Global Perspective of Nitrate Flux in Ice Cores

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3-20-1995

Global perspective of nitrate flux in ice cores

Qinzhao Yang University of New Hampshire - Main Campus

Paul A. Mayewski University of New Hampshire - Main Campus

Sallie I. Whitlow University of New Hampshire - Main Campus

Mark S. Twickler University of New Hampshire - Main Campus

Michael Morrison University of New Hampshire - Main Campus

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Recommended Citation Yang, Q., P. A. Mayewski, S. Whitlow, M. Twickler, M. Morrison, R. Talbot, J. Dibb, and E. Linder (1995), Global perspective of nitrate flux in ice cores, J. Geophys. Res., 100(D3), 5113–5121, doi:10.1029/ 94JD03115.

This Article is brought to you for free and open access by the Earth Sciences at University of New Hampshire Scholars' Repository. It has been accepted for inclusion in Earth Sciences Scholarship by an authorized administrator of University of New Hampshire Scholars' Repository. For more information, please contact [email protected]. Authors Qinzhao Yang, Paul A. Mayewski, Sallie I. Whitlow, Mark S. Twickler, Michael Morrison, R. Talbot, Jack E. Dibb, and Ernst Linder

This article is available at University of New Hampshire Scholars' Repository: https://scholars.unh.edu/ earthsci_facpub/115 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 100, NO. D3, PAGES 5113-5121, MARCH 20, 1995

Global perspective of nitrate flux in ice cores QinzhaoYang, 1 Paul A. Mayewski,1 SallieWhitlow, 1 Mark Twickler,1 MichaelMorrison, 1 Robert Talbot,2 JackDibb, 1 and Ernst Lindera

Abstract. The relationshipsbetween the concentrationand the flux of chemical species(Cl-, NO3- , SO42-,Na +, K+ , NH4+ , Mg2+ , Ca2+) versussnow accumulation rate were examined at GISP2 and 20D in Greenland, Mount Logan from the St. Elias Range, Yukon Territory, Canada, and Sentik Glacier from the northwest end of the Zanskar Range in the Indian Himalayas. At all sites, only nitrate flux is significantly(a = 0.05) relatedto snowaccumulation rate. Of all the chemicalseries, only nitrate concentration data are normally distributed. Therefore we suggest that nitrate concentrationin snowis affectedby postdepositionaJexchange with the atmosphereover a broad range of environmentalconditions. The persistent summer maxima in nitrate observedin Greenland snow over the entire range of record studied(the last 800 years)may be mainlydue to NO• releasedfrom peroxyacetyl nitrate by thermal decompositionin the presenceof higher OH concentrationsin summer. The late winter/early springnitrate peak observedin modern Greenland snowmay be related to the buildupof anthropogenicallyderived N Oy in the Arctic troposphereduring the long polar winter.

