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doi:10.1016/j.at

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Atmospheric Environment 41 (2007) 5031–5043

www.elsevier.com/locate/atmosenv

What is causing high ozone at Summit, Greenland?

Detlev Helmiga,�, Samuel J. Oltmansb, Thomas O. Morsea, Jack E. Dibbc

aInstitute of Arctic and Alpine Research, University of Colorado at Boulder, UCB 450, Boulder, CO 80309, USAbEarth Systems Research Laboratory, National Oceanic and Atmospheric Administration (NOAA),

325 Broadway, Boulder, CO 80305, USAcInstitute for the Study of Earth, Oceans, and Space, University of New Hampshire, Durham, NH 03824, USA

Received 14 September 2005; received in revised form 22 May 2006; accepted 30 May 2006

Abstract

Causes for the unusually high and seasonally anomalous ozone concentrations at Summit, Greenland were investigated.

Surface data from continuous monitoring, ozone sonde data, tethered balloon vertical profiling data, correlation of ozone

with the radionuclide tracers 7Be and 210Pb, and synoptic transport analysis were used to identify processes that contribute

to sources and sinks of ozone at Summit. Northern Hemisphere (NH) lower free troposphere ozone mixing ratios in the

polar regions are �20 ppbv higher than in Antarctica. Ozone at Summit, which is at 3212m above sea level, reflects its

altitude location in the lower free troposphere. Transport events that bring high ozone and dry air, likely from lower

stratospheric/higher tropospheric origin, were observed �40% of time during June 2000. Comparison of ozone

enhancements with radionuclide tracer records shows a year-round correlation of ozone with the stratospheric tracer 7Be.

Summit lacks the episodic, sunrise ozone depletion events, which were found to reduce the annual, median ozone at NH

coastal sites by up to �3 ppbv. Synoptic trajectory analyses indicated that, under selected conditions, Summit encounters

polluted continental air with increased ozone from central and western Europe. Low ozone surface deposition fluxes over

long distances upwind of Summit reduce ozone deposition losses in comparison to other NH sites, particularly during the

summer months. Surface-layer photochemical ozone production does not appear to have a noticeable influence on

Summit’s ozone levels.

r 2007 Published by Elsevier Ltd.

Keywords: Tropospheric ozone; Snowpack-atmosphere gas exchange; Snow photochemistry; Synoptic transport

1. Introduction

Ozone has a fundamental role in atmosphericoxidation chemistry (Crutzen and Zimmermann,1991; Thompson, 1992), both as the most importantprecursor of the OH radical as well as being anoxidant itself, contributing to the atmospheric

e front matter r 2007 Published by Elsevier Ltd.

mosenv.2006.05.084

ing author. Tel.: +1303 492 2509.

ess: detlev.helmig@colorado.edu (D. Helmig).

destruction of organic molecules. Recent researchon snow-photochemical processes in sunlit snow hasrevealed a previously unexpected abundance ofchemical reactions and oxidation chemistry in sunlitsnowpack (Domine and Shepson, 2002). Newfindings on chemical reactions occurring within thesnowpack are presented in other contributions tothis special journal issue (Beyersdorf et al., 2007;Sjostedt et al., 2007). Several studies have shownthat ozone plays an intimate role in snowpack

ARTICLE IN PRESSD. Helmig et al. / Atmospheric Environment 41 (2007) 5031–50435032

chemistry. Ozone loss in the snow is driven by solarirradiance. The diurnal cycle and light dependencyof ozone in interstitial air shows the oppositebehavior as many other important trace gases,including NO, NO2, C2–C4 alkenes, aldehydesand hydrogen peroxide (Sumner and Shepson,1999; Dibb et al., 2002; Peterson and Honrath,2001; Swanson et al., 2002; Jacobi et al., 2002, 2004;Helmig et al., 2007a). Although many questionsregarding snowpack chemistry remain unanswered,it appears plausible that ozone itself plays animportant role in formation and destruction ofother trace gas species in interstitial air.

Ozone concentrations in the surface layer atSummit, Greenland are remarkably high (Helmiget al., 2007b). Mean and median annual ozone inthe available 2000–2005 surface record were 47.0and 46.1 ppbv, respectively, which was 15–20 ppbvhigher than at any of the other 3 NorthernHemisphere (NH) and 20–25 ppbv higher than at 6Antarctic stations that were included in this study.Peak 1-h ozone mixing ratios of 69, 66, 81 and79 ppbv were observed during 2000, 2001, 2002 and2004, respectively, again 20–40 ppb higher than atany other polar site.

