Nitrate trace determinations in snow and firn core samples of ice shelves at the weddell sea, Antarctica

Atmospherrc

Encironmenl

Vol.

22, No.

3. PP. 537-545.

1988.

0004-6981/88 s3.00+ 0.00 Pergamon Prers pk.

Printed in Great Britam.

NITRATE TRACE DETERMINATIONS IN SNOW AND FIRN CORE SAMPLES OF ICE SHELVES AT THE WEDDELL SEA, ANTARCTICA JOHANN NEUBAUER

and

KLAUS

G. HE~MANN*

Institut fiir Anorganische Chemie der Universitat Regensburg, Universitiitsstrage 31, D-8400 Regensburg, Federal Republic of Germany (First received 10 April 1987 and received for publ~~at~~~2 September 1987)

Abstract-The definitive method of isotope dilution mass spectrometry was applied to determine nitrate traces in surface snow and fim core samples of different ice shelves along the Weddell Sea, West Antarctica and in precipitations near the Antarctic Peninsula. Three of a total number of seven depth profiles we analyzed showed weak seasonal variations with a trend to nitrate co~ntrat~on maxima in summer and minima in winter. The average nitrate concentration of the depth profifes down to 220 cm lay in the range of 38-93 ng g - 1which agrees with other ice shelf analyses. The highest levels in our ice shelf depth profiles are in the same range as those analyzed at the South Pole. No marine in~ue~ has been found for the nitrate concentration in contrast to the situation for chloride. The mean nitrate concentrations in new snow, in old surface snow and in firn core samples were 176, 107, and 60 ng g-l, respectively, indicating a substantial decrease with time. The results clearly demonstrate that a re-emission of HN03 after deposition in surface snow by evaporation or photochemical decomposition must occur and that there is not a simple correlation between the nitrate concentration in Antarctic snow and in the Antarctic atmosphere. A relatively high nitrate concentration of 266 ng g- 1was found in a hoar-frost sample. The same range of nitrate concentrations as in surface snow samples was found in pr~ipi~tions of snow around the Antarctic Peninsula and in precipitations over the South Atlanticat places without substantial anthro~~~~ influence. A similar nitrate background’ exists in these regions and, therefore, possibly the same primary source in the troposphere is responsible for these comparable nitrate levels. Key word index: Nitrate, Antarctica, snow, fim cores, seasonal variations, deposition and re-emission of

HI%&, isotope dilution mass spectrometry.

One major aspect of the investigation of trace compounds in pr~ipitati~n~ and fim core sampies of Antarctica is the question of the primary sources of

these compounds. In addition, a better knowledge of the global pathways of trace substances in the atmosphere might result. Theoretical considerations by Junge (1977) demonstrate a good correlation of compounds in snow to atmaspheri~~mposit~on. Dick and Peel (1985) have made experimental studies of the air-snow relationship of trace elements in Antarctica. The major cations and anions in Antarctic snow are H’, Na’, NH;, Cat”, Mg’+, K+, SO:-, Cl- and NO; (Legrand and Delmas, 1984, 1985; Delmas, 1986). Parker et al. (1978) estimated that the mean annual accumulation of fixed N as nitrate on the Antarctic ice sheet is 2.73 x 10’ t using analysis data from the South Pole, The ion balance of Antarctic snow demonstrates that the nitrate ions mainly exist in the form of HNO, (Legrand and Delmas, 1986). In the atmosphere HNO, is generally derived from NO,. Because of its refatively long lifetime, HNO, is the most abundant N species at low altitude in the

