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4

Uptake of methanesulfonate and sulfate in icePAI-YEI WHUNG and ERIC S. SALTzMAN, Rosenstiel School of Marine and Atmospheric Science, University of Miami,

Miami, Florida 33149-1098GERARDO W. GROSS, Department of Geoscience, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801

Methanesulfonate (MSA) is an atmospheric oxidationproduct of dimethyl sulfide; the latter is produced by

phytoplankton in the surface ocean. Because of its biogenicorigin, MSA can be used as a biological tracer in studies ofatmospheric sulfur chemistry. The ratio of MSA to total non-seasalt sulfur has been used as an indicator for various atmos-pheric sulfur sources (Legrand et al. 1991; Whung et al. 1994).Due to differences in physicochemical properties of MSA andsulfate, postdepositional processes could alter this fraction.Specifically, we address the question of whether there is a dif-ference in the distribution coefficients (Conc. jce I COflC.water)of MSA and of sulfate. We also study the effect of ammonia onthe uptake of MSA and sulfate in ice.

Ice columns 1=20-25 centimeters (cm), 4=3.8 cm] aregrown from dilute solutions of MSA or sulfate (initial concen-trations 10-' to 10-1 IV). The initial volume of liquid solution isalways 350 cubic centimeters. Controlled unidirectionalgrowth imposes a steep temperature gradient and a nearlyconstant freezing rate on the system [0.2 centimeters per hour(cm/hr)]. Continuous stirring keeps solute concentration inthe melt nearly uniform, and it minimizes the trapping ofsolute in grain boundaries. This method primarily determinesthe limits of solute solubility in the ice matrix itself. It maybecome a proxy for solute distribution in antarctic ice in caseswhere solute rejected from the matrix is trapped in grain

boundaries (Mulvaney, Wolff, and Oates 1988). Slices of about1.5-cm thickness were analyzed by ion chromatography (fig-ures 1 and 2). As a measure of solute uptake we compute thedistribution coefficient (see table) by a curve-fitting algorithm(Gross, Wong, and Humes 1977). Distribution coefficient isthe ratio of solute concentration on the solid side to that onthe liquid side of an equilibrium phase boundary. For the pur-poses of the fit, the cone section (figure 1) and an initial solutetransient, if present (figure 2), are excluded from the fit, andthe curve is extrapolated to the starting coordinate (zerolength) of freezing. Concentrations are expressed in units ofgram-equivalent weight (IV) and in parts per million (ppm) ofsolution or of melted ice.

For the acid form of the two species, the following trendsare observed in the table. Except for MSA columns grown fromconcentrations less than iO N, the distribution coefficientdeclines with increasing concentration in the liquid. Averagesulfate concentration in the ice is higher, up to 40 times, thanthe MSA concentration for starting solutions of similar normal-ity. One remarkable exception was the 1.7x10 1 NMSA solution(column number 318-046) for which the ratio was reversed:about 17 times higher in the MSA column; we suggest thatappreciable intergranular trapping of MSA must have occurred.

The presence of ammonium, even in considerably lessthan stoichiometric proportion (see table, column number

COLUMN 319-020

CONELO

ISLIDE SLIDE

DIEL SLIDE

NOTE: ALL CUTS 0.2 cmFigure 1. Typical slicing diagram for an ice column. Slices taken for grain-size analysis ("Slide") and for dielectric measurements ("Diel") areshown, but results will be discussed elsewhere.

ANTARCTIC JOURNAL - REVIEW 199473

-3

-4Cot. 318-070: 410 N (NH4)2SO4

-4.5

-5

-5.5 4Col. 318-076: 4x10' N H,SO4 j

-61510152025Distance in Ice [cm)

CDC.,

CD

Ca

C',

C3C00C)0-J

-5.5a,C)

C,)

dC00C)0-J

-6\4/

-6.5

-7 \ -'-7.5

Col. 319-020: 3x10 4 NHCH,SO,

-8 1 5101520

Distance in Ice [cm]

Both MSA and sulfate columns showevidence of strongly non-steady-statesolute uptake (large scatter of data, largedifferences between columns grownfrom similar concentrations of the samesolute, e.g., columns 318-055 and -040).These effects are greatly attenuated insolutions containing ammonium hy-droxide (NH40H). Dielectric measure-ments, to be reported elsewhere, showthe effects of grain-boundary trappingand of hydrogen-bonding by ammoni-um. To pin down these phenomena

Figure 2. Typical plots of solute concentration vs. ice-column length, and eruptake by ammonium. Each point represents the concentration measured IA weighted average concentration of each column (see table) is computeslice concentrations weighted by the slice volumes. The initial high-conprominent in the MSA columns, is excluded from curve-fitting.

318-082), greatly enhances the uptake of both MSA and sul-fate (figure 2). This effect has also been observed with fluo-ride, chloride, and nitrate. We propose that the polar ammo-nium group facilitates the hydrogen-bonding of anions to theice lattice.

ihancement of solute more precisely, we propose me mappingn one slice (figure 1). of impurity distribution in laboratory-:1 from the measured grown ice by scanning electron micros-centration transient, copy and energy-dispersive x-ray micro-

analysis (Mulvaney et al. 1988).We thank Jeanne Verploegh and

Sandra Swartz of New Mexico Bureau of Mines and MineralResources Chemistry Laboratory for the analysis of four sul-fate columns. This research was supported by National Sci-ence Foundation grants OPP 93-22518 to Rosenstiel Schoolof Marine and Atmospheric Science, University of Miami,

Solute uptake by ice columns grown from dilute MSA and sulfate solutionsNOTE: N = normality (gram-equivalents per liter); ppm = parts per million; k 0 '= distribution coefficient extrapolated to the starting pointoffreezing (Gross et al. 1977); kay = weighted average concentration of a column divided byfinal melt concentration.

