1
Episodes of High Surface Ozone Amounts at South Pole During Summer and Their
Impact on the Long-term Surface Ozone Variation
Samuel J. Oltmans
1
, Bryan J. Johnson
1
, and Detlev Helmig
2
1
NOAA Earth System Research Laboratory, Global Monitoring Division, 325
Broadway, Boulder, Colorado, 80305, USA
2
Institute of Arctic and Alpine Research, University of Colorado, Boulder, Colorado
80309, USA
Abstract: Long-term surface ozone and ozone profile measurements are used to investigate the character of summertime ozone behavior at South Pole. Summer ozone profiles show a significant gradient more than 40% of the time in which mixing ratios at the surface are at least 8 parts per billion by volume (ppbv) higher, and may exceed 20 ppbv higher, than mixing ratios several hundred meters above the surface. These ozone gradients are linked to very stable conditions in the boundary layer. The frequency of occurrence of these surface ozone enhancements has varied with time with the most recent 10-year period showing a greater number of occurrences. Although the summer enhancements have influenced the overall long-term pattern of change in surface ozone, they are not the only factor. The earlier decline in surface ozone amounts that continued into the mid 1990s was influenced by changes in other seasons as well. Surface ozone measurements from the 1960s show that summer enhancements were a significant feature of the record at South Pole during this period. Measurements at a lower elevation inland location (Byrd Station), not on the Antarctic Plateau, do not show large summer ozone chemical production events indicating that this phenomenon is primarily confined to the plateau.
Introduction
The discovery of an active photochemistry in the summer boundary layer over the
Antarctic plateau (Davis et al., 2001, 2004) has fueled ongoing investigations of the character, causes and significance of this phenomenon (Crawford et al., 2001, Jones and
Wolff, 2003). Because an important consequence of this photochemical processing is the production of elevated ozone amounts in the near surface boundary layer over an extensive portion of the Antarctic continent encompassed by the higher elevation plateau, the possible implications for the tropospheric ozone budget are of particular interest.
In this work several aspects of summertime ozone behavior are investigated. The
South Pole balloon ozonesonde measurements are used to characterize the boundary layer ozone profile during the November-December-January austral summer, and to determine the relative frequency of occurrence of several categories of ozone vertical gradient

2 strength. Using a methodology developed for this work, the long-term surface ozone record at South Pole is used to determine a time series of the annual occurrence (a combination of their frequency and duration) of summertime ozone enhancements. The implications of the changing frequency of occurrence of the summer enhancements for the overall long-term surface ozone variation are also explored. In addition several years of surface ozone data from the 1960s from South Pole and several other Antarctic sites are investigated to demonstrate that ozone chemical production events are likely restricted to the Antarctic plateau and that the South Pole events are part of an even longer historical pattern than that encompassed by the continuous record dating from
1975. Although the focus of this work is to better quantify the occurrence of summer high ozone events, changes in the temporal pattern of these events are discussed in light of several mechanisms that have been proposed for longer-term changes in surface ozone at
South Pole.
The earliest analysis of surface ozone observations in Antarctica (Oltmans and
Komhyr, 1976) noted the unique behavior represented by the summer South Pole measurements compared to that at other Antarctic locations. Their conclusions linked the boundary layer meteorology and summer surface ozone enhancements but did not consider the possibility of ozone production near the surface. Crawford et al. (2001) linked the active photochemistry discovered during the December 1998 Investigation of
Sulfur Chemistry in the Antarctic Troposphere (ISCAT) campaign at South Pole with the enhanced boundary layer surface ozone. Jones and Wolff (2003) examined the frequency of enhanced ozone events and found a change over time with an increasing frequency of periods with enhanced ozone that they attributed to changing actinic flux related to stratospheric ozone depletion. For this study the record of surface ozone measurements that begins in 1975 is extended through the summer season of 2005-2006. Identification and quantification of the magnitude of enhanced ozone events are based on a projected seasonal pattern that in the absence of the enhancements in ozone would decline more or less linearly from the end of the seasonal maximum in October to the seasonal minimum in February. This projected seasonal pattern is determined for each year so that other possible contributors to a longer-term change in surface ozone amounts at South Pole have a reduced impact on the analysis.

