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Atmospheric Environment 42 (2008) 2817–2830
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Nitric oxide in the boundary-layer at South Pole during theAntarctic Tropospheric Chemistry Investigation (ANTCI)
Detlev Helmiga,�, Bryan J. Johnsonb, Matt Warshawskyc, Thomas Morsea,William D. Neffb, Fred Eiseled, Douglas D. Davise
aInstitute of Arctic and Alpine Research (INSTAAR), University of Colorado at Boulder, Boulder, CO 80309-0450, USAbEarth System Research Laboratory, National Oceanic and Atmospheric Administration (NOAA), Boulder, CO 80305, USA
cAzeoTech Inc., Ashland, OR 97520, USAdNational Center for Atmospheric Research, Boulder, CO 80307, USA
eGeorgia Institute of Technology, Atlanta, GA 30332, USA
Received 16 November 2006; received in revised form 22 March 2007; accepted 26 March 2007
Abstract
The vertical distribution of nitric oxide (NO) was investigated by profiling from a tethered balloon platform during the
2003 Antarctic Tropospheric Chemistry Investigation (ANTCI) at South Pole (SP), Antarctica. The lower atmosphere was
probed between the surface and 120m height by pulling air from an inlet attached to the balloon through a thin-wall,
135m-long Teflon sampling line and by analyzing NO in this airflow with a ground-borne monitor. Losses and conversion
of NO during the 2–4-min residence time in the sampling line were on average on the order of 6–16%, providing a feasible
approach for the measurement of vertical NO profiles under SP conditions. NO was found to be highly variable within the
lowest 100m of the atmosphere. Greatly enhanced NO mixing ratios were constrained to a shallow (20–50m height) air
layer nearest to the surface, above which NO rapidly dropped to its mixed boundary layer background levels. Concurrent
measurements of ozone and meteorological conditions provide insight into linkages between the ongoing snowpack and
boundary layer nitrogen oxides (NOx ¼ NO+NO2) and ozone chemistry. Since [OH] and [HO2] are non-linearly coupled
to absolute levels of NOx, their concentrations and the rate of ozone production are expected to similarly show appreciable
changes on small vertical scales during conditions with enhanced [NOx].
r 2007 Elsevier Ltd. All rights reserved.
Keywords: Antarctic plateau; Snowpack–atmosphere gas exchange; Snow photochemistry; Tethered balloon profiling; Nitric oxide; Ozone
1. Introduction
Atmospheric chemistry at the South Pole (SP) hasbeen found to deviate from other polar sites in anumber of important aspects. Foremost, ambient
e front matter r 2007 Elsevier Ltd. All rights reserved
mosenv.2007.03.061
ing author. Tel.: +1303 492 2509;
6388.
ess: [email protected] (D. Helmig).
concentrations of nitric oxide (NO) are much higherthan at other polar locations (Davis et al., 2001,2004). NO mixing ratios in excess of 500 pptv havebeen recorded during the austral summer at SPnumerous times. Further experiments, including studiesin the Arctic, have revealed insights into sourcesand sinks of nitrogen oxides (NOx¼NO+NO2)in the polar environment. Photodenitrification ofnitrate in sunlit snow was shown to be a primary
.
ARTICLE IN PRESSD. Helmig et al. / Atmospheric Environment 42 (2008) 2817–28302818
source for NO formation (Honrath et al., 1999,2000a, b). This process results in positive, upwardsfluxes of NO out of the snow (Jones et al., 2001;Honrath et al., 2002; Oncley et al., 2004). ReportedNO surface fluxes at SP, although somewhat higher(1.5–3 times), were still of similar magnitude as atSummit, Greenland and Neumayer, Antarctica.However, resulting ambient air NO concentrationsat SP have been noted to be significantly higher thanat other polar sites. Factors that have beenidentified to contribute to the unique NO buildupat SP include the presence of 24-h, un-modulatedsunlight and the fact that SP is located at the base ofa large drainage field on the edge of the Antarcticplateau. Under these conditions, NO can accumu-late in air which is transported downslope overseveral days to the SP. Enhancements in NO areparticularly linked to episodes with suppressedatmospheric mixing (e.g. increased atmosphericstability and decreased wind speed) in the surfacelayer. Finally, quite unique in a remote environmentsuch as the SP, is the presence of non-linear NOx
chemistry, where the lifetime of NOx is found to bedependent on the concentration of NOx (Daviset al., 2004).
Besides high NO levels, other gases involved intropospheric radical cycling were found to beanomalously enhanced. 24-h average hydroxylradical concentrations were on the order of2.5� 106 molecules cm�3 (mean value, Mauldinet al., 2004). Under these elevated NOx and HOx
(HO+HO2) conditions chemical HOx–NOx cyclingresults in significant production of ozone (Crawfordet al., 2001; Chen et al., 2004; Helmig et al., 2007), arather unexpected condition for this pristine polarenvironment.