Ozone in the surface layer was found toequilibrate with firn air on time scales of a fewhours (Peterson and Honrath, 2001; Helmig et al.,2007a). Given the high ambient ozone levels atSummit, ozone concentrations in the snowpack arealso expected to be higher than inferred ozoneconcentrations at other polar sites. Up to 50 ppbv ofozone were measured at 1m depth in the Summitsnowpack (Helmig et al., 2007a). Higher ozonelevels suggest that photochemical oxidation chem-istry in the Summit snowpack is similarly enhancedcompared to other polar locations.

In this manuscript we investigate possible reasonsand contributing factors for high ozone at Summitby analyzing experimental data from tetheredballoon soundings, surface data records, radio-nuclide tracers and trajectory analysis.

2. Experimental

Surface layer ozone: Surface layer ozone fromcontinuous measurements at Barrow (711190N,1561360W, 8m above sea level), Summit (721340N,381290W, 3212m), Westman Island, Iceland(631340N, 201290W, 127m) and Zeppelinfjellet(781540N, 111530E, 475m) were obtained and

analyzed as detailed elsewhere (Helmig et al.,2007b).

Ozone sonde data: Electrochemical ozone sondedata were retrieved from the World Ozone andUltraviolet Data Center archive (www.woudc.org/).Coordinates for Alert are 821320N, 621430W, 10m;for Resolute 831100N, 941541W, 30m; for Syowa691000S, 391350W, 29m; and for the South Poleobservatory 891590S, 241480W, 2810m.

Tethered balloon data: The temporal and verticaldistribution of ozone over the Summit researchcamp was investigated using a tethered balloon dataset from June 2000. Instrumentation and otherdetails of these measurements were presentedpreviously (Helmig et al., 2002). Profile data wereaveraged to 1-m height intervals. Missing datapoints were interpolated. Data for heights 4500mabove the surface were omitted. Continuous surfaceobservations, where available, were included for abetter, higher-resolution description of surface layerconditions.

Meteorological data: Data for wind speed andwind direction at the Summit camp during 2004were recorded at the ‘Science Trench’. Furtherdetails of these measurements are provided else-where (Helmig et al., 2007a).

Radionuclide tracers: The activity of the radio-nuclide tracers 7Be and 210Pb was measured inbulk aerosol samples that typically were collectedover 2-d intervals. Further details are given by Dibb(2007).

Trajectory and potential vorticity (PV) analyses:The back trajectories to Summit and WestmanIslands were computed from the NCEP/NCARReanalysis Data Set (Kalnay et al., 1996). Thetrajectory model (Harris et al., 2005) determines thevertical position of the air parcel explicitly using thevertical wind field in the analyzed data set (three-dimensional trajectories). The model has a fixed 1-htime step that interpolates between the 6-h analyzeddata set available on a 2.51 horizontal grid. Thetrajectories were computed for an altitude 500mabove the station to minimize the influence of localterrain effects. PV (in PV units of1 PVU ¼ 1.0� 10�6m2 s�1Kkg�1) was calculatedfrom the same data fields for the 290K potentialtemperature height. Further transport analyseswere obtained by backward calculations with theparticle dispersion model FLEXPART (see: (http://zardoz.nilu.no/�andreas/STATIONS/SUMMIT/)).Further details of FLEXPART are provided inStohl et al. (1998, 2005).

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40

50

60

70

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90

0 50 100 150 200 250 300 350

Calendar Day

Ozo

ne

(p

pb

v)

Summit Barrow Zeppelinfjellet

Fig. 2. Hourly ozone at Summit during 2004 compared to a 10-

year average of ozone at Barrow and Zeppelinfjellet. Springtime

ozone depletion events were removed from the Barrow and

Zeppelinfjellet records to allow for a better comparison of

seasonal cycles.

