the Kiel Bight. The contributions of sea salt, mineral dust and anthropogenic emissions to each of these elements were assumed to be represented by the concentrations of indicator elements, which were Na, Al and Zn, respectively. Based on this assumption a multiple regression analysis was applied to the concentration data. The results showed that atmospheric sea salt contributed significantly only to Sr and, ofcourse, Na. Considerable portions of Al, K, Ca, Ti. Cr. Mn, Fe, Rb, Sr and Ba were derived from mineral dust. Anthropogenic sources were responsible for total V, Ni, Cu, Zn, As and Pb, and there was an anthropogenic component for most of the other elements.

Moreover, the anthropogenic contribution was characterized by a nearly constant composition with respect to Ca, Ti, Cr. Mn, Fe, Cu. Zn, As, Rb, Sr, Ba and Pb. indicating that trace metals over the Kiel Bight are mainly derived from one source area. This conclusion was confirmed by correlating anthropogenic trace metal concentrations with the wind direction. A 40” wind sector directed to the south of the sampling site was identified as the major pathway for the transport of anthropogenic trace metals to the Kiel Bight.

Key word index: Trace metals. Baltic, source identification, atmospheric transport,

INTRODUCTION

The significance of atmospheric deposition for trace metal budgets for the Baltic Sea and its subareas have been reported by several authors (e.g. Rodhe et al., 1980; Andreae and Froelich, 1984; Bruegmann, 1984; Larsen and Pheiffer Madsen, 1984; Brzezinska and Garbalewski, 1980). In addition to a determination of the atmospheric fluxes, an identification of the sources is of great importance. This paper presents an ap- proach to estimate the influences of different sources on atmospheric trace metal concentrations over the Kiel Bight.

If an extensive set of concentration data for a sampling site is available, statistical methods, such as factor analysis, may be applied. A basic requirement for this type of analysis is that a characteristic, relatively unchangeable composition for each of the effective sources must exist. The statistical treatment of concentration data presented here also requires such an assumption. The sources considered are mineral dust, sea salt and anthropogenic emissions, each of which is represented by an indicator element, Al, Na and Zn, respectively. On the basis of this concept, a multiple regression analysis is applied to the concen- tration data of 59 weekly air filter samples collected over the Kicl Bight. The results characterize the chemical composition of the sources and lead to an

*Present address: GKSS-Forschungszentrm Geesthacht, Max-Planck-StraRe, 2054 Geesthacht. Federal Republic of Germany.

estimation of the contribution of the different sources for each trace metal.

Additionally, the data were analyzed by multiple regression to determine whether a relationship exists between atmospheric concentrations and the wind direction during sampling. From the obtained results of this analysis the main source areas for several elements could be identified.

SAMPLING AND CHEMICAL ANALYSIS

During 1981-1983 59 air filter samples were col- lected from a lighthouse in the Kiel Bight. This lighthouse is located on an artificial island about 20 km from the city of Kiel which has approximately 300,000 inhabitants and is only slightly industrialized. Emissions of trace metals near Kiel are mainly due to power plants, traffic and a refuse incinerator. The surrounding areas as well as the Danish islands to the north of Kiel Bight are heavily cultivated for agricul- ture. The nearest industrial center with diverse emis- sions is located in the suburban Hamburg area, about 100 km S of the sampling site. The large industrial areas of the F.R.G. and the G.D.R. are about 500 km from Kiel.

During weekly sampling about 100-200 m3 ol’ air

was drawn through a double filter consisting of cellulose (Whatman 41) and PTFE (Sartorius. SM 11807). Samples were analyzed by neutron acti- vation for Na, Al, V and Mn, and after digestion with HNOa, by total reflection X-ray fluorescence

1276 BERND SCHNEIDER

(Michael& et af., 1984) for K, Ca, Ti, Cr, V, Mn, Fe, Ni, CU, 2% As, Se, Rb, Sr, Ra and Pb. For a more detailed description of sampling and analytical techniques see Schneider (1984a).

RESULIS

The arithmetic mean concentrations and the 1 U- range of atmospheric trace metals over Kiel Bight are shown in Table 1 (columns 1,2). These values are based on the trace metal content of 59 individual samples. In the following section two aspects of the data set are considered:

1. What are the chemical characteristics for the ef- fective sources and to what extent do they contribute to the trace metal concentrations over Kiel Bight?

