chemical composition of falling snow at dumont d'urville, antarctica

Journal of Atmospheric Chemistry 14:31-42, t 992. © 1992 Kluwer Academic Publishers. Printed in the Netherlands.

C H E M I C A L C O M P O S I T I O N O F F A L L I N G S N O W AT D U M O N T D ' U R V I L L E ,

A N T A R C T I C A

Francois MAUPETIT and Robert J. DELMAS Laboratoire de Glaciologie et Gdophysique de l'Environnement BP 96 38402 Saint Martin d'Hdres cedex (France)

ABSTRACT. Fourteen samples of fresh falling snow were collected at Antarctic coastal base Dumont d'Urville in 1984. The samples have been analysed for major ions (including MSA) by ion chromatography and acid titration. The results are relevant to the chemical composition of background precipitation in polar marine conditions. The seasalt aerosol contribution is dominant. All samples are found to be acidic in the range 3-16 laeq/l. The calculated non-seasalt sulfate (nssSO42-) concentration is significantly negative for 3 of the 14 samples. NssSO42 is found to be relatively high in summer and fall. MSA also exhibits the same pattern probably linked to local marine biogenic activity and/or atmospheric photochemical processes. The MSA to nssS042- ratio is in good agreement with values reported for coastal Antarctic ice cores and subantarctic aerosol. The background mean value for nitrate concentration is 1.1 ~eq/l but two very strong spikes (up to 16 peq/l) are observed. The first seems to be linked with long range transport of continental air masses while the second (in winter) is clearly due to a sudden input of nitric acid, possibly from the stratosphere. This paper represents a preliminary approach to a larger air and snow monitoring to be developped at this site.

Key words : Antarctica, snow chemistry, sulfate aerosol, nitrate deposition.

1. In t roduct ion

The composition of atmospheric aerosol in remote areas has attracted increasing attention in

recent years in relation with global environmental problems such as acid precipitation, climate

changes and ozone depletion (in the stratosphere) and formation (in the troposphere). The

chemical composition of precipitation (rain or snow) at remote locations is of prime importance

in understanding the sources, transformation and deposition of the natural aerosol. In this respect,

the Antarctic atmosphere is of particular interest. We report here on the chemical composition

of falling snow at a coastal site (Dumont d'Urville, Ad61ie Land) in Antarctica.

32 FRAN(~OIS MAUPETIT AND ROBERT J. DELMAS

Marine biogenic activity has been recognized as a major source (on a global scale) of gaseous

sulfur compounds (mainly DMS) which are transformed in the atmosphere mainly into sulfuric

acid, the most important component of natural background aerosol, and into methanesulfonlc

acid (MSA). Aristarain et al (1982), Mulvaney and Peel (1988) and Wagenbach et al (1988) have

already emphasized the role of sulfur compounds in the chemistry of snow or aerosol at coastal

Antarctic sites. Some of these authors and also Gjessing (1984 ; 1989) have pointed out that

seasalt aerosol could be fractionated at these high latitudes. Increasing interest has recently been

shown in nitrate deposition in Antarctic snow in relation to denitrification of the lower

stratosphere by polar stratospheric cloud sedimentation and its possible link to the Antarctic

ozone depletion (Legrand and Kirchner, 1988 ; Mayewski and Legrand, 1990).

Very few measurements have been performed on falling snow, particularly during winter. In this

work we have attempted to investigate several snowfalls with the aid of a number of short

duration samples collected during a one-year period. Such measurements will need to be more

frequently repeated over several years before final conclusions can be drawn. Nevertheless they

represent a preliminary approach to a better understanding of subantarctic aerosol chemistry.

2. Experimental techniques

2.1. COLLECTION SITE AND SAMPLING PROCEDURE

The Dumont d'Urville (DDU) base is located in Adtlie Land, East Antarctica (66°42'S,

140°00'E), on a rocky island (41 m.a.s.l) at a few kilometers from the Antarctic continent

(figure 1).

Great precautions were taken in sample collection. Falling snow was sampled directly

in precleaned plastic bags on days of very low wind speed to prevent sampling of drifting snow

and tearing of the plastic bags by wind. The polyethylene bags were precleaned in our laboratory

in Grenoble by rinsing with ultrapure water and then thermically sealed. Bags were opened in

DDU at the beginning of the snowfall by an operator wearing clean gloves and kept open with

plastic sticks for the duration of the snowfall on the roof of a laboratory. At the end of the

snowfall, the bags were heat-sealed and kept in a frozen state until they were melted for analysis

in our Grenoble laboratory. Even if an exact evaluation of the contamination due to the base

activities is difficult, we think that the samples chemical composition was not disturbed by such

a local pollution source.

