the sources and composition of mercury in pacific ocean rain

Journal of Atmospheric Chemistry 14: 489-500, 1992. © 1992 Kluwer Academic Publishers. Printed in the Netherlands.

THE SOURCES AND COMPOSITION OF MERCURY IN PACIFIC OCEAN RAIN

by R.P.Mason, W.F. Fitzgerald and G.M. Vandal Department of Marine Sciences The University of Connecticut Groton, CT 06340 USA

ABSTRACT. Here we report measurements of total Hg (HgT), reactive Hg (HgR), and methylmercury (MMHg) in precipitation from the equatorial Pacific Ocean, collected during a cruise in January and February 1990, and from a mid-continental location in the rural temperate lacustrine northcentral Wisconsin environs. The concentrations of Hg T (14.4 -+ 6.5 pM), Hg R (8.9 -+ 4.5 pM) and MMHg (<50 fM) found in equatorial Pacific rain were less than the average concentrations found in Wisconsin. In general, the results indicate that although particulate Hg is a small fraction of the total atmospheric Hg burden, it is the major contributor to Hg in precipitation. Furthermore, deposition could be an important source of Hg R to the equatorial Pacific Ocean. In contrast, deposition is not a significant source of MMHg to either the equatorial Pacific Ocean or the remote seepage lakes of Wisconsin. This implies that methylated mercury is formed in situ in these systems.

1. Introduction

Precipitation is the major source of mercury (Hg) to ocean waters. Gill and Fitzgerald (1987) estimated the pluvial flux to the open ocean at 1.7X109 g/yr or about 10 megamole (Mm)/yr and fluvial inputs at 1 Mm/yr. The total annual Hg input to the atmosphere is currently estimated at 25-30 Mm, of which human-related sources contribute approximately 30-60% (Fitzgerald, 1989; Nriagu and Pacyna, 1988). Fossil fuel combustion and waste incineration yield the major anthropogenic emissions of Hg (Nriagu and Pacyna, 1988). Recent studies in the equatorial Pacific Ocean (Kim and Fitzgerald, 1986; Mason and Fitzgerald, 1990) and in freshwater systems (Vandal et al., 1991) have demonstrated that evasion of elemental mercury (Hg °) at the water surface is an important natural flux of Hg to the atmosphere.

Elemental Hg, the principal form of atmospheric Hg, has a residence time estimated at approximately one year (Fitzgerald, 1989). It is readily mixed intrahemispherically and a distinct latitudinal gradient is present over the ocean (Fitzgerald, op. cit.). Atmospheric oxidation may be an important mechanism for the conversion of sparingly soluble Hg ° to the soluble species found in wet deposition (Iverfeldt and Lindqvist, 1986). However, neither

490 R. P. MASON, W. F. FITZGERALD AND G. M. VANDAL

the relative importance of atmospheric oxidation of Hg ° nor the extent of competing reduction reactions (Munthe, 1991) is currently known.

Although particulate Hg is a small fraction of the total atmospheric Hg burden, it is the major contributor to Hg in precipitation. In addition, dry depositional fluxes of Hg can be important in certain regions, such as the mid-continental northcentral Wisconsin lake environs (Fitzgerald et al., 1991). In open ocean regions, dry deposition represents a small percentage of the total Hg deposition (Fitzgerald, 1989).

Here, we are reporting results of Hg speciation measurements in precipitation from the remote equatorial Pacific Ocean and from northcentral Wisconsin. This comparative examination using Hg species provides new insights concerning the atmospheric cycling, availability, and reactivity of Hg in precipitation and natural waters. Such information is lacking in total Hg determinations.

Mercury speciation determinations in natural waters have evolved through the adoption of trace-metal-free clean laboratory protocols (Gill and Fitzgerald, 1985; Fitzgerald, 1989) and as a result of recent analytical advances (Bloom and Fitzgerald, 1988; Bloom, 1989). The concentrations of reactive mercury (HgR), total Hg (HgT) and monomethylmercury (MMHg) in wet deposition can now be determined accurately. Reactive Hg consists of Hg species that are readily reduced by SnC12 under acidic conditions. This fraction is comprised primarily of labile inorganic and organic complexes of Hg(II), and of labile particulate associations. Total Hg is a measure of the total dissolved and particulate Hg in an unfiltered sample. Strong oxidation with bromine monochloride (BrC1) is used to destroy stable Hg associations, converting Hg to a readily reducible form for analysis.