3
Measurements
The primary sources of data used in this study are the continuous, long-term South
Pole surface ozone record that began in 1975, the ozonesonde profiles that began in 1986 but are used in this study from 1991, and surface ozone measurements from several locations in Antarctica (see Figure 1 for station locations). The South Pole data have been used extensively (Schnell et al., 1991; Crawford et al., 2001; Jones and Wolff, 2003;
Oltmans et al., 2006; Helmig et al., 2007a) to investigate long-term changes and the summer enhancement phenomena. The first two years of surface ozone measurements were made using an electrochemical sensor similar in design to the ECC ozonesonde discussed below. The inlet was located approximately 3 meters above the snow surface.
Beginning in 1976 a Dasibi Model 1003-AH was added and run alongside the electrochemical sensor through 1979 (Oltmans and Komhyr, 1986) when the electrochemical sensor was removed from service. In 1977 the surface ozone analyzers were installed in an elevated building and sampling was done from a 10 meter, high flowrate sampling stack. The effective height of the air intake was about 15 meters above the snow surface. This inlet height has been maintained within about

2 meters depending on snow accumulation and a move to a new building in 1998. In 2000 a TEI
Model 49C ozone analyzer was installed and operated alongside the Dasibi Model 1003-
AH through 2005. The calibration level of the station instruments has been maintained by periodic intercomparison on an approximately three-year schedule with a network standard instrument that has been regularly intercompared with a U.S. National Institute of Standards and Technology (NIST) maintained standard reference photometer (SRP).
Overall accuracy and long-term precision are estimated at

5 %.
The surface ozone observations made in the 1960s at several locations in Antarctic were obtained using a variety of instrument types. The calibration records and the absolute scaling of these data have not been preserved (Oltmans and Komhyr, 1976).
Measurements made with several types of analyzers with overlapping records do not give sufficient confidence that these data can be combined with modern records to extend the trend record backwards. These data do, however, provide important information on the

4 character of variations seen during this period and an opportunity to glean some information on the spatial characteristics of summer ozone behavior in Antarctica.
The ozonesonde profile measurements were obtained using the electrochemical concentration cell (ECC) ozonesonde (Komhyr, 1969). Characteristics of the performance of this instrument are detailed in Johnson et al. (2002). Prior to 1991 the ozonesonde profile data were obtained with an analog data recording system from which the data were extracted at a relatively low altitude resolution of about 250 meters.
Beginning in 1991 the digital ozonesondes provided data every 7.5 seconds or at an altitude resolution of about 40 m for the nominal balloon rise rate of 5 m s
-1
, chosen so that a full ozone profile through the stratosphere could be obtained in a reasonable period of time. In 1998 the system was upgraded to obtain data every 1.2 seconds. Because, as will be shown later, the variations seen in the surface record are confined to a layer <500 m above the surface, the higher vertical resolution provided by the data since 1991 give a clearer picture of the vertical extent and variation within the boundary layer than was possible with the earlier data set (Crawford et al., 2001). As discussed later, and shown in greater detail in Johnson et al. (2007), the time response of the ozonesonde sensor needs to be kept in mind when examining the vertical structure of the enhancement events.
As part of the Antarctic Tropospheric Chemistry Investigation (ANTCI) campaign conducted during Summer 2003 standard release balloon ozonesondes as used in the operational program at South Pole were compared with a slowly ascending and descending ozonesonde attached to a tethered balloon. A full discussion of the results of these comparisons is contained in Johnson et al. (2007). An example of one such comparison done on December 18, 2003 is shown in Figure 2. Because the released sonde showed some noisiness in the early portions of the sounding due to the jostling of the instrument when it was released, the release ozonesonde data have been smoothed slightly with a running 5-point (5 one second data points) smoothing. The ozone profile obtained from the ozonesonde on the tethered balloon showed a much sharper transition at the top of the low-altitude enhanced layer than was seen in the release balloon.
Because of the slow ascent rate of the tethered balloon, the response time of the ozone sensor is not a factor and the altitude of the sharp transition in the tethered balloon profile can be considered the true top of the enhanced ozone layer. In this example the transition