* To whom correspondence should be addressed. AB*2:3-c

537

atmosphere (Ehhait and Drummon~ 1982). HNO, is removed from the atmosphere within a characteristic period of 2-6 days, during which wet deposition is more efftcient than dry deposition. The most sign&ant sources of NO, are those from the earth’s surface, e.g. fossil ‘fuel burning, soil release, biomass burning, as well as those in the. atmosphere, e.g. oxidation processes of N species in lower oxidation states, aircraft emissions, lightning and NO, transport from the stratosphere (Logan, 1983). The irn~r~nt role of NO, as a precursor of HNO, in the atmosphere has been discussed in a number of articles (Crutzen, 1979; Ehhah and Drummond, 1982). Up to now, a semiquantitative picture of total nitrate deposition has only been obtained for the Northern Hemisphere (Ehhalt and Drummond, 1982). These results show that high deposition of NO; exists over both the industrialized continents and the tropical land masses, whereas little deposition is found over the oceans. Maximal concentrations of nitrate analysed in precipitations at middle northern latitudes are caused by fossil fuel combustion. Nitrate concentration falls sharply south of 30% because. of weaker NO, sources in the Southern Hemisphere. Most of the nitrate investigations in Antarctic snow and firn cores have been carried out with samples collected at the South Pole (Laird et al., 1982; Legrand

538

JOHAUC NEMAUFR and KLAIJS G. HEUMANI-X

and Delmas, 1984; Zeller and Parker, 1981) and in East Antarctica (Herron. 1982; Legrand and Delmas, 1985. 1986: Zanolini et af,, 1985). No systematic nitrate data are known for regions in West Antarctica. Different sources are discussed to explain the measured nitrate concentrations in Antarctic snow and ice. The primary sources of nitrate in the Antarctic atmosphere are largely unclear up to now, in contrast to the situation for the other most abundant trace ions. Anthropogenic influences on the nitrate concentration have been found in recent Greenland ice samples but not in those from Antarctica (Herron, 1982). Geophysical phenomena are considered to be responsible for high nitrate concentrations in samples of the ice sheet. For example, Wilson and House (1965) believed that the dissipation of energy by aurora leads to the formation of nitrite and nitrate in the upper atmosphere ofAntarctica. Laird et al. (1982) as well as Zeller and Parker (1981) estimated a positive correlation between nitrate production in the Antarctic atmosphere and solar activity. Such an effect was not found by Herron (1982) investigating Greenland snow samples. Rood et al. (1979) and Zanolini et af. (1985) correlate high nitrate ~on~ntrations in samples of depth profiles in the ice to the appearance of the

historical supernovae. This could not be confirmed by the investigations of Herron (1982) and Risbo et al. (1981) in the Greenland ice sheet, although the Northern Hemisphere was predominantly exposed to the historical supernovae. The largest single source of global atmospheric NO, is considered to be oxidation of biogeni~lly produced N,O (Crutzen et al.. 1975). This process may be insigni~~nt in Antarctica where low rsN/14N values in nitrate suggest a non-biogenic origin (Wada et al., 198 1). Volcanic activities have also been discussed as a possible source for nitrate. However, no significant correlations between nitrate concentrations in Antarctic ice and volcanic activities have been found (Legrand and Delmas, 1985, 1986). Parker and Zeller (1980) suggestad that marine aerosols are a probable source of nitrate in the Antarctic atmosphere, whereas Herron (1982)and Delmas (1982) demonstrated that the ocean is not the source-r possibly only a minor source-of background nitrate in Antarctica. The fact that the real sources ofnitrate in Antarctica are largely unknown and a matter of inconsistent discussion shows that only systematic investigations can contribute to a solution ofthis problem. Therefore. one aim of our analyses was to gain more systematic knowledge of nitrate concentrations in snow samples of' the western Antarctic region. In some investigations significant seasonal variations of the nitrate concentration in Antarctic fim core samples have been found, e.g. at the South Pole (Legrand and Delmas, 1984). On the other hand, analyses of other firn core sampies could not contlrm this seasonal variation (Aristarain et ai.. 1982; Parker et at., 1982; Zanolini et al., 1985). We investigated this seasonal effect in a number of depth profiles. In addition, our analyses tested different

hypotheses, which were controversially discussed in the past, e.g. the influence of marine aerosols and 01’snow accumulation rates on the nitrate concentration in Antarctic snow. It must also be taken into account that some of the disagreements among nitrate results are not a matter of fact but the result of inaccurate analyses in the low concentration range of nitrate (Herron, 1982; Legrand and Delmas, 1984). We therelore applied the definitive method of isotope dilution mass spectrometry (IDMS), which results in relatively accurate analyses data. it’ contamination can be controlled.