Monomethylmercury is determined by direct derivitization using tetraethylborate (Bloom, 1989) and therefore is a measure of all labile MMHg present. The derivitization determination also measures the labile inorganic mercury {Hg(II)}. The ethylation reaction is performed at a pH of 5 (Bloom, op. cit.) while the SnCI 2 reduction determination is done on an acidified sample. Thus, the two techniques do not always provide the same measure of the labile ionic mercury fraction, especially in a water sample which contains substantial amounts of dissolved organic matter. However, we have found that the Hg R and Hg(II) determinations are equivalent for rainwater and snow samples.

These techniques were used to determine the Hg speciation in rain collected in the equatorial Pacific Ocean as well as rain and snow obtained during 1989 and 1990 as part of the Mercury In Temperate Lakes Program in northcentral Wisconsin (Fitzgerald et al., 1991). In general, this comparative study shows that wet deposition is an important source of HgR, while precipitation fluxes of MMHg are not significant to either remote oceanic regions (Mason and Fitzgerald, 1990) or to mid-continental lakes (Fitzgerald et al., 1991). The current data demonstrate that atmospheric sources of MMHg account for a small fraction of the MMHg in freshwater and oceanic fish (Fitzgerald and Watras, 1989; Mason and Fitzgerald, 1990).

2. Experimental

2.1 Sampling procedures: Deposition

A rain collector was mounted on top of a 25 ft aluminum tower on the bow of the ship NOAA M A L C O L M BALDRIDGE during the 1990 NOAA MB-90-01-RITS cruise in the

SOURCES AND COMPOSITION OF MERCURY 491

equatorial Pacific Ocean. The design of the tower and the sampling protocols were similar to those used by the Sea-Air Exchange Program (SEAREX) on the R/V MOANA WAVE (Duce, 1989; Fitzgerald, 1989). The tower was used exclusively for the collection of gaseous, particulate and rain samples. A Teflon R rain funnel, constructed from a molded Teflon R sheet, was contained in an acrylic housing, with a removable acrylic lid. The funnel and housing were constructed so that rain entering the funnel contacted only Teflon R parts (Fig. 1) which were rigorously acid-cleaned prior to use. The collection bottle was screwed directly into a machined Teflon R connector at the bottom of the funnel. The acrylic housing was supported by an aluminum frame so that the top of the funnel was approximately 1.5 m above the platform of the sample tower.

Five rain collections were made in the oceanic region between the latitudes of 15°S and 5°N, and the longitudes of 150°W and 180°W, on the second leg of the cruise (Fig. 2). The ship's radar was used to adjust the track and speed of the vessel so that the rain collection period could be maximized without contamination from the ship (Fitzgerald, 1989). The direction of the ship was maintained such that the rain was intercepted as it passed over the bow.

ACRYLIC I LID = I

ACRYLIC BOX

-7

I

TEFLON SHEET

TEFLON FUNNEL

TEFLON CONNECTOR

TEFLON BOTTLE

Figure 1: Schematic of the rain collector. The dimensions of the rain collector are: Teflon R funnel diameter 30 cm; acrylic housing dimensions 45 X 45 X 60 cm.


Rain and snow samples were collected in northcentral Wisconsin as part of the Mercury in Temperate Lakes Program (Fitzgerald et al., 1991). The samples were collected on an event basis by trained personnel using techniques designed to avoid trace metal contamination. In between rain collections the acrylic housing of the rain collector was covered by large clean poly-bags. Prior to each collection, the funnel was rinsed and prepared for rain collection (Fitzgerald et al., op. cit.).

