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ARTICLE IN PRESS
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doi:10.1016/j.at
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Atmospheric Environment 38 (2004) 5375–5388
www.elsevier.com/locate/atmosenv
South Pole NOx Chemistry: an assessment of factorscontrolling variability and absolute levels
D. Davisa,�, G. Chena,b, M. Buhra,c, J. Crawfordb, D. Lenschowd, B. Leferd,R. Shetterd, F. Eiseled, L. Mauldind, A. Hogane
aEnvironmental Science and Technology Buildings, School of Earth and Atmospheric Sciences, Georgia Institute of Technology,
311 frest st, Atlanta, GA 30332, USAbNASA Langley Research Center, Hampton, VA, USA
cSonoma Tech and Air Quality Design, Golden, CO, USAdAtmospheric Chemistry Division, National Center for Atmospheric Research, Boulder, CO, USA
eRetired, formerly at US CRREL, Geochemical Sciences Division, USA
Received 5 April 2003; accepted 12 April 2004
Abstract
Several groups have now shown that snow covered polar areas can lead to the release of NOx to the atmosphere as a
result of the UV photolysis of nitrate ions. Here we focus on a detailed examination of the NO observations recorded at
South Pole (SP). Topics explored include: (1) why SP NOx levels greatly exceed those at other polar sites; (2) what
processes are responsible for the observed large day to day NO concentration shifts at SP; and (3) possible explanations
for the large variability in NO seen between SP studies in 1998 and 2000. As discussed in the main body of the text, the
answer to all three questions lies in the uniqueness of the summertime SP environment. Among these characteristics is
the presence of a large plateau region just to the east of SP. This region defines one of the world’s largest air drainage
fields, being nearly 1000 km across and having elevation of � 3km: In addition, summertime SP surface temperatures
typically do not exceed �251C; leading to frequent cases where strong near surface temperature inversions occur. It
experiences 24 h of continuous sunlight, giving rise to non-stop photochemical reactions both within the snowpack and
in the atmosphere. The latter chemistry is unique at SP in that increasing levels of NOx lead to an enhanced lifetime for
NOx; thereby producing non-linear increases in NOx: In addition, the rapid atmospheric oxidation of NOx; in
conjunction with very rapid dry deposition of the products (HNO3 and HO2NO2), results in a very efficient recycling of
NOx back to the snowpack. Details concerning these unique SP characteristics and the extension of these findings to the
greater plateau region are discussed. Finally, the relationship of NOx recycling and total nitrogen deposition to the
plateau is explored.
r 2004 Elsevier Ltd. All rights reserved.
Keywords: Antarctica; South Pole; Photochemistry; NO; NOx snow emissions; ISCAT
e front matter r 2004 Elsevier Ltd. All rights reserved.
mosenv.2004.04.039
ing author. Fax: +1-770-414-1221.
ess: douglas.davis@eas.gatech.edu (D. Davis).
ARTICLE IN PRESSD. Davis et al. / Atmospheric Environment 38 (2004) 5375–53885376
1. Introduction
As discussed in Davis et al., 2000 (this issue), the year
2000 Investigation of Sulfur Chemistry in the Antarctic
Troposphere (ISCAT) field study was configured sub-
stantially different from ISCAT 1998 due to the
unexpected findings of the 1998 study. Most important
among these were finding significant quantities of NO
being generated from the UV photolysis of nitrate ions
within the snowpack (e.g., Davis et al., 2001 and
references therein). Similar observations, but involving
lower concentrations, have also been reported by
Honrath et al. (1999, 2000), Jones et al. (2000), and
Ridley et al. (2000). In addition, laboratory photochemi-
cal studies involving liquid phase solutions of nitrate have
provided confirming results (Mack and Bolton, 1999).
Of great importance in the 1998 study was establish-
ing that in addition to highly elevated NO levels (e.g.,
median value of 225 pptv), hydroxyl radical concentra-
tions were also greatly enhanced (24 h average,
� 2� 106 molec cm�3). This NOx–HOx chemical cou-
pling led to yet a final surprise, during summer months
in near surface air there is the net photochemical
production of O3 (Crawford et al., 2001).
Given the unexpected observations of 1998, a large
number of questions related to NOx remained at the
outset of ISCAT 2000. Having now completed ISCAT
2000, many of these questions have now taken on a
sharper focus. High on this list are: (a) what atmospheric
factors are most responsible for the large day-to-day
variability seen in SP NO levels; (b) what factors are most
responsible for the order-of-magnitude higher values of
NOx seen at SP versus other polar sites; (c) what processes
within the snowpack are responsible for the formation of
NO, NO2; and possibly other NOy species; (d) what
factors are responsible for the large difference seen in NO
levels between the 1998 and 2000 ISCAT campaigns; (e)
what are the levels of NO on the Antarctic plateau itself;
(f) what is the NOx flux from SP surface snow; and
finally, (g) what is the magnitude of the primary nitrogen
source to this region and from where does it originate?
This paper will primarily focus on two issues: the
exceptionally large variability seen in NO levels at SP; and
why the median values of NO at this site are nearly an order
of magnitude higher than at any other polar site currently
investigated. The important issue related to defining the
magnitude of the NOx flux at SP has been addressed in a
companion paper by Oncley et al. (this issue).
2. NO measurement technique and model description
2.1. Measurement technique
As during ISCAT 1998 (Davis et al., 2001), the
primary NO sampling location for ISCAT 2000 was the
second floor of the Atmospheric Research Observatory
(ARO) building operated by NOAA/CMDL. The
standard sampling configuration involved a sample inlet
line that extended out from the ARO building � 1m: Itwas located � 10m above the snow surface on the side
of the building facing the prevailing wind. For a very
limited time (i.e., 26 November–1 December), a second-
ary sampling location during ISCAT 2000 was the SP
MET tower, located � 90m from ARO. (Note, the
ARO building as well as the MET tower are both
upwind of any station contamination.) The NO instru-
ment when used in the MET tower study was housed in
a specially constructed environmental enclosure and had
an inlet equipped with a solenoid valve to enable
sampling at three different elevations on the tower
(i.e., 0.5, 4.7, and 21.8m) at different times. The clear 14
in OD Teflon sampling lines were of equal length (23m)
so as to minimize any differences in wall losses which
were themselves quite small (o3%). During the last 10
days of ISCAT 2000 (20–30 December), sampling was
also carried out at three different locations. The first
location was the standard position on the second floor of
ARO, and the second was located 16m above the snow
surface on the ARO roof. The third sampling location
had two positions, either 0.5m above the snow surface
or 0.01–0.9m below the surface. The low elevation
sampling operations were physically located 5m in front
of the ARO building, thus ensuring sampling on the
windward side of ARO.