SAMPLING

AND EXPERIMENTAL

The samples were collected during two expedition legs ot the German polar research ship ‘FS Polarstern’ in 1985 (ANT III/3): Punta Arenas-German Antarctic station ‘Georg-van Neumayer’Filchner ice shelf-Capetown, January-March 1985: ANT IVi2: Rio de J~ejr~An~rctic Peninsula-Punta Arenas, November 1985). The sampling near the Antarctic Peninsula (South Shetland Islands) was carried out on board the ship. The collecting system was placed on the fore-deck approximately 20m above sea-level. The plastic collecting system consists of an inclined funnel (diameter = 35 cm) which ends in a cleaned PE storage vessel. The collection system could freely rotate in connection with a plastic vane which faced the open end of the funnel into the wind. In this way, a sufficiently high sample amount could also be collected during snowsto~s~ With respect to the sensitivity of our method, 50-100 g snow has to be collected for one analysis by IDMS to be able to get accurate results in the lowest ngg-’ range. Sampling on board the ship was only carried out if the relative wind direction was opposite the direction of the ship to prevent contaminatjon from the exhaust gases of the ship. In our experience, due to consistent nitrate analyses on board the ship, one should observe the direction of the ship’s exhaust gases during the total sampling and keep the sampling time < 1h. The samples coflected on the ice shelves were taken at positions up to 31 km away from the ice edge. Surface snow samples-up to a depth of 3 cm-were put directly into precleaned plastic boxes with special care to prevent contamination. Samples from deeper layers (down to 220 cm) were obtained by ex~vation of a snow-pit using pure plastic tools or with a pre-cleaned plastic drill (we only analyzed samples from the second drilling; the first drilling was performed for cleaning). Firn segments were cut with a diameter of 7.6 cm. These depth profiles of tirn core samples were divided into 10 cm long units and they were stored in a plastic box shaped to fit the fim segments. A segment 1Ocm from the snow surface taken with the drill and an identical parallel sample taken with plastic tools resulted in identical nitrate concentrations which shows that inaction by the drill can be neglected. Samples processed on board the ship were melted a few hours before the chemical treatment of the sample took place. Otherwise, the tightly closed boxes were vacuum packed in plastic foil and stored at - 20°C until these samples were processed in our laboratory at the University of Regensburg. The nitrate analyses were carried out by isotope dilution mass soectrometrv (IDMSl. The principles of this definitive method are desc%e-d el&whek (He&inn, 1986, lot%), whereas the special topics of nitrate determinations by IDMS are given by Heumann and Unger (1983). A 15NO; spike solution was used for the isotope diiution process. Negative thermal ionization mass spectrometry was applied as a selective and sensitive ionization method to determine the

Nitrate trace determinations from ice shelves in Antarctica i5N/14N isotope ratios (Unger and Heumann, 1983; Heumann et al., 1985).To prevent contamination, the chemical procedure was carried out in a laminar flow box under clean-room conditions. To determine the ‘blank’, the total analysis procedure (including the contact with the plastic tools and vessels) was carried out without a snow sample but with increasing amounts of ‘super-pure’ water (de-ionized water, which was distilled three times in a quartz vessel). In this way the nitrate contribution from the super-pure water could be eliminated and the ‘blank’ value was obtained to be (0.14 f 0.03) fig NO;. Investigations on board the ship and in our Regensburg laboratory have shown that there is no significant difference in the ‘blank’. Considering the uncertainty of the blank of s = + 0.03 pg nitrate and using sample amounts of about lOOg, detection limits (3 x s definition) for the nitrate trace determination of 0.9 ng g-i could be obtained. This detection limit is considerably lower than the lowest nitrate concentration analyzed in Antarctic snow up to now. Due to the fact that the definitive method of IDMS is applied the nitrate results of snow samples should be accurate, which means that former doubts about the accuracy of such low nitrate determinations are not as relevant as in the case

when other methods are used.