Previous measurements of NOx at SP have beensurface-based, with sampling line inlets outside ofthe Atmospheric Research Observatory (ARO)building or mounted at several heights on anadjacent tower for flux gradient measurements.These previous measurements have not allowedgauging the complete vertical distribution of NOx
relative to the atmospheric boundary layer (ABL).In fact, earlier estimates of the ABL depth (Oncleyet al., 2004) suggested depths greater than 40m andas high as several hundred meters, well beyondtower heights at SP. In addition, given the fact thatNO is emitted from the snow, that concentrationbuildups have always been seen during conditionswith suppressed vertical mixing, and that theaverage NOx lifetime is estimated at 10 h (Davis
et al., 2004), sharp vertical gradients would appearlikely to develop over the snow surface. As notedabove, with the lifetime of NOx increasing atconcentrations above �200 pptv, rapid enhance-ments in the levels of NOx are further promotedwhen ABL depths are shallow.
These questions motivated the desire for detailedvertical profile measurements of NO, preferablywith high resolution near the surface and under thewide range of surface layer conditions that areencountered at SP, and particularly during times ofsurface layer NO enhancements. To achieve this,tethered balloon measurements appeared to be apromising approach, as this technique allows for theprobing of the lowest layers of the atmosphere athigh temporal and vertical resolution when operat-ing from a fixed site. In the Antarctic TroposphericChemistry Investigation (ANTCI) 2003, these bal-loon profiles could also be compared againstvertical profiles of NO derived from Twin Otterobservations over the plateau (Davis et al., 2007).
The range of conditions under which tetheredballoons can be operated recently has been signi-ficantly widened with the invention of Sky-Docballoon platforms, which combine properties ofballoons and kites. These platforms can be operatedunder conditions of calm winds as well as underhigh wind conditions, when most blimp systems fail.However, gas measurements from tethered balloonsrequire light-weight, battery-powered instrumentpayloads. There are only a few gases that can bemeasured with in situ, balloon-borne instrumentsthat meet these requirements. The most frequentapplications are for water vapor by capacitancemeasurement and for ozone using electrochemicalozone sondes (e.g. Helmig et al., 2002; Acevedoet al., 2004; Johnson et al., 2007). Vertical profilesfor other species have also been obtained bycollecting whole air samples for later analysis ofe.g. CO2 (Kuck et al., 2000), volatile organiccompounds (VOC) (Greenberg et al., 1999 andreferences therein), SO2, NO, NO2 and ozone (Chenet al., 2002). Target analytes from air collected ataltitude have also been concentrated on solidadsorbent tubes with programmable balloon-bornesamplers (Helmig et al., 1998; Greenberg et al., 1999)for later analysis in cases where analytical instrumen-tation itself was too heavy and too power-consumingto be deployed on the balloon itself. Measurements ofNO2 and NOx have been accomplished with aballoon-borne luminol-chemiluminescence instrument(Glaser et al., 2003). This instrument, weighing about
ARTICLE IN PRESSD. Helmig et al. / Atmospheric Environment 42 (2008) 2817–2830 2819
12 kg, was raised to a height of 800m with a 54m3
tethered balloon.For the anticipated conditions at SP none of the
published balloon techniques appeared feasible formeasurement of vertical NOx profiles since: (1) theuse of smaller payload/size balloons was highlydesirable (expenses for balloon operation increaseexponentially with balloon size, particularly underthe extreme logistical and weather conditions at SP);and (2) the expected sensitivity of these previouslyemployed NOx measurement techniques would havebeen insufficient to measure the full range of NOconcentrations expected at SP (e.g., at timeso10 pptv). These constraints motivated us to investi-gate the feasibility of sampling air with a longsampling line, with an inlet attached to and raisedwith the balloon and with analysis of NO by aground-based, high sensitivity NO+ozone chemi-luminescence analyzer. This manuscript provides adescription of this unusual sampling approach andof the findings obtained from this experiment duringANTCI.
2. Experimental
2.1. Site description
The tethered balloon experiment was conductedat the Amundsen–Scott research station at SP from10–31 December 2003. The launch site was �300meast (sector) of the geographic SP, �130m southeast(sector) of the ARO building and�200m east (sector)of the ANTCI sodar measurements reported by Neffet al. (2007). The NO vertical profile experimentswere performed between 17 and 28 December 2003(Day of Year (DOY) 351–362).
2.2. Tethered balloon platform
Two helium-filled Sky-Doc tethered balloons(Floatograph Technologies, Marion, IN) were usedas profiling platforms. Detailed technical informa-tion on the balloon systems as well as theinstrumentation used for supporting measurementsof meteorological conditions and ozone are pro-vided elsewhere (Helmig et al., 2002, 2007; Johnsonet al., 2007).