D. Helmig et al. / Atmospheric Environment 41 (2007) 5031–5043 5033

3. Results and discussion

3.1. Diurnal and seasonal surface ozone

Surface ozone measurements at Summit began inJune 2000. Only two years (2001, 2004) have nearlycomplete records as during other years the Summitcamp was not operated through the winter season.The averaged monthly distribution from the avail-able data is shown in Fig. 1. The seasonal ozonemaximum is observed during April–June, whenboth mean and median ozone are above 50 ppbv.The months with the lowest ozone are Novemberand December. The mean seasonal ozone amplitude(Helmig et al., 2007b) is �12 ppbv. This seasonalcycle is rather unusual for NH remote sites, astypically ozone photochemical losses maximizeduring the summer period, resulting in ambientozone minima during mid- to late summer.

The seasonal ozone cycle at Summit is furtherinvestigated in Fig. 2. For a comparison, ozonesurface records from Barrow (1994–2004) andZeppelinfjellet (1990–2000) were each averagedand plotted with the Summit 2004 data. Springtimeozone depletion events cause episodic periods withsignificantly reduced ozone at Barrow, and to alesser extent at Zeppelinfjellet. Sunrise ozone deple-tion is a coastal phenomenon and does not affectozone at Summit to a noticeable degree. Springtimeozone depletion events were estimated to lower theannual mean ozone at Barrow by �3 ppbv (Helmig

Fig. 1. Distribution of 1-h ozone data at Summit in 2000–2004.

The diamond is the mean, the horizontal bar inside the box is the

median, the box is the inner 50th percentile (25th and 75th

percentiles) and the whiskers are the inner 90th percentile (5th

and 95th percentiles) of all observations.

et al., 2007b). Data from such events were removedfrom the Barrow and Zeppelinfjellet records toallow for a more representative presentation of theirseasonal cycles. The comparison of these 3 dataseries reaffirms the difference in ozone seasonality atSummit compared to the 2 other Arctic, low-altitude locations. The seasonal ozone cycles atBarrow and Zeppelinfjellet are quite similar, how-ever the winter maximum at Barrow occurs �1month later and the seasonal amplitude at Barrow is�5 ppbv higher than at Zeppelinfjellet.

The seasonal ozone cycle is determined by the neteffect of chemical ozone source and sinks, transporteffects and deposition terms. Barrow is surroundedby Arctic Tundra towards the E–S–W sectorsduring spring and summer, while Zeppelinfjellet islocated on the island of Spitzbergen with acomparatively small continental footprint. Ozonedeposition rates to tundra are expected to be similarto those measured to grass land, which are in therange of 0.2–0.7 cm s�1 (Wesely and Hicks, 2000). Incontrast, ozone deposition fluxes to oceans andsnow-covered landscapes are much smaller, in therange of o 0.01–0.05 cm s�1 (Wesely and Hicks,2000; Helmig et al., 2007c). Consequently, ozonedeposition losses during the snow-free period innorthern Alaska (May–October) are expected to behigher at Barrow than at Zeppelinfjellet, resulting ina stronger drawdown, i.e. sink of ozone, and inreduced ambient ozone concentrations during thesummer months.

The hourly Summit data (Fig. 2) show anabundance of positive and occasional negative

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spikes. Remarkably, most negative spikes have amuch shorter duration. We investigated selectednegative spikes during times when ozone fluxgradient measurements with multiple monitors andsupporting meteorological instrumentation wereundertaken and have found that such events arosefrom camp exhaust (e.g. from the station generator)being transported to the measurement site. Thecomplete record is presented here to illustrate therare occurrence of the sampling of camp contam-ination. No attempt was undertaken to investigateand eliminate the relatively infrequent negativeozone spikes as required information for filteringthe data is not available for the entire ozone record.It was also determined that these occasionalcontamination periods do not have a significantimpact on the data analysis and interpretation.Periods with ozone enhancements typically lastmuch longer, on the order of 1–20 d. A higherabundance of ozone enhancement episodes appearsto occur during the spring and summer months. Theseasonal ozone cycle shows a slower and weakerdecline of ozone during the summer than at Barrowand Zeppelinfjellet.

Summertime ozone sinks at Summit are expectedto be weaker due to lower ozone deposition lossescompared to the two (in particular Barrow) othersites. Summit is surrounded by 300–1200 km ofglacial ice sheet and extensive ocean surface beyond.

