Trace metal and major ion composition of precipitation at a North Sea coastal site

Environment Vol. 23, No. 12, pp. 2751-2759, 1989. Printedin Great Britain.

Atmospheric

0

lloo4-6981/89$3.00+ 0.00 1989 PergamonPress plc

TRACE METAL AND MAJOR ION COMPOSITION OF PRECIPITATION AT A NORTH SEA COASTAL SITE DAFS

P. W. BALLS Marine Laboratory, Victoria Road, Aberdeen, AB9 SDB, U.K.

(First received 16 January 1989 and received for publication 7 June 1989)

Abstract-Major ion and trace metal (Zn, Cu, Cd, Pb, Fe, Mn) concentrations have been determined in 32 rainfall events at a North Sea coastal site over a period of 14 months (June 1987-July 1988). Precautions have been taken to avoid trace metal contamination. Trace metal depositions are positively correlated to excess sulphate deposition; the exception is Cd for which no clear relationship is evident. This is probably due to contamination of the samples with respect to Cd. The highest trace metal concentrations are associated with small rainfall events and with the occurrence of fog and mist. Trace metal concentrations (volume weighted average) are in general agreement with earlier data. For Cd and Zn, however, they are lower and consequently it is suggested that estimates of annual deposition be revised downwards. Key word index: Trace metals, wet deposition, North Sea.

INTRODUCTION

Atmospheric deposition has been identified as an important mechanism whereby contaminants are introduced to marine waters. The mechanism is important for trace metals, e.g. Pb (Flegal and Patterson, 1983) and also for organic compounds including pesticides (Richards et al., 1987) and polycyclic aromatic hydrocarbons (den Hollander et al., 1986). In the open ocean atmospheric deposition may determine the composition of inorganic particulate matter in surface waters (Buat-Menard and Chesselet, 1979). In coastal areas despite the relatively greater riverine inputs the atmospheric flux is also important but uncertain, as highlighted in the Quality Status Report on the North Sea (1987). Several studies have examined the trace metal content of precipitation around the North Sea and these have been reviewed by van Aalst et al. (1982). Dehairs et al. (1982) concluded that inputs of Cu, Zn, Pb and Cd to the North Sea from the atmosphere exceed by an order of magnitude the combined inputs of the rivers Scheldt, Rhine and Meuse while for Fe and Mn the two inputs are approximately equal. Cambray et al. (1975) examined the trace metal composition of rain water from North Sea coastal sites and also at some inland locations. They generally observed higher concentrations at coastal than at inland sites, this was described as the ‘maritime effect’. The sea surface microlayer is enriched in trace metals relative to bulk sea water (Pattenden et al., 1981) and the origin of the maritime effect was proposed to be the generation, by bursting bubbles, of a trace metal enriched aerosol from the sea surface. Subsequent deposition of this aerosol could possibly explain

higher trace metal concentrations in precipitation at coastal sites. The above mechanism has been demostrated to introduce back to land radionuclides initially discharged to coastal sea water (Cambray and Eakins, 1980; Eakins and Lally, 1984; Walker et al., 1986). Using field determinations of trace metal enrichments in the sea surface microlayer it has however been concluded by Church et al. (1975), that the maritime effect is unlikely to contribute significantly to rain water concentrations observed on the Atlantic coast of the U.S. or at Bermuda. To date most atmospheric flux studies around the North Sea have collected bulk deposition over some fixed time period (typically a month). An alternative explanation for elevated trace metal concentrations in coastal precipitation is contamination by dry deposition; for sea salt components this will be greatest near the coast. Reviews of precipitation collection procedures have emphasized the importance of excluding dry deposition and if possible of sampling on an event basis (Galloway and Likens, 1976 ; Galloway, 1978). Several studies have demonstrated that there are chemical differences between rainfall collected on a weekly and on an event basis (de Pena et al., 1985; Sisterson et al., 1985). A further important consideration in the collection of precipitation for trace metal analysis is the avoidance of contamination (Galloway and Likens, 1976; Tramontano et al., 1987). The present work set out to collect precipitation on an event basis at a North Sea coastal site taking precautions to avoid contamination. In addition the deposition of acidic components was examined since, with a few exceptions (Asman et al., 1981; Church et al., 1984, Jickells et al., 1984, Ross, 1987), little attempt

2751

P. W. BALLS

2152

has been made to relate trace metal depositions those of acidic components.