Introduction tion of nitrate data retrieved f•om worldwide ice cores. Ice core records from polar regions provide a unique The relationships between snow accumulation rate recordof environmentaland climaticchange [e.g., Dans- and nitrate concentration/fluxare important because gaard and Oeschger,1989; Lorius et al., 1989; Mayewski they can be used to assesthe dominant processesof ni- et al., 1993a, 1994] and illustrate the influenceof an- trate incorporation into snow. The seasonal variation thropogenic emissionson the chemistry of the remote of nitrate concentration in snow is presumably related atmosphere[e.g., Neffel et al., 1985; Maycwski et al., to the source of nitrate precursor in the atmosphere. 1986, 1990a; Maycwskiand Legrand,1990]. They have Previousstudies conducted by Herron [1982],Legrand also contributed to our understanding of atmospheric and Delmas[1986], and Legrandand Kirchner[1990] in- chemistry,atmospheric circulation and biogeochemical vestigated relationships between nitrate concentration cycling[e.g., Legrandel al., 1988, 1991; Delmas and and flux versus snow accumulation rate in Greenland Legrand, 1989; McElroy, 1989; Mayewski et al., 1993a, and Antarctica. However, there is a discrepancy in 1994]. their conclusions.Herron [1982]concluded that nitrate Nitrate is one of the major chemical speciesin po- concentration decreasedwith increasingsnow accumu- lar snow. As such, measurements of nitrate in ice cores lation rate, whereasLegrand and Delmas [1986]and may provide information for understanding the com- Legrandand Kirchner[1990] found that nitrate concen- plex atmosphericnitrogen cycle [e.g., N½ubau½rand tration was independent of snow accumulation rate. In Heumann, 1988; Legrand and Kirchner, 1990; Mayew- this paper we reevaluate this relationship using high- ski and Legrand, 1990; Mayewski et al., 1990a; Mulvaney resolution snow chemical series from central Greenland and Wolff, 1993]. However,the sources,transport path- as well as other seriesfrom southern Greenland, north- ways, and scavengingprocesses of nitrate depositedon west Canada, and central Asia. glaciersare not wellunderstood [.Wolff, 1995]. Under- Annual nitrate summer peaks in snow have been re- standing these relationshipswill improve our interpreta- ported for both Greenland and Antarctica [Finkel et al., 1986; Mayewski et al., 1987, 1990b; Davidson et al., 1988; Neubauer and Heumann, 1988; Legrand and 1Glacier ResearchGroup, Institute for the Study of Earth, Kirchner, 1990; Whirlow et al., 1992; Mulvaney and Oceans and Space, University of New Hampshire, Durham. Wolff, 1993]. However, the causefor the timing of 2AtmosphericChe•nistry ResearchGroup, h•stitute for the the peak is not yet well understood. We investigated study of Earth, Oceans and Space, University of New Hampshire, cause(s)for the peak in Greenlandsnow to improveour Durham. 3MathematicsDepartment, University of New Hampshire, understandingof the relationshipbetween seasonal vari- Durham. ations of nitrate measured in snow and possiblesources of NO• in the atmosphere. Copyright1995 by the AmericanGeophysical Union. Methodology Paper number 94JD03115. In this paper we first examine the relationship be- 0148-0227/95/94 JD-03115505.00 tween the concentration of major cations and anions

5113 5114 YANG ET AL.' NITRATE FLUX IN ICE CORES

(CI-, NO3-, SO42- , Na+, K+, NH4+, Mg2+, Ca2+) nuclear fallout events. Maximum dating error is 4-1 and their flux versussnow accumulation rate using36 year throughout most of the cores[Lyons and Mayew- detailed sections(averaging 10 samplesper year over ski, 1983; Holdsworth and Peake, 1985; Mayewski et sectionsranging from 2 to 10 years)sampled from the al., 1986, 1993b]. Flux (F) of eachchemical species is GISP2 corelocated at Sununit, Greenland,to seeif ni- calculated based on that individual annual mean con- trate behaves differently from other major ions. The centration(•) timessnow accumulation rate (A) of the same analysis is performed on several other glacio- sameyear (F = C x A). chemicalseries, namely, 20D from southernGreenland; Mount Logan from the St. Elias Range; Yukon Terri- Results and Discussion tory, northwest Canada; and Sentik Glacier from the northwesternend of the ZanskarRange in Ladakh,In- dian Himalayas. Ice core sites and some basic infor- Chemical Concentration/Flux versus Snow Accumulation Rate mation about sites and sampling details are presented in Figure i and Table 1. Although the Sentik Glacier The relationship between chemical (CI-, NOa-, nitrate time seriesis shorter than the others,it is the SO42-, Na+, K+, NH4+, Mg2+, Ca2+) concentration only continuoushigh-resolution chemical record avail- (pg/kg)and snow accumulation rate (kg m -2yr -•) was able from a low-latitude/high-elevationsite. examinedusing the GISP2 chemicaldata sets(exclusive Samplesretrieved from GISP2, 20D, and Mount Lo- of the anthropogenicperiod) (Figure 2). On thebasis of gan wereprocessed in a coldroom at temperaturesthat the data shown in Figure 2, we examined each individ- did not exceed-12øC by individualswearing precleaned ual speciesusing statistical methods of polynomial and polyethylenegloves, nonparticulating clean suits, and linear regressionto determine if there is a relationship. particle masks. All tools and containers used for sam- From Figure 2 it is clear that none of the concentra- pling were precleanedin ultrapure water. Container tion seriesof chemicalspecies are statistically related blanks prepared on a frequent basisshowed that these to snow accumulationrate. This suggeststhat the an- containerswere free of contamination.All analyseswere nual mean concentration of chemicalspecies in snow is performedby ion chromatographyusing a DionexTM independent of the annual snow accumulation rate at model4040 with techniquesdescribed previously [e.g., Summit, Greenland. Mayewski et al., 1990b; Buck et al., 1992]. Sam- In Figure3 wecompare chemical flux (kg km -2 yr-•) ples retrieved from Sentik Glacier were processedin versussnow accumulation rate (kg m -2 yr-•). A table the field, and nitrate was measuredusing a Technicon of t values(Table 2) providesthe significanceof the AutoAnalyzer TM system[Lyons and Mayewski, 1983; slope coefficients for all chemical flux versus snow ac- Mayewskiet al., 1984]. cumulation rate relationships in Figure 3. Nitrate has Each core was dated by usinga varietyof different the highestt valueand correlationcoefficient (r value). seasonalindicators (e.g., major anionsand cations,sta- For the other species,t values exceed the critical value ble isotopes),historically documented volcanic events of 1.645(significance level ct - 0.05), but their r values plus total beta activity signaturesrelated to known are lower than 0.4, suggestingthat lessthan 20% of to-