0

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30

35

0 25 50 75 100 125 150 17

Day

Delta O

zone (

ppbv)

Fig. 3. Amplitude of the daily change in ozone (maximumminus minim

2004 full year record is shown with filled, grey symbols. For the three ot

year, open symbols are used. Also shown are best fit polynomial curve

As mentioned earlier, all of these land cover typeshave low ozone uptake rates compared to vegetatedlandscapes. Consequently, summertime ozone de-position losses are expected to be low at Summitcompared with other Arctic locations. Furthermore,due to the elevation, ambient air at Summit remainscolder and dryer during the summer, which isexpected to result in lower summertime ozone lossesfrom photolysis (followed by the O(1D)+H2Oreaction) compared to lower sites such as Barrowand Zeppelinfjellet.

The frequency distribution of sudden changes insurface ozone was further investigated by calculat-ing the daily amplitude of the ozone change. Bothmodeling exercises and statistical analysis of ozonesurface data have shown that diurnal, photochemi-cal ozone cycles at Summit are small, no more than�1 ppbv (Helmig et al., 2007b). Consequently, anydiurnal changes larger than 1 ppbv must be fromtransport of air with lower or higher ozone toSummit. The results of this analysis in Fig. 3 showthat during November–February daily changes insurface ozone generally are smaller than during thespring and summer months. This analysis impliesthat transport events have a stronger influenceduring the spring and summer than during othertimes of the year. The extent and origin of ozonetransport was investigated further with verticalprofile data, analysis of radionuclide tracers, and

5 200 225 250 275 300 325 350

of Year

2000

2002

2003

2004

Poly

um hourly value on a given day) at Summit during 2000–2004. The

her years, where measurements were only available for part of the

s that were fitted through all four data series.

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by synoptic transport analysis, which will bepresented in the following sections.

3.2. Vertical ozone distribution

The average of 10 years of year-round ozone-sonde profile data (based on 300–600 soundingsdepending on the location), binned in 250m heightintervals, for the Arctic sites Alert and Resolute andthe Antarctic stations Syowa and South Pole areshown in Fig. 4. Ozone generally increases withaltitude as can be seen in the profiles from these 4sites. Differences in the profiles within both the 2Arctic and the 2 Antarctic sites are small. However,a prominent gradient between the NH and SH isevident. Sea-level surface ozone at the two NH sitesis �5 ppbv higher than at Syowa. This differenceagrees well with the previously reported hemi-spheric, surface polar ozone gradient (Helmig etal., 2007b). With increasing altitude, the NH–SHgradient steadily increases, reaching �20 ppbv atSummit’s elevation of 3212m. The median annual

Fig. 4. Average ozone profiles at the NH sites Alert and Resolute

and the Antarctic stations Syowa and South Pole for 1995–2004.

The elevation of Summit is indicated by the horizontal line and

the median multi-year Summit ozone mixing ratio by the ‘X’.

Summit surface mixing ratio of 46.1 ppbv isindicated by the ‘X’ in this figure. This comparisonshows that the annual mean and median ozone atSummit are almost exactly the same as free tropo-spheric ozone levels measured by the ozonesondes.Ozone at Summit also compares well with meanozone values from other, NH high-altitude sites,i.e. 42.5 ppbv at Mauna Loa Observatory, Hawaii(3397m), 51.5 ppbv at Zugspitze, Germany(2962m), 50.0 ppbv at Jungfraujoch, Switzerland(3580m), 46.0 ppbv at Izana, Tenerife, Spain(2800m) and 48.0 ppbv at Niwot Ridge, Colorado(3020m) (mean ozone values were calculated fromdata available in the World Data Centre forGreenhouse Gases (http://gaw.kishou.go.jp/wdcgg.html)).