SAMPLING

to

AND ANALYSIS

Samples were collected in the grounds of Girdleness lighthouse, Aberdeen (Fig. 1). The lighthouse is situated on the seaward side of a treeless golf course and is ca 4 km from the city centre of Aberdeen close to the district of Torry. Although a site further from the city would have been preferable, one close to the Laboratory was chosen so that collectors could be deployed and collected at short notice. Whether or not the city directly influenced the rainfall composition is not clear but there was no obvious relationship between trace element depositions and occasions when the air had passed over the city. There are no major sources of atmospheric contamination in the locality other than those of a domestic nature, the nearest large oil burning power station is at Peterhead 45 km to the north. Polythene funnels (42 cm diameter) were mounted in plastic coated metal frames 1.5 m above a grass covered area and attached to 5 L polythene containers. The large diameter funnels permitted the collection of

Rattray

sufficient sample from light rainfall events (l-2 mm) while the high volume receiver ensured that major events were also collected satisfactorily. Approximately 35% of the total rainfall during the study period was collected. Samples for trace metal analysis were collected in acid washed (1% HNO,) equipment which was subseqently rinsed with double distilled water. For major ion analysis samples were collected in a duplicate set of equipment which was rinsed with double distilled water. Cleaned equipment was stored in polythene bags, collection vessels were weighed prior to deployment and reweighed on return to the laboratory. Dry deposition effects were minimized by deploying samples as soon before and collecting them as soon after rainfall events as possible. Typical deployment times were 2436 h. After weighing, the pH of the major ion sample was determined and the remainder of the sample stored in a polythene bottle at 4” C for major ion analysis. The pH was determined using a combination electrode (Russel) with a Corning 113 meter. The system was calibrated using buffers and a mixture of dilute HNO, and H,SO, (pH range 3 -4.4). Four series of standards were prepared containing varying amounts of artificial

Head

Fig. 1. Location

of sampling

site

2153

Trace metal and major ion composition

sea water to more closely match the matrix of the samples (150, 625, 1250 and 2500 PM Na). Sodium, Ca and K were determined by atomic emission and Mg by atomic absorption (Perkin-Elmer 5000). Flow injection techniques were used for the calorimetric determination of SO:-, Cl- and NO;, manufacturers (Tecator) application notes were followed. A manual technique (based on the formation of indophenol blue) was used for the determination of NH,, (Solorzano, 1969). Samples for trace metal analysis were acidified with concentrated HNO, (3 ml !- ‘, Fisons ‘Primar’) without filtration and stored in acid washed polythene bottles at 4” C. The trace metals Zn, Pb, Cu, Cd, Fe and Mn were determined by GFAAS (Perkin-Elmer Zeeman/3030). Furnace conditions were chosen to ensure that the same response was observed with spiked samples as with standards, L’vov platforms were used in all analysis. In order to obtain a consistent response for Cd it was necessary to add a matrix modifier to samples (5 ~1 5% ammonium dihydrogen orthophosphate to 30 ~1 sample). Samples were acidified individually at the time of collection, the reproducibility of acid handling and dispensing, also variations between different batches of acids, are likely sources of error in the data. Some

indication of the magnitude can be obtained from the variability of the blank (acidified distilled water). Typical blank values (pg /- ‘) for Fe, Zn, Cu, Mn, Pb and Cd were 5.8, 1.6, 1.0, 0.07, 0.37 and 0.06 respectively. For Fe, Zn, Cu and Mn the blanks varied by up to ca 20% while for Cd and Pb variations of up to 50% were observed. For all metals duplicate analyses within a batch agreed to within 10%. Variations between batches however was greater being up to ca 20% in some cases.