5 layer was only about 100 m thick rather than the 300 m implied by the release ozonesonde. Under the assumption that the ~200 m altitude difference in the top of the enhanced ozone layer between the release ozonesonde and tethered sonde was a result of the sensor response, the measured ascent rate of 5 m/sec means that it took about 40 s for the release sonde to achieve the same ozone value as that seen by the tethered sonde, which translates to a response time of ~40 s (200 m divided by 5 m s
-1
). This would indicate somewhat better performance than the quoted time of 60 s for a 90% response to a step change (Komhyr et al., 1995). Since the regular South Pole station release balloon profiles were obtained on only an approximately weekly schedule during the summer months, they only present snapshots of the overall time variation of ozone in the near surface layer.
Results
The unique character of the summertime surface ozone behavior can be seen in the comparison of the seasonal variation at South Pole and at an Antarctic coastal station
(Figure 3). The seasonal pattern at Arrival Heights is typical of other coastal locations in
Antarctica (Oltmans and Komhyr, 1976; Helmig et al., 2007a). At Arrival Heights (see
Figure 1 for station locations) spring (Aug.-Sep.-Oct.) surface values begin to decline toward a seasonal minimum in January. At South Pole on the other hand periodic events with the highest ozone values of the year are seen during this period. The seasonal minimum is also delayed until February. During March to October the two locations have similar ozone amounts but they differ from one another during the late spring and summer. This summer reduction of ozone toward the summer is seen throughout the S.H.
(Oltmans and Levy II, 1994) and is associated with strong photochemical loss processes during the summer in an environment where low nitrogen oxide concentrations prevail
(Ayers et al., 1992).
Characteristics of Profiles
The regular probing of the vertical structure of ozone over the South Pole reveals the wide variations that occur in the boundary layer during the summer (Nov.-Dec.-Jan.).

6
Two profiles are shown in the individual panels of Figure 4 that contrast a case with a large boundary layer ozone enhancement and one with no enhancement. In the November
25, 1996 profile ozone (upper graph) drops about 15 ppbv from the surface value of 37 ppbv through a layer ~300 m thick (this takes into account the lag in the ozonesonde response time). In contrast the January 1, 2001 profile (lower graph) shows essentially no vertical gradient in the lowest 1000 m of the atmosphere. There is also a noticeable difference in the thermal and moisture profile structure in these contrasting cases. A significant temperature inversion layer is discernable in the November 25 profile that is also reflected in the moisture content (the frost-point temperature computed from the radiosonde humidity measurement). The inversion (temperature increasing with altitude) in the November 25 profile extends to an altitude of 3100 meters above mean sea level
(msl) - or about 300 meters above the surface - while in the January 1 profile, where there is no surface ozone enhancement, there is not a well-developed inversion. It appears that the enhanced ozone layer extends about 100 m above the top of the inversion. This result is attributable to the mismatch between the time response of the temperature sensor
(nearly instantaneous) and that of the ozonesonde, which has a somewhat slower response as noted earlier. In all of the cases that were investigated where an ozone gradient was present the layer was confined to about the lowest 400 m above the surface.
In these two examples it is not the absolute difference in temperature that is important, since this primarily reflects the seasonal change in temperature, but the thermal structure, which reflects the strong atmospheric stability. Solar irradiance measurements (that represent incoming solar radiation almost exclusively at visible wavelengths) show a period of continuous high irradiances indicative of generally clear sky conditions leading up to the November 25, 1996 profile. On January 1, 2001 the day prior to the measurement shown in the lower graph indicates a reduction in the solar irradiance due to the presence of clouds. At the time of the sounding itself, however, there was likely minimal cloudiness. The other notable difference in these two cases is the local wind direction in the period prior to the soundings and at the time of the sounding. In
November 1996 steady but light winds from the east were present for several days while in January 2001 winds, though light, were consistently from the north or north-northwest.
As has been noted earlier (Crawford et al., 2001) and in more recent work (Helmig et al.,