RESULTS AND DISCUSSION Depth profiles

539

The highest nitrate concentration analyzed in a segment of the firn cores is 160 ng g- ‘, the lowest one is 18 ngg-‘. The range between the minimum and maximum nitrate concentration and the means of all depth profiles are summarized in Table 1. The means are in the range of 38-93 ng g-’ with a mean of the means of 60 ng g- *. Other nitrate results for surface snow, firn core and ice samples analyzed in various regions of Antarctica by different laboratories are summarized in Table 2 for comparison. Our results are in good agreement with the data of the Ross ice shelf (Herron, 1982), with the data of Gjessing (1984) who analyzed samples from the Riiser Larsen ice shelf and with those from Unger (1984) on the Ekstrom ice shelf about 30 km south of GvN. Snow samples from inland Antarctica usually show higher nitrate concentrations than those from the ice shelves. This can possibly be attributed to the lower snow accumulation rate in inner Antarctica compared with the coastal regions (Herron, 1982). The relevance of this effect will be discussed later. However, two of our depth profiles on the Riiser Larsen ice shelf with means of 88 and 93 ng g- ’ demonstrate that not only Antarctic inland samples can contain relatively high nitrate levels but also those from the coastal region.

The nitrate trace concentrations of seven depth profiles are represented in Figs 1 and 2. Because lo-cm

Seasonal variations

long segments of the firn cores were used, the resolution of the concentration profiles is given in units of 10 cm. Figures l(a) and l(b) show the results of two depth profiles from the Riiser Larsen ice shelf whereas Fig. 1(c) represents the results which were obtained for a firn core on the Ekstrom ice shelf near the German station ‘Georg-von-Neumayer’ (GvN). The position of GvN is marked in Fig. 4. Figures 2(a)-2(c) show three depth profiles which were obtained at places 5, 10 and 15 km from the ice edge of the Riiser Larsen ice shelf. These three sampling points were positioned on a line perpendicular to the ice edge. Figure 2(d) represents the results in a fin core of the Filchner ice shelf near the German summer station. The average snow accumulation rate near GvN was determined to be about 75 cm a-l (Reinwarth et al., 1982). We found clearly delineated ice layers in our firn cores on the Ekstrom and the Riiser Larsen ice shelf with average distances of KM0 cm from one another. These ice layers could only be the result of a surface melting process during Antarctic summers and can successfully be used to determine the yearly snow accumulation if only a depth of a few meters has to be dated. It follows from this that the depth profiles in Figs 1 and 2 represent snow accumulations between 1 and 4 years. It is interesting to note that the analysis of one ice layer, which was collected in February 1987 on the Riiser Larsen ice shelf (S71”28’, Wll”50’) in a depth of 25 cm, showed a nitrate concentration of 128 ng g- ‘. This is a substantially higher concentration than the average ofall depth profiles. Elevated nitrate and dust concentrations have already been found in summer ice layers in Greenland (Finkel et al., 1986).