CH3S03- (methansulfonate)6.9E-46.3E-48.6E-41.3E-36.9E-37.7E-35.8E-34.8E-25.5E-1

318-058318-079319-020319-023a318-082318-055318-040318-043318-046

1.4E-41.7E-42.9E-42.7E-41 .4E3b1.5E-31.6E-31.4E-21.7E-1

12.916.127.325.8

131 b144149

1,33316,271

65.659.682.0

124656735553

4,57452,495

1.1 E-51.4E-56.4E-56.OE-43.6E-42.OE-42.5E-55.2E-69.7E-5c

7.3E-90.00073.5E-59.OE-90.00099.8E-55.6E-80.0051.OE-47.8E-70.0741.7E-32.5E-60.248.1E-41.6E-60.152.6E-41.4E-70.0148.4E-52.5E-70.0242.8E-55.3E-55.15.8E-6

SO4 (sulfate)

319-0171.7E-4

319-0151.1 E-3

319-0129.3E-3

318-0732.OE-2

318-067a2.1E-2

318-0764.1E-2

318-070a4.1E-2

319-0091.OE-1

aAmmonjum salt.

7.954.8E-423.0

51.23.9E-3189

448 4.OE-21,920

977 8.6E-24,128

990 1.2E-15,712

1,978 2.2E-110,560

1,980 1.9E-18,880

4,896 5.3E-125,344

b44 ppm NH 40H added to initial liquid.

3.9E-70.0191.7E-34.OE-70.0192.3E-44.9E-60.246.3E-42.4E-60.121.1 E-41.9E-49.05.9E-32.5E-60.123.6E-52.5E-412.04.9E-33.2E-60.151.7E-5

CAnomalous value.

8.1E-41.OE-41.2E-42.8E-51.6E-31.1 E-51.4E-36.1E-6

ANTARCTIC JOURNAL - REVIEW 1994

74

3.544.555.56

5

4

[C]

2

0U)

C)0-J

and ATM 89-21289 to New Mexico Institute of Mining andTechnology.

References

Gross, G.W., P.M. Wong, and K. Humes. 1977. Concentration depen-dent solute redistribution at the ice-water phase boundary. III.Spontaneous convection. Chloride solutions. Journal of ChemicalPhysics, 67(11), 5264-5274.

Legrand, M., C. Feniet-Saigne, E.S. Saltzman, C. Germain, N.I. Barkov,and V.N. Petrov. 1991. Ice-core record of oceanic emissions ofdimethylsulphide during the last climate cycle. Nature, 350(63 14),144-146.

Mulvaney, R., E.W. Wolff, and K. Oates. 1988. Sulphuric acid at grainboundaries in antarctic ice. Nature, 33 1(6153), 247-249.

Whung, P.-Y., E.S. Saltzman, M.J. Spencer, P.A. Mayewski, and N.Gundestrup. 1994. A two hundred year record of biogenic sulfur ina south Greenland ice core (20D). Journal of Geophysical Research,99(D1), 1147-1156.

Dielectric response of ice grown from dilute sulfate solutionsGERARDO W. GROSS and ROBERT K. SvEC, Department of Geoscience, New Mexico Institute of Mining and Technology,

Socorro, New Mexico 87801PAI-YEI WHUNG, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida 33149-1098

Sulfate (H2SO4) in polar ice reflects atmospheric circulationpatterns at the time of deposition; if related to specific vol-

canic events, it can date ice layers. Electrical conductivitymeasurement (ECM) and dielectric profiling (DEP) tech-niques provide a rapid means for logging the distribution ofsulfate (and other aerosols) in ice cores (Moore, Paren, andOerter 1992).

The present work aims at obtaining baseline data ondielectric response of sulfate in ice columns (=20-25 centime-ters (cm), 4=3.8 cm] grown in the laboratory under quasi-equilibrium conditions. Trapping of solute in grain bound-aries is minimized by continuous stirring of the melt. For sul-fate concentration and electrical measurements, the columnswere sliced in approximately 1.5-cm increments.

On selected [10-13-millimeter (mm) thick] slices dielectricrelaxation is measured at 31 frequencies in the range 1 hertz to100 kilohertz at 13 temperatures between -1°C and -85°C. Alinear blocking layer, that is, a 0.13-mm thick foil of a dielectriclow-loss material (Teflon) is inserted between each electrodeand the sample. With linear blocking layers, electrode reactioneffects are suppressed and replaced with a constant seriescapacitance (Gross and McGehee 1988). Inversion of the datayields ice parameters uncontaminated by electrode effects.

In the figures, three dielectric relaxation parameters(principal relaxation time, high-frequency conductivity, andstatic or direct-current conductivity), measured on six icesamples of different sulfate concentrations, are plottedagainst reciprocal absolute temperature (Arrhenius plots).

The principal relaxation timethe principal relaxation time (figure 1), is the character-istic response time of ice molecules reorienting them-

selves in an electric field. t2 is dependent on both temperatureand solute concentration. For pure ice, it falls on the solid line,but it is depressed when, with decreasing temperature,"extrinsic" interactions of solute molecules with the icedipoles become dominant. Three such extrinsic effects areobserved in figure 1:

Figure 1. Arrhenius plot of the principal relaxation time, t2. Ice sam-ples (starting with sample number 187010) grown, respectively, from10-1 N (approximately 4,900 ppm sulfate), 10-2 N, 10- N, and 10-4 NH 2 SO4; 2 and 400-2 N (NH 4)2SO4 solutions. Concentrations areexpressed in units of gram-equivalent weight per liter of solution (N) orin parts per million (ppm) of melted ice.

ANTARCTIC JOURNAL - REVIEW 199475