As in 1998, the 2000 study used an NO chemilumi-
nescence instrument that was significantly modified in
design relative to commercial instruments (e.g., see
Davis et al., 2001). The typical sampling flow rate for
this system was 1 lmin�1: Two standard addition
calibrations and zero air (artifact) tests were performed
each day. The NO calibration gas used in the current
study was intercompared with other NIST traceable
standards both before and after the mission with very
good agreement being found. The 2s detection limit for
the chemiluminescence instrument at SP was estimated
at � 6 pptv:In addition to the primary chemiluminescence NO
instrument cited above, a second commercial chemilu-
minescence NOx–NO instrument (Thermo Environmen-
tal Instruments Model 42S) was also available during
the 2000 study. This instrument provided an indepen-
dent check on the 1998 and 2000 ISCAT NO observa-
tions. It was independently operated by other ISCAT
investigators. Although this system had a much higher
NO detection threshold (e.g., 50 pptv), measurements
with a good signal/noise ratio were frequently possible
due to the elevated levels of NOx at SP. When high levels
of NO did occur, the results from the commercial
instrument were typically overlapped within the un-
certainties of each respective instrument. Because of its
reduced sensitivity, however, during much of the 2000
ARTICLE IN PRESSD. Davis et al. / Atmospheric Environment 38 (2004) 5375–5388 5377
study this instrument was used to continuously monitor
NO and NOx ðNOþNO2Þ levels within the snowpack
(e.g., see Section 4.1.1). The latter instrument was
always housed in the ARO building.
Both the primary NO and the TECO instruments
were used in measurements of NO in interstitial air. The
procedure used in this sampling involved first removing
a plug of snow down to some predetermined depth
followed by the insertion of a Teflon sampling line down
to this depth and then the packing of snow around this
line. The sampling line itself was fitted with a filter to
prevent the entrance of any loose snow. Very low flow
rates (1 lmin�1) were used in these studies to prevent
sampling surface air.
2.2. Model
The photochemical box model used in this study was
similar to that described previously by our group
(Crawford et al., 1999). This model assumes all
calculated species to be in steady state, and includes
250 reactions. It is typically constrained by observa-
tional values for O3; NO, CO, H2O; CH4; NMHC,
pressure, and temperature; however, it can also accom-
modate observational constraints based on measure-
ments of OH, HO2;HNO3;HO2NO2;H2O2; CH2O; andHONO.
Gas kinetic rate coefficients were taken from Demore
et al. (1997) and Atkinson et al. (1992) with updated
values being used when available. The photolysis
coefficients were derived from in situ actinic flux
measurements (Davis et al., 2000, this issue). Thus,
variation in values due to shifts in the overhead O3
column density and/or clouds/fog conditions were
accounted for. Since no diurnal UV flux variations
occur at SP during the Austral summer, photochemical
steady state was assumed for all model calculated
species. NO2 could therefore be readily calculated from
the known levels of NO, O3; and JNO2: Note, in some
places in the text the quantity NOx (e.g.,
NOþNO2 ¼ NOx) is used. When this atmospheric
NOx relates to the primary NO instrument (used for
ISCAT 1998
12/24 12/25 12/26 12/27Date
0
100
200
300
400
500
600
NO
(pp
tv)
NOT(22m)-T(1.6m)
(a) (b
Fig. 1. Time series plots of observed NO and vertical temperature dif
2000. Note, the trend in DT illustrates the strong correlation between
90% of all atmospheric measurements), the required
NO2 value has been calculated from the cited model.
When NOx is reported for in snow measurements, the
TECO instrument (with converter) was used to provided
independent measurements of NO and NOx:
3. Comparison of ISCAT 1998 and 2000 atmospheric NO
data and related variables
NO sampling during the 1998 study occurred over the
time period of 30 November 1998 to 4 January 1999.
For the 2000 study, it started on 15 November and
ended 30 December. As revealed in the representative
data plots presented in Figs. 1a and b, NO values during
both the 1998 and 2000 ISCAT studies ranged over
nearly two-orders-of magnitude. (For the full ISCAT
1998 and 2000 data sets the reader can go to: http://
nsidc.org/usadcc/.) As one of the most isolated sites on
the planet, the SP environment was assumed to be
virtually free of NO prior to the ISCAT observations
(Schnell et al., 1991). Yet, as seen in Figs. 1a and b, NO
levels range from 10 to 600 pptv and can be seen shifting
between these extremes in times of less than 24 h. When
evaluated in terms of monthly medians, SP NO (as well
as NOx) mixing ratios are found to be nearly an order of
magnitude higher than at other polar sites. Even at SP
itself, a comparison of one field study to another (e.g.,
ISCAT 1998 versus ISCAT 2000) reveals monthly
medians that differ by more than a factor of 2 (i.e.,
1998 median, 225 pptv; 2000 median, 86 pptv).
When the 1998 and 2000 December data are plotted in
the form of wind roses (Figs. 2a and b), the difference
between them does not appear to be a strong function of
wind direction. It also does not appear to be strongly
related to major shifts in wind speed. For both years the
bulk of the data tends to lie in the 0–90� quadrant with
the second most populated sector being 270–360�: Thewind field pattern shown in Figs. 2a and b is interesting
in that for both years it shows that ‘‘downslope’’ flow of
cold–dry air draining off the plateau toward the coast
dominates (Parish, 1988). In fact, for most years this
ISCAT 2000
12/13 12/14 12/15 12/16Date
-0.500.511.522.53
T(22m
)-
T(1.6m
)(
C)
NOT(22m)-T(1.6m)
)
ference ðDTÞ between 22 and 1.6m for (a) ISCAT 1998 and (b)
atmospheric stability and the presence of high NO levels.
ARTICLE IN PRESS
ISCAT 2000
0
45
90
135
180
225
270
315
02
2
4
4
6
6
8
8
10
10
ISCAT 1998
0
45
90
135
180
225
270
315
02
2
4
4
6
6
8
8
10
10
NO L evel(pptv)
300+240 to 300180 to 240120 to 18060 to 1200 to 60
(a)(b)
Fig. 2. SP wind rose plots for ISCAT 1998 and 2000: polar plot of wind speed and wind direction color coded with observed NO levels.