7
2007b) ozone enhancement episodes are favored during times with winds from the easterly sector coming over the plateau.
A composite picture formed from the 215 analzyed vertical profiles (Figure 5) shows that enhanced surface ozone was a common feature, where 6% of the profiles had a strong ozone gradient (delta O
3
> 17 ppbv), 35% had a moderate gradient (8-17 ppbv),
35% had a weak gradient (2-7 ppbv) with higher ozone near the surface while 24 % had insignificant gradients (< 2 ppbv) in the lowest 400 m. In the cases when a gradient in ozone was present (even in the cases with a weak gradient) the temperature structure in the lowest ~300 m shows a temperature inversion of several degrees Celsius on average.
Such temperature gradients generally were not present for the no-ozone gradient cases
(Figure 5 – lower panel). When there is no ozone gradient the boundary layer has ozone mixing ratios, temperatures and moisture values that are similar to the values seen above the boundary layer indicating that the near-surface layer is well mixed with the layer above. Thus the thermal stability of the lowest part of the troposphere, marked by colder temperatures at the surface on average than for the no ozone gradient case, is an important ingredient in the development of enhanced ozone in this layer. Above ~3500 m above msl on average the ozone and temperature profiles for the enhanced ozone and no ozone cases are nearly identical.
Surface Ozone Events
The hourly ozone mixing ratios for January 2001 and December 2003 (the time of the
ANTCI 2003 campaign) at South Pole (Figure 6) show the large variability typical of the summer months. Maximum hourly concentrations exceed 40 ppbv while minimum values are near 20 ppbv.
In order to quantify the frequency of enhanced ozone events during the summer at
South Pole a projected seasonal pattern is formulated. This is based on the decline from
October through February found at other Antarctic locations. As can be seen in figure 2 the average ozone amounts at Arrival Heights and South Pole during the period from
March through October closely match one another. Similarly, the ozone record from the inland, but non-plateau, location of Byrd station (Figure 9 – inset) follows a pattern similar to the coastal sites with surface observations (Helmig et al., 2007a). These

8 stations regularly reach minimum values of 10-15 ppbv in January-February that are not observed at South Pole. At South Pole average and inner 50 th
percentile values during
February and March are nearly identical (Figure 2a). The February value over the plateau may reflect a modest buildup of ozone dispersed over the plateau as a result of photochemical ozone production seen most dramatically in the ozone enhancement events.
The projected seasonal ozone values for November through January are a simple linear interpolation between the October value for a year and the February value for the following year. Since the February mean is on average nearly identical with March and does not achieve the low values seen at non-plateau locations, this is a conservative estimate of the non-chemically perturbed (non-enhanced) pattern for the projected decline at South Pole. A single projected value for each of the months November, December, and
January is used for each summer season (i.e. the projected decline is not calculated on an individual daily basis). Attempts at a seasonal fit based on the February through October values did not produce a significantly different result from using a linear interpolation.
By calculating a projected seasonal pattern for each summer any longer-term measurement drifts in the time series are minimized, although it is not expected that significant drifts are present. Using a projected seasonal cycle is a somewhat less conservative approach than that used by Jones and Wolff (2003) that recognizes the strong seasonal decline that is seen at non-plateau locations and that is also reflected in the troposphere above the boundary layer at South Pole as well as at the coastal ozonesonde station at Syowa (figure not shown).
The projected seasonal value for each month was subtracted from each hourly value for the month. Both the positive and negative deviations larger than 1 ppbv were accumulated. The number of deviations was normalized by dividing by the total number of hourly observations during the month. This reflects changes in both the number and duration of these events. The normalized frequency of the positive and negative deviations is shown in Figure 7 for each summer month (November, December, and
January) and the summer season for each year. The value for the season is plotted with the January year (e.g. for the season November 2003 – January 2004 the value is plotted in 2004). There are some differences in the pattern of the deviations depending on the