A well defined seasonal variation of the nitrate concentration with maxima in the Antarctic summer and minima in the Antarctic winter-as it was found by Legrand and Delmas (1984) at the South Polc+could not be observed in all of our depth profiles represented in Figs 1 and 2. Due to a yearly snow accumulation rate of SO-60 cm in the regions where we collected the iirn core samples and due to the fact that our collection took place in the Antarctic summer (January and February 1985), weak trends with summer maxima and winter minima could only be derived from the results represented in Figs 1(b), 2(c) and 2(d), respectively. Our results show that it is not the geographic position of sampling in Antarctica itself that is responsible for whether seasonal variation is found or not. The preliminary aerosol investigations with filter sampling by Wagenbach (Personal communication, 1986) near GvN showed well-defined seasonal variations of nitrate with a maximum during October-November (measured in 1983 and 1984). The fact that we still found a weak trend for seasonal variation in only three of seven depth profiles demonstrates that there is not a simple correlation between the nitrate concentration in Antarctic atmospheric particulates on the one hand and the deposited nitrate in Antarctic snow on the other hand. With respect to our depth profiles, nitrate depositions from October-November 1984 should be within the firn segment between 10 and 20cm. No concentration maximum is measured in these parts of the depth profiles. If maximum concentrations of nitrate in the Antarctic atmosphere could be confirmed for the period October-November, where a relatively low

JOHAMY

NEUBAUER

and KLALJS G. HEUMAMV

somplmg

60-

1

-

S7Y22'

poslt~on W20'2E'

r-

LO-_

n

S70°53' WOP'27'

60

0

0

20

LO

60

80

100

120

1LO

160

180

200

220

depth lcml -

Fig. 1. Nitrate concentration profiles in fim core samples at two positions on the Riiser Larsen [distances from ice edge: 16 km and 31 km; date of sampling: 22 January 1985 and 1 February 1985;Fig. l(a) and l(b)] and one site on the Ekstriim ice shelf [distance from ice edge: 20 km; date of sampling: 19 January 1985; Fig. l(c)]. Sample amount = 100 g per 10 cm long firn core segment.

snow accumulation rate exists, then one consequence of our results is that dry deposition of nitrate does not play an important role in the total content of this anion in Antarctic snow. This agrees with the general knowledge that wet deposition of nitrate on the earth’s surface is more efficient than dry deposition (Ehhalt and Drummond, 1982). Other evidence for the fact that dry deposition-at least during a period of a few days-does not influence the nitrate concentration in snow is our analysis of two surface snow samples which originated from the same precipitation (controlled by helicopter observation tlights) but were collected just after the snowfall at a place 1 km away from the ice edge and 3 days later (at a place 31 km away from the ice edge). Between the two different samplings no snow drifting occurred which normally can influence such a result. Roth snow samples show nearly identical nitrate concentrations of 57.6 ng g-’ and 57.5 ng g-l, respectively. Obviously, an equivalent ‘background’ level

of nitrate in the atmosphere of this region (Riiser Larsen ice shelf) leads to the same concentration. Marine itrfluence

The results just mentioned also make obvious that there is no measurable marine influence on the nitrate concentration in Antarctic snow because the two points of sampling are at different distances from the ice edge. This result can also be seen in Figs 1 and 2 which do not show any significant trend dependent on the distance of the sampling site from the ocean. Figure 3 represents the results of nitrate and chloride analyses of surface snow samples dependent on the distance from the ocean on a line perpendicular to the ice edge. The sampling positions at distances of 5, 10 and 15 km, respectively, are identical to those for the results represented in Figs 2(a)-(c). The evident decrease of the chloride concentration with increasing distance from the ice edge shows the tremendous marine influence on

Nitrate trace determinations 160

541

from ice shelves in Antarctica

20 1

sampling position s 73019 w 200 35’

sampling position s 730 17’ w2w 37’

s 73021’ W20Q33’ n

0

20

Lb

60

0

20

10

60

80

depth lcml -

depth tcml B

Fig. 2. Nitrate concentration profiles in firn core samples at three positions on the Riiser Larsen [S, 10 and 15 km distant perpendicular to the ice edge; date of sampling 12 February 1985,Figs 2(9)-2(c)] and one position on the Fiicbner ice shelf [distance from ice edge: 20 km; date of sampling: 6 February 1985; Fig. 2(d)]. Table 1. Nitrate concentrations analyzed in flm core samples on different ice shelves at the Weddeli Sea (sample amount = 100 g from 10 cm tong segments) Nitrate concentration (ng g- ’ ) Position of samohna Ekstrom ice shelf (GvN): S70”53’WO8”27 Riiser Larsen ice shelf: S73”17’W20”37 S73”t9’W20Q3S S73”21’W20”33 S7302~W20”28 S74”Ol’W21”W Filchner ice sheK: S77O~W~O38

Depth (cm)

Range min.-max.