RESULTS

The major ion concentrations from 32 rainfall events are given in Table 1. Excess SO:- concentrations have been calculated using the Na data and the SO:-/Na ratio in sea water, Riley and Chester (1971). The ion balance is defined as C ([cations] -[anions]) C ([cations + [anions]) ’ The highest concentrations of sea spray constituents were generally observed during autumn and winter in association with higher wind speeds, a

Table 1. Summary of major ion concentrations 1987-1988

Date of sampling 5-l/6 14-1617 16-1717 26-2711 28-2917 a-718 12-13/8 15-17/8 19-2018 25-2618 31/8-l/9 7-8/10 l&16/10 27-28/10 ll-12/11 23-24/l 1 4-7112 19-21/12 &l/l 7-8/l l&18/1 25-26/l 28-31/l 22-2212 23-2512 l&16/3 22-2313 8-l l/4 24-2514 262715 20-2116 4-5/l

Rainfall (mm) 10 3.3 15 1.5 1.4 2.6 5.1 4.6 1.1 10 3.3 22 14 13 5.3 2.1 0.9 3.8 12 1.8 0.6 3.8 14 0.5 5.6 18 2.1 1.0 3.3 3.3 2.2 9.8

WI

CNKJ CNW

Ccl1

CSOd CSg41 CMgl PI

WI

Ion balance (%I -0.1 -2.8 2.9 -1.2 2.5 3.8 5.1 -7.1 3.8 -2.9 -3.5 4.1 2.4 0.1 0.5 - 1.5 0.3 -2.8 -1.1 0.2 0.2 0.4 -1.0 7.0 - 1.0 -2.3 1.7 0.8 0.3 -7.3 1.8 8.5

(PM)

(PM)

(PM)

(PM)

(PM)

(PM)

(PM)

(/M

WI

[Na] WV

39 302 45 6 46 20 55 3 3 21 44 19 26 23 17 1 21 22 11 3 24 28 36 2 1 27 81 2 12 45 23 19

6 235 14 16 24 25 25 6 11 11 21 8 9 20 I 8 68 10 14 10 18 3 8 12 21 6 31 23 33 25 17 4

22 329 14 4 35 9 31 2 18 13 40 10 21 30 25 13 100 9 32 18 32 35 52 15 I 30 235 22 60 55 15 9

1210 4540 310 964 222 520 500 76 227 495 2160 693 532 696 2820 1840 4770 327 1700 1470 2580 3180 2590 138 3020 1250 5300 3800 2040 62 21 255

75 3.75 32 52 28 44 48 3 15 35 135 49 38 55 166 95 216 33 98 80 158 172 145 13 160 77 296 218 123 32 15 14

13 142 16 3 11 17 22 0 3 10 24 13 10 19 21 1 31 16 11 5 26 9 22 6 5 13 24 23 18 29 14 1

114 389 30 68 23 54 55 6 25 42 219 76 54 77 290 189 499 34 174 156 254 321 260 16 280 123 530 356 229 I 4 27

31 155 8 19 9 14 18 1 14 9 64 30 14 22 80 40 134 7 46 40 138 68 74 14 87 40 185 132 71 3 5 7

23 12 6 21 5 10 10 1 7 I 42 13 9 14 54 34 89 6 32 29 93 60 49 2 56 23 141 78 37 1 4 4

1020 3610 210 830 190 475 465 47 198 404 1630 629 480 583 2390 1480 3910 260 1390 1220 2020 2720 2210 130 2470 987 4650 3250 1650 56 26 232

P. W. BALLS

2754

similar phenomenon was reported by Cape et al. (1984). During southerly gales sea spray was sometimes deposited directly into the sampling apparatus resulting in Cl- concentrations > 10 mM. Since such samples are not representative of precipitation they have been excluded from the data set. Trace metal concentration data are summarised in Table 2 together with volume weighted averages, the latter are compared with the results obtained at Rattray Head, Aberdeenshire by Davies (1977) and Cambray et al. (1979) in Table 3. The data of Church et al. (1984) for the Atlantic coast of the U.S. are also included as are those of Ross (1987) for southern Sweden although these data are not directly comparable to the present results, they were obtained using stringent anti-contamination procedures and thus represent a useful reference level.