Fourteen samples were taken under those conditions in 1984. Sample 1 thawed out in

the plastic bag during sampling because of the ambient temperature (+2°C).

CHEMICAL COMPOSITION OF FALLING SNOW 33

i 0

Antarctic

-9 0 E -

Dumont d'Urville 180

i - -

Figure 1. Map of Antarctica showing Dumont d'Urville sampling site

2.2. MEASURING TECHNIQUES

Samples were analysed for Na +, NH4 +, K +, Mg 2+, Ca 2+, C1-, NO3-, SO4 2" and CH3SO 3- (MSA)

using a Dionex model 2010i ion chromatograph. The working conditions used by Legrand et al

(1984) were slightly modified for the analysis of organic acids and MSA (Saigne et al, 1987).

The analytical accuracy is typically + 10% (Legrand et al, 1984). Due to the high Na + values

encountered, a few samples were diluted for monovalent cation measurements (Na +, NH4 + and

K + are measured in the same chromatograph run).

Acidity was measured using a titration technique with an accuracy of + 0.5 peq/l

(Legrand et al, 1982).

To avoid any contamination by sample contact with ambient trace gases and particularly

ammonia (Legrand et al, 1984), samples were melted in the bag just prior to analysis, sucked

through the walls of the plastic bag using a clean syringe and needle (Kirchner, 1988) and

injected directly into the ion chromatograph.

3. Results

The results are presented in table 1. We have excluded from this list ammonium data and the

values obtained for sample 1.


Ammonium concentrations are surprisingly high for an samples (in the range 0 to 7 peq/l if

we exclude sample 1). As previously said (2.2.), precautions were taken to prevent contamination

of the sample by ambient ammonia. The dilution of certain samples (see 2.2. above) lowered the

concentration range of NH4 ÷, increasing its sensitivity to contamination during this operation.

The particular case of sample 1 was also examined : it shows a very high concentration of NH4 ÷

(95 peq/l) which is not balanced by the alkalinity of the sample. These observations led us to

the conclusion that the aerosol at DDU may have been locally contaminated with NH4 ÷ by the

close vicinity of a penguin colony and/or that the samples may have been contaminated during

sample analysis. Thus, NH4 ÷ concentrations are not reported here and were not taken into

account in the following discussion. Note that in any case they do not contribute significantly

to the total ionic budget.

The ionic balance of the precipitation chemical composition was checked using a linear

regression calculation on the sum of the cations versus the sum of the anions. A good balance

was shown by the slope (1.01) of the regression line (r=-0.99), even if a weak cation excess (+

3.8) was revealed by the positive intercept of this regression line.

4. Discussion

4.1. MARINE INFLUENCE

Due to the coastal location of DDU, the chemical composition of precipitation is dominated by

a marine influence. We chose Na ÷ as the seawater reference species and then assumed that all

Cl-, K ÷ and Mg z÷ came from seasalt, the slope of linear regression between those dements and

Na t (1.16 ; 0.0211 ; 0.234 respectively) being very close to the seawater ratio (1.16 ; 0.0218 ;

0.227, Wilson, 1975).

The seasonal nature of seasalt inputs to Antarctica has been reported (e.g., Legrand and

Delmas, 1984 ; Wagenbach et al, 1988). It is difficult to speak of seasonal variations on the

basis of this small data set covering a single year but we can point out that the maximum of Na ÷

in precipitation was found in winter and the minimum in summer (figure 2).

4.2. NssSO42- IN COASTAL ANTARCTIC SITES

T h e nssSO42 concentrations were calculated from the expression :

n s s S O 4 2 = 5042- - 0.121 N a ÷ (1)

C H E M I C A L C O M P O S I T I O N O F F A L L I N G S N O W 35

.8

c5

Z

. 4

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36

Conco~r~ions (it Eq.1-1

10 / I I I

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6

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/ I I I d F M A

1 5 0 - I I I

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1011

75

0 _~ , t l F M I A

I I I I I I I

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I I I I I I 1 M J J A S 0 N D

I I I I I I

! J,

FRAN(~OIS M A U P E T I T A N D R O B E R T J. D E L M A S

Concentr=ions (~ Eq.1-1

15 1 ~ I I I

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0 . I I I . I • F M A

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/

O ( d I F I M I A

i I i I i I --

. . , . , I , , . , ,I M J J A S 0 N D

!

~:1 I I I:1 I 0 N D

Figure 2. Ionic concentrations (peq.l -~) of Na +, H ÷, HNO 3- and nssSO4 2.

using Na ÷ as the seawater reference species and with concentrations expressed in peq/l. Four of

the fourteen samples present negative nssSO42- concentrations.