2.3. Long sampling line experiments
Vertical NO profile measurements were per-formed with a long sampling line from an air inlet
that was attached to the balloon at one end and acustom-built NO chemiluminescence instrument atthe other. This tubing was made of light-transparentPFA Teflon (0.78 cm o.d., 0.64 cm i.d., 135m length,McCoy, Fort Collins, CO) and was equipped with aPFA inlet funnel (Savillex Corp., Minnetonka,MN), which housed a PTFE (polytetrafluoroethy-lene) membrane filter (Millipore Corp., Bellerica,MA). The Teflon tubing and the inlet filter wereconditioned in the laboratory before the field trip bypurging with 250 ppbv of ozone-enriched air for twodays. The inlet was attached 6m below the balloonto the tether line. The slack of the sampling tubingwas laid out on the snow surface along a straight,�50m flagged line at times when the balloon wasnot at maximum profiling height.
Air was pulled through the sampling line con-tinuously while the balloon was raised and loweredto a maximum altitude of �120m. The surface endof the sampling line ran into a temporary, balloonlaunch building that was located right next to thewinch. The sampling line was connected to amanifold that allowed sampling of air with eitheran ozone monitor (Thermoenvironmental Corp.(TEI) Model 49C, Waltham, MA), the NO monitor(see details below) or with both instrumentssimultaneously (which was done during most times).The sampling flow rate was determined by thesampling pumps of these two analyzers and was�1.2 Lmin�1 (TEI) or 2.4 Lmin�1 (both instru-ments combined). Under these conditions thesample residence time in the sampling line wascalculated at 4.2 and 2.1min, respectively. Thisresidence time was confirmed by measuring thedelay time between sample injection and signaloccurrence with NO-spiked sample air. Betweenballoon flights a shorter sampling line (0.64 cmo.d.� 0.48 cm i.d.� 10m length PFA) and the longballoon line inlet were placed side by side on a 2-m-high tower upwind of the winch and balloonbuilding, and sample air was alternated betweenthese two inlets every 5min. These measurementswere used to continuously monitor the loss andconversion rates of NO in the long sampling line.
2.4. NO measurements
The chemiluminescence method used to measureNO is a well characterized technique that has beenintercompared successfully with other techniques onseveral occasions (Hoell et al., 1987, Gregory et al.,1990). The technical configuration of the instrument
ARTICLE IN PRESSD. Helmig et al. / Atmospheric Environment 42 (2008) 2817–28302820
was similar to the one described in our earlierpublications. Calibration involved the use of stan-dard addition of NO and was performed once a day.Artifact tests using zero air were also carried outonce each day. The 2s detection limit for a 1-mintime resolution was 5 pptv. Data were acquiredevery 5 s and averaged to 1min means. Theaccuracy and precision of the 1-min surface NOmeasurements are estimated to be on the order of710%. Due to uncertainties in the applied corre-ctions, balloon profile data from the long samplingline measurements (see more discussions below) areestimated to have an accuracy of 715–20%.
2.5. Data analysis
Instantaneous balloon heights during profilingexperiments were calculated by the pressure changemeasured with a pressure sensor attached to theballoon and referenced to the average ‘‘beforelaunch’’ pressure, while descent balloon heightcalculations were referenced to the surface pressuremeasured after completion of the descent profile.Using the balloon height values the vertical balloonprofile was visually associated with the NO data andwas corrected for the time delay in the samplingline. The height values were then associated with therespective NO mixing ratios (after correction forsampling line losses, see below) to construct thevertical NO profiles. Uncertainties in the heightdetermination are estimated to be 5m. Missing NOvalues at certain heights were filled in by interpola-tion between available data points to yield 1-mresolution vertical NO profiles. Balloon and the
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357.5 357.6 357.7
Day of Y
NO
(pp
tv)
Fig. 1. Comparison of 12 h of NO measurements (5min data) by altern
with both inlets located side-by-side at 2m above the surface.
surface time series data were combined in colorcontour plots applying a smoothing algorithm.