DISCUSSION

pH and acidic components

Hydrogen ion concentrations ranged from 1 to 300 PM with a volume weighted mean of 30 nM

Table 2. Summary

Date of sampling 5-l/6 14-1617 l&17/7 26-27/l 28-2917 6-718 12-13/8 1517/8 19-20/8 25-2618 31/8-l/9 7-8/10 14-16/10 27-28/10 1lLl2jll 23-24/l 1 4-7/l 2 19-21/12 667,‘l 7-8/l 16-18/I 25-2611 28-31/l 22-2212 23-2512 14-16/3 22-23;‘3 8-l l/4 24-2514 262115 2&21/6 445,‘7 Volume weighted *Contamination

corresponding to a pH of 4.52. The pH range previously published for the area is 4.24.3 based on 1978-1980 data, Warren Spring Laboratory (1983). It has been reported that the pH of event collected precipitation is to some extent dependent on the deployment time (Sisterson et al., 1985; de Pena er al., 1985). In the present work pH determinations were made immediately after collection, by contrast some of the data used in the area average above originates from monthly collections. The apparent difference between the present work and the average value quoted from previous work could therefore be the result of a sampling artefact, but there are alternative explanations. Harriman and Wells (1985) have reported a decrease in H+ concentration in precipitation at Pitlochry, Scotland, a decrease from ca 60 PM in 1979 to ca 35 PM in 1983 was observed. It is conceivable therefore that the higher average pH observed in the present work relative to that reported previously is due to a more general trend. The pH of precipitation is determined by the balance between contributing anions (xs[SO:-] and [NO;]) and cations ([NH:]), i.e. [H’] + [NH;] = 2xs[SO:-]

of trace metal concentrations

+ [NO;].

1987-1988

Rainfall (mm) 10 3.3 I5 1.5 1.4 2.6 5.1 4.6 1.1 IO 3.3 22 14 13 5.3 2.7 0.9 3.8 12 1.8 0.6 3.8 14 0.5 5.6 1X 2.1 1.0 3.3 3.3 2.2 9.8

5.1 29 3.5 5.7 6.4 2.4 3.3 0.6 5.4 1.1 5.2 2. I 2.0 4.3 2.6 1.0 2.9 2.2 2.3 3.1 12 4.2 5.5 3.6 3.5 2.9 14 20 9.6 3.6 3.2 4. I

average

4.0

suspected.

I .45 0.76 0.54 5.20* 1.40 2.52* 0.54 0.10 1.52 1.20 0.56 0.24 0.18 0.17 0.24 1.09 0.85 0.10 0.17 0.65 9.10* 0.41 1.00 1.10 2.45* 0.50 1.13 2.48* 0.83 0.43 0.38 0.29 0.68

9.3 8.7 1.3 3.8 3.1 2.1 0.9 0.3 1.4 0.2 1.2 0.6 1.7 0.7 1.2 1.5 29* 0.9 0.9 1.7 2.5 3.5 3.2 2.3 1.8 7.2 13 4.5 1.3 I .O 2.9

45 240 21 60 I IO 47 46 9 160 I2 220 26 26 5x 75 x4 9X 47 130 110 670 310 220 220 180 x7 430 370 I IO 70 80 31

2.3

8X

I0

x.9 88 3.7 21 28 x.3 17 2.5 50 4.2 I3 6.2 5.1 12 X.0 5.1 30 5.3 x.5 IO 63 1x 25 51 I3 9.6 51 95 39 I2 27 14 I!

I.8 16 1.1 3.2 3.5 2.5 1.8 0.8 4.7 3.9 6.7 1.7 I.1 3.7 3.4 2.6 6.6 1.5 3.0 3.9 13 I0 8.1 5.9 5.5 3.0 18 21 8.6 1.9 3.6 1.4 3.x

Trace metal and major ion composition

2155

When the values listed in Table 1 are substituted into this expression there is a deficiency of positive charges in ca 70% of cases. This deficit can largely be explained by taking into account excess Ca’+ concentrations, i.e. [H’] + [NH:] +2xs[CazC] =2xs[SO:-] + [NO;]. Excess Ca2 + concentrations were calculated from the Ca:Na ratio in sea water (Riley and Chester, 1971), and substituted into the expression above. With this correction the charge balance was within 20% in over 80% of samples. The occurrence of excess Ca’ + ions is attributed to weathering of soil or rock derived particulate matter in the aerosol during which H+ ions are substituted for Ca2 + ions. Trace metals