30øW 30øE 120øE 60øE \ !

90 ø 180 ø

! / \ 150 øW 150 øE 120øW 60øW

Figure 1. Locationmap showingthe samplingsites at Summit,Greenland; 20D, southGreen- land; Mount Logan;Sentik Glacier;and southpole. YANG ET AL- NITRATE FLUX IN ICE CORES 5115

Table 1. Ice Core Site Details Core GISP2 20D• MountLogan Sentik Glacier South Pole a Location 72.58øN, 65.01øN, 60.63øN, 35.00øN, 90.00øS 38.47øW 44.87øW 140.50øW 76.01øE Elevation, meters above sea level 3207 2616 5340 4908 2880 Accumulationrate, m yr-1 water equivalent 0.23 0.41 0.33 0.60 0.085 Mean annual temperature, øC -31.5 -20.2 -28.9 -3 -50 Total samplesanalyzed 1524 1357 1853 103 759 Samplingresolution, samples/year ~10 ~6.3 ~6.4 ~6.2 •4.3 Periodcovered by core(AD) 1989-1200e 1984-1767 1980-1688 1980-1963 1980-1805 aMayewski el al. [1986, 1990a]. øHoldsworthand Peake[1985]. CMayewskiel al. [1984], Lyons and Mayewski [1983]. aLegrandand Kirchner [1990]. eRepresentedby 36 uniformly distributed sectionsthroughout the period. Each sectionranges from 3 to 10 years. tal flux variance can be explained by a linear fit to the The best fit linear regressionof nitrate flux to snow relationship. Only nitrate flux has a statistical linear re- accumulation rate for Summit, Greenland is lationship with snow accumulationrate over the range r - 0.75 + 0.071A (r - 0.81) (1) measured(120 to 350kg m -2 yr-1 waterequivalent and whereF is nitrate flux in kg km-• yr-1, and A is the 9.3 to 25.6kg km-2 yr-1 of NO3-). snowaccumulation rate in kg m-• yr-•.