2.2 Sampling Procedures: Gaseous and Particulate Mercury

Atmospheric gaseous and particulate Hg samples were collected continuously during the equatorial Pacific cruise. Total gaseous Hg (TGM) was trapped on gold columns, while speclation samples were collected using Carbotrap R columns (Bloom and Fitzgerald, 1988). Particulate Hg was collected using quartz wool plugs. All sampling trains had a quartz column on the inlet to remove the particulate fraction. Sample volumes for TGM and speciation determinations were between 0.5 and 2 m 3, while larger volumes were needed for particulate measurements (2-10 m3).

la.I e'~ I--

2 0 N

ION

10S

RAIN COLLECTIONS DURING EQUATORIAL PAClRC OCEAN CRUISE

i

" : " : ,,CENTI~a~.

PACIFIC OCEAN P ~

SAMOA "E] " ~ TAHITI I - - J

I I I I I 1 7 0 W 150W 1 3 0 W 1 1 0 W 9 0 W

LONGffUDE

\

2 0 S 170E 7 0 W

Figure 2: The cruise track of the Malcolm Baldridge in the equatorial Pacific during the MB-90-01-RITS cruise. The location of each rain collection is marked by a triangle. Note that two collections were made in the vicinity of the Equator at 180°W.


2.3 Analytical procedures: Deposition

Rain samples from the Pacific were either analyzed immediately or were stored frozen in hermetically sealed, double-bagged Teflon R bottles prior to analysis. All analyses of Hg R, Hg T and MMHg were performed on board within a week of collection. Figure 3 outlines the analytical protocols. Subsamples were taken for speciation determinations and the remaining sample was acidified to a final concentration of 0.5% HCI. In Wisconsin, rain samples were subsampled under clean room conditions after collection. The subsample for Hg R and Hg T measurement was acidified prior to shipment to the University of Connecticut for analysis. Snow samples and samples for MMHg determination were stored and transported frozen and unacidified. All samples from Wisconsin were analyzed at the University of Connecticut.

A 250 ml aliquot of unfiltered, acidified precipitation was reduced using 10% SnC12 solution to determine Hg R (Gill and Fitzgerald, 1987a). Quantification was by cold vapor atomic fluorescence spectroscopy (CVAFS; Bloom and Fitzgerald, 1988). Total Hg was determined after strong oxidation of samples with BrCI (Bloom and Crecelius, 1983) and neutralization with hydroxalyamine prior to SnC12 reduction, gold trapping, and CVAFS detection. The blanks for the Hg R and Hg T determinations were, respectively, 0.5 pM and 2 pM.

P R E C I P I T A T I O N . 250 mL - - -

ACIDIFIED

BrCl OXIDATION 1 250 mL ACIDIr:IED

2 0 0 mL BUFFERED AT pH 5

SnCI= REDUCTION ETHYLATION

I

T R A P W I T H G O L D T R A P P I N G CARBOTRAP

J I A T O M I C F L U O R E S C E N C E D E T E C T I O N

Hg .r Hg R Hg(ll) MMHg

Figure 3: Flow diagram illustrating the analytical methods used for the determination of reactive (HgR) , total (HgT) and methylmercury (MMHg) in precipitation.


For MMHg, tetraethylborate was added to a 100-200 ml aliquot of buffered rainwater (pH 5) to produce the methylethylmercury derivative, which is volatile. In the procedure, ionic Hg is converted to diethylmercury. After a reaction period, the Hg derivatives were stripped from solution and trapped on a carbon absorbent. The trapped species were then separated using cryogenic gas chromatography and were quantified by CVAFS (Bloom, 1989). Typically, the analytical blank was 2 pg for MMHg or 50 fM for a 200 ml sample.

As discussed in the Introduction, we have found no difference for precipitation samples between the two measurements of reactive ionic mercury (SnC12 reduction and ethylation), and therefore no distinction is made between the two determinations in this paper. All rain samples from Wisconsin were analyzed using SnCI 2 reduction, while the value of Hg R for snow samples from Wisconsin (Table 2) was determined by the ethylation technique. The ethylation method was used for the majority of the measurements in the Pacific [Hg(II) in Table 1]. Dimethylmercury (DMHg), if present, would also be detected by the procedure used for MMHg. However, DMHg is highly reactive in the gas phase (Niki et al., 1983; Thomsen and Egsgaard, 1986) and has not been detected by us in gaseous atmospheric samples collected in Wisconsin, and during the 1990 equatorial cruise, nor has it been found in precipitation.