Data presented here are hourly averages for December only. Note: wind direction measurements may be subject to large uncertainties
for speeds under 2m s�1:
0 2 4 6 8 10Wind Speed (m/s) Wind Speed (m/s)
0
100
200
300
400
500
600
NO
(pp
tv)
0 2 4 6 8 100
100
200
300
400
500
600
NO
(pp
tv)
-40 -38 -36 -34 -32 -30 -28 -26 -24Dewpoint (deg. C )
0
100
200
300
400
500
600
NO
(pp
tv)
-40 -38 -36 -34 -32 -30 -28 -26 -24Dewpoint (deg. C )
0
100
200
300
400
500
600
NO
(pp
tv)
(a) (b)
ISCAT 1998 ISC AT 2000
(c) (d)
-34 -32 -30 -28 -26 -24 -22 -20Temperature @ 1.6m (°C)
0
100
200
300
400
500
600
NO
(pp
tv)
-34 -32 -30 -28 -26 -24 -22 -20Temperature @ 1.6m (°C)
0
100
200
300
400
500
600
NO
(pp
tv)
(e) (f)
Fig. 3. Binned 1998 and 2000 ISCAT data used in correlation plots of observed NO vs. wind speed, (a) and (b); NO vs. surface dew
point, (c) and (d); and NO vs. surface temperature (e) and (f). Symbols and error bars represent median values and inner quartiles (25th
and 75th percentiles). Data used in the plots are 10min averages for December only.
D. Davis et al. / Atmospheric Environment 38 (2004) 5375–53885378
ARTICLE IN PRESSD. Davis et al. / Atmospheric Environment 38 (2004) 5375–5388 5379
‘‘downslope’’ flow prevails 85–90% of the time (Hogan
et al., 1993 and references therein), and gives rise to
extremely low dew points at SP. The plots do suggest,
however, that there was a somewhat larger percentage of
‘‘upslope’’ flow (180–350�) in 1998 than 2000; but, this
population when compared to the total data set is a
rather small.
For both years one also sees a similar trend in NO
values versus wind speed. That is, the highest NO values
tend to be found at some of the lowest wind speeds. This
relationship is better illustrated here in the form of
Figs. 3a and b where ‘‘binned’’ NO and wind speed data
(30 points) have been used. From Fig. 3a it can be seen
that 1998 median values range from 105 to 425 pptv, and
show a clear trend of decreasing values with increasing
wind speed. For 2000, with the exception of the lowest
wind speed bin, a similar trend can be seen though the
range of values is smaller (i.e., 75–160 pptv).
The perturbation seen in the low wind speed ISCAT
2000 data is interesting in that although it does not
involve a large data population (i.e., 30 points), it does
suggest that some form of shift most likely occurred in
the meteorology between studies. This point is further
emphasized in the form of Figs. 3c and d. These plots
show the ISCAT 1998 and 2000 NO data plotted against
dew point, and reveal a significant difference in trends.
For example, in 1998 some of the highest NO values
occur when the corresponding dew-point values are the
lowest. During ISCAT 2000, no such simple trend is
evident. In fact, the highest NO values occur in the
middle of the dew-point range, e.g., �35 to �32� C: Bycontrast, when the comparison is between ambient
temperature and the NO concentration (i.e., Figs. 3e
and f), a very similar trend is evident for both 1998 and
2000 though the 1998 data set encompasses a much
(b12/29/00 12/30/00
Date
0
500
1000
1500
2000
2500
3000
NO
and
NO
x(p
ptv
)
(a)
Snow NOx
Snow NO
Fig. 4. Observed temporal variation in NO and NOx for: (a) snow
December 2000. The large value for the ratio of NOx to NO at �20
depth. At �1 cm this ratio approaches the same value as that in the a
end of 29th December 2000 is due to the sampling site in front of th
illustrating the potential impact that can result from overhead clouds
larger temperature range than for 2000 (e.g., �34 vs:�31 and �21 vs: �24).
4. Discussion
From the ISCAT 1998 and 2000 observations cited
above, it is quite apparent that although some very
important similarities can be found between the two NO
data sets, there are also some significant differences.
Perhaps more importantly, very large differences have
been found between the ISCAT results and those
reported at other polar sites. Below, these and related
issues are explored.
4.1. Major factors controlling South Pole NOx
Given that the major source of NOx at SP is its release
from the snowpack following the UV photolysis of
nitrate ions, it follows that the near surface atmospheric
concentration of NOx at any point in time should be a
function of at least four factors. These would include: (1)
the net flux of NOx from the snowpack ðF ðNOxÞSAÞ; (2)the net NOx advection flux ðF ðNOxÞAdvÞ; (3) the atmo-
spheric lifetime of NOxtNOx; and (4) the depth of
atmospheric mixing (i.e., planetary boundary layer
depth, PBL). Here we explore the effect of each of these
factors at SP; and ,when possible, compare the relative
magnitudes of these factors at different polar sites.
4.1.1. Snowpack NO–NOx levels and NOx flux
Representative SP snowpack measurements of NO
and NOx during ISCAT 2000 are shown in Fig. 4a.
These data were recorded with the sample inlet
� 20 cm below the snow surface. They unequivocally
12/29/00 12/30/00
Date
0
100
200
300
400
NO
(pp
tv)
)
Air NO
and (b) atmosphere (NO only) on a representative day, 29th
cm primarily reflects the attenuation of solar radiation at this
tmosphere. Note, the dip in NOx and NO at the beginning and
e ARO building falling into the shadow of the ARO building,
.
ARTICLE IN PRESS
90.0S
70.5S 64.5S
74.5N
82.5N
South Pole Neumayer PalmerStation
Summit,Greenland
Alert, Canada
0
50
100
150
200
250
300
350
400
NO
x(p
ptv
)
Fig. 5. Comparison of NO observations recorded at several
different polar sites. Data shown were taken from Davis et al.
(2001 this issue), Jones et al. (2000), Jefferson et al. (1998),
Yang et al. (2002), and Ridley et al. (2000). Note, the NOx
value for SP was that estimated from NO2 calculated from a
photochemical box model as described in the main text.
D. Davis et al. / Atmospheric Environment 38 (2004) 5375–53885380
demonstrate that the snowpack at SP is a major source
of atmospheric NOx: For example, a typical sampling
day on 29 December reveals � 1500pptv of NOx and
600 pptv of NO. This NO value is nearly seven times
larger than the median value estimated from all ISCAT
2000 data. More importantly, as shown in Fig. 4b, over
the same daytime period the difference between snow-
pack and atmospheric levels of NO varies by a factor
ranging from 2 to nearly 60. Since the atmospheric ratio
of NO:NO2 at SP is typically around 2:1, it follows that
the snowpack-to-atmospheric concentration difference
for NOx would, in most cases, be even larger.