3. Results and discussion
3.1. Sampling line tests
Under the used sampling flow rate and tubingdimensions air is transported from the inlet to theinstrument within �2.1min. During this time, NO isexpected to react with ozone in the sample air with arate constant of k ¼ 7.1� 10�15 cm3molecule�1 s�1
at �25 1C (Atkinson et al., 2006). Thus, given an O3
concentration of 40 ppbv, we calculate that as muchas 50% of NO could be converted to NO2 duringthe transport from the inlet to the monitor providedthere was no photolysis of NO2 during this sametime period. The actual conversion rate wasexperimentally examined by placing the inlet ofthe long line side-by-side with the shorter line on the2-m tower. The sample residence time through theshorter tubing was �5 s; yielding a theoreticalNO-NO2 conversion rate of 3%. Air flow wasalternated from these two inlets every 5min betweenballoon profiles (altogether, about 120 h of thesemeasurements were obtained). An example of 12 hof observations of this type is shown in Fig. 1.A drop in signal was observed whenever sample waspulled through the long line. The relative loss in theNO signal (e.g., long line vs. short line) was found tobe quite reproducible, and for the data shown is onthe order of 10–15%. This change is clearly less thanexpected based on the assumption of no regenera-tion of NO from NO2 photolysis, as the expected
357.8 357.9 358.0
ear 2003
Short Surface Line
Long Balloon Line
ating the air flow through the long sampling line and a short line
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ratio of NO from the short line versus long linesampling (that is derived from the sample residencetime calculation) would be �2 (97%/50%). Themuch lower loss of NO, not surprisingly, points tothe transmission of significant NO2 actinic fluxthrough the thin-wall Teflon tubing and to aregeneration of NO from photodissociation ofNO2 during transport through the sampling line.As detailed above, the sampling line was made ofthin-wall (wall thickness 0.7mm) light-transparentTeflon and it was placed on the snow surface whereit was exposed to solar radiation and light reflectedfrom the snow. We conclude that photostationaryconditions inside the sampling line were such that amuch smaller fraction of NO was converted to NO2
during transport from the balloon inlet to themonitor than what would be expected during darkconditions. These tests also give an upper limit forthe overall loss of NOx in the long line. If NO2
would have been lost at a higher rate than NO, thenmore loss of NO would have been expected due toNO equilibrating with NO2. Consequently, thesedata show that the overall loss of NOx in the longline must have been within or below the overallobserved reduction seen in the NO signal. All datafrom the side-by-side comparison were investigatedwith respect to the dependency of the NO conver-sion rate towards the ozone mixing ratio in the airsampled. A regression analysis of these datademonstrated that the loss rate for NO wasdependant (at P499.9%) on the amount of ozonein the collected air sample, with the regression lineshowing a 6–16% reduction in NO at 20–50 ppbv.As a result of these tests, all NO data collected
0
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Day of Yea
NO
x (p
ptv)
Fig. 2. Nitric oxide and ozone in the surface layer (at 4m (DOY 350.0–
December 2003.
through the long sampling line were adjusted usingthe measured level of ozone and the regressionequation; the calculated correction factor increasedfrom 1.07 at 20 to 1.20 at 50 ppbv ozone. The NOloss in the short line was presumed negligible giventhe much shorter residence time in this tubing andtherefore this correction approach was not appliedto the short line data.
3.2. Surface NO
The time series for ambient NO at ground-levelduring the time period of the balloon experiment isshown in Fig. 2. An episode having substantiallyenhanced NO did occur during the period of theballoon operation during ANTCI. While ambientNO remained below the 100 pptv range during thefirst and last few days of this experiment, duringDOY 354–359 (20–25 December) substantial in-creases, reaching in excess of 600 pptv were ob-served as winds decreased below 3m s�1 for thisextended period. Since several similar 500–600 pptvevents have previously been reported (Davis et al.,2001, 2004, 2007), the fluctuations seen in the datashown in Fig. 2 can be viewed as typical of what canoccur during the summertime at SP.
3.3. Tethered balloon data
A total of 34 vertical tethered balloon profiles(ascent and descent) of NO were collected duringDOY 351–362. These continuous up and downprofiles were supplemented by several experimentswhere the inlet was raised in �15m steps and
358 359 360 361 362 363 364
r 2003
0
10
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40
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O3 (
ppbv
)
Surface NO
Surface O3
357.2m) and 2m (DOY 357.2–364.0) at South Pole during 15–29
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AscentDescent
Dec. 25 (DOY 359.44)
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NO (pptv)
Hei
ght (
m)
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100
120
0 100 200 300 400
NO (pptv)
Hei
ght (
m)
AscentDescent
Dec. 20 (DOY 354.99)
Fig. 3. Vertical profiles (illustrated profiles are 5-point running means) of NO as measured with the tethered balloon long sampling line
experiment during the period of enhanced NO buildup (left, DOY 354.99, 20 December) in comparison to an episode when NO surface
layer mixing ratios were much lower (right, DOY 359.44, 25 December). Indicated flight times are the apex times (balloon at maximum
height). Both graphs show data from the balloon ascent and descent, which were on the order of 30min apart.
D. Helmig et al. / Atmospheric Environment 42 (2008) 2817–28302822
‘parked’ at a given height for 5–10min, as well as byan experiment where the long sampling line inletwas kept at 110m height for 3 h with alternatingsampling between this height and the surface. A fewselected data examples will be given here toillustrate the conditions and variability of thevertical NO distribution.
Vertical NO profile data obtained during 20–21December (DOY 354–355) and 25 December(DOY 359) are shown in Fig. 3. For both days,good agreement between the ascent and descentdata was seen. On 20 December, NO mixing ratioswere largely enhanced near the surface. NOdropped monotonically with height, most dramati-cally in the 0–40m layer. At 100m height, less than1/8 of the surface concentrations remained. A muchdifferent condition was encountered 5 days later on25 December. Here, NO mixing ratios were muchlower (�90 pptv) and NO was homogenouslydistributed throughout the limited portion of theABL that could be sampled with the tetheredballoon system.