9 month. For November there is a more consistent pattern of higher positive deviations relative to negative ones beginning in 1992. In December there is a tendency for the earliest years to have a greater frequency of positive deviations followed by a period of about equal positive and negative deviations with again a tendency for a greater occurrence of positive deviations beginning in 1992. January is dominated by several years in the 1980s when negative deviations occur very often. The average variation for the summer season shows that for the season as whole there is more of a balance between the positive and negative departures in the 1980s and a dominance of positive departures after 1992. There appears to be a tendency toward positive departures in the first four years of the record although it should be noted that during the first two years the measurements were made more than 10 meters closer to the snow surface than in later years. Given the steep vertical ozone gradient during the enhancement events this could be a significant factor (Helmig et al., 2007b) in producing more positive deviations in those years.
Perspectives From Earlier Antarctic Surface Data
Surface ozone measurements from the 1960s give a perspective on the summer pattern for an earlier period at South Pole and at several other sites. Examples of hourly values for two months in the 1960s at South Pole (Figure 8) show a similar pattern to the current summer variations (caution needs to be exercised in comparing the magnitude of the mixing ratios from the earlier observations with the later data). Extending the analysis of the deviations for the summer monthly values as done for the 1975-2005 data to the earlier period (Figure 7) suggests a pattern similar to what was observed in the 1970s with a rough balance between positive and negative deviations.
From an examination of the Arrival Heights observations it was clear that an Antarctic coastal location does not show the abundant summer enhancements seen at South Pole.
Byrd Station is located inland but is not on the higher elevation plateau (1540 m at Byrd vs. 2840 m at South Pole). Although Byrd remains snow covered throughout the year it does not see the large enhancements seen at South Pole during the summer (Figure 9) and the seasonal pattern is similar to the coastal locations with ozone declining more or less monotonically, approaching the minimum by December (Figure 9). In a transect from

10
Byrd to the South Pole in Summer 2002 (Frey et al., 2005) enhanced surface ozone amounts were not seen until crossing 85S when an elevation of 2400 m was reached. It seems likely that the unique conditions of the higher elevation plateau are a necessary ingredient for the occurrence of enhanced ozone at the surface. It appears that local ozone production at South Pole itself is not sufficient to produce the degree of ozone enhancement that is regularly observed at South Pole in the summer. Transport of air for several days over the plateau region conducive to ozone production and confinement near the surface has been noted to be a factor contributing to the high ozone levels observed at
South Pole (Helmig et al., 2007b).
Discussion and Conclusions
The long-term measurements of surface ozone and the ozone balloon profiles from
South Pole demonstrate the dominating influence of summertime enhanced ozone events in the boundary layer in determining the unique character of the seasonal pattern at this site. In the most dramatic cases ozone at the surface is 20 ppbv higher than at the top of the layer that is less than 500 m thick. Much more common are surface enhancements in the range of 5-15 ppbv that are responsible for driving the overall pattern of surface variations. Situations with enhanced surface ozone are characterized by stable boundary conditions with a low level inversion that corresponds (after accounting for the time response of the ozonesonde) with the enhanced ozone layer (Figures 4 and 5). The characteristics of the profiles are quite consistent with the results of Crawford et al.
(2001).
The cause of the summer ozone enhancement episodes is linked to the unique photochemistry associated with the high boundary layer NO
X
concentrations found during these events, sometimes reaching 100s of parts per trillion (Davis et al., 2001,
2004; Helmig et al., 2007c). The high NO
X
concentrations found over the plateau are in contrast with amounts of a few parts per trillion that have been measured over the
Southern ocean on the coast of Antarctica (Jones et al., 2000). The source of the elevated
NO
X
to the atmosphere is likely photochemical production from nitrates deposited in the snow (Honrath et al., 1999; Jones et al., 2000, 2001). Based on measured constituent and meteorological conditions during the summer at South Pole significant ozone production