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40 0 , I : I • I , I , I : 50 100 150 200 250 300 350 5O 100 150 200 250 300 350

SnowAccumulation Rate (kg/m 2y) Figure 2. Chemicalconcentration (Na +, NH4+, K+, Mg2+, Ca2+, CI-, NO3-, S042-) versus snow accumulation rate for the GISP2 detailed sections. 5116 YANG ET AL.' NITRATE FLUX IN ICE CORES

i i ß i i i I • o I i i 4. NH 4 ß o o o o ß ß øøoo ß ß . ß .;. 7% ß • :ø.o o: ø eO•ooød• ø ß . ,,0. ß t•oß ß ,, . ? %' •ff%00 . ß 6' ß d• o i , i . i , i , i ß I • I a I , I , I

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25 NO3- ß ßø ßß 20S042' 20 .. 15 000% • ß ß 10 ,?:'?ß - 5 , I , I , I ß I t ßøo ß ß ß 5o 100 1•0 2oo 2•0 3oo 350 50 100 150 200 250 300 350

SnowAccumulation Rate (kg/m2 y) Figure 3. Chemicalflux (Na+, NH4+, K+, Mg2+, Ca2+, CI-, NOs-, S04•-) versussnow accumulationrate for the GISP2 detailed sections. The solid line is the least squaresfitted regressionline to each species.

Herron [1982]investigated snow pits and ice cores nitrate concentration was not related to snow accumu- recovered from different sites in Greenland and Antarc- lation rate, but nitrate flux was related to snow accu- tica and suggestedthat the relationshipbetween nitrate mulation rate by the following expression: concentrationand snow accumulationrate may be ex- pressed as F - 0.99 + 0.082A - o.9) (3) Our data sets(Figure 2) showthat nitrateconcentra- NO;- GA-« (2) tion is independentof snow accumulation,in harmony with the findingsof Legrandand Delmas [1986]and where C is a constant, and A is the snow accumulation Legrandand Kirchner [1990]. Furthermore,we found rate in kg m-2 yr-1. that the relationship between nitrate flux and snow ac- Legrandand Delmas [1986] and Legrandand Kirch- cumulation at Summit, Greenland, agreesclosely with her [1990]found a discrepancyin Herroh's[1982] re- the relationshipat southpole [Legrandand Kirchner, lationshipfor Antarctica. Their data suggestedthat 1990]. Table 2. Calculatedt-Distribution Values and Correlation Coefficients (r) for the RegressionLines Displayed in Figure 2 Na+ NH• K+ Mg2+ Ca2+ C1- NOa- SO42- GISP2 n=117 t value 1.71 2.78 1.75 4.22 4.67 2.92 12.54 3.13 r 0.15 0.3 0.16 0.36 0.39 0.25 0.82 0.28 n, Data points used in regression. YANG ET AL.: NITRATE FLUX IN ICE CORES 5117