3.3. Spatial and temporal ozone distribution

The vertical and temporal distribution of ozonebetween the surface and 500m height above thesurface during June 2000 is shown in the colorcontour plot in Fig. 5. During this 18-d period, twodistinct transport events were observed that broughtair with elevated ozone to Summit. During the firstevent (Day of Year (DOY) 158–162) ozone increasedto 470ppbv. Most of the enhanced ozone passedover Summit at 4250m height with only a smalleffect on surface ozone, except at the end of the period(DOY 162) when surface ozone increased to465ppbv for several hours (Helmig et al., 2002).The second transport event during DOY 169–171 hadmaximum ozone of 470ppbv in the 100–400m layerand lasted for �2d. Surface ozone rose to �60ppbvduring and shortly after this period. A correlationbetween high ozone and low water vapor in selectedvertical profiles was previously reported (Helmig et al.,2002). Here, we combined all available vertical watervapor partial pressure balloon data and water vapormeasurements from a tower (2m) in a contour plotanalysis (Fig. 6). In the surface layer, water vapormixing ratios vary due to the diurnal cycle of solarheating and nighttime cooling of the snow surface.Even though the sun does not set below the horizon atSummit during June, amplitudes of the temperatureswings in the surface layer are significant, on the orderof 5–20 1C between day and night (Helmig et al.,2007a). These diurnal temperature changes, which arecontained in a shallow surface layer, cause convectivemixing and a diurnally growing mixed boundary layer(Helmig et al., 2002; Cohen et al., 2007). During mostdays, the temperature and water vapor changes occur

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100

0

160 165 170

Day of Year 2000

Heig

ht [m

]

2.0

1.5

1.2

0.8

0.4

H2O VP (mBar)

Fig. 6. Water vapor partial pressure at Summit between the surface and 500m during 4–21 June 2000. Black dots indicate the distribution

of data points that went into this contour plot analysis.

500

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200

100

0160 165 170

Day of Year 2000

Heig

ht (m

)

70

65

60

55

50

45

40

Ozone (ppbv)

Fig. 5. Ozone at Summit between the surface and 500m during 4–21 June 2000. The black dots indicate the distribution of data points that

went into this contour plot analysis.

D. Helmig et al. / Atmospheric Environment 41 (2007) 5031–50435036

within the lowest 50–250m of the atmosphere (Figs. 6and 7). A comparison between the ozone and watervapor graphs (Figs. 5 and 6) illustrates an anti-correlation between ozone and water vapor in airabove the surface layer. All high-ozone events thatwere detailed above coincide with low water vaporboth in the vertical and temporal domains. Sincetropospheric water vapor mixing ratios generallydecline with altitude, and are much lower in the

stratosphere, it is likely that the two distinct plumesthat brought high ozone-containing air to Summitduring June 2000 originated in the high troposphereor represent stratospheric intrusion events. PV fieldsover Summit during DOY 158–162 increased from abackground of 0–1 to values of 3–4 PVU on 9 June,6:00h (DOY 161.25) at the 290K potential tempera-ture height (100–300m above the surface). Thischange that occurred coincident with the largest ozone

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500

400

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100

0

160 165 170

Day of Year 2000

Heig

ht [m

]

300

295

290

285

280

Pot Temp (K)

Fig. 7. Potential temperature at Summit between the surface and 500m during 4–21 June 2000. Black dots indicate the distribution of data

points that went into this contour plot analysis.

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ozo

ne

(p

pb

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7B

e (

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-3/1

0)

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7B

e/2

10P

b

2004

2003

2002

2000

7Be

Be/Pb

Fig. 8. Comparison of the monthly means of the diurnal

amplitude of ozone fluctuations (2000–2004 data series shown

by open symbols and solid lines) with the seasonal variation of

the radionuclide 7Be activity and the 7Be/210Pb ratio (average of

58 monthly medians between 6/97–7/05).

D. Helmig et al. / Atmospheric Environment 41 (2007) 5031–5043 5037

maximum and lowest water minimum seen in theballoon data further supports the assumption oftransport of air with recent high troposphere orstratospheric origin during this period. Average windspeeds at 200–400m height during DOY 159–162 andDOY 169–171 were 4–6 ms�1. From the duration ofthese elevated ozone events and their average transportvelocity the horizontal extends of these two eventswere estimated at 900 and 1500km, respectively.

These observations are in stark contrast tovertical tethered balloon data from South Poleduring December 2003. At South Pole, sustainedconditions with surface layer air that had up to25 ppbv-enhanced ozone were observed for severaldays. High ozone clearly originated in the surfacelayer and was trapped, under stable boundary layerconditions within 100–200m near the surface.During the 20-d experiment, all enhanced ozoneoriginated near the surface and no indications forozone transport from higher atmospheric layerswere found (Helmig et al., 2007d). Analysis ofturbulence data for surface layer stability andmixing layer depth has shown highly variable andrapidly changing boundary layer conditions atSummit. During the summer months, averagemidday sensible heat fluxes were �10Wm�2 andresulting convective mixing regimes drove a diur-nally growing mixed boundary layer (Cohen et al.,2007). These conditions are contrasted by observa-tions from South Pole, where December sensible

heat fluxes were variable, remained negative forextended periods of time, and were � �2Wm�2 onaverage (median value for December 2003). AtSouth Pole stable surface layer conditions can besustained for several days under low wind speedconditions (Neff et al., 2007).