The emission rate of NOx from the snowpack at SP
has been addressed by Oncley et al., (this issue), with the
finding that over a 6 day period of time (26 November to
1 December 2000) the flux was relatively constant. These
investigators used the modified Bowen ratio technique
involving simultaneous measurements of the eddy-
covariance surface heat flux along with the temperature
and NO concentration at two elevations. The estimated
NOx flux over the stated time period had an average
value of 3:9� 108 molec cm�2 s�1: (In this study, the
initially derived NO flux was converted to a NOx flux
from photochemical considerations where photochemi-
cal equilibrium values of NO2 were estimated for SP
conditions.) This SP value can be contrasted with that
estimated at Neumayer, Antarctica by Jones et al.
(2001). These authors measured the gradient in NOx at
two heights (e.g., 2 cm and 2.5m) over a two day period
in February. Their results were based on using an
estimated eddy diffusivity value that gave a 24 h
emission flux for NOx of 1:3� 108 molec cm�2 s�1: A
third site, Summit, Greenland, has also reported a NOx
flux value. In the latter case the flux estimate was based
on vertical gradient measurements of NOx at 1 and 2m
above the snowpack in conjunction with simultaneous
measurements of the atmospheric turbulence (Honrath
et al., 2002). The 24 h average vertical flux estimated via
eddy covariance for 5 June to 3 July 2001 was 2:5�108 molec cm�2 s�1:A comparison of these three independent fluxes
suggests that the differences do not scale with the
known snow nitrate ion concentrations and estimated
UV irradiance for each site. For example, if the flux were
controlled by the product of snow nitrate concentration
and nitrate photolysis rate, Summit, Greenland should
have recorded the highest value. This would reflect the
fact that snow nitrate levels are somewhat higher there
(e.g., 3 nmol g�1vs: 2nmol g�1at SP in the top 45 cm
(Dibb et al., 2004)) and that the measurements were
carried out at the time of summer solstice. Neumayer, on
the other hand, because of the study’s February
execution date would be expected to have one of the
lower NOx flux values; however, snow nitrate levels at
this site are closer to those found at Summit rather than
SP. Given this, the Neumayer flux also does not seem to
fit into any simple analysis that compares sites based
only on UV irradiance and nitrate levels. Currently,
therefore, too many uncertainties still remain as related
to the overall NOx production rate, its release to the
atmosphere, and in the reliability of the flux measure-
ments themselves to be able to assess the real differences
between sites.
What is apparent from the above assessment is that
the NOx flux values from three different sites vary by
only a factor of three. By contrast, the differences in the
median atmospheric NOx mixing ratio at SP versus
other polar sites are far larger than what might be
predicted from any adjusted differences in the NOx
fluxes (see Fig. 5). Thus, while differences in the values
of F ðNOxÞSA between sites is one of the contributing
factors to the observed difference in concentration, it is
most likely not the dominant one.
4.1.2. Planetary boundary layer depth
All other things being equal, for a fixed surface-to-
atmosphere NOx flux value, the larger the atmospheric
volume element filled the lower will be the resulting
steady-state concentration. Placed in the context of the
SP study, this means that the observed NOx concentra-
tion should be inversely proportional to the depth of the
planetary boundary layer (PBL). The PBL factor
therefore emerges as a critical variable in understanding
the large scale variations in the NOx mixing ratio.
Estimates of the PBL depth during ISCAT 2000 were
made using wind turbulence data collected with sonic
anemometers mounted on the SP MET tower. These
data when processed in conjunction with empirically
derived relationships relating the integral scales resulted
in the estimated PBL values cited in this study. For those
cases involving stable meteorological conditions, the
PBL depth was defined as that height at which the
ARTICLE IN PRESS
(b)(a)
1000
800
600
400
200
0
PB
L D
epth
(m
)
11/26 12/1 12/6 12/11 12/16 12/21 12/26
DateDate
h(3.5 m) h(7.2 m) h(unstable)
300
250
200
150
100
50
0
NO
(pp
tv)
11/26 12/1 12/6 12/11 12/16 12/21 12/26
Fig. 6. Time series plots of observed NO and mixing depth values derived from observed vertical velocity spectrum at 3.5 and 7.2m as
measured on the SP MET tower. The curves labeled h(3.5m) and h(7.2m) are the estimated mixing depths from the spectral peaks
recorded at the corresponding sensor elevations on the MET tower. The h(unstable) profile has been generated by values estimated
from the integral length scale for unstable conditions (for details see Oncley et al.,, this issue).
0 0.004 0.008 0.012 0.016
1/PBL (1/m)
0
50
100
150
200
250
NO
(pp
tv)
500m 250m 167m 125m 100m 83m 71 m
Fig. 7. Correlation plot of binned NO data vs. the reciprocal of
the mixing depth. Symbols and error bars signify averages and
one standard deviation, respectively.
D. Davis et al. / Atmospheric Environment 38 (2004) 5375–5388 5381
turbulence level was 110
that at the surface. (For further
details on the PBL evaluations the reader is referred to
Oncley et al.,, this issue.)
The dominating role of the PBL depth in influencing
the observed NO mixing ratio can be most easily seen in
Figs. 6a and b and also in Fig. 7. Figs. 6a and b
qualitatively show that the highest levels of NO correlate
with the lowest PBL values; and conversely, some of the
lowest NO values can be seen correlating with the
highest values of PBL. Fig. 7, on the other hand,
presents a quantitative assessment of this relationship in
the form of a regression plot of the data displayed in
Figs. 6a and b. For purposes of clarity, Fig. 7 has used
‘‘binned’’ NO and PBL values (i.e., every 30 observa-
tions). In this case, a plot of the NO mixing ratio against
1/PBL reveals a very strong correlation between the two
variables, i.e., R2 ¼ 0:93:Given that the PBL depth plays such a critical role in
defining SP NO levels, it is reasonable to ask whether
this factor alone might not provide the basis for
understanding the large concentration difference seen
in NO at different polar sites. In this regard, an analysis
of the ISCAT 2000 data produced an estimated median
PBL value of 193m. The lowest values reported fell into
the range of 50–60m, and were shown to be associated
with stable meteorological conditions. Recall, in the
above text, these values were defined based on the
turbulence level having decreased to 110
of the ground
level turbulence value. However, an indication that PBL
depths at SP might be even shallower comes from some
earlier acoustic sounding results reported by William
Neff (private communication) as well as from the 1998
ISCAT NO data set. In the case of the 1993 acoustic
data, these summertime results revealed PBL values as
low as 20m. The ISCAT 1998 NO data, on the other
hand, lead to an estimated median NO valve that is 2 12
times larger than that from the 2000 study. One
argument for explaining the higher 1998 NO median,
therefore, would be that the median PBL depth for this
study was substantially lower than during 2000.