To investigate the apparent uniquenessof the GISP2 in Antarctica [Mayewskiand Legrand,1990]. The nor- nitrate data compared to other chemical species, we mally distributed nitrate concentration may be a con- examinedtheir concentrationprobability plots [Looney sequenceof postdepositionalalteration, potentially via and Gulledge,1985]. The "straightness"of the proba- exchangebetween the surfacesnow and the atmosphere bility plot is testedby usingtheir correlationcoefficients [Neubauerand Heumann,1988]. Hence we suggest that [Looneyand Gulledge,1985; Minitab, 1993]. Nitrate is the exchange of nitrate between the snow and the at- the only specieswhose concentration data are normally mosphereresults in sufficientsmoothing of the seriesto distributed at a significancelevel of 95% in Greenland make them normally distributed(Table 3). snow(Table 3). Two depositional styles dominate the incorporation To test if a similar relationshipbetween nitrate flux of chemicalspecies into snow,namely, wet and dry [Bar- and snow accumulationrate existsat our other study tie, 1985; Davidson,1989]. Equations(1)and (4)-(6) sites, we used chemical data seriesfrom three additional have small ((10%) y-interceptvalues relative to the sites:20D (exclusiveof the anthropogenicportion of the individual mean nitrate flux of 17, 24, 17, and 74 kg record), Mount Logan, and Sentik Glacier. At all three km-2 yr-1 in the snowat GISP2, 20D, Mount Logan, sites the relationship between nitrate flux and snow ac- and Sentik Glacier, respectively. If the value of the y in- cumulation rate has the highest correlation coefficient tercept of these equations reflects the flux contributed (significantat the95% level) of any of the species (Table by dry depositionalprocesses [Legrand and Kirchner, 4). 1990],then nitrate is incorporatedinto snowpredomi- The best fit linear regressionequations of nitrate flux nantly by wet depositionat thesesites. However, as dis- to snow accumulation rate at the three sites are cussed previously, the nitrate mean annual concentration is affected by the postdepositionalprocesses. For 20D: F = 1.3+ 0.053A (r = 0.71) (4) nitrate the y- intercept value may not be an indication of the contribution of dry depositionin snow. MountLogan: F = -1.9+0.054A (r = 0.79) (5) Since the annual mean concentration of nitrate is in- SentikGlacier: F = 1.8+ 0.080A (r = 0.74) (6) dependent of snow accumulationrate at our study sites Since there is a relatively strong linear relationship and at south pole [Legrandand Kirchner, 1990], the between nitrate flux and snow accumulation rate at all presence of a statistically significant linear relationship three sites, we investigated the distributions of nitrate between nitrate flux and snow accumulation rate sug- concentration versus the other chemical speciescon- gests that the supply of atmospheric nitrate is large centration at these sites. The normality of chemical enough so that it is never depleted over the accumu- concentration probability plots from 20D, Mount Lo- lation range studied here. If this is the case, we would gan, and Sentik Glacier were tested as describedfor the suggestthat annual mean nitrate concentrationin snow GISP2 data set (Table 3). Nitrate is the only chemi- is proportional to its annual mean concentration in the cal speciesfor whichthe correlationcoefficient is higher atmosphere despite postdepositional process, as long than the criticalvalue (at significancelevel of 95%)for as the alepositionalenvironment does not vary signif- each of the data sets. icantly. Nitrate concentrations in snow at these sites are dis- Sources for Nitrate Summer Peak tinct from the other chemical speciesconcentrations in that they are normally distributed. A recent surface As mentioned previously, one of the important char- snow chemistrystudy at Summit, Greenland[J. Dibb, acteristics of nitrate concentrations in Greenland and unpublisheddata, 1994]indicates that the summersur- Antarctic snow is that there is a persistent nitrate sum- face snowpackloses nitrate at some time after deposi- mer peak [Finkel et al., 1986; Mayewski e! al., 1987, tion, as has been reported for lower accumulation sites 1990b; Neubauer and Heumann, 1988; $teffensen, 1988;

Table 3. Normality Tests for Chemical Concentration Data Distribution Na+ NH4+ K+ Mg2+ Ca•+ C1- NOa- S04•- GISP2 n-117 r critical-0.988" r 0.903 0.717 0.831 0.966 0.909 0.931 0.996 0.907

20D n-174 r critical-0.990 r 0.926 0.822 0.649 0.935 0.916 0.917 0.991 0.954

Mount Logan n=220 r critical=0.992 r 0.683 0.825 0.847 0.884 0.834 0.954 0.995 0.955

Sentik Glacier n=16 r critical=0.942 r 0.893 ß ß ß ß 0.915 0.965 ß n, Data points used in test. Asterisk, species not measured. a(After Table 13.1 in Minitab ReferenceManual [1993]. 5118 YANG ET AL.: NITRATE FLUX IN ICE CORES

Table 4. Calculatedt-Distribution Values and Correlation Coefficients (r) Between ChemicalFlux and Snow AccumulationRate at 20D, Mount Logan, and Sentik Glacier Na+ NH4+ K+ Mg•'+ Ca2+ C1- NOs- SO42- 20D n-174 t value 6.98 2•22 2.38 6.59 5.66 7.81 12.88 7.39 r 0.43 0.15 0.16 0.41 0.36 0.47 0.66 0.45

Mount Logan n-220 t value 4.95 6.21 6.4 4.95 5.48 6.79 18.89 11.29 r 0.32 0.39 0.4 0.32 0.35 0.42 0.79 0.61

Sentik Glacier n=16 t value 1.59 ß ß ß ß 2.01 4.26 ß r 0.38 ß ß ß ß 0.46 0.74 ß n: Data points used in regression. Asterisk, species not measured.