The conditions leading to a large gradient of NOin the ABL were further investigated in theexperiment shown in Fig. 4. Over 3 h, every 5minthe sampling air flow was alternated between theshort line, 2-m-tower inlet and the long line inlet onthe balloon, which was raised to 110m. NO nearthe surface fluctuated between 150 and 280 pptv(dropping towards lower levels by the end of thisexperiment). During this same time, NO at 110mwas less variable and remained much lower, on the
order of 30–50 pptv. These data show that the largefluctuations seen in the surface data (Fig. 2) likelyreflected conditions in the very lowest layer of theatmosphere (first few tens of meters above thesurface) only, while at heights above 50m muchlower and less variable NO levels were encountered.This conclusion is further manifested by the analysisshown in Fig. 5, where all available surface data andballoon profiles were included in a color contourdisplay of NO mixing ratios between the surface and100m height. Most of the NO concentrationchanges are constrained to a thin surface layer,about 20–50m deep. It is interesting to note that thethickness of the layer with enhanced NO concentra-tions slowly increased from DOY 355–358; thecalculated growth rate of the layer with enhancedNO over this time was �0.35mh�1. Please note thatduring DOY 357.0–357.4, winds were low(1.5–2.0m s�1 at 13m) and wind direction (sector110–1211) was slightly from outside of the clean airsector (defined as sector 340–1101). A modestenhancement over background levels seen in theSO2 data (recorded at the ARO) and some spikesobserved in the NO data suggest that data recordedduring this period may have had a modestcontribution from camp operations.
3.4. Which conditions cause high NO?
A series of unique conditions at SP have beennoted to foster the buildup of high NO in thesurface layer (see discussion in the introduction
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Balloon Inlet
Balloon Inlet Height
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2nd BalloonLaunch
2 Inlets Side-by-Side
2 Inlets Side-by-Side
Fig. 4. Approximately 4 h of alternating NO measurements from two inlets. First, two inlet lines were sampled side-by-side near the
surface. Next the long sampling line inlet was lifted to 110m and air was alternated between the raised balloon inlet and the tower inlet
every 5min. The balloon pressure sensor ran out of battery power �20min after reaching 110m. After �3 h, the balloon was brought back
to the surface, equipped with a new battery and another vertical profile was measured with continuous sampling (instead of alternating)
from the balloon inlet. After the completion of that profile, air was again alternated between the long and short sampling line.
Fig. 5. Vertical and spatial distribution of NO in the surface layer at SP during 17–28 December 2003. Thirty four vertical NO balloon
profiles and continuous surface layer measurements were combined in this graph. The distribution of individual balloon and surface data is
illustrated by the dotted black lines. Please note that on occasion large gaps in vertical balloon profile data (up to 1–2 days) were
interpolated by the contour plotting algorithm (IDL). Given the sparseness of data in the interpolated regions, resulting descriptions of the
NO distribution will have a much reduced certainty in such cases. The dotted white data time series depicts the atmospheric boundary
layer (ABL) depth (derived from the sodar measurements) during those times when the ABL dropped below 100m.
D. Helmig et al. / Atmospheric Environment 42 (2008) 2817–2830 2823
section). Besides the surface and tethered balloonmeasurements of NO, the ANTCI experiments alsoyielded vertical profile observations of ozone,meteorological parameters, and estimated mixinglayer heights from sodar soundings (Neff et al.,2007). Ozone measurements were performed both
with lightweight balloon-borne electrochemicalozone sondes as well as by monitoring ozone inthe air flow collected during the long-samplingline experiment. Therefore, overall more frequentozone profiles were obtained and ozone data extendto higher altitudes (500m above the surface)
ARTICLE IN PRESSD. Helmig et al. / Atmospheric Environment 42 (2008) 2817–28302824
(Helmig et al., 2007). Analyses of these data providefurther insight into the conditions that cause thesituations with elevated NO in the surface layer.
Time series data of NO and ozone surfacemeasurements are compared in Fig. 2. These dataseries show some noteworthy similarities anddifferences. The first notable 10–12 ppbv increasein ozone on DOY 352–353 did not show aconcurrent change in NO. In contrast, the increasein ozone on DOY 354 is followed by a steep rise inNO. While ozone increased from �19 to �41 ppbvwithin 10 h, NO climbed from �30 to �250 pptv.For further interpretation inferred boundary layerheights from the sodar measurements were includedin Fig. 5. These data show that during the firstozone increase (DOY 352–353) boundary layerheights dropped briefly o50m, but remained4100m during most of this period. In contrast,during the second, more steeply and longer lastingincrease in ozone (DOY 354–359) the increase inozone was paralleled by a rise in NO and by theboundary layer height dropping from the previously4100m to lower depths of 20–40m. The transitionperiod is shown in more detail in Fig. 6, where NO,ozone, and boundary layer depths are plottedtogether in the same graph. Early in the day, themixing layer extended beyond the range of the sodar(4180m). As the boundary layer stability started toincrease (as reflected by the decreasing boundarylayer depth) ozone increased almost monotonically.It is striking that NO did not increase until theboundary layer collapsed to less than 50m, whichoccurred about 6 h later. Subsequently, ozone
Fig. 6. Atmospheric boundary layer (ABL) depth as inferred from sod
(asterisk) during the transition to the high-NO episode on DOY 354.