Mean value

220

18-117

38

50 70 70 145 160

46-134 18-62 54-160 22-74 37-83

88 47 93 45 53

50

30-93

55

the ~n~ntration of this anion. Nitrate is not affected by the marine aerosol in a measurable way. Because a marine influence on the nitrate ~n~ntration should be finely measurable in the coastal regions, our results and those from other laboratories (Herron, 1982; Gjessing, 1984) refute assumptions that the ocean can su~t~tially contribute to the nitrate content of Antarctic snow (Parker and Zeller, 1980). During the

same time period of our sampling on the ice shelves, nitrate concentrations in pack-ice of the Weddell Sea in the range of 0.04-1.6 pgg-’ were analyzed (Di~km~n, Personal ~mmuni~tion, 1985). if a similar range of nitrate concentration is assumed in the surface layer of the ~tarcti~ Ocean, the chloride concentration exceeds that of nitrate by a factor of (l-50) x lo* due to the known chloride composition of

542

JOHAN&NEUBAUER and KLAUSG.

Table 2. Average nitrate ..-I __-_ Region --~. South Pole

Terre Ad&e Byrd Station James Ross Island Ross ice shelf Riiser Larsen ice shelf Ekstriim ice shelf Different ice shelves at the Weddell Sea * U.Y.s~trophotometri~ t ton chrumatog~phy.

NEUMANN

in SUrfaCe snow, tirn core and ice samples from different regions of Anldrctica ---p-l_l_ .~-“..__~___ _____ ,_____ Average nitrate Age of Analytical concentration (ng g ’ ) samples method Reference ~---____.-._-~ - ---- ----.--.._.I..__..-.-.~..____._l__“___ __ _ 80 Firn cores up to Phot.* Parker et ui., 1981 a depth of 4-S m 102 Sampling in 1980/81, Phot. Laird er al., 1982 up to 53 years old 89 Samples from ict Legrand and L?elmas, 1984 1959-1969 9-86 Sampling in 1982183, IC Legrand and Delmas, 1985 surface snffw 19-66 Old ice samples, IC Palais and Legrand, 1985 approx. tOs years 29 Samples from IC Arjstara~n et ul., 1982 t964- 1979 39-41 Samples from IC Nerron, 1982 1971- 1977 35-66 Sampling in 1978/?9, Phot. Gjessing, 1984 only a few years old 51 Sampling in 1983, tDMS Unger, 1984 up to 2 years old 38-93 Sampling in 1985, lDMS This work up to 4 years old

Concentrations

method or photometric method after reduction with ~dmium.

i

I

s

10

distance

from

ice

15 shelf

edgtfkm]

---c

Fig. 3. Nitrate and chloride concentrations in surface snow samples de~ndent on the distance from the ice edge fRiiser Larsen ice shelf; sampling positions see Figs Z(a)-_(c)],

the ocean, Sea-spray is the dominant source for chloride in marine aerosols and, therefore, also in Antarctic snow. If sea-spray is also the mast ~m~rtant primary source for nitrate in ~tarctic snow, the concentration level of this anion should be a factor (l-50) x lo4 lower than that for chloride. As can be seen from the results represented in Fig. 3 there is only a difference in the ~n~ntration of these anions up to a factor of about IO. The signi~ant infiuence of the distance from the coast on the sea-salt ~ncentration of Antarctic snowas it is represents in Fig. 3 for chloride--agrees with the inv~tigatjons of Herron and Langway (1979) on the Ross ice shelf. Legrand and Delmas (1985) suggested that the critical parameter for the transport of