At Summit, PBL values have been measured based on
the use of tethered balloons (Helmig et al., 2002).
However, these investigators only estimated values
under neutral or unstable meteorological conditions.
In fact, during the month of the study (June 2002), the
data show that when the solar intensity was low (e.g.,
20:00–07:00 h) stable meteorological conditions were the
dominant condition. Over the daylight period of
09:00–18:00 h, the temperature inversion that developed
on most nights was found to give way to neutral to
slightly unstable conditions, giving rise to PBL values of
10–250m. However, the fact, that PBL values were most
likely very small between 20:00 and 07:00 h would
ARTICLE IN PRESS
0 100 200 300 400 500 600 700 800
NOx (pptv)
5
10
15
20
25
30
Life
time
(hou
rs)
Fig. 8. Correlation plot of model estimated NOx lifetime as a
function of NOx levels using binned data. Model values are
based on actual SP observations. NOx values have been defined
from measured NO and model calculated NO2: Symbols
indicate average values and error bars represent one standard
deviation.
D. Davis et al. / Atmospheric Environment 38 (2004) 5375–53885382
appear to be inconsequential in terms of their in-
fluence on NOx: Since UV photolysis of nitrate was
shut down during these times, there would be
no generation of NOx; hence NOx concentration levels
would approach zero as has been shown by Helmig
et al. During the 9 hours in which there was significant
solar flux to promote photochemistry, we estimate
that the median PBL depth at Summit was in the
range of 70–80m. Thus, this would place Summit’s
PBL median value at a depth nearly two times
lower than that cited above for ISCAT 2000, but
perhaps not much different than our guesstimate for
ISCAT 1998.
PBL values at the coastal station Neumayer appear to
have never been quantitatively assessed. Jones et al.
(2001) in their evaluation of the NOx flux for the month
of February (1999) gave an estimated value of 300m,
but the basis for this assignment is unclear. These
authors have noted that considerable uncertainty
remains in the value assigned to this parameter as
related to the Neumayer site. Koenig (private commu-
nication) has noted that there can be two completely
different weather regimes at Neumayer, each of which
have quite different impacts on the PBL depth. When
the weather is influenced by strong easterly flow there is
the complete absence of surface inversions. By contrast,
during times of strong katabatically flow from the south
there can be associated with it extreme surface inver-
sions. The latter, however, tend to be less likely during
the summer months. However, at another western
Antarctic coastal site (i.e., Palmer Station), a series of
acoustic sounder data collected during the months of
January and February of 1994 produced a median PBL
value of 150m (William Neff, private communication,
also see Davis et al., 1998). Thus, if these Palmer PBL
data were to be used as a best estimate of the Neumayer
site, the difference between SP and Neumayer would be
less than a factor of 1.5.
In summary, although the PBL data at SP has
revealed that this factor is the dominant one controlling
the NO mixing ratio at this site, it is not abundantly
obvious with the limited PBL data available at other
sites how this factor alone can explain the large
difference in NOx levels observed at different polar
sites. Clearly, as explored in the text below, the
relationship between solar flux and the PBL depth will
need to be considered as will the coupling with other
possible variables.
4.1.3. NOx chemical lifetime and horizontal transport
As noted earlier in the text, both the chemical lifetime
and the transport characteristics of NOx can influence
the levels of this species. Here, both variables are
discussed collectively (e.g., using box modeling results)
since it is the coupling between them that generates the
largest impact on NOx:
It is well known that under typical tropospheric
conditions the lifetime of NOx is controlled by its
reaction with OH (R1). However, under elevated NOx
conditions, things become more complex in that NOx
regulates the concentration of OH. Thus, NOx can
influence its own lifetime (see Fig. 8). The relationship
between HOx;NOx; and its lifetime is most easily seen in
terms of sequence of reactions (R1Þ ! ðR4)
OHþNO2 þM ! HNO3; ðR1Þ
HO2 þNO2 þM ! HO2NO2; ðR2Þ
HO2 þNO ! OHþNO2; ðR3Þ
HNO3;HO2NO2 ! surface deposition: ðR4Þ
From this set of reactions, one sees that NO2 is removed
by both (R1) and (R2). For SP conditions, the shortest
lifetime for NOx is found when it is at its lowest
concentration (e.g., Fig. 8). This is contrary to what one
finds at a non-SP polar site where reaction (R1) forms
the dominant loss channel for NOx: The reason both
(R1) and (R2) define significant NOx loss processes at
SP is that the ambient temperature is sufficiently low
that the thermal decomposition of HO2NO2 is slowed to
the point that it can readily be deposited to the
snowpack (Slusher et al., 2002). As such, (R2), which
depends on the HO2 level rather than OH, becomes the
dominant NOx loss process at low levels of NOx:However, with further increases in NOx; (R3) leads to
large decreases in HO2 and equally large increases in OH
as shown in Fig. 9 (Chen et al., 2000 this issue; Mauldin
et al., 2000, this issue). The net effect is that the
reduction in the loss rate of NOx via (R2) is offset by the
increase in the loss rate via (R1). Modeling runs, in fact,
ARTICLE IN PRESS
0 50 100 150 200 250 300 350 400 450 500
NO (pptv)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
OH
(1E
6 m
olec
/cm
3 )
Fig. 9. Binned data used in a plot of observed OH vs. observed
NO during ISCAT 2000. Symbols correspond to median values
and error bars denote inner quartiles (25th and 75th percen-
tiles). OH data are those reported by Mauldin et al. (this issue).
0 20 40 60 80 100 120
Time (hr)
Time (hr)
0
100
200
300
400
500
600
NO
x(p
ptv
)
PBL = 10PBL = 20PBL = 40P BL = 120P BL = 240
(a)
0 20 40 60 80 100 1200
100
200
300
400
500
600
NO
x(p
ptv
)
PBL = 10PBL = 20PBL = 40PBL = 120PBL = 240
(b)
Fig. 10. Model simulated accumulation (pptv) of NOx in a
moving parcel of air above a SP snow surface as a function
NOx lifetime and mixing depth. Panel (a): variable NOx lifetime
as shown in Fig. 8 and (b) constant NOx lifetime at 7 h.
Note, the NOx lifetime is estimated from: ([NOx])/
(k[OH][NO2]+(k[HO2][NO2]–k[HO2NO2)), where the ‘‘k’’
terms are the corresponding reaction coefficients or thermal
decomposition constant.