2. What is the influence of the wind direction on trace metal concentrations over Kiel Bight?

Mineral dust, sea salt and anthropogenic emissions are considered as potential sources for trace metals over Kiel Bight. Their impact on trace metal concentra- tions is expected to vary highly between the samples and to be strongly affected by the wind conditions during sampling. An evaluation of the contribution by each of these sources is made through use of indicator elements. These are elements whose concentrations may bc attributed to a single source only. Moreover, it is assumed that the contribution by each of the sources is characterized by an approximate constant elemental composition. Atmospheric Al is mainly associated with mineral dust, and it is therefore considered as an indicator element for this source (Rahn, 1976). AS a uniform composition is assumed for mineral dust, its contribution to any element (X,) will form stable ratios (M,) with respect to total Al (Al,):

M, = X,/AI,

or X,= M,xAl, [Ii

For atmospheric sea salt particles, Na is commonly m use as an indicator element (Duce and Hoffman, 19761, In this presentation a slight contribution by mineral dust to Na, is taken into account. Sea salt derived Na {Na,) is obtained from Na, by subtracting Al, multi- plied by the crustal Na/AI ratio (Mason, 1966). The element ratios for atmospheric sea salt (S,) are given by:

S, = X,/Na, Of

X, = S, x Na,. (2)

Ratios S, are not necessarily identical to those in bulk seawater, as chemical fractionation may occur during sea spray formation (e.g. Weisel et al., 1984; Schneider, 1985).

Anthro~genic emissions are largely responsible for the concentrations of Zn, Cu. As and Pb over Kiel Bight. An estimate based on AI and Na concentrations in the samples indicates that the contribution of mineral dust or sea salt is negligible (C 5 %).

Moreover, Zn, Cu, As and Pb are found in nearly uniform proportions in each sampie, as ifhrstrated in Fig. 1. These findings can be used to postulate characteristic ratios (A,) for the anthropogenic por- tion of any element (X,) using Zn as an indicator element:

or A * = XJZn,

X .= A,xZn,. (31

Considering potential ~ntributjons by mineral dust, sea salt and anthropogenic emissions to each element, the total concentration (X,) consequently is given by:

x,=x,+x,+x,. (4)

Table 1. Arithmetic mean ~~nt~tio~s (X,), lo-range and estimated contri- butions by mineral dust, sea salt and anthropogenic emissions [ “/.I

X, (ng m-‘) la-range Mineral dust Sea salt Anthrop. em.

Na 1404 44&2370 10 90 - Al 394 68-720 100 - - Zn 8-106 - - 100 K

3: 124-476 39 - 61

? 435 27 83-787 5-49 65 88 - 35 12 V 9.7 4.9-15 Cr 2.9 0.2-5.6 ;

- 100 - 82

Mn 15 4-26 49 - 51 Fe 369 59 - 41 Ni 4‘0 1.9-6.3 - - 100 CU 1.3-14 - 100 As

::: 0.2-5.4 z - 100

Se 1.6 0.6-2.6 - - 66 Rb 1.4 0.3-2s 58 - 42 Sr 4.7 2.0-7.4 47 21 32 E&i 7.9 2.1-14 s5 - 45 Pb 53 14-92 - - 100

Source characterization for atmospheric trace metals over Kicl Bight 1277

Inserting Equations (l)-(3) to Equation (4) it follows: determine M,, S, and A, the concentration data are

X,= (M,x Al,)+(S,x Na,)+(A,x Zn,). (5) subjected to a multiple regression analysis on the basis of Equation (5), which is completed by an invariable

X,, Al,, Na,and Zn,are measured quantities for each term (intercept) X0. Values for Xe are due to contri- sample, whereas M,, S, and A, are element specific butions to X,, which are not linked with the abun- ratios for the corrkponding sources. In order to dance of Al, hIa or Zn.

(a)

Zn 22O-

hg m-31

ZOO-

l&J-

160 -

140 -

120-

cu [n9 m-31

(b) 240 -

Zn

220-

Ins m-31 xx)-

As In9 m-31

Fig. I (a)-(b).

1278 BERND SCHNEIDER

6 , I 1 , I 5 1 I *

40 80 80 100 120 UO WO l80 200 220 240

Pb be m-31

Fig. 1. Relationship between anthropogenic tmce metals and calculated regression lines for Zn vs Cu. Zn vs As and Zn vs Pb.

Table 2 shows the multiple correlation coeflicients (rifand thec&uIated values for M,and &that havea level of significance > 95 %. Values for X0 are included only in the case that they amount to more than 18 % of the mean totai concentration.

Only for Sr could a marine contribution be detected, characterized by an S, of 6.5 x IO-*.