the bulk of sea-salt aerosol is elevation rather than distance from the coast. The chIoride results represented in Fig. 3 demonstrate that the sea-salt ~n~ntration can decrease more than a factor of ten witbin 15 km from the coast line. This shows that distance from the ocean is also a critical ~rameter for the chloride transport mechanism. ~j~ere~ces in the nitrate ~on~ent~afion @“new and old S?lOW It is remarkable that the three surface snow samples (Fig. 3) collected at the same lotions as the samplesof the depth proNes represented in Fig. 2(a)_(c) always contain higher nitrate con~~trations than the first lo-cm segment of the f?rn cores. We therefore analyzed

543

Nitrate trace determinations from ice shelves in Antarctica different surface snow samples from ice shelves. The results of these samples are represented in Fig. 4. They yielded nitrate concentrations in the range of 31-384 ng g-l with an average value of 143 ng g-‘, which is much higher than the mean of 60 ng g-t for all samples from the depth profiles. New snow samples collected around the South Shetland Islands show the same concentration range as those from the ice shelves. It is interesting to point out that we also analyzed comparable nitrate concentrations in precipitations over the South Atlantic (sampling was carried out in November 1985, whereas the samples on the ice shelves were collected in January and February 1985) at places without substantial anthropogenic influences. This suggests that a similar nitrate ‘background’ level in the atmosphere-and, therefore, possibly the same primary source in the troposphere-is responsible for the comparable nitrate results of these regions. If one splits the analysis results of surface snow samples from the ice shelves into those for new snow (up to a few hours after snowfall) and those for old snow (at least some days old), the calculated average nitrate concentrations are summarized in Table 3. A tremendous decrease in the average nitrate concentration with increasing age of deposition is to be observed. Figure 5 represents histograms of abundance distribution curves for nitrate concentrations (one concentration interval corresponds to 25 ng g- ’ ) analyzed in snow samples of different ages after snowfall. The maximum of the abundance distribution curve for the samples of the depth protiles lies between 25 ng g-l and 50 ng g-t which includes 43% of all firn core samples. The nitrate concentrations in depth profile samples were always less than 150 ng g- ‘. The maxi-

Table 3. Average nitrate concentrations in Antarctic snow samples dependent on the age of the deposition Average nitrate concentration (ng g- ’ )

Deposition age Old depositions of nitrate in depth profiles Old depositions of nitrate in surface snow Fresh depositions of nitrate in surface snow

I

60 107 176

60

-z a -

LO

2

60

I

1

:ji,

mf;dwMm_

0

50

100

150

200

250

NO; concentration

300

[“g/g]

350

LOO

-

Fig. 5. Abundance distribution curves for nitrate concentrations in Antarctic snow dependent on the age of the deposition.

mum concentration in the distribution curve for old surface snow shifts to a higher level. Thirty per cent of these samples show nitrate concentrations between 125 and 150 ng g-’ ; 10 % of these samples lie in the range of 200-225 ngg-‘. Whereas we could always find samples with nitrate concentrations < 50 ng g-i in firn cores and in old surface snow, this concentration interval is completely missing for new snow. In this case, 23 % of all samples show concentrations between 225 and 300 ng g-l; 15% of these samples lie in the range of 35O-400 ng g-l. The last two concentration levels were never reached in samples of old snow.