This calculation of nssSO42- can be problematic for remote marine sites (Keene et al, 1986)

where seasalt concentrations are high since the nssS042 amount represents a small difference

between two large numbers. We made a simple error calculation on this estimation :

AnssSO42- = ASO42- + 0.121ANa ÷ (2)

Assuming that for a marine site, the S042- fraction coming from seasalt is of the same order

of magnitude as the total SO42- -

A n s s S O 4 2" = 2 A S O 4 2" ( 3 )

With an experimental error of 10% we finally obtain the following error expression:

~ I s s S O 4 2" = 0 . 2 S O 4 2" ( 4 )


This simple calculation led us to the conclusion that only 3 samples present a significant

negative nssSO42 concentration (nssSO42 > AnssSO42-). The same conclusion is reached using

Mg 2÷ as the seawater reference species, as proposed by Keene et al (1986), or using the

equations recommanded by Hawley et al (1988). This effect cannot be explained by an input of

terrestrial Na ÷ (see 4.1.) or by a variation of the sulfate to sodium ratio which has been well

documented (Keene et al, 1986). Thus, this negative nssSO42- concentration cannot be attributed

to a calculation artifact.

Such results have already been reported from coastal sites of Antarctica (Delmas et al,

1982 ; Aristarain et al, 1982 ; Gjessing, 1984 ; 1989 ; Wagenbach et al, 1988 ; Minikin and

Wagenbach, 1990). The only satisfactory explanation proposed to explain this phenomenon is

a sulfate fractionation during seasalt aerosol production due to the low temperature, leading to

a lower sulfate to sodium ratio compared to seawater ratio (Wagenbach et al, 1988 ; Gjessing,

1989). Samples with negative nssSO42- concentrations present a depleted SO42 to Na ÷ ratio

(0.079 to 0.110) compared to the seawater ratio (0.121). Such a depletion, suppc, rting this

explanation, has already been reported (Gjessing, 1989 ; Minikin and Wagenbach, 1990). This

phenomenon mainly occurs in winter when the seasalt concentration is higher (figure 2), an

observation similar to the seasonal pattern obtained over a three-year period by Wagenbach et

al (1988) at the German Antarctic coastal base G.v. Neumayer (figure 1). We can conclude that

negative nssSO42 concentrations are significant and frequently obtained in coastal Antarctic areas

under cold temperatures, high windspeeds and high seasalt concentration conditions. This effect,

leading to an underestimation of nssSO42- during winter, can be problematic for the calculation

of the MSA to nssSO42" ratio at this time.

The seasonal pattern of nssSO42- in DDU seems to be characterised by higher values in

summer and fall and low or even negative values during winter (figure 2).

4.3. METHANESULFONIC ACID (MSA)

MSA was also measured on those samples. Concentrations range from 0 to 44 ppb with a mean

value of 6.5 ppb (table 2), and MSA to nssSO42 ratios range from 0 to 0.36 (table 2). Those

values (relatively high MSA and low nssSO42- concentrations) are in good agreement with

coastal Antarctic ice core data (Saigne and Legrand, 1987 ; Legrand et al, 1991) and subantaretic

aerosol or precipitation data (Berresheim, 1987 ; Pszenny et al, 1989). This comparison suggests

that the source of MSA in coastal Antarctic precipitation is mainly local (the subantarctic ocean),


The seasonal pattem of MSA (figure 3) is somewhere similar to that of nssSO42with maxima

in summer and fall probably linked to marine biogenic activity and/or atmospheric

photochemical processes.

38

4.4. NITRATE

ppb

12

10

8

I I I I

44

ppb F M A M

I I I I I

_! ,1, I,, J A S O N

I i ~ I I I I 1--

3011

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-I00

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0 4 0

0.:30

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FRAN(~OIS MAUPETIT AND ROBERT J. DELMAS

Figure 3. Ionic concentrations

(ppb) f MSA and nssSO42- and

mass ratio of MSA to nssSO42-.

Concentrations of NO 3 in DDU precipitation are characterised by relatively low values (most

of them lower than 1 peq/1 and a mean value of 1.1 peq/l without samples 6 and 10) disturbed

by two very high peaks (15.6 and 16.2 !aeq/l) for samples 6 and 10 respectively (table 1). This

mean value is in good agreement with that of background precipitation in Antarctica (Legrand

and Delmas, 1984 ; 1986).