Applying M,, S, and A, to Equations (l)-(3), the contributions by the corresponding sources to each trace metal may be estimated for any sample. The results referring to the mean concentrations are listed in Table 1 (columns 3-5). For reasons to be discussed later, Ko, V0 and Nio are attributed to anthropogenic contributions, whereas Se0 may be due to other sources.

The second important question examined concerns the relationship between trace metal concentrations and meteorological conditions over Kiel Bight. In this presentation the considerations are confined to the wind direction at height of about 10 m above sea level. Hourly wind records conducted by the Deutscher Wetterdienst have been available.

The wind rose is arbitrarily divided into nine 40” wind sectors starting with 0”. It is assumed that each of

these sectors accounts for a characteristic trace metal concentration (X,, X1,. . . . , X9). These concen- trations will be observed when the wind blows from one of these sectors all the time during sampling. But as during weekly samflmg winds may be highly variable, usually more than one sector will contribute to the observed concentration X,. The contributions by any

Table 2. Muitiple~~e~tion ~~jen~ rt and caicu- iatcd values for M, = X /Al,, A, = X /&I, and X0 obtained from the rtgrw!on analysis (&r S, see text)

ri MS A, X0 (rum-‘)

& 0.83 0.30 1.3 108 0.88 0.66 2.5 -

Ti 0.96 0.059 0.054 V 0.83 - 0.074 G Cr 0.94 1.1 x 10-a 0.034 - Mn 0.99 0.018 0.13 - Fe 0.97 0.53 2.5 - Ni 0.82 - 0.038 1.7 CU 0.95 - 0.12 -- As 0.98 - 0.052 - se 0.87 - 0.018 0.52 Rb 0.98 2.2 x lo-’ 0.011 - Sr 0.91 5.3 x 10-S 0.025 - Ba 0.90 9.4 x 10-S 0.054 - Pb 0.95 - 0.75 -

sector is then given by the product Xi x hi where hi is the relative frequency of winds coming from sector i during sampling. Hence, X, may be expressed by:

X,=CXiXhi+X,* (6)

X, takes into account possible portions of X, that do not depend on the wind direction.

After determination of the wind frequency data hi for each sample a multiple regression analysis is performed on the basis of Equation (6) for elements, which are derived from a single source. The resulting

Source characterimtioa for atmospheric trace metals over Kiel Bight 1279

Xi will give information on the importance of the and Duce (1972) and Hoffman et al. (1974X different wind sectors for trace metal concentrations respectively. over Kiel Bight. It may be estimated from the ratios K/Na and

In Table 3 results are shown for Al and Na, which Ca/Na in bulk seawater, that about 10-15x of

represent mineral dust and sea salt, and for Zn, Cu, As, measured K, and Ca, should be of marine origin, but

Pb, V and Se, which are mainly of anthropogenic these contributions are obviously too small to be origin. Additionally, the characteristic concentrations recognized by the multiple regression analysis. for the indicator elements, Al, Na and Zn, are shown as The regression analysis of Na, data with respect to shadowed areas in Fig. 2. The closed circles around the the frequency of wind sectors reveals a correlation sampling site represent the invariable term X,.

Table 3. Multiple correlation coefficients (rs) and characteristic concentrations (Xi)

DISCUSSIONS AND CONCLUSIONS of wind sectors, for which a correlation with Al, Na. Zn, Cu. As. Pb. V and Se was

1. Seo salt particles

A marine contribution to atmospheric trace metals over Kiel Bight could be detected only for Sr. On an average about 20 Y0 of total Sr was sea salt derived. The calculated Sr/Na ratio (6.5 x lo-*) agrees reasonably with the Sr/Na ratio of bulk seawater (7.1 x 10e4, Broecker and Peng, 1982). These results suggest that no significant fractionation of Sr occurs during sea spray formation. Similar findings for Sr and other earth alkaline elements have been reported for marine aerosols from Hawaii and the N Atlantic by Hoffman

‘u Xi (ng m-‘)

(a)

Al 0.70

Na 0.46 Zn 0.71 cu 0.67 As 0.69 Pb 0.67 V 0.43 se 0.65

x, = 790; x. i= 1200 x, = 1090; x, = 94 X, = 2820; X, = 625 X s = 426; X, = 20 xs = 53; x, = 3.0 X, 3: 22; X, = 0.8 X, = 324; X, = 24 X’s = -16; X, = 12 Xs = 8.2; X, = 0.82

7

r ’

I ,_ 100 km 1 Na H 1000 ng m-3

L Berlin , I To E 80

t so 100’

1 110 120 139 140

Fig. 2 (a).

1280 BERND SCHNEIDER

Al t-i SOOng me3

Berlir 1 1 I I ?

90 100 110 120 130 140 1

56

Fig. 2 (b).

between Na,and sector 7, which is from the North Sea (Table 3, Fig. 2). Transport of Na from other sectors, in particular from the northwest (sector 8), appears as a relatively high constant term X,. This fact and the low correlation coefficient (r v = 0.46) are most likely a consequence of neglecting wind speed in the model. As wind speed determines the production of atmospheric sea salt particles, the hypothesis of characteristic Na, concentrations for the wind sectors has only limited validity.

2. Mineral dust

The multiple regression analysis indicates that min- eral dust contributes significantly to atmospheric trace metals over Kiel Bight (Table 2). Contributions ranged from 100 % for Al (by definition) to 18 % for Cr (Table 1). The calculated elemental ratios M, = X,/AI, are rather similar to the corresponding ratios for crustal rocks (Mason, 1966). Enrichment factors:

EF ,x,,s, = (X,IA4Li,I(XI~L,,,

are given in Table 4. Deviations from unity for some elements may be due to the fact that mean values for crustal rocks do not take into account mineralogical

peculiarities of individual source areas. For example, the Rb content in the fine grained fraction (< 20 pm) of River Elbe sediments (Schneider, 1984b) shows a

J%rUS, of 1.94, which is nearly identical with EF,,,, = 2.00 obtained for Rbin mineral dust. The River Elbe passes through an area expected to act as a main source for mineral dust over the Kiel Bight (see the following discussion of wind sector analysis) and its sediments will mirror the mineralogy of this region. This in- dicates that the calculated enrichment of Rb in mineral dust is not an artifact but a consequence of the characteristic composition of rocks and soils in the source area. Hence, one should not rely only on data for mean crustal composition when estimating crustal fractions for atmospheric trace metals.

The correlation between AI, and the frequency of wind sectors is characterized by a multiple correlation coefficient (r, = 0.70) that is considerably higher than for Na,. Although the generation of mineral dust depends on wind speed, just as the production of sea salt particles does, the effect on rW is smaller. This may be a consequence of a lower variability in wind speed for continental derived winds than for winds coming from the North Sea.

Three wind sectors (3,4, 5) account for the bulk of

Source characterization for atmospheric trace metals over Kicl Bight 1281

North Sea

wb

5 /\ Kiel .~~~. -

‘....::.. .:..::::.,

..:. . . . . :: . . . . $

t 5

Baltic Sea

I 5 , 100 km I Zn 1-l 200 ng m-3

Berlin

1 I I I 1 1 1 I 70 E 80 90 loo 110 120 130 140

Fig. 2. Wind sectors that determine the concentrations of Al, Na and Zn over Kiel Bight, closed circles represent wind independent concentration levels. The

radii are a measure for the characteristic concentrations.

Table 4. Enrichment factors EF,, for the crus- tal fraction of trace metals over kiel Bight

0.94 Fe 0.85 1.47 Rb 2.00

Ti 1.09 Sr 1.15 Cr 0.92 Ba 1.81 Mn 1.50

mineral dust over the Kiel Bight (Table 3, Fig. 2). In accordance with the distribution of land masses they are directed towards eastern and central Europe. The characteristic concentrations for Al diner only slightly. The lowest value is obtained for sector 3, which partly covers the Baltic Sea area. Winds from the SW (sector 6) are not correlated with Al,. This may be due to the fact that these winds may represent both continental and marine air masses. Possible transport of mineral dust from this sector as well as from sectors 7-9 and l-2 are reflected by Al,, which amounts to 24 % mean Al.

3. Anthropogenic sources

The most critical point in the model applied here is the assumption that the anthropogenic component always shows the same elemental composition, ex- pressed by constant ratios A, = XJZn,. The degree to which the different elements meet this requirement may be inferred from the multiple correlation coef- ficients ri. Additionally, constant ratios A, require values for X0, which should be low compared to the mean concentrations if contributions by additional sources (e.g. biogenic, volcanic) are absent.

On the basis of these criteria the results of the multiple regression analysis (Table 2) show that there exists a reasonable compositional stability for Ca, Ti, Cr, Mn, Fe, Cu, As, Rb, Sr, Ba and Pb relative to Zn:X,, values for these elements are negligible and the multiple correlation coefficients are higher than 0.90, except for Ca.

An explanation for this strong relationship is ob- tained from the regression analysis of the anthropogenic-only derived elements Zn, Cu. As and Pb with respect to the frequency of wind sectors. Table 3 and Fig. 2 show that the concentrations of Zn, Cu. As and Pb are only correlated with sector 5

1282 BERND SCHNEIDER

(160-200”), which is directed to the F.R.G. Multiplication of the mean frequency of winds from this sector (z 10%) by the characteristic concentra- tions Zns, CUE, Ass and Pb, result in a mean contribution of 63% to the total Zn, Cu, As and Pb concentrations. The major portion of these trace metals therefore is derived from a relative narrow wind sector representing a rather limited source area. Hence, anthropogenic emissions from this area will be well mixed after being transported to the Kiel Bight and account for approximate constant ratios A, = X,/Zn, for most of the elements.

Thirty-seven percent of mean Zn. Cu, As and Pb could not be assigned to any wind sector and appears as the invariable term X,. Rahn and Lowenthal (1984) use a set of six elemental ratios to identify different source regions in Europe and N America. Converting these ratios to Zn as reference element, W Europe derived aerosols are characterized by As/Zn and noncrustal Mn/Zn ratios of 0.058 and 0. IS, respect- ively. These values agree well with the corresponding A, for As (0.052) and Mn (0.13) calculated here. In contrast, the ratios As/Zn and noncrustal Mn/Zn for E Europe (Rahn and Lowenthal, 1984) amount to 0.14 and 0.19. respectively. This supports the previous finding that transport of anthropogenic trace metals to Kiel Bight mainly occurs from the S (sector S)and that the contribution by easterly or southerneasterly winds should be small.

In Table 5 the A, values from Table 2 are compared with the corresponding ratios for emissions in the F.R.G., which have been reported by Pacyna et al. (1984). For Ca, Ti, Fe, Rb, Sr and Ba no emission data are available, whereas K. V, Ni and Se will be discussed separately. Except for Cr. a comparison of the data reveals a surprising relationship between emission characteristics and atmospheric concentrations. Emissions in the neighbouring countries of the F.R.G., except Belgium, show different elemental ratios (Pacyna et al., 1984). This is an additional indication that a considerable portion of anthropogenic trace metal over Kiel Bight is received from the F.R.G.

Moreover, Lannefors et al. (1983) report trace metal concentrations for Sjoangen (S-Sweden), which are sorted with respect to transport sectors. Ratios Cu/Zn and Pb/Zn referring to transport from central and W Europe amount to 0.12 and 0.98, respectively. These values are also in accordance with the corresponding

Table 5. Values A, For Cr. Mn. Cu. As and Pb (from Table 2) compared with the corresponding ratios for

emissions in the F.R.G. (Pacyna et al., 1984)

Cr/Zn Mn/Zn Cu/Zn AsfZn Pb/Zn

A,. Kiel Bight Emission, F.R.G.

0.034 0.18 0.13 0.18 0.12 0.13 0.052 0.067 0.75 0.80

A, of 0.12 and 0.75. respectively, observed over Kiei Bight (Table 2).

The previous discussion did not comprise K, V. Ni

and Se, because the regression analysis with respect to the indicator elements revealed high values for X,, In

the case of K, V and Ni this will be due to additional anthropogenic contributions. V and Ni are highly correlated according to their common source, which is the combustion of oil. K may be derived from the combustion of lignite in E Europe. For example, Kemp (1984) found a considerable long-range transport of K from the SE to Denmark.

Obviously.contributions to anthropogenic K, Vand Ni from sectors other than sector 5 are important and their ratios to Zn differ significantly from those of sector 5. Consequently, the multiple regression coef- ficient ri will decrease and a term X0 will be obtained.

These assumptions are confirmed by the regression analysis of V with respect to the wind data (Table 3). Apart from a negative correlation with sector 8, which of course is not real, no preferred wind direction appears. The wind independent term X, (12 ng m-‘) nearly equals the mean V concentration (9.7 ng m - 3).

In contrast, a distinct correlation of Se concen- trations with sector 5 is obtained (Table 3). completed by a wind independent term Se, of 0.82 ng rne3. This value is of the same order of magnitude as Se, (0.52 ng m-‘), suggesting that there exists a back- ground level of particulate Se over Kiel Bight. This Se fraction might be due to a release of vapor phase Se from the biosphere (Mosher and Duce, 1983) and a subsequent interaction with particulate matter.

Acknowledgemenr-This work was supported by the German Ministry for Science and Technology (BMFT) under the Grant No. MFU OS25/3. J am indebted to U. Rabsch and P. Krischker, Institut fiir Meereskunde, Kiel. for making available the analytical facilities ol’ the isotopic laboratory. Moreover, I have lo thank H. Gralll (br helpful discussions and R. Fuhrhop for making the statistical computations.

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