The extent to which a comparison can be made between the average trace metal concentrations observed in this work and those reported preiously is limited since: (a) Previous studies have collected bulk precipitation samples over monthly intervals (which include dry deposition) and not on an event basis. (b) Geographical differences in composition are anticipated thus direct comparison is restricted to data obtained locally. (4 In earlier work samples were filtered prior to acidification and storage, in the present study they were not. The acidification of unfiltered samples is likely to result in the leaching of additional metals from the particulate material and may consequently produce higher concentrations. Lum et al. (1987) subjected atmospheric particulates to a three-stage sequential leaching technique. For Cd, Pb and Zn they reported that > 85%, >43% and >26%, respectively of the total (as determined by an aqua regia/HF digest) metal was water soluble, higher proportions were released at pH4. Unless therefore there is a heavy loading of particulates, direct acidification is unlikely to have a major effect on concentration. With the exception of Fe, and possibly Mn, the results from the present study are comparable to or generally lower than the earlier data for the North Sea (Table 3). This is particularly noticeable for Zn, an element for which there is a major contamination problem. Higher Fe concentrations are attributed to leaching from particulates which were not removed by filtration. Iron and Mn are closely correlated (r = 0.82) with an Fe:Mn ratio of 22-30. The ratio in soils is ca 40, (Martin and Whitfield, 1983), higher Fe concentrations may therefore be indicative of the presence of soil in samples. The Cd concentrations given here are generally higher than those reported from the Atlantic coast of

2756

P. W. BALLS

the U.S. and southern Sweden. Some of the Cd concentrations in Table 2 are extremely high (asterisked) and contamination is suspected. To ensure analytical quality samples were analyzed on more than one occasion, results were reproducible suggesting that any contamination occurred during the sampling stage. The obvious contamination in some samples casts some doubt on the rest of the Cd data, they should therefore be viewed with some caution and regarded as an upper estimate. Limited information is available on the relationship between trace metal concentrations in precipitation and rainfall volume. It has been reported however that concentrations are highest during the initial stages of a rainfall event (Georgii et al., 1984; Ambe and Nishikawa, 1986). Although no attempt was made during the present study to determine the changes in concentration during a single event there was a large range in the volume of precipitation collected from individual events. In Fig. 2 trace metal concentrations are plotted against volume collected. It is evident that higher concentrations are generally associated with smaller rainfall events. This is in agreement with the work mentioned above and suggests an effective scavenging of the atmospheric aerosol during the initial stages of

precipitation. As regards trace metal deposition per event the greater volume tends to compensate for the lower concentration and the highest depositions are generally observed with the larger events (Fig. 3). This is most evident for Pb and Zn whilst for Cd there is no clear relationship. Cambray et al. (1975) observed higher trace metal concentrations at coastal sites around the North Sea than at those inland and, in order to explain this observation they suggested a ‘maritime effect’. Using the approach taken by Church et al. (1984) it is possible to demonstrate that for the maritime effect to begin to make an important contribution to trace metal concentrations in precipitation the enrichment factor of the surface microlayer (relative to bulk sea water) needs to be l-2 orders of magnitude greater than those observed in the field (Pattenden et al., 1981; Hardy et al., 1985). It is concluded therefore that the maritime effect is not significant and that trace metal concentrations are controlled by natural and anthropogenic releases on land. Few studies have examined the relationships between wet deposition of trace metals and acidic components. At Bermuda, however, Jickells et al. (1984) observed higher trace metal concentrations in precipi-

100

32 0

75

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preclpltation

O 18

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0

0

A? 0

0

I 0

24 ; ?

0

A 0

24

mm 10.0

32

of

24

10

precipitation

r 0

0 7 5-

24 c Y

12

6

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6

5.0.