60

Weddell

Sea

Nitrate

I

I

60

LO

20

W

Fig. 4. Nitrate concentrations in surface snow samples in the area of the Weddell Sea.

loss from

snow

results show that there is a substantial loss in the nitrate content of Antarctic snow with increasing age of deposition. Up to now only wet and dry depositions of nitrate from the Antarctic atmosphere on the surface of the ice sheet have been discussed, but These

544

JOHANN NEUBALJERand KLAUS Ci. HEUMA~YN

not a possible re-emission of nitrate into the atmosphere. Our results clearly demonstrate that re-emission of nitrate into the atmosphere must occur. Evaporation of HNO, from surface snow into the atmosphere and phot~hemical decomposition of HNO, are the two most probable reactions which can reduce the initial nitrate concentration. Possible evaporation to return deposited nitrate from the earth’s surface into atmospheric HN03 was suggested by Ehhalt and Drummond (1982). On the other hand, the photochemical decomposition of HNOj by the reaction given in Equation ( I) is well known (Wagner et al., 1980; Ehhalt and Drummond. 1982): h x $1

HN03 .-- m----tNO1 + OH.

If)

The photoreaction of Equation (1) can possibly also explain why nitrate concentration maxima in summer and minima in winter have not been found in all depth profiles of firn core samples although one must assume that a photochemical step is involved in the formation of HNO, from its gaseous precursors NO,. During the Antarctic summer the higher production rate of HNOJ in the atmosphere followed by wet deposition can be compensated in some cases by a higher re-emission initiated by light. Our nitrate results with new and old snow samples show that the situation for an exact interpretation of nitrate depositions in the Antarctic ice sheet is morecomplicated than it was assumed up to now. Further extensive investigations must therefore be carried out for a correct interpretation of the nitrate accumulation in Antarctica. Nitrate

concentration

in

a hoar-frost

sample

Different meteorological conditions during nitrate deposition and during a possible return of nitrate to the atmosphere must substantially influence the nitrate content of Antarctic samples. To give an example, we found a very high nitrate concentration of 266 ng g- ’ in a hoar-frost sample collected on the Filchner ice shelf, which is much higher than the corresponding average value of 176 ng g - ’ for fresh snow (Table 3). This result correlates with the suggestion of Herron (1982) about the nitrate concentration dependence with respect to the snow accumulation rate. The accumulation rate of hoar-frost is lower than in the case of a ‘normal’ snowfall, which explains the high nitrate concentration in hoar-frost samples assuming approximately constant ‘background’ nitrate concentrations in the atmosphere. Recently, comparable observations in samples of hoar-frost and fog have also been made in Europe (Hiitter, 1985).

CONCLUSION

This work presents systematic analyses of nitrate traces in ice shelf samples of West Antarctica. The most

important result of our investigation is the fact that the nitrate accumulation in Antarctic snow and ice is not only influenced by wet deposition but also by a possible re-emission of nitrate from surface snow to the atmosphere. This contributes to a better knowledge of the natural cycle of nitrate and its precursors and, with respect to our results, its successorsin Antarctica, which is still largely unknown and. therefore, must be a topic of further investigations.

Acknowledgements-We thank the Deutsche Forschungsgemeinschaft for financial support which was granted within the *‘Schwerpunktprogramm Antarkt~sfor~hung”. We are grateful to the “Alfred-Wegener”Institut fur Polarforschung“ in Bremerhaven and to the crew of the polar research ship “FS Polarstern” for all assistance.

REFERENCES

Aristarain A. J., Delmas R. J. and Briat M. (1982) Snow chemistry on James Ross island (Antarctic Peninsula). J. geophys. Res. 87, 11,004-l 1,012. Crutzen P. f. (1979)The role of NO and NO* in thechemistry of the troposphere and stratosphere. Ann. Rev. Earth Planet.

Sci. 7, 443472.

Crutzen P. J., lsaksen 1. S. A. and Reid G. C. (1975) Solar proton events: stratospheric sources of nitric oxide. Science N.Y. 189, 457459. Delmas R. J. (1982) Antarctic sulfate budget. Nature 299, 677-678. Delmas R. J. (1986) Antarctic precipitation chemistry. In Chemisfr~of~u~t~~hase

Atmospheric

Systems,

pp. 249.-266.

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