3.4. Radionuclide tracer comparisons

The activity of the radionuclide tracer 7Be hasbeen shown to be a selective tracer for air ofhigh troposphere/low stratosphere origin. Vertical

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mixing above Summit and its seasonal and high-frequency variability has been studied in detail using7Be as well as 210Pb, which, in contrast to 7Be, is anindicator for continental transport (Dibb, 2007).Averages of monthly median 7Be as well as7Be/210Pb with the monthly averages of the diurnalozone changes are combined in the same graph inFig. 8. Both, diurnal ozone changes and 7Be activityshow coinciding seasonal minima (February) andmaxima (mid- to late summer) as well as similarannual amplitudes, however it is apparent that 7Beshows a 2–3 months sooner and more rapid declinethan the ozone amplitudes. Besides stratospherictransport events 7Be signals are also modulated bydown mixing into the surface layer. The observedcorrelation between surface ozone and 7Be may thusonly indicate the mixing efficiency into the surfacelayer and not necessarily a common source. A morerobust indicator is the 7Be/210Pb ratio (Wagenbachet al., 1988). These data, which are also included inFig. 8, also correlate well with the ozone ampli-tudes. This comparison further supports the sug-gested connections between ozone transport eventsand 7Be and its sources.

The seasonal dependency of this relationship isfurther investigated in Fig. 9. A relative ‘ozoneenhancement’ over the seasonal, lower altitude,polar NH ozone background was first calculatedby subtracting from the hourly Summit data themean of the seasonal ozone at Barrow and

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50

60

0 50 100 150 200

7Be (fCi m-3)

Delta O

3 (

ppbv)

Fig. 9. Comparison of the relative enhancement of ozone at Summit (d

(mean of Barrow and Zeppelinfjellet)) and concurrent measurements of

right graph (b) shows the regression line slopes (with 95% confidence

Zeppelinfjellet (Fig. 6). The 7Be data typicallyrepresent a 2-d integrated average sample. Thehourly ozone data were matched with the 7Besampling periods and the maximum ozone valuesduring the 2-d radionuclide sampling were corre-lated with the 7Be activity (Fig. 9). Although thedata pairs are somewhat scattered, these twoparameters show a statistically significant (P 495%) correlation with a regression line slope of0.1170.04 ppbv fCi�1m3). The right side panelshows a monthly regression line slope analysis ofthe 2004 data. During all months of the yearregression line slopes are positive, indicating thatenhanced ozone events are correlated with 7Be.During 7 months, this correlation is statisticallysignificant at P495%. The same correlation analy-sis was done between the enhanced ozone eventsand the activity of the continental tracer 210Pb.These 2 data series were found to have no statisticalcorrelation with an R2 coefficient of 0.0018; theregression line slope was 0.2370.81 ppbv fCi�1m3.

3.5. Synoptic ozone transport

Comparison of the Summit ozone record withdata from Westman Island (located at the southerntip of Iceland) and trajectory analysis were used toidentify a transport event that brought polluted airwith elevated ozone from continental Europe toSummit. On 12 August, a sudden ozone increase of

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ifference between ozone at Summit and the seasonal background7Be. All available 2004 data are included in the left panel (a). The

intervals) for the monthly analyses.

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Fig. 10. Surface ozone at Westman Island, Iceland (a) and Summit (b), Greenland during August 2004. Air transport, as derived from

FLEXPART simulations, to Summit during the four episodes marked with letters A–D in the Summit graph is shown in Fig. 12.

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01 03 05 07 09 11 13 15 17 19 21 23 25 27 29 31

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7B

e (

fCi

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)

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b (

fCi

m-3

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7Be med

210Pb

210Pb med

Fig. 11. Radionuclide tracer measurements at Summit during August 2004. The staggered horizontal lines indicate the 2004 annual

median 7Be and 210Pb values.