11 can be expected (Crawford et al., 2001) although modeled production may not fully explain the magnitude of the enhancements that have been measured (Crawford, 2001;
Helmig 2007b).
The year-to-year variability of the surface ozone enhancement events (Jones and
Wolff, 2003) is one of the intriguing features of the record with a tendency for a more consistent pattern of enhancements events starting in Summer 1992-1993 (Figure 7).
During the 1980s, which was a period of fewer enhancement events, there were a larger number of lower ozone events that were particularly prominent in January but were also more frequent in the other summer months as well. This is not inconsistent with the suggestion of Schnell et al. (1991) that the decline in South Pole surface ozone seen up to that point might be associated with a greater frequency of summer transport of air parcels from the Antarctic coast with lower ozone amounts. The Schnell et al. (1991) association of these lower ozone amounts with ozone losses in the troposphere from the influence of stratospheric ozone depletion on the seasonal tropospheric photochemical ozone loss at the periphery of Antarctica does not seem to be borne out, however, given the continuing stratospheric ozone depletion but the recovery of South Pole summer surface ozone. A more direct link with changing surface ultraviolet levels has been postulated by Jones and
Wolff (2003) where enhanced ultraviolet radiation as a result of diminished overhead column ozone is able to produce a more active photochemical production of nitric oxide from nitrate deposited in the snow (Crawford et al., 2001). Stratospheric ozone depletion at South Pole is strongest in October but generally persists to a significant degree through much of November. As pointed out by Jones and Wolff (2003) available UV radiation rises significantly through the summer. In January the difference between the total column ozone amount in the pre and post ozone hole years is much smaller than during the earlier austral summer months and thus the influence on UV reaching the surface is greatly reduced (Jones and Wolff, 2003). However, the pattern of more numerous enhancements in the most recent 14 years seems to be present in January as well as the other summer months. The ability to detect the increase in the occurrence of ozone enhancements in January as well as in November and December is one of the key findings of this work and suggests that the possible link with stratospheric ozone changes could be complex. Because the ability to sustain the surface enhancements is closely tied

12 to the boundary layer stability characteristics (Crawford et al., 2001; Neff et al., 2007), any long-term changes in the overlying synoptic scale wind pattern could also lead to changes in the frequency of occurrence of enhancement events (Neff, 1999; Thompson and Solomon, 2002; Helmig et al., 2007b). The 25-year record of solar irradiance measured at South Pole (Dutton et al., 2006) has shown a pattern of long-term change that might be indicative of changes in circulation patterns. Irradiance values declined in the 1970s, remained relatively low in the 1980s and rebounded in the 1990s. The decline in solar irradiance was linked to increases in cloudiness (Dutton et al., 1991; Schnell et al., 1991) that reflects the transport of moister air from the coast of Antarctica to South
Pole during the 1980s. The changes in the measured irradiance, which does not include the relevant ultraviolet wavelengths, cannot be used to determine the direct effect of changes in the ozone photochemical environment. The cause of the change in frequency of summertime enhanced surface ozone events may thus result from the complex interplay of a changing radiation environment and long-term meteorological characteristics such as cloudiness or alteration in the stability characteristics of the nearsurface layer resulting from weakened synoptic scale wind regimes (Neff, 1999).
At South Pole there has been an overall pattern of long-term change in surface ozone mixing ratios (Schnell et al., 1991; Crawford et al., 2001; Oