Armosphrr~c Enwonnenr, Vol. IS, pp. 579-582.

‘0 Pergamon Press Ltd. 1981. Printed in Great Bntain

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PREDICTION OF MARINE ATMOSPHERIC DEPOSITION RATES USING TOTAL 7Be DEPOSITION VELOCITIES*

ERIC A CRECELKJS

Battelle, Pacific Northwest Laboratory, Marine Research Laboratory, Sequim, Washington 98382, U.S.A.

(First received 2 May 1980 and in jinalJ~rtn 11 Augusf 1980)

Abstract-Samples of atmospheric particulate matter and total atmospheric deposition were collected for a one-year period at a rural site on the Washington coast. Samples were analyzed for 13 stable elements and ‘Be. The ratios of elements to ‘Be in air and total deposition samples were similar. The total deposition rates of elements measured were used to predict total deposition velocities of those elements in other areas, then compared to actual measurements at those locations.

INTRODUCIION

Atmospheric deposition is felt to be an important source of materials to the world’s aquatic environ- ments. This has been demonstrated for lakes and coastal waters (Cambray et al., 1979; Hodge et al., 1978; and Winchester, 1972) with reference to nutrients and pollutants. However, because of the lack of suitable sampling platforms, there are few measure- ments of atmospheric deposition to the open ocean. One study was performed by Cambray et al. (1979) on a natural gas platform in the North Sea, and Duce et al., (1976) calculated estimates of the rate of deposition as a source of elements to the oceans. As a part of a program to determine the relative importance of atmospheric deposition as a source of elements to the oceans, the study reported here was undertaken to evaluate using 7Be as a method for determining these inputs.

The ‘Be method assumes that the total deposition velocity (t Vg)t of any element will be proportional to the t Vg of 7Be. Beryllium-7 was chosen as a tracer because it is primarily attached to submicron air particulates and, therefore, should have the same total deposition velocity as anthropogenic pollutants pre- sent in the atmosphere on submicron particulates, and its total deposition velocity is relatively easy to meas- ure. In addition, there have been measurements of total ‘Be deposition velocity at many locations in the open ocean (Young and Silker, 1974; 1980). Therefore, it is assumed that, if a relationship between the t Vg of 7Be

and the t Vg of an element can be determined by field measurements at coastal locations, then the t Vg of that element can be predicted for areas of the open ocean. Total deposition can be estimated by measuring the

* This paper is based on work performed for the U.S. Department of Energy under Contract EY-76-C-06-1830.

t The total deposition velocity is defined as tVg = F/C, where F = amount of element deposited per unit time per unit area, and C = average concentration of airborne element.

average concentration of airborne elements at sea and applying an experimentally derived t Vg.

Two assumptions used in making the estimates are: (1) the ratio oft Vg of an element and t Vg of ‘Be is constant at different geographical locations, and (2) the methods used to measure total deposition rates are satisfactory. One should realize that there are possible problems with both these assumptions. Local sources of elements may have unique particle sizes and dry deposition velocity has been shown to be particle size dependant (McMahon and Denison, 1979; Gatz, 1975; and Sehmel and Sutter, 1974), and there is no standard deposition collection device. just how representative the artifical surfaces are relative to natural surfaces is not known.

SAMPLING AND ANALYSIS

Measurements were made of the air concentration and total deposition rate (both dry and wet fallout) of airborne elements at Quillayute, Washington, a coastal site located 3 km from the ocean beach on the northern Washington coast (50 m above sea level). The land surrounding the site is low relief and forested, the rainfall is 268 cm per year (NOAA Quillayute Weather Station), and the winds are generally westerly. The site was chosen because of its remote location and wind patterns. No wind directional control ofesampling was used. Wind back trajectories indicate that this site should be representative of the air chemistry along the Washington coast (Fox and Ludwick, 1976).

Samples of atmospheric particulate matter and total at- mospheric deposition were collected 4 m above the ground. Air particulates were collected at a flow rate of 8.5 m3 min-’ using 16 cm diameter IPC filter material. The linear velocity of air through the filter was maintained at greater than 300 mmin-’ to insure high filtering efficiency for submicron particles (Stern et al., 1960). Air filters were changed ap proximately once a week, and total deposition samples were changed at two-week intervals. A polyethylene bucket was used for collecting total deposition. After collection the deposition sample was spiked with rubidium, as an internal standard, acidified to pH 1 with Ultrex@nitric acid, filtered, and the filtrate was then evaporated to near dryness and mounted for analysis. More than 90% of the elements analyzed in the total deposition samples were dissolved by the acid before filtering in order to concentrate the elements in a

579

580 ERIC A. CRECELIUS

small area for improved analytical sensitivity. The Rb internal standard is used to correct for counting geometry when the samples are analyzed by energy dispersiie-X-ray fluorescence (XRF) usina the method of Nielsen (1977). The air filters and the hid leached particulates from’the deposition samples were also analyzed by XRF. The ‘Be activity in air filter and deposition samples was quantified by gamma counting, using a Ge(Li) diode with an anticoincidence shield (Wogman, 1969).

The average air filter blank was determined and subtracted from the loaded air filters. The blank was less than 2 7; of the load filters except for Cu, V, Se, and As for which the blank was less than 10%. The total deposition bucket blank was determined by adding a titer of high purity water to the bucket after a rain sample had been removed from the collector. The high purity water was then processed as a rain sample. The average deposition bucket blank was subtracted from each deposition sample. The blank contributed less than 1 p; of a sample for all elements except Cr, Br, V, Se and As for which the blank contributed less than lo”,,.

RESULTS AND DISCUSSION

Table 1 shows the concentration of 14 elements in the air, the total deposition rate, and the deposition velocity (t Vg) as measured at Quillayute, Washington. The air concentrations are similar to those reported for rural continental (Crecelius ef al., 1980) and rural coastal areas (NAS-NRC, 1978) and approximately twice the levels reported for the North Atlantic open ocean (Buat-Menard and Chesselet, 1979). Although the Quillayute air concentrations are elevated com- pared to open ocean concentrations, when wind direc- tional control was used, the air concentrations were much lower, similar to those values reported for open ocean sampling (Ludwick PZ al.. 1977). The total deposition rates determined at Quillayute are lower

than those reported by Cambray et al. (1979) Hodge ef al. (1978) and Andersen et 01. (1978) for more indus- trial areas. The total deposition velocities (fk’g) at

Quillayute for the elements in Table 1 ranged from 0.30 to 4.3 cm s- ‘. These t Vgs are within the range of

dry deposition velocities reported for held measure- ments reviewed by McMahon and Denison (1979).

The ‘Be f Vg measured at Quillayute (I .O cm s ’ ) is identical to that measured off the Washington Coast by Young and Silker (1980). They determined the I C;cg for ‘Be by measuring the ‘Be inventory in the ocean water column. This agreement suggests that the air and

deposition sampling methods were appropriate for air particulates containing ‘Be. Sized air particulates collected at Quillayute showed that 82”” of the ‘Be was associated with the i 1.1 pm fraction (Ludwick et

al., 1975). The ‘Be t Vg measured at Quillayute approximates

(within a factor of 4) the I Vg of the other elements measured. The estimated t V’g of ‘Be over much of the ocean ranges from 0.4 to 2.0 cm s- ’ (Young and Silker, 1980) and generally follows the precipitation patterns,

increasing with increased precipitation. The rainfall at Quillayute is relatively high (268 cm) and distributed over many days of the year. Therefore, total deposition may be dominated by washout or rainout as opposed to dry fallout which would be predominant in an arid climate.

Using the data from Table 1 and the literature, an estimate of elemental deposition rates for Fe, Pb, and

Cu were made for several marine sites. Five marine sites were selected: Sequim, Washington, located on the Strait of Juan de Fuca (Crecelius, unpublished data); La Jolla, California, located on the Pacific Ocean (Hodge et al., 1978); Ensenada, Mexico, located on the

Pacific Ocean (Hodge ef al., 1978); North Sea, includ- ing five sites surrounding this water body (Cambray YI al., 1979); and the Tropical Atlantic (Duce er u1., 1976). All five sites had measured air chemistry data and four of the five had measurements of total deposition. The Atlantic site had only estimates of deposition rates. Three elements (Fe, Pb and Cu) were chosen for the

calculations, each of these elements representing air particulates of different origins. Iron is associated with

Table I. Air chemistry data measured at Quillayute, Washington between February 1976 and January 19778

_

Total Concentration Depositron rate deposition velocity

(ngm-‘) (mgm-‘a--‘) (ems-‘) __-

K 87,42 Ca 130+68 Br 8.9 ) 4.0 Ti 21+13 Cr 0.76 f 0.46 Mn 6.3 + 4.4 Fe 227,138 V 1.7 f 1.0 Pb 19&15 As 1.6k2.4 Se 0.28kO.13 cu 2.2 * 0.85 Zn 31* 13 ‘Be 0.25+0.13dpmm~’

66+58 2.3 118+ 162 2.9 5.5 + 4.7 1.9 5.5 + 8.7 0.83

0.16kO.25 0.66 1.1 io.9 0.55 27rf:31 0.37

0.16 f 0.19 0.30 2.6 Ifr 3.9 0.43

0.48 * 1.4 0.95 0.036 + 0.027 0.38

3.0 +4.8 4.3 21+ 18 2.1

80,900+72,800dpmm~2a~’ 1 .o

l Arithmetic mean and standard deviation of 24 deposition samples and 48 air filters.

Prediction of marine atmospheric deposition rates using total ‘Be deposition velocities 581

crustal materials, Pb with anthropogenic sources, and When the Fe t Vg is increased to that of ‘Beand applied

Cu may be associated with marine origin (Cattell and to the air data of the five sites, a more accurate

Scott, 1978). prediction is obtained (Table 2).

The deposition rates for the five sites were calculated in two different manners. The first method used the elemental t Vg measured at Quillayute and ratioed for the ‘Be t Vg at the different sites using Young and Silker (1980) data. For example, Fe t Vg at Quillayute

was 0.37 cm s- ’ and the t Vg of ‘Be in La Jolla was

0.5 cm s- ’ (500,;; of the ‘Be t Vg at Quillayute), thus, an adjusted Fe t Vg of 0.185 cm s - ’ was used for La Jolla. The second method was to use the estimated ‘Be t Vg for the particular site for all the elemental t Vgs. For example, 0.5 cm s- ’ was used for Fe, Pb and Cu at La

Jolla. In Table 2 the results are compared with measured deposition rates at four of the sites and a predicted rate at the fifth site.

The deposition rates predicted by the ratio method in Table 2 are too low for Fe. This is probably because

the Fe t Vg measured at Quillayute (0.37 cm s- ‘) was lower than would be expected for a crustal element.

Table 2. Predicted and measured deposition rates of three elements at five different marine sites (mg rn-‘a- ‘)

The Pb deposition rates predicted in Table 2 are

within a factor of two of the measured rates. This implies that Pb tVg at Quillayute is similar to that at other locations. When ‘Be r Vg was substituted for Pb t Vg, the predictions were not as accurate as those

calculated using the Quillayute value (Table 2). The Cu deposition rates predicted by using the Cu

tVgof4.3cms~’ are within a factor of three. When the

‘Be t Vgs are used to calculate the Cu t Vg, rates were lower than measured and half the cases showed closer agreement than using the Quillayute Cu t Vg.

The correlation between predicted and observed

deposition rates was estimated by linear regression statistical analysis for the data in Table 2. The corre- lation was slightly better when using the ‘Be t Vg (r = 0.54). than when using Quillayute elemental I Vgs (r = 0.51) ratioed to the estimated ‘Be t Vg. Both correlation coefficients were significant at the 95’:” confident level.

Location Fe Pb Cu

A comparison can be made between predicting

atmospheric deposition using ‘Be t Vg versus washout

rates. Buat-Menard and Chesselet (1979) used air particulate chemistry data (Fe-100 ng m- 3, Pb- 9.9ngmm3, and CuA.79 ng m-3) from shipboard

sampling and assumed a ‘Be t Vg of l.Ocm s- ’ to calculate the deposition rate to the North Atlantic for Fe, Pb and Cu to be 32, 3.1,0.25 mg me2 a-‘, respect- ively. The calculations by Duce et al. (1976) of

deposition of Fe, Pb, and Cu to the Tropical Atlantic using washout rates, shown in Table 2, are within a factor of 2.

In summary:

(1) The elemental deposition rates at a specific marine sitecan usually be predicted within a factor of 5 using elemental air particulate data and applying the predicted ‘Be t Vg for the specific site to all elements.

(2) Although the t Vg of elements differ for each site, the range of t Vg for coastal sites with dramatically different climate is probably between 0.2 and 2 cm s- l. Since ‘Be t Vg over the ocean ranges from 0.4 to 2.0cm s- ‘, ‘Be t Vg can predict the deposition rate of most elements to within a factor of 5.

(3) Agreement exists between the deposition rates predicted by ‘Be and those predicted by washout rates for the Tropical Atlantic.

Sequim, Washington Measured

(Crecelius, unpublished data) Predicted by

ratio method* Predicted using rvg = 1.0

Southern California Measured

(Hodge et al., 1978) Predicted by

ratio method* Predicted using

t vg = OS? Ensenada, Mexico

Measured (Hodge er al., 1978)

Predicted by ratio method*

Predicted using I vg = 0.5t

North Sea Measured

(Cambray et al., 1979) Predicted by

ratio method* Predicted using

rvg = 1.ot Tropical Atlantic

Predicted by washout (Duce et a/., 1976)

Predicted bv ratio method*

Predicted using f vg = 0.5t

42

13 7.3 2.6

36 17 0.6

500 28 11

36 38 11

96 88 2.5

220 11 4.7

122 10 14

331 24 3.2

253 27 13

29

79

30

7.6

21

4.7

13

31

1.4

0.5

1.1

0.9

10

2.4

0.3

0.9

0.2

l Predicted by ratio method used the r Vg measured at Quillayute (Fe = 0.37, Pb = 0.43, and Cu = 4.3) and ratio to the ‘Be tVg estimated for each site by Young and Silker (1980).

* Predicted by using the ‘FkrVg estimated by Young and Silker (1980) for all three elements.

(4) In order to greatly increase the accuracy of atmospheric input rates to the ocean, a great deal of research would be necessary at several different clima- tic sites remote from large land masses. A deposition

collection device that represents the sea surface would be preferable to a bucket type collector.

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