3. Results

Table 1 contains a summary of the Hg speciation data collected in the equatorial Pacific Ocean. Reactive Hg concentrations (Hg R or Hg(II) in Table 1) ranged from 5.5 to 15.5 pM for the Pacific Ocean collections, and averaged 8.9 _+ 4.5 pM (Fig. 4). Total concentrations averaged 14.4 -+ 6.5 pM (9.0-22.5 pM). The average values for Hg T and MMHg from the equatorial Pacific are lower than those observed by us in Wisconsin in 1989 (Fitzgerald et all., 1991) and 1990 (this work). These data are summarized in Table 2. The average Hg T was similar during both rain seasons (52.5 +- 24.0 and 49.3 + 20.8 pM) while the average Hg R was higher during 1990 (41.0 -+ 20.6 pM) than during 1989 (13.7 _+ 10.6 pM).

While we recognize that the data are yet limited, it is instructive to compare the oceanic and mid-continental measurements. Statistically, the Pacific and Wisconsin Hg T data sets are different (Mann-Whitney test, p=0.003). For HgR, the Pacific measurements are statistically different from the 1990 Wisconsin rain data (p=0.005). The differences in Hg R between the open ocean and northcentral Wisconsin in 1989 are not definitive, most likely due to the smaller number of Hg R determinations (4 in the Pacific and 6 in Wisconsin in 1989).

Our previous measurements of Hg in oceanic rain range from 10-20 pM for remote regions of the Pacific Ocean to 50 pM for Southern New England (Fitzgerald, 1989). Table 3 contains a summary of precipitation measurements in the Pacific Ocean prior to the 1990 cruise. The samples collected at the two remote tradewind island sites and from the Tasman Sea have similar average concentrations to the current data set. The samples from the North Pacific regions have substantially larger concentrations. This increase is most likely a result of increased continental and anthropogenic input to this region. Interestingly, the total concentrations observed in northcentral Wisconsin (Table 2) are similar to those obtained for rain from the northeast Pacific Ocean.


DATE LOCATION VOL. Hg R TOTAL MMHg Hg(II) (LAT. & (L) (pM) (pM) (fM) (pM) LONG.)

2/6/90 5°N, 180°W 1.2 22.5 < 50 7.0

2/9/90 0 ° , 180°W 0.9 5.5 6.5

2/10/90 0 ° , 180°W 0.9 9.0 <50 7.5

2/14/90 7°S, 180°W 0.4 16.5

1/30/90 12°S, 157°W 1.1 17.5 < 50 15.5

AVER. 14.4 8.9

S.D. 6.5 4.5*

* The average includes the Hg R determination.

Table 1: The speciation of Hg in equatorial Pacific Ocean precipitation during January and February 1990.

25

20

Z o 15

I - - z 10, bJ O z o o 5,

TOTAL Hg k-~ REACTIVE Hg

0 o 0 o 7Os

LATITUDE OF RAIN COLLECTION

12°S

Figure 4: The reactive and total mercury concentrations measured for the five rain collections during the 1990 equatorial Pacific Ocean cruise.


SAMPLE n* Hg R (pM) TOTAL (pM) MMHg (pM) PERIOD

RAIN 1989 12 13.7 _+ 10.6 52.5 _+ 24.0 0.78 -+ 0.34

RAIN 1990 9 41.0 _+ 20.6 49.3 +_ 20.8 0.37 -+ 0.16

SNOW 88/89 6 17.5 _+ 12 30.0 _+ 4.5 0.24 -+ 0.11

SNOW 89/90 3 8.0 _+ 0.75 14.9 _+ 3.9 0.52 _+ 0.20

* n = number of deposition events sampled.

Table 2: Summary of the average concentration of Hg species observed in wet deposition from northcentral Wisconsin. Data for 1989 are taken from Fitzgerald et al., (1989).