D. Davis et al. / Atmospheric Environment 38 (2004) 5375–5388 5383
show that the NOx lifetime remains relatively stable at
approximately 7 h for NOx levels as high as 170 pptv
(Fig. 8). Beyond this, the OH concentration becomes
sufficiently high that (R1) becomes a major loss path!-
way for HOx: Thus, additional increases in NOx cause
further OH reductions (see Fig. 9), thereby resulting in
an increase in the NOx lifetime. The NOx level at which
this shift in lifetime occurs depends critically on the
magnitude of the primary sources for HOx: The larger
the primary source, the more NOx is required before
(R1) becomes the dominant loss. At SP the primary
HOx source (e.g., Oð1DÞ þH2O) and photolysis of snow
emissions of H2O2 and CH2O (Chen et al., 2000, this
issue) is so small that the peak in OH levels occurs at
relatively low NOx (e.g., 100–170 pptv). This is not true
at other polar sites where primary HOx sources tend to
be an order of magnitude larger (see e.g., Yang et al.,
2002). Since it is onlyNOx that is in excess of those levels
corresponding to the OH peak that result in NOx
lifetime increases, non-linear increases are much less
likely to occur at other polar sites.
The impact of tNOxon the build-up of the NOx
concentration with time is illustrated here in Figs. 10a
and b. In a, the model run was based on a fixed NOx
surface flux corresponding to the average value reported
by Oncley et al., (this issue). In this case tNOxis allowed
to vary as the amount of NOx accumulates in time for
different PBL depths as an air parcel passes over a
snowfield having a constant NOx flux. Quite apparent is
that the final NOx concentration is a strong function of
the absolute NOx level, and hence, on the PBL depth. By
contrast, as shown in b, when the same flux is used but
tNOxis fixed at its low value of 7 h. Little difference is
seen between the two simulations for PBL depths of
40m and greater, but for these conditions the equili-
brium NOx mixing ratio remains below 200 pptv and the
NOx lifetime remains short. For PBL depths of 20 and
10m, the difference between simulations is dramatic. In
this case, equilibrium levels of NOx exceed 200 pptv
which allows for non-linear growth in NOx for the
simulation where the lifetime is allowed to vary (a). This
demonstrates the importance of PBL depth in achieving
non-linear increases in NOx:Evidence supporting this non-linear behavior can be
found in several data segments of the ISCAT 1998 and
2000 studies. For example, at the 10m PBL level shown
in a, model runs suggest that NO concentrations as high
as 600 pptv could easily be reached within � 16h: Infact, levels as high as 600 pptv were observed on 15
December 2000, and during ISCAT 1998 such levels
were found on no less than seven occasions (e.g., 9, 18,
24, 25, and 30 December and 1, 2 January). In nearly all
cases the time required to reach these levels was 16 h or
less.
Although the above examples make the case that the
NOx lifetime and the PBL depth are two of the critical
factors responsible for the highly elevated values of NO
at SP, still other aspects of the SP setting need further
ARTICLE IN PRESSD. Davis et al. / Atmospheric Environment 38 (2004) 5375–53885384
exploring. In the simple accumulation model cited
above, the build-up of NOx shown in a was envisioned
as taking place over time until the loss rate for NOx was
comparable to its rate of release from the surface.
However, in an actual plateau setting, a wind field would
be present which would continually shift the location of
the initial volume element. Thus, for the results of a to
be representative of a plateau setting, the additional
assumption is required that the NOx emission rate not
vary. Using as our example the case of a 20m PBL depth
with a wind speed of 5m s�1; the flux field would have to
be constant over approximately a 3 day period. This
would involve a distance of � 1200 km: At SP � 90% of
the air draining into the site comes from the Antarctic
plateau. This plateau reaches out to � 1000km in
several directions. Thus, the probability is high that on
a given day the NOx measured at SP reflects the
accumulation of NOx from at least one day. By contrast,
at Summit, Greenland, being only 100 km across, the air
arriving is either from a coastal area (upslope flow) or is
free tropospheric flow. In either case, the expected NOx
level would be low. Concerning the coastal station
Neumayer, the most common summertime setting
involves strong easterly flow of maritime air. This would
again suggest low levels of incoming NOx: A similar
summer scenario should hold for Palmer Station. As
reported by Jefferson et al. (1998) and by Jones et al.
(2000), background ambient levels of NOx at both sites
are indeed low, e.g., p10 pptv:At SP maximum levels of NOx most often appear
under stable meteorological conditions. The resulting
shallow PBL depths lead to rising NOx levels that can
then ramp up steeply during the 24 h of constant SP
sunlight. In this setting, the rapid build-up of NOx is
greatly augmented by its non-linear lifetime. Under
these low wind conditions, therefore, only local NOx
emissions influence NOx levels. A sudden break-up of
this shallow PBL, due to abrupt increases in wind speed,
can then lead to equal, if not faster, decreases in NOx:However, at SP the extent of the drop in concentration
can also be modulated by the degree to which the
arriving air parcel has accumulated NOx from the
plateau. Thus, the SP presents a far more complex
picture for NOx than most other polar sites.
4.2. ISCAT 1998 vs. 2000 NO levels
As noted in the overview paper for ISCAT 2000
(Davis et al., 2000, this issue), and as illustrated here in
Figs. 3a–f, significant differences can be found in several
meteorological parameters recorded during ISCAT 1998
and ISCAT 2000. During the month of December, for
example, the 2000 study reported higher wind speeds,
lower dew points, and a significantly narrower tempera-
ture range than in the 1998 investigation. In 2000 there
were also significant discontinuities in the wind speed
and dew point profiles when plotted against NO.
Collectively, these differences point to the likelihood of
a noticeable difference in the synoptic-picture for the
Antarctic plateau/SP region in 1998 versus 2000.