Davidson, 1989; Whirlow et al., 1992; Mulvaney and and reservoir of NO• for the northern high latitudes Wolff, 1993]. However,the mechanism(s)that causes due to its longerresidence time (up to monthsin win- this peak is not well understood. To investigatethe ter) and strong thermal decompositioncharacteristics sourceof the nitrate in snow,we start our investigation [Cox and Roffey,1977; Singh, 1987; Honrath and Jaffe, of the cause(s)for the annualnitrate summerpeak in 1992;Singh et al., 1992a].Singh and Hanst [1981], and Greenlandsnow by examiningcontrols on the formation Singhand Salas[1983] suggested that PAN not only is of seasonalvariations in nitrate in the atmosphere. associatedwith polluted air but also is an important Nitric acid (HNO3) is formedprimarily by the reac- constituentof the natural atmosphere.The production tion of NO• + OH in the presenceof sunlight and is rate of OH radicals varies seasonally,with a maximum well knownas a main sinkfor atmosphericNO• [Logan, in summerin the troposphere[Warneck, 1988]. Hon- 1983; Warneck,1988; Ehhalt et al., 1992]. HNO3 is also rath and Jaffe [1992]reported a well-definedseasonal formed by the nighttime reaction betweenN•O5 and cycleof NOy, with a maximum(560-620 parts per tril- H•O [Russelle! al., 1986; Mozurkewichand Calvert, lion, by volume)in earlyspring and minimum (median 1988]. Various processesand sourcescan contribute 70 parts per trillion, by volume)in summerat Barrow, to the nitrate budget at high latitude, includingcom- Alaska. The transition occurs sharply in April, when bustionof fossilfuel (especiallyin the northernhemi- a sevenfoldto eightfolddecrease in NOy concentration sphere[Logan, 1983; Ehhal! e! al., 1992]),biomass burn- wasobserved. Davidson et al. [1993]also conclude that ing, lightning, ammonia oxidation and soil exhalation, the minima in summer of atmosphericCO and CH4 at in addition to processesacting within the stratosphere Dye-3, Greenland, is causedin part by high concentra- (galacticcosmic rays, N•O oxidation[Levy et al., 1980]). tions of OH whichare relatedto relativelyhigh airborne A few scientistshave investigated the possibleorigin concentrations of NOx and 03. of nitrate in polar precipitation, suggestingan impor- To explain the summer peak seen in nitrate concen- tant role for lightning and N20 stratosphericoxidation tration in polar snow we proposethe following: Dur- [Legrandand Kirchner, 1990; Wolff, 1995]and some- ing the winter, cold temperature and low insolation at times processesacting in the high atmosphere[Parker middle to high latitudes favor long distancetransport and Zeller, 1979;Zeller et al., 1986]. Usinga modeling of PAN into polar regions. Further, since there is no approach,Legrand et al. [1989]suggested that light- sunlight during the polar winter, the secondarysources ning, soil exhalation, and N•.O stratosphericoxidation of NO• and OH are limited. Under such conditions, are likely major contributors to the natural Antarctic snow falling during the polar winter has low nitrate nitrate budget. The major sourcesof NO• in the north- concentrations due to a lack of NO• and OH. With ern hemisphere troposphere are combustionof fossilfu- the appearanceof sunlight and increasingatmospheric els, lightningformed at high altitude/low latitude, and temperaturesduring the polar spring,the NOx released NO exhalation from soils [Logan, 1983; Legrandand from PAN reacts with OH to form HNO3, yielding a Kirchner, 1990;Mayewski e! al., 1990a]. summer nitrate peak in Greenland snow. With the rather short residencetime of NO• in the The lesspronounced summer nitrate peaks at Mount troposphere(less than a day in summerand a few days Logan and Sentik Glacier may be a consequenceof the in winter [Singh,1987]), it is unlikelythat it is directly types of air massesreaching these sites. Air masses transported from primary sources to interior central reaching Mount Logan and Sentik Glacier are pristine Greenland. Most of the nitric acid and particulate ni- free tropospheredue to high elevations[Monaghan and trate is believedto be removedduring transport [Bar- Holdsworth, 1990, Mayewski et al., 1993b; Wake et al., tie, 1985; Lyons et al., 1990; Bottenheimet al., 1993]. 1993]. Additionally, thesesites maintain somesunlight However,the formationof peroxyacetylnitrate (PAN) during winter so that the nitrate formed/releasedfrom has recently received considerableattention as a carrier PAN is depositedthroughout the year. YANG ET AL.: NITRATE FLUX IN ICE CORES 5•9