Recently Dick et al., (1990) reported an average concentration of 14 pM Hg for Antarctic snow; a value of the same order as our remote ocean measurements (Tables l&3), and the snow from Wisconsin (Table 2). Bloom and Watras, (1989) report similar concentrations for a few rain and snow collections in Wisconsin, and in Washington State. A summary of data from various authors reported values between 5 and > 100 pM (Lindqvist and Rodhe, 1985). While the more recent measurements are comparable to our data, we suspect contamination during sample collection and manipulation is an explanation for many of the higher values.

REGION REGIME TOTAL Hg (pM)

# OF SAMPLES

NE PACIFIC N. PACIFIC 45 _ 20 5 (44- 55°N, WESTERLIES 151-158Ow)

38ON, 167Ow WESTERLIES 85 1

ENEWETAK ATOLL NE TRADES 14 _ 8 3 (11ON, 165OE)

AMERICAN SAMOA SE TRADES 22 _+ 12 4 (13°S, 170Ow)

TASMAN SEA S. PACIFIC 19 _+ 4 6 (35Os, 170OE) WESTERLIES

TABLE 3: Concentrations of total Hg in rain over the Pacific Ocean (from Fitzgerald, 1989).


The differences in the Hg content between the equatorial Pacific and mid-continental rains correspond to the atmospheric particulate Hg distribution between these regions. For example, the average particulate H g concentration was 2.6 pg/m 3 during the 1990 cruise in the equatorial Pacific and 22 pg/m~for Wisconsin (from 1989 data; Fitzgerald et al., 1991). This suggests that differences in the concentration and speciation of mercury in rain is related to differences in the concentration and form of atmospheric particulate mercury.

The MMHg concentration was below the current detection limit of 50 fM for all analyses of equatorial Pacific rain (Table 1) and as a result MMHg accounts for less than 1% of the Hg T. Concentrations of MMHg in northcentral Wisconsin deposition are typically between 0.2 and 1 pM (Fitzgerald et al., 1991 and Table 2) and account for between 1 and 7% of the total Hg. The relatively higher concentrations of MMHg in continental rain, and the undetectable amounts in open ocean rain suggest that MMHg in atmospheric deposition has a continental source, and that ocean derived particulate contains little MMHg.

The reactive portion of the open ocean rain was the main constituent and comprised, on average about 70% of the total. Three of the four rain events where both measurements were made had similar %HgR/Hg T values (89%, 85%, and 83%)[Table 1]. However, the collection at 5°N, 180°W had only 31% Hg R. Although the data set is as yet limited, the Hg R concentrations are within a narrow range (Table 1) suggesting that the difference is due primarily to an increased input of unreaetive Hg to the more northerly site; most likely of continental origin. This explanation is consistent with the location of the sampling stations as all collections, except that of 2/6/90, were on or south of the Equator (Fig. 2). Differences in the aerosol composition between the two Hemispheres is well known (Arimoto et al., 1989; Buat-Menard et al., 1989) and Hansen (1990) found a sharp discontinuity at the Intertropical Convergence Zone (ITCZ) separating higher Northern

+ 3 + Hemispheric concentrations of carbon black (20_ 8 ng/m ) from lower concentrations (9_ 4 3 ng/m ) in the Southern Hemisphere. Thus, the differences in the relative composition of

Hg R and Hg T could reflect the differences in the sources of particulate to the atmosphere.

4. Discussion

We have hypothesized that the Hg R fraction represents an active substrate immediately available to biota for methylation, conversion into Hg °, and for other reactions (Mason and Fitzgerald, 1990). Therefore, in regions where other inputs of Hg are small, precipitation is an important source of labile Hg. In the open ocean, as well as seepage lakes in Wisconsin and elsewhere, deposition to the water surface is the predominant flux of labile Hg into the system. Calculations for Little Rock Lake, a small seepage lake in northcentral Wisconsin, show that Hg R in rain readily accounts for the annual Hg accumulation in biota (Fitzgerald and Watras, 1989; Fitzgerald et al., 1991). The non-reactive fraction, which requires strong oxidation to release the Hg from its bound state, is thought to be unreactive on the timescale of small lake processes. However, in the open ocean environment, this may not be the case and this fraction could be an additional source of labile Hg to the system.