leveled out in the 40–50 ppbv range over the next4 days while NO continued to increase. Thedistinctly different behavior of NO and ozone canalso be seen in the vertical profile data for DOY 355(Fig. 7). Here, high NO values are confined to ashallow mixing layer (o40m) within a shallowsurface inversion whereas ozone is nearly well-mixed to the base of a second, elevated inversionlayer, which was present just below 200m. Ofinterest in the ozone profile is also the slightenhancement of a few ppbv in the lowest few tensof meters coincident with the higher levels of NO.Above 200m, ozone falls off to levels that are moretypical for background conditions over the SP. Atthe end of the episode, NO mixing ratios begandropping rapidly around noon on DOY 358 inresponse to increasing wind speed and a deepeningof the mixing layer. However, surface ozoneremained elevated for another 12 h. The differencesin the vertical scales of the enhancements of NO andozone over the entire period are also evident fromthe comparison in Fig. 8. Here, we show an excerptof the longer ozone data set presented in Helmiget al. (2007) to illustrate the vertical and temporaldistribution of ozone during the period of con-current ozone and NO measurements. In comparingthese two graphs, it is striking that the depth of thelayer with increased ozone is consistently deeperthan that for NO. While NO levels drop steeplywithin the lowest 50m, ozone enhancements wereobserved throughout the 200–300m layer. Theseobservations are consistent, however, with the factthat the lifetime of ozone is �60 times longer than
ar measurements (filled circles), ozone (open diamonds) and NO
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Fig. 7. Behavior of NO and ozone (03:00–04:30 h, DOY 355 (21 December)) as a function of mixing layer and inversion layer depth during
the high-NO episode. The depth of the boundary layer, as inferred from the sodar soundings is illustrated by the shaded area in the left
graph. The ozone enhancement within this layer was estimated by extrapolating the slope of the ozone profile within the well-mixed
portion of the ozone profile to the ground level (shaded area in the center graph).
D. Helmig et al. / Atmospheric Environment 42 (2008) 2817–2830 2825
that for NOx (see more detailed discussion of thispoint in text below).
3.5. Meteorology– chemistry connections
Balloon data of temperature, wind speed andwind direction were used to calculate verticalRichardson number profiles. These calculationsshowed that highly stable mixing conditions, causedby low wind speeds, lack of wind sheer and strongsurface layer temperature inversions, prevailedduring DOY 354–359. There were sustained sunnyconditions with no cloud cover during these days.Richardson numbers 40.1 extended through theentire 500m column that was probed with theballoon. ABL heights (Fig. 5) during the period ofenhanced NO were consistently between 20 and40m. Neff et al. (2007) concluded that verticaltransport through this 20–40m deep boundary layercould be described by diffusion rates, with estimatedtime scales on the order of 6–12 h typically.
Davis et al. (2004, 2007) have previously shownthe inverse relationship between atmospheric in-stability, inferred boundary layer height and NOconcentrations and chemistry. Previous publicationshave also investigated chemical sources of NOx and
the role of NOx in HOx chemistry at SP. Theseinterpretations have been based upon availablesurface layer observations of radicals, and concen-trations of their precursors (most importantly O3,CH2O, and H2O2), which, due to snowpack emis-sions (for the latter two), are enhanced in thesurface layer at SP. Collectively, along with theradiative properties of the atmosphere, these datahave been used as input to photochemical modelingexercises. Important findings from these studies, asreported by Crawford et al. (2001), Davis et al.(2004, 2007), Chen et al. (2001, 2004) and Oncleyet al. (2004) include the following:
(1)
The one snowpack NOx flux experiment at SPresulted in an average value at 3.9� 108molecules cm�2 s�1. This measurement wasnearly 3 times larger than fluxes measured byJones et al. (2001) at Neumayer, Antarctica andabout a factor of 1.5 times higher than forSummit, Greenland (Honrath et al., 2002).
(2)
Frequently conditions at SP are such that [OH]is found to increase linearly with NO until80–100 pptv of NO are reached. The upperrange of [OH] values appears to be 2–3.5� 106molecules cm3. At higher NO the chemistry
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Fig. 8. Comparison of balloon vertical profile data of NO and ozone in the surface and boundary layer at SP during ANTCI. Ozone
profiles were extended to higher altitudes with balloon-borne ECC sondes while the NO measurements were limited to the lowest �100m
due to the length of the sampling line. Black dots indicate the distribution of measured data points that were used to generate the
interpolated color plots.
D. Helmig et al. / Atmospheric Environment 42 (2008) 2817–28302826
becomes quite non-linear and typically [OH]declines due to NOx acting as a HOx removalspecies.