As reportedby Finkel et al. [1986]and Whirlowet al. ing the longpolar winter. NO• released/formedfrom [1992],our snowpit chemistrystudy around the GISP2 a componentof NO• (N2Os,PAN, HO2NO2)reacting sitereveals that thereis sometimesa late winter/early with OH to form HNOs in the atmosphereis believed spring nitrate peak in modern Greenland snow. Since to cause this peak. this earlier peak has not been observedfrom our GISP2 detailedsections and Finkel et al.'s [1986]data before Acknowledgments. We wouldhke to acknowledgethe the year 1900 A.D., it may result from the input of assistanceof the Polar Ice CoringOffice (University of anthropogenicmaterial. In general,any NOy species Alaska,Fairbanks) and the GreenlandIce SheetProject 2 (NO, NO2, HO2NOe,HNOs, the PANs,HNOe, NeOs, ScienceManagement Office (University of NewHampshire). NOs, and gaseousnitrates [Singh,1987]) may end up The commentsof reviewersare greatly appreciated. Sup- port for this work was provided by the National Science as nitrate to be removedfrom the atmosphere. PAN Foundation. concentrationsare elevatedin the pollutedair [$ingh, 1987; Warneck,1988] which is transportedand accu- mulatedin "Arctic haze" duringwinter. PAN has also References beenreported to be a significantcomponent of NOy Atkinson,R., A.M. Winer, and J. N. Pitts Jr., Estimation in the Arcticboundary layer during spring [Bartie and of night-time N•O• concentrationsfrom ambientNO2 and Bottenheim,1991]. But NO• releasedfrom PAN is re- NOsradical concentrations and the role of N2Osin night- strictedin the polar regionsdue to lowertemperatures time chemistry,Atmos. Environ., œ0,331-339, 1986. in late winter/earlyspring [Singh et al., 1992b]. Battle, L. A., Scavengingratios, wet deposition,and in- cloudoxidation: An applicationto the oxidesof sulphur In addition to the role of NO• and PANs, NeO5 and nitrogen, J. Geophys.Res., 90, 5789-5799,1985. chemistry may also play a role in the formation of the Barrie,L. A., and J. W. Bottenheim,Sulfur and nitrogen in modern spring nitrate peak in Greenland snow since the Arctic atmosphere,in Pollution of the Arctic Atmo- the low light to dark conditionsfavor the formation of sphere,edited by W. Sturges,Elsevier, New York, 1991. N205 [Calvertet al., 1985;Leaitch et al., 1988].There Bottenheim,J. W., L. A. Barrie, and E. Atlas, The parti- are two potentialways for N•O• to yield nitrate spring tioningof nitrogenoxides in the lowerArctic troposphere peaksin snow. First, NOe is releasedfrom NeO• due during spring1988, J. Atmos. Chem.,17, 15-27, 1993. Buck, C. F., P. A. Mayewski,M. J. Spencer,S. Whitlow, to an increasedtemperature in spring[Atkinson et al., M. S. Twickler, and D. Barrett, Determination of ma- 1986],and it then reactswith OH to form HNO•. Sec- jor ionsin snowand ice coresby ion chromatography,J. ond, NeO5 scavengesto HNO• on aqueousaerosols in Chromatogr., 59•, 225-228, 1992. late winter/spring [Calvert et al., 1985; Mozurkewich Calvert, J. G., A. Lazrus, G. L. Kok, B. G. Heikes, J. and Calvert, 1988]. Althoughthere are no N20• data G. Walega, J. L. Cantrell, and C. A. Cantrell, Chemical mechanismsof acidgeneration