D. Helmig et al. / Atmospheric Environment 41 (2007) 5031–5043 5039

�25 ppbv, lasting �1.5 d was observed (Fig. 10(b))at Summit. Ozone peaked at 75.0 ppbv at 10:00 h on12 August, which was the second highest readingduring 2004. Similarly enhanced ozone was ob-served in the ozone record from Westman Island,which is 1200 km distant from Summit for anextended period on 9–15 August (Fig. 10(a)). AtSummit elevated ozone readings began on 11August, are interrupted on 13–15 August, and

reappear on 16–17 August. The radionuclidemeasurements during this period (Fig. 11) show adrop in 7Be and a concomitant increase in 210Pbbetween 10 and 12 August. Both of these trends areindicative of increased transport from a continentalregion. The intermittent drop in ozone during 13–14August is paralleled by a temporary decrease in210Pb and a rise in 7Be, probably due to anintermittent change in air transport. What follows

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are another 3 d of elevated ozone with enhanced210Pb tracer levels. It is remarkable that both 7Beand 210Pb remained above their annual mediumvalues (47.7 and 2.4 fCim�3, respectively) duringthis mid-August episode, which shows the effectiveventilation/turnover of the Summit surface layerduring this time.

The back-trajectory analysis and FLEXPARTcalculations further expand upon the radionuclideobservations. The trajectory data show strong flowfrom the northern coast of Western Europe andEngland reached Greenland in about 4 d andascended the Greenland ice sheet to Summit fromthe S/SW on 12 August. The trajectories to Summitat 500m height on 12 August pass near Iceland, butthe near-surface source of air bringing high ozone toIceland originates a bit further east on the Balticcoast. During this period recorded winds at Summitwere predominantly from S to SW with sustainedwind speeds of 8–10m s�1 in the surface layer (windspeed at 10m height). Back trajectories for 13August, 6:00 h to 15 August, 12:00 h show that airarriving at Summit originated in northern Canadaduring this time. On 15 August, 18:00 h backtrajectories switch back to England and Western

Fig. 12. FLEXPART calculations of column-integrated source–recepto

four periods (A–D) correspond to the ozone minima and maxima as in

function to emission input. The units of this analysis reflect the verticall

Europe. Similar conclusions were obtained fromFLEXPART. Fig. 12 shows the source–receptorrelationship for 4 selected 3-h periods that corre-spond to 2 of the ozone minima and 2 of themaxima in Fig. 10(b). These graphs illustrate thatair arriving at Summit during the ozone peaks on 12August as well as on 16 August had strong mid- andeast-European influence whereas the intermittentminima on 10 and 13 August were associated withair transport from the North Canadian Arctic. It isof interest to note that for the first ozone peakFLEXPART shows that most of the transport toSummit passed right over southern Iceland, inagreement with the delay in the ozone peaksobserved at Westman Island and at Summit (Fig.10). In contrast, for the 16 August ozone event,which is only evident at Summit but not in theWestman Island data, the center of the trajectoriespasses �1000 km further south with little impact onsouthern Iceland.

FLEXPART shows significant increases inanthropogenic SO2, NO2 and CO tracers at Summitduring 11–12 and 15–16 August that resulted fromEuropean emissions that had been transported withthese air flows over 4–8 (11–12 August), respectively

r relationships for four 3-h time windows in August 2004. These

dicated in Fig. 10(b). The units of this analysis reflect a sensitivity

y integrated residence time at a given location (Stohl et al., 2005).

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over 7–11 d (15–16 August) to Summit. Takentogether, radio-nuclide data, surface wind data,trajectory and FLEXPART analyses provide aconclusive explanation that continental air with ahigh loading of anthropogenic air pollutants,transported over 6–10 d from Central to WesternEurope, is the likely cause for these mid-August10–20 ppbv-enhanced ozone events at Summit.