Typically during the summer months a low pressure
center over the Ross Sea Ice Shelf dominates the flow
pattern to the South Pole (Harris et al., 1992, Neff et al.,
1978, 1999). This more typical situation, was present
during the ISCAT 1998 study. The flow pattern in this
case can best be characterized in terms of free tropo-
spheric winds from west of Greenwich meridian and
near surface winds from east of Greenwich meridian
with a strong temperature inversion typically being
present in the boundary layer. By contrast, during
ISCAT 2000 there was a high pressure center positioned
over the polar plateau in December while the low
pressure center over the Ross-Sea Ice-Shelf was mostly
absent (Hogan et al., unpublished results). Free tropo-
spheric winds therefore tended to be near, or east of,
Greenwich meridian, as were the surface winds, reflect-
ing the flow about this high pressure center. This near
coincidence of boundary layer and free tropospheric
wind directions weakens surface temperature inversions
(e.g., Obremski et al., 1988); thus, isothermal or
standard lapse temperature structures have a higher
probability of occurring. The latter now appears to have
generated deeper mixing, and hence, reduced NO mixing
ratios in 2000. Reflecting the strong possibility that
1998, on average, had more cases of very low PBL
depths, Fig. 11 displays the NO concentration plotted
against DT ; where the latter is the temperature gradient
measured on the 22m MET tower. The argument here
would be that the greater the positive value of DT the
lower the PBL. In this case one can see substantially
more data in 1998 at DT values X2:0:
4.3. Relationship between South Pole and polar plateau
NOx levels
In Section 4.1 we addressed the question: what factors
are responsible for the uniqueness of SP NO and NOx
levels relative to other polar sites. Here we examine a
related question, namely, do the conclusions reached for
SP apply to other locations on the Antarctic plateau?
The short answer is that we now believe that the SP
conclusions will apply to much, but not all, of the
plateau. The equivocation shown here reflects our earlier
text in Section 4.1.3. There it was stressed that the role of
day–night solar cycling on NO levels could be very
significant. Under nighttime conditions, speculation is
that not only does the photodenitrification mechanism
completely shut down; but once reactivated due to the
on-set of sun-rise, the PBL depth would also increase
(e.g., see Helmig et al., 2002). Thus, NO levels would
unlikely reach the high concentrations seen at SP. This
sequence of events appears to be borne out by the
ARTICLE IN PRESS
-1 -0.5 0 0.5 1 1.5 2 2.5 30
100
200
300
400
500
600
NO
(pp
tv)
(a)
-1 -0.5 0 0.5 1 1.5 2 2.5 3
T(22m) - T(1.6m) (deg C)T(22m) - T(1.6m) (deg C)
0
100
200
300
400
500
600
NO
(pp
tv)
(b)
Fig. 11. Relation between observed NO levels and temperature gradient as measured on the SP MET tower, e.g., T(22m)–T(1.6m) in�C: Binned data are for: (a) ISCAT 1998; (b) ISCAT 2000.
D. Davis et al. / Atmospheric Environment 38 (2004) 5375–5388 5385
studies reported by Jones et al. (2001) and Honrath et al.
(2002), both of which involve sites experiencing solar
cycling. Their results suggest that most of the reservoir
NOx in the snowpack is lost during dark hours, possibly
due to heterogeneous processes operating within the
snowpack. With the return of daylight, the NOx
generating mechanism is re-initialized, but new reservoir
levels of NOx then need to be re-established before
significant releases can occur to the atmosphere. This
would suggest that the levels of NO observed on the
plateau might be latitude dependent, leading to the
further conclusion that some areas of the plateau may
never experience the effects resulting from a non-linear
NOx lifetime. If so, these areas would be expected to
have much lower average values of NOx than observed
at SP.
Based on the assumption that the photolysis fre-
quency for labile nitrogen in the snowpack would be
similar to that for JðHNO3Þ (Cotter et al., 2003; Wolff
and Jones, 2002), the latitude at which a substantial roll-
off (e.g., a factor of 2) in photolysis rate might be
observed is estimated to be � 77� S: This would mean
that � 23of the Antarctic plateau is likely to experience
highly elevated NOx levels. However, considering the
fact that only 50–70 pptv of NO is required to drive OH
from levels of 3� 105 to 2–3� 106 molec cm�3; even
very modest levels of NO at these lower latitudes could
still have a major impact on the oxidizing power and/or
O3 generating capacity of near surface air over the
plateau (e.g., see Crawford et al., 2001; Chen et al., 2000
this issue).
4.4. Importance of NOx recycling to the net deposition of
nitrogen
It is now clear from the observations of ISCAT 1998
and 2000 that snowpack emissions of NOx have a
profound effect on the photochemistry within the PBL.
But it is also of interest to examine whether this release
of nitrogen is large enough to have a significant
impact on the total nitrogen deposited to the surface
at SP and the greater plateau region. Based on SP core
samples, Legrand and Kirchner (1990) estimated the
mean deposition flux for nitrate at SP (over the last
hundred years) to be 9.5 kg NO3 km�2 yr�1 (or
2.1 kgNkm�2 yr�1). This number, however, must be
viewed as a net deposition flux given the photochemi-
cally driven emission of NOx as observed during ISCAT
1998 and 2000. This net deposition flux can be compared
with the NOx emission flux reported here (i.e.,
3:9� 108 molec cm�2 s�1). If one assumes that this
emission flux is fairly constant during the 4 months of
significant sunlight (e.g., 130 days with solar zenith
angles o80�) at South Pole, the annual NOx emission
flux would be 1:0kgNkm�2 yr�1; which is roughly half
the value of the net deposition flux. (Note, assuming a
constant flux across the entire plateau, the 6� 106 km2
surface area of the plateau would release a yearly flux of
0.006TgN.)
Despite the photochemical liberation of roughly half
as much nitrogen as is annually buried at SP, much of
this nitrogen is expected to be redeposited to the snow
based on the short lifetime for NOx (e.g., median value
of � 8h). As shown in Fig. 12, NOx released from the
Antarctic plateau is rapidly recycled back to the oxidized
form, HNO3 and HO2NO2: But, both HNO3 and
HO2NO2 have relatively short lifetimes due to their
efficient deposition to the snow surface (e.g., � 3h;Slusher et al., 2002). Thus, NOx reappears as snowpack
nitrate in less than a day. Given the persistence and
speed of katabatic windflow (e.g., 2–10m s�1), NOx may
be transported up to several hundred kilometers down-
slope before redeposition occurs. However, for katabatic
flow occurring along the outer perimeter of the plateau
(within 12
day flow time) there would result a net
loss of nitrogen from the plateau as there would be for
vertical mixing which would move NOy into the free
troposphere.
ARTICLE IN PRESS
Fig. 12. Nitrogen budget schematic illustrating the recycling of reactive nitrogen on the Antarctic plateau as well as possible nitrogen
loss processes that would lead to a requirement for a new primary nitrogen source.