(3)
The atmospheric lifetime of NOx is regulated byHOx radical levels. When NOx is low its lifetimeis reasonably constant. However, at elevatedlevels of NOx (4200 pptv), NOx can removesuch large amounts of HOx that this allows theNOx lifetime to steadily increase. The resultof this feedback is that the NOx lifetimebecomes a function of the NOx concentrationitself. Thus, the NOx chemical lifetime increasesfrom a low 7.5 h at NOx levelso200 pptv to 18 hat NOx levels of 600 pptv. And still largerlifetimes can occur as NOx concentrationscontinue upward.
(4)
Net photochemical ozone production resultsfrom the active photochemistry occurring inthe surface layer. Modeling exercises yieldedproduction rates that can account to4–6 ppbv day�1. The most determining reactionpathway is viaNOþHO2 ! NO2 (R1)
NO2 þ hn! NOþO (R2)
OþO2 þM! O3 (R3)
ARTICLE IN PRESSD. Helmig et al. / Atmospheric Environment 42 (2008) 2817–2830 2827
with emissions of H2O2, CH2O, and CH4
oxidation being the most important precursorsdefining primary primary HOx/ROx sources.
For comparison, 24-h average ozone productionrates for Summit, Greenland were estimated to belower at 2–3 ppbv day�1 (Yang et al., 2002). It hasbeen pointed out that ozone production at SP isexpected to be more efficient also due to, amongother things, the presence of unmodulated 24-hsunlight during the summer and the longer durationof sustained stable boundary layer conditions(Cohen et al., 2007; Neff et al., 2007). Similar tothe NOx atmospheric lifetime, net ozone productionshows a non-linear behavior with increasing [NO],reflecting the non-linear relationship between NOand OH and hence HO2. For the specific conditionsencountered during the ISCAT 2000 field study, asteep increase in the ozone production rate is seenbelow 100 pptv of NO; the production rate thenmaximizes between 100 and 250 pptv and declinestowards higher [NO] (Chen et al., 2004). The steepvertical NO gradients seen in the balloon data, andthe strong dependence of the NOx lifetime and thenet O3 production rate on NOx concentrationsimply that, under conditions when strong NOprofiles are present (e.g. DOY 353–359, Fig. 5),these aforementioned processes may similarly havea height dependency above the snow surface.Consequently, large changes in the NOx lifetime,[OH], [HO2], and ozone production are expected onscales of a very few meters in height. Given theexpected decline of the ozone production rate at[NOx] 4250 pptv, under the frequently encounteredconditions of surface-layer NOx mixing ratios of4250 pptv, ozone production would not be at itshighest nearest to the surface for these specifictimes. Instead, a modest increase in OH and ozoneproduction might be expected with increasingheight, reaching a maximum at the height where[NO] is in the range of 80–150 pptv, for instancewithin the 20–50m layer under conditions such asduring DOY 355–359 (Fig. 5).
The balloon vertical ozone observations showedthat ozone generally decreased gradually withheight (Fig. 8, and Helmig et al., 2007), withoutshowing any maximum at a distinct height abovethe surface, as would be expected from the predictedvertical gradients of the ozone production rate(see discussion above). It needs to be pointed outthat the suggested increases in OH and net O3
production rate with altitude were seen in model
runs reported by Davis et al. (2007), when using anensemble of NO data from the ANTCI aircraftexperiment as input to the model. Their limitedinput data set used an average value (e.g. 325 pptv)for near-surface NO over areas of the Antarcticplateau that were probed by the selected ANTCIaircraft flights. Similarly high vertical gradients inozone production would consequently be expectedunder the highly enhanced NO levels (4300 pptv)that were observed near the surface during DOY355–359. At lower near-surface values for NO(e.g., closer to the SP average of 200 pptv) therewould be almost no gradient in ozone productionwith height up to the point where a major drop inNO levels occurs. As related to O3 production andvertical concentration changes in O3, another factorthat comes into play is the lifetime and the relativeratio of the production rate over the trace gasbackground concentration. As already mentionedabove, the chemical lifetime of NOx is in the rangeof 0.3–1 days, depending on the NOx concentration,versus �30 days for ozone. Under the stabilityregimes during DOY 355–359, the mean verticaltransport time through the on average 30 m-thickboundary layer was estimated to be on the order of8.972.9 h (Neff et al., 2007). This time scale is of asimilar magnitude as the estimated NOx lifetime,but relatively short compared to the ozone lifetime.Consequently, there will be sufficient time for ozoneto be transported out of the surface layer while mostof the NOx chemistry and recycling is happeningclosest to the surface. This behavior is expected toalso result in much steeper vertical concentrationgradients for NO (compared to ozone), an effectthat is clearly evident from the data comparisonsin Fig. 8.