3.6. Photochemical ozone production

Recent publications have investigated photoche-mical oxidation chemistry over the glacial snow atSouth Pole and Summit, respectively. These studieshave concluded that significant photochemicalozone production would be expected under theenhanced levels of oxidized nitrogen species andother photochemically important compounds thatare present in the surface layer during the sunlitperiods (Dibb et al., 1998; Davis et al., 2001, 2004).Modeled interquartile ozone production rates wereon the order of 2–3 ppbv d�1 and 3–5 ppbv d�1 forSummit (Yang et al., 2002) and South Pole(Crawford et al., 2001; Chen et al., 2004), respec-tively. Since this ozone production is driven byphotochemistry, production rates and ambientdiurnal ozone cycles are expected to maximizeduring and shortly after solar noon. However,analysis of diurnal ozone cycles at Summit did notshow any significant increases in ozone duringnoon/afternoon. It has been pointed out thatphotochemical ozone production in the surfacelayer may possibly be offset by a diurnally variableand photochemically driven ozone deposition rate(Helmig et al., 2007a). Ozone production rates onthe center of the Greenland ice sheet in the range ofthe model results would not be expected to make astrong contribution to the ambient ozone levels atSummit, as transport times of air moving acrosscentral Greenland seldom exceed 1–2 d, which is tooshort for a large ozone buildup of locally andphotochemically produced ozone. During summer,solar irradiance at night drops to �1/10 ofnoontime values, which will shorten the overalldiurnal ozone production period. The balloon datado not show ozone enhancements in the surfacelayer, where ozone production is expected to be thehighest. Consequently, much in contrast to SouthPole, local ozone production does not appear tohave an important contribution to the high ozonelevels at Summit.

4. Conclusions

The analyses of processes that produce thevariability of ozone at Summit provide a pictureof the various contributors to the overall ozonedistribution in the NH free troposphere. Summit, atan altitude of 3212m appears to be quite represen-tative of free tropospheric ozone amounts in the NHpolar regions based on a comparison with ozonesonde profile measurements at Alert and Resolute.These data also emphasize the strong asymmetry inthe tropospheric ozone distribution between the NHand SH; substantially higher ozone amounts areobserved in the NH, particularly in the mid-tropo-sphere.

Sunrise ozone depletion events that are a char-acteristic feature in the surface ozone records fromNH polar coastal stations are not observed atSummit. The lack of this springtime ozone sink isestimated to elevate the annual mean ozone atSummit by up to �3 ppbv over the mean ozone atthe lower altitude, coastal locations. Ozone surfacedeposition rates over the glacial ice sheet and thesurrounding oceans are expected to be low year-round, causing reduced mixed boundary layerozone deposition losses in air that is transportedto Summit.

Transport of air with elevated ozone from thehigh troposphere/low stratosphere is an important,year-round process that contributes to surface-layerozone enhancements. The frequency of transportevents maximizes during spring and summer and islower during the late fall and winter period. Theidentification of several events that brought pol-luted, continental air with enhanced ozone fromwest/central Europe to Summit indicates thatsynoptic transport of polluted air plays a role inthe Summit ozone budget. However, the correlationanalysis of enhanced ozone events with two radio-nuclide tracers shows a higher correlation withupper troposphere/lower stratospheric transportthan with continental air.

Neither pollution transport events nor transportfrom the stratosphere have been found to be animportant ozone source at South Pole. The diurnalirradiance cycle at Summit and the smaller size ofthe Greenland ice sheet (compared to Antarctica)foster more spatially and temporally heterogeneous(less stable) boundary layer conditions, which isdriving a more vigorous gas exchange between thesurface layer and the free troposphere. Theseconditions reduce photochemical ozone production

ARTICLE IN PRESSD. Helmig et al. / Atmospheric Environment 41 (2007) 5031–50435042

regimes as observed at South Pole. In summary, ouranalysis implies much different ozone sources andreasons for enhanced ozone conditions at Summitthan those that are of importance at South Pole.

Acknowledgments

This research was supported through the UnitedStates National Science Foundation (Office of PolarPrograms, Grants #9530579, 9813312, 0137538 and0240976). L. Cohen and F. Bocquet (University ofColorado, Boulder) helped with the preparation ofsome of the figures. K. Steffen (University ofColorado, Boulder) provided automated weatherstation data for Summit that were included in thecolor contour plots. We thank A. Stohl, NorwegianInstitute for Air Research (NILU) for makingavailable the FLEXPART trajectory analysis. Lo-gistical support for field work was provided byVECO Polar Resources and the US 109th AirNational Guard. We thank the Danish Commissionfor Scientific Research for granting permission toconduct this research at Summit.

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