D. Davis et al. / Atmospheric Environment 38 (2004) 5375–53885386
Returning to the down-slope redeposition scenario,
such a flow pattern suggests that there might be a
gradient in net deposition across the plateau. Mayewski
and Legrand (1990) have expressed difficulty in explain-
ing the large geographical variation of nitrate across the
Antarctic plateau with core samples from SP containing
as much as five times more nitrate than samples from
Vostok and Dome C. However, SP is located on the
lower edge of the plateau and typically receives down-
slope flow from the greater plateau. Thus, redeposition
of nitrogen from upslope emissions could offset the local
emission at SP. By contrast, Vostok and Dome C are
located much further up the plateau and experience little
to no downslope flow (see Parish, 1988). Relative to SP,
then, they would experience much less redeposition to
offset the emission of NOx:Evidence for redeposition can be found in the
measurements of nitrate within the first few centimeters
of snow. Measurements from SP indicate that nitrate
decreases by roughly an order of magnitude between the
surface and 10 cm (Dibb et al. (2004), and references
therein). Similar observations have been reported for
Dome C (Rothlisberger et al., 2000). The emission of
nitrate photolyzed within the upper several centimeters
of snow followed by the rapid redeposition of nitrate at
the surface would provide a plausible explanation for
the sharp gradient in nitrate observed by these
investigators.
Accumulation rate is another factor relevant to
understanding differences in nitrate between core
samples from SP, Vostok, and Dome C. Annual
accumulation at Vostok and Dome C is 2–3 times lower
than that at SP (Mayewski and Legrand, 1990). Thus,
fresh snow at the former sites resides in the upper 10 cm
for a longer period of time, thereby allowing photo-
chemical processes to release a larger fraction of the
initially deposited nitrate. The prolonged exposure to
the photodenitrification process, coupled with the more
limited opportunity for redeposition, would at least
qualitatively appear to explain much of the large
difference found in nitrate levels at SP relative to
Vostok and Dome C.
The collective processes of photo-induced emission of
NOx; followed by chemical oxidation and transport, and
finally redeposition would seem to allow for a major
redistribution of any nitrate initially deposited to the
Antarctic plateau. Differences in the length of exposure
to photochemistry as a function of accumulation rate
would also appear to significantly influence the degree of
nitrate redistribution. Differences in ice core samples as
well as estimates for nitrate deposition and NOx
emission flux estimates support the contention that this
redistribution appears to influence a large fraction of the
nitrate annually deposited to the plateau. Such a
redistribution further complicates the use of nitrate as
a proxy species for interpreting major geophysical events
ARTICLE IN PRESSD. Davis et al. / Atmospheric Environment 38 (2004) 5375–5388 5387
and climate changes as deduced from ice cores collected
from various plateau sites.
5. Summary and conclusions
The ISCAT 2000 field study has generated the most
extensive SP NO data base yet. These data have
provided significant answers to many of the questions
remaining after ISCAT 1998. Two independent instru-
ments operated by independent investigators have
unequivocally established that near surface SP NO
levels reach extraordinarily high levels (e.g., 600+pptv)
during the summer months. These measurements also
convincingly demonstrated that the source of NO is its
emission from the snowpack. The chemical process
responsible at SP has clearly been identified as photo-
chemical in nature, as it has also been shown by other
investigators at other polar sites. Many of the details
concerning the overall mechanism, however, still remain
conjecture. The data further indicate that at SP
photochemical processes within the snowpack operate
24 h a day. It follows that modulation of this subsurface
production of NOx can occur from both shifts in
overhead cloud coverage as well as from changes in the
overhead ozone column density (e.g., see Davis et al.,
2001; Mauldin et al., 2000 this issue).
One of the major tasks undertaken in this work has
been to address the question: why does SP generate such
high concentration levels of NO relative to other polar
sites? The answer now appears to lie in the unique
interplay that occurs between four factors: (1) 24 h of
continuous sunlight, (2) regularly occurring meteorolo-
gical conditions leading to very shallow PBL depths, (3)
being downwind from one of the world’s largest air
drainage fields, and (4) having a HOx–NOx chemical
environment where increasing NOx levels leads to a non-
linear response in NOx lifetime. Of the polar sites that
have been investigated, none but the SP reaches
sufficiently high levels of NOx that factor (4) becomes
significant. At no other polar site is it likely that surface
air parcels accumulate NOx over time periods of one to
two days. And, at no other polar site do shallow PBL
depths occur on a regular basis at the same time that
there is relatively continuous solar activity.
One of the strongest correlations found between the
NO mixing ratio and a meteorological parameter was
that involving the PBL depth ðR2 ¼ 0:93Þ: Thus, dailyshifts in the PBL depth were a major factor responsible
for the frequently observed abrupt changes in the
concentration level of NO. This has led to our
speculating that a systematic shift in the average PBL
value between the 1998 and 2000 study is mostly likely
the basis for the much higher NO median value during
ISCAT 1998. This hypothesis appears to be generally
consistent with the synoptic picture generated for the
Antarctic continent for the years 2000 and 1998. For
example, unlike in 1998, during December of 2000 high
pressure dominated over the entire polar plateau. This
produced near surface and free tropospheric winds over
the plateau that had nearly the same direction, a
meteorological setting leading to a weakening of the
surface temperature inversion, and hence, greater
vertical mixing.
Two final conclusions from the current analysis
address first the question of what the similarity might
be between the SP NO levels and those on the Antarctic
plateau. At this point in time, there are no direct
observations of NO or NOx over this expansive region.
Speculation here is that given the solar flux and nitrate
levels available on the plateau, high values of NO should
be found over a very large fraction of the plateau. The
second conclusion involves the importance of NOx
recycling to the plateau. It now appears that the
recycling of NOx due to its photo-induced emission
from the snowpack can explain most of the sharp
decreases seen in nitrate levels with depth at SP and
other plateau sites. It also appears to offer an explana-
tion for the variation seen in the average buried nitrogen
level found at SP versus higher elevation sites like Dome
C and Vostok.
Looking to the future, clearly the expansion of the
current SP observations to include a significant portion
of the Antarctic plateau must be viewed as a high
priority. Equally important will be collecting detailed
vertical profiles of NO and other species at SP to assess
the extent of the hypothesized oxidizing canopy over the
Antarctic plateau. And finally, a renewed effort is
needed to define the primary sources of plateau reactive
nitrogen as well as the loss processes operating in this
unique environment.
Acknowledgements
The authors would like to thank NOAA’s CMDL
personnel for their ready support of all our ISCAT
efforts at the ARO facility at South Pole, with special
thanks going to Pauline Roberts. The author D.D.
Davis would also like to express his appreciation to the
National Science Foundation’s Office of Polar Programs
(Grant # OPP-9725465) and the Division of Atmo-
spheric Chemistry for their partial support of this
research. Finally, D.D. Davis would like to acknowledge
the efforts of Joe Arnoldy in helping with the graphics of
this paper.
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