Other analyses (Neff et al., 2007; Helmig et al.,2007) have also concluded that the rapid changes insurface layer concentrations at SP during December2003 coincided with changes in air transport, whichcaused air from different source regions withdifferent surface conditions and chemical historiesto be transported to SP. According to these studies,the elevated ozone observed during DOY 355–359likely had a significant contribution from ozoneproduced in a region upstream of SP. Accordingly,the sudden increase of ozone on DOY 354 resultedfrom a combination of two conditions, firstly arapid change in air flow from W to E that broughtdownsloping air from the Antarctic plateau to SP,and secondly, the fact that this air had beentransported in a shallow surface layer for several
ARTICLE IN PRESSD. Helmig et al. / Atmospheric Environment 42 (2008) 2817–28302828
days prior to reaching SP. Two earlier periodsduring ANTCI with even higher NO levels werelikely preceded by still longer times with NE-SEflow and stable boundary layer conditions (Neffet al., 2007).
An important effect of the NOx–HOx/ROx–O3
coupling and the short chemical lifetimes (relative totransport scales) is that constant flux approachesare unreasonable as flux divergences with height areexpected under such conditions. This phenomenonhas been described previously for other environ-ments and it has been pointed out that fluxmeasurements from a fixed height above the surfacewill result in underestimation of surface fluxes(Lenschow and Delany, 1987; Heal et al., 2001;Oncley et al., 2004). Further analysis of the verticalconcentration profiles of measured species (ozone,NOx and meteorological parameters in our case)and deduced photochemistry requires incorporationof available data into an (at least) one-dimensionalmodel (Heal et al., 2001). The application of both a1D and 3D model to the ANTCI data for furtherassessment of the vertical and spatial distribution ofoxidizing chemistry over SP and Antarctica ispresented by Wang et al. (2007).
4. Conclusions
Tethered balloon vertical profiling with airsampling through a 130-m-long sampling lineprovided a good tool for probing the distributionof NO in the surface and lower boundary layer atSP. NO was found to be vertically and temporallyhighly inhomogeneous. Large enhancements of NOwere constrained to the lowest 50m of the atmo-sphere. Many of the previous NO measurements atSP were done from the second floor at the AROwith inlets at 10–17m height above the snowsurface. The steep gradients in the lowest 20m seenin the balloon data show that these measurements insome cases yielded significantly lower values (underextreme stable conditions possibly by up to a factorof 2) than what would have been obtained frommonitoring with inlet heights closer to the surface.
The concentration of surface layer NO is con-trolled by atmospheric stability, emission rates,advective accumulation rates, and the variablelifetime of NOx. The highest enhancements gene-rally were observed during strong inversions, whichwere present during periods with calm to light windsand frequently coincided with downsloping air flowfrom the Antarctic plateau. The strong dependencies
of NOx lifetime, OH and ozone production onabsolute NOx concentrations and the large changesof NO with height infer that all three of theseparameters under the right conditions, will havenotable dependencies on height on scales of a few totens of meters. While the NO enhancements in thesurface layer are the most notably, the balloon aswell as the ANTCI aircraft data (Davis et al., 2007)show that NO levels in the 50–100 pptv range are anomnipresent conditions in the boundary layer abovethe Antarctic plateau. Of particular importance isthe recycling of reactive nitrogen on the plateau andits relationship to the steady-state concentration ofNOx. These NO and inferred NOx levels aresignificantly higher than what would previouslyhave been anticipated for the polar environment.This increased NOx will result in a much enhancedoxidizing chemistry in the summertime lowerAntarctic troposphere (see more discussions inDavis et al., 2007; Wang et al., 2007).
Many questions about the photochemistry in andabove the snow at SP remain unanswered. Thefindings from this study show that vertical profilingunder conditions when steep gradients are presentcan provide a powerful tool for the study of thesechemical processes and equilibrium conditions.Under stable atmospheric conditions at SP, profil-ing of the lowest 50m provides opportunities toprobe repeatedly photostationary conditions underchanging chemical reactant concentrations. Thissituation resembles an outdoor chemical kineticsexperiment under fully natural conditions, where,rather than changing reactant concentrations insidea reaction chamber, a similar effect can be achievedby moving observations along a vertical scale fromthe surface in a very short amount of time to 50m.SP offers unique opportunities for photochemicalstudies. Further and expanded chemical verticalprofile measurements, e.g. with tower gradientinlets, instruments on a moving elevator, bytethered balloon probing or aircraft experiments,will therefore be helpful means for filling existinggaps in descriptions of the SP photochemistry.
Acknowledgments
This research was supported through the UnitedStates National Science Foundation (Office of PolarPrograms, grant #0230046). Any opinions, findings,and conclusions expressed in this material are thoseof the authors and do not necessarily reflect theviews of the National Science Foundation. A. Drexler
ARTICLE IN PRESSD. Helmig et al. / Atmospheric Environment 42 (2008) 2817–2830 2829
and J. Seiffert helped with the balloon experiment atSP. We thank Raytheon Polar Services and the US109th Air National Guard for logistical support.The SP staff’s efforts in accommodating thetethered balloon experiment are highly appreciated.
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