O

:: 0

0 0

2.5

t

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0

0

6

12

mm of precfpltatlon

18

24

0

6 mm

8 0

0 000

0

12 of

0

18

preclpltatlon

Fig. 2. Relationship between trace metal concentration and rainfall amount: Pb, Zn, Cu and Cd.

0

,

24

2151

Trace metal and major ion composition 16

7

0

0

12. 0 9

e-

0

00

0

a0

0 0

0

6

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mm of

0

24

18

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mm of

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16 -

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6

10

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0 1.e -

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Fig. 3. Relationship between trace metal deposition and rainfall amount: Pb, Zn, Cu and Cd.

tation in association with increased acidity. This was further related to the advection of air masses from the east coast of the U.S. In Fig. 4 excess SO42- depositions are plotted against the deposition of Pb, Zn, Cu and Cd. For Pb and Zn there is a high degree of correlation (r = 0.887 and 0.770), respectively. With the exclusion of one data point Cu depositions also show a loose relationship to those of excess SOi- (r g OSSS), for Cd however there is no clear relationship (r = 0.295). These results suggest that some trace elements have similar sources to those of acidic components and/or that they are removed in a similar way during precipitation. A correlation matrix for the deposition of trace metals and excess SOi- and other major components is given in Table 4. In general depositions are well correlated, the noticeable exception is for Cd, comment has already been passed on the reliability of this particular data set. Using the volume weighted average trace metal concentrations and rainfall data it is possible to calculate a flux and compare this with previous estimates. The results are summarized in Table 3. The

fluxes from the present study are at the lower end of the previously reported range and remarkably similar to those reported for the Atlantic coast of the U.S. The exception is Fe, the reasons for this have been discussed above. It is unlikely that the present fluxes are underestimates but it is conceivable that overestimation will result if contamination occurs during sampling or analysis. Any bias form contamination is exaggerated when relatively few samples are analyzed, e.g. monthly collections over a year. It must be emphasized that the results from one station in the northern North Sea cannot be extrapolated to estimate a flux for the North Sea as a whole. It can be tentatively suggested, however, that on the basis of these data any revision of the flux for the North Sea would be in a downward direction. A major problem with extrapolating coastal data to offshore areas is the problem of how much rain falls at sea and also whether its composition is similar to that on land. It is generally recognized that less rain falls at sea than on land (Cambray et al., 1979); uncertainty in the rainfall figure is the biggest single error in the estimation of a tIux. Model predictions have suggested that

2758

P. W. BALLS 60

/

xs

(mg)

SO4

xs SO4

tmgl

2.4

0

-0

2

4 xs

SO4

6

0

(mg)

xs SO4

(mg)

Fig. 4. Relationship between trace metal and excess sulphate deposition: Pb(r=0.887), Zn(r=0.770),Cu (r=0.588)and Cd (r=0.295).

Table 4. Correlation matrix(r) for deposited

NH; ;.$3 xs so: Mn Zn Fe CU Cd

-

trace metals and excess sulphate, data included)

Pb

Cd

CU

Fe

Zn

Mn

0.699* 0.863* 0.890; 0.887* 0.782* 0.896* 0.607* 0.628* 0.376

0.024 0.198 0.321 0.295 0.4103 0.309 0.4112 0.565*

0.182 0.440$ 0.510~ 0.405 0.4323 0.505t 0.400

0.204 0.658* 0.394 0.505t 0.925* 0.761*

0.566: 0.86s 0.704* 0.770* 0.898*

0.394 0.792* 0.565* 0.688*

acidity, nitrate and ammonia

XsSO:0.794* 0.836* 0.928*

H+ 0.7911 0.812*

(all

NO; 0.753%

*p
deposition decreases rapidly in an offshore direction (Krell and Roeckner, 1988). In order to improve further our estimates of atmospheric deposition to marine waters there is an urgent need for the collection of precipitation at sea to determine both its amount and composition.

Acknowledgements-The author is grateful to Drs T. Jickells and M. Tranter for valuable comments on an early version of this paper. The cooperation of the lighthouse keeper, Mr Rosie, is acknowledged as is sampling and analytical assist: ante from Jonathan Kemp. Comments by Howard Ross at the review stage were most helpful and constructive.

Trace metal and major ion composition REFERENCES

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