in the troposphere,Nature, setsavailable from the ambientGreenland atmosphere 317, 27-35, 1985. and we are unableto providequantitative relationship Cox,R. A., and M. J. Roffey,Thermal decomposition of per- betweennitrate in snowand N•O• in the atmosphere, oxyacetylnitratein the presenceof nitric oxide, Environ. thesetwo processesmay alsobe important in explain- Sci. Technol., 11, 900-906, 1977. ing the differencebetween the preanthropogenicnitrate Dansgaard,W., andH. Oeschger,Past environmental long- summerpeak and the anthropogenicera nitrate spring term records from the Arctic, in The Environmental Re- peak. cordin Glaciersand Ice Sheets,edited by H. Oeschgerand C. C. LangwayJr., pp. 28%318,John Wiley, New York, 1989. Conclusions Davidson,C. I., Mechanismsof wet and dry depositionof atmosphericcontaminants to snowsurfaces, in The Envi- By examiningthe GISP2, 20D, Mount Logan,and ronmentalRecord in Glaciersand Ice Sheets,edited by H. SentikGlacier ice corechemical species, we find that Oeschgerand C. C. LangwayJr., pp. 29-51, John Wiley, the meanannual concentrations of all chemicalspecies New York, 1989. Davidson,C. I., J. R. Harrington,M. J. Stephenson,F. P. are independentof the annual snow accumulationrate. Small, F. P. Boscoe,and R. E. Grandly, Seasonalvari- Only the nitrate concentration data seriesreveal a nor- ations in sulfate, nitrate and chloride in the Greenland mally distributed pattern. Based on previouswork ice sheet: Relationto atmosphericconcentrations, Atmos. [Neubauerand Henmann, 1988; Maycwski and Legrand, Environ., 23, 2483-2493, 1988. 1990],we suggest that thecause of thispattern is post- Davidson, C. I., et al., Chemical constituents in the air and depositionalalternation. As a consequence,nitrate flux snowat Dye-3, Greenland,I, Seasonalvariations, Atmos. Environ., 27(A), 2709-2722,1993. hasa relativelystrong linear relationship to the rate of Delmas,R. J., and M. Legrand,Long-term changes in the snow accumulation at all sites studied. concentrationsof majorchemical compounds (soluble and The presenceof a nitrate summerpeak for the past insoluble),in The EnvironmentalRecord in Glaciersand several thousand years in Greenland snow is believed Ice Sheets,edited by H. Oeschgerand C. C. LangwayJr., to be associatedwith the higherOH radicalconcentra- pp. 319-342, John Wiley, New York, 1989. tions and an availableNO• sourcereleased from PAN Ehhalt, D. H., F. Rohere,and A. Wahne, Sourcesand dis- in summertime. tribution of NO• in the upper troposphereat northern midlatitudes,J. Geophys.Res., 97, 3725-3738,1992. The observednitrate peak in earlyspring in modern Finkel, R. C., C. C. Langway Jr., and H. B. Clansen, (anthropogenicera) Greenland snow may be dueto the Changesin precipitationchemistry at Dye 3, Greenland, buildupof N O• as Arctichaze in the tropospheredur- J. Geophys.Res., 91, 9849-9855, 1986. 5120 YANG ET AL.: NITRATE FLUX IN ICE CORES

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Jack Dibb, Paul A. Mayewski, Michael Morrison, (ReceivedApril 25, 1994; revisedOctober 19, 1994; Mark Twickler, Sallie Whirlow, and QinzhaoYang (cor- acceptedOctober 21, 1994.)