For sample 6, the ionic balance is excellent and allows us to reconstruct the sample chemical

composition. It is clear that NO 3 is not present in this sample only as HNO 3 as is generally the

case in Antarctic precipitation. The only composition of the non-seasalt fraction that can be

deduced is a mixture of seasalt components altered by the interaction of NaCI and H2SO 4 :

2 NaCI + H 2 S O 4 . . . . > 2 HCI + Na2SO 4 (5)


TABLE 2. Concentrations (in ppb) of MSA, nssSO42 (calculated from Na *) and mass ratio of

MSA to nssSO42

Sample MSA nssSO42 MSA/nssSO42

2 4 92 0.04

3 3.5 16 0.22

4 1.5 < 0 /

5 7.5 145 0.05

6 0 522 0

7 44 300 0.15

8 4 24 0.17

9 0 < 0 0

10 2.5 35 0.07

11 0 < 0 0

12 2.5 < 0 /

13 6 17 0.36

14 10 56 0.18

and terrestrial salts (nssCa 2÷ and nssMg 2+ combined with nssSO42- and NO3 ). Furthermore, this

NO 3 peak can be linked to an occasional long range transport of continental material reported

already for DDU using radon 222 as a marker (Polian et al, 1986) and in ice cores of East

Antarctica where during the glacial age, terrestrial dust was high and nitrate present as a salt


Unfortunately, the radon 222 measurements were not performed at the time of our sampling,

and no meteorological data were available to confirm this hypothesis.

The case of sample 10 seems to be different. Even if the ionic balance of this sample is not

obtained, the relatively high acidity (13.7 ~cq/l) indicates the presence of HNO 3, nssSO4 z- being

very low (0.7 Iaeq/l). This means a strong input of HNO 3 occurring in winter. Origins and

sources of NO 3- in polar precipitation is still an open question but it has been recently proposed

that most of Antarctic NOr comes from the oxidation of NOx produced by lightning in the low


latitude troposphere (Legrand and Delmas, 1986 ; Legrand and Kirchner, 1990) and by N20

oxidation within the middle stratosphere (Legrand and Kirehner, 1990). Studies on recent polar

precipitation have revealed an increase of NO 3" in winter snow at the South Pole (Legrand and

Kirchner, 1988) and a possible recent NO 3 increase at several Antarctic sites (Mayewski and

Legrand, 1990) which could be the result of denitrification of the lower stratosphere by

sedimentation of polar stratospheric clouds, possibily linked to spring ozone depletion (Legrand

and Kirchner, 1988 ; Mayewski and Legrand, 1990). At the German Antarctic coastal base G.v.

Neumayer, an aerosol filter study revealed a bimodal seasonal variation of NO 3- with maxima

in July-September and November-December (Wagenbach et al, 1988). A more detailed study of

this seasonal variation at this site point out a trimodal signal with late winter and spring peaks

attributed to polar stratospheric cloud sedimentation (Pfeilsticker and Wagenbach, 1990). Thus,

even if we have no direct evidence for a stratospheric fallout of HNO 3 at DDU for this snowfall

event, we cannot totally discard this deposition mechanism which seems to occur under

particular conditions in Antarctica.

5. Conclusion

Despite the small number of snowfall events collected, the following conclusions, which should

be taken as a preliminary interpretation, may be presented :

most of the samples exhibit satisfactorily balanced ionic compositions. The seasalt contribution

is dominant due to the coastal position of the sampling site. All the samples are found to be

acidic in the range 3-16 peq/l. NssSO42 concentration was significantly negative for 3 of the 14

samples. The MSA to nssS042 ratio is found to be in good agreement with reported values from

subantarctic region aerosols and coastal Antarctic ice cores suggesting a local marine biogenic

activity source for MSA. A typical Antarctic background concentration was found for nitrate

but was occasionally disturbed by sudden inputs. One seems to be associated with the long range

transport of continental air masses while the other one could be representative of stratospheric

denitrification that seems to have been recorded in recent winter and spring Antarctic

precipitation.

Both continuous air and snow monitoring at this site would be extremely valuable in order to

allow a more thorough and conclusive interpretation. This work is in current development.

6. Acknowledgments

We thank P. Nisol for sample collection. We are grateful for discussions with M. Legrand. This

work was supported by the French Centre National de la Recherche Scientifique (CNRS) and

the French Ministry of the Environment. Field work was supported by Terres Australes et

Antarctiques Franqaises (TAAF) and Exp6ditions Polaires Franqaises (EPF).

CHEMICAL COMPOSITION OF FALLING SNOW

7. References

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42 FRANI~OIS MAUPETIT AND ROBERT J. DELMAS

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chemical composition of falling snow at dumont d'urville, antarctica

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