The dissolution of certain atmospheric particulate trace metals in surface seawater and in rain has been reported recently. Maring and Duce (1989) found that all the particulate Cu collected during the low-dust season at Enewetak Atoll in the North Pacific was of oceanic origin, and that mineral dust accounted for about 30% of the particulate Cu during the high dust season. Almost all of the particulate non-aluminosilicate Cu from Enewetak


was soluble in water (Maring and Duce, 1989). In addition, the mode of deposition, wet or dry, had little effect on the fate of atmospheric particulate Cu in seawater. Furthermore, anthropogenlcally derived particulate Cu did not contribute significantly to the remote particulate aerosol at this location. In contrast, all the non-aluminosilicate Pb was of anthropogenic origin (Maring and Duce, 1990). Arlmoto et al. (1989) discuss the sources of trace metals found in marine air and indicate that the sources of Cu and Pb are different. The ratio of natural to anthropogenic sources of Hg is similar to that of Cu (Nriagu, 1979). For Pb, nearly all inputs are anthropogenic (Maring et al., 1989, and references therein). The speciation determinations in this work suggest that greater than 80% of the particulate aerosol found in the equatorial Pacific south of the ITCZ is soluble in rain (determined as HgR), while the solubility of particulate Hg in rain appears to decrease north of the Equator. Thus, particulate Hg in marine aerosol may show solubility characteristics that are similar to that of particulate copper.

In summary, the determinations of reactive, total and MMHg in remote ocean rain have provided important information concerning the sources of Hg to aquatic systems. Wet deposition is not a significant source of MMHg to the equatorial Pacific Ocean. Similarly, results indicate that the atmospheric input of MMHg to Little Rock Lake, a small seepage lake in Wisconsin, is insufficient to account for the MMHg in fish and dearly indicates in situ production of MMHg in these systems (Fitzgerald et al., 1991). Atmospheric deposition is an important source of Hg R to the open ocean, as well as seepage lakes.

The present determinations of Hg T agree with the previous data from open ocean regions (summarized in Fitzgerald, 1989) and suggest that Hg in equatorial Pacific Ocean rain is primarily in a reactive form. Increases in atmospheric particulate levels due to anthropogenic emissions will lead to increases in Hg, and other trace metals, in deposition. In addition, increases in Hg ° emissions could also lead to higher Hg concentrations in rain as a result of increased atmospheric oxidation of Hg °. More research is required to identify and to elucidate further the sources of MMHg, Hg R and unreactive Hg in rain. This work has provided new information about the atmospheric cycling and deposition of Hg to aquatic systems.

6. Acknowlegements

The help and support of the captain, officers, crew, scientists and survey technicians of the NOAA M A L C O L M BALDRIDGE is greatfully acknowledged. In particular, we thank G. Harvey for inviting us to participate in the cruise, A. Pzenny for organizing and erecting the tower, Jane Knox for assistance, and all who were involved in rain collections. We acknowledge the contributions of Dave Good and Gary Grenier who constructed the rain sampler and other equipment. The use of the facilities at the Trout Lake Limnological Station, Center for Limnology, University of Wisconsin-Madison is gratefully noted. Rain, snow and atmospheric samples in Wisconsin were coUected on our behalf by Steve Claas, Kent Hatch, Bill Fitzgerald, and others. The Pacific Ocean research was supported by the Research Foundation of the University of Connecticut, while the continuing investigation in Wisconsin was supported by the Wisconsin Department of Natural Resources and the Electric Power Research Institute (RP2020-10). Additional support was provided by a


NATO International Collaborative Research Grant (0160/88). This is contribution # 239 of the Marine Sciences Institute, The University of Connecticut.

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Bloom, N.S. (1989). Determination of picogram levels of methylmercury by aqueous phase ethylation, followed by cryogenic gas chromatography with atomic fluorescence detection. Can. J. Fish. Aquat. Sci. 46 1131-1140.

Bloom, N.S. and Watras, C.J. (1989). Observations of methylmercury in precipitation. Science of the Total Environment 87/88 199-207.

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the sources and composition of mercury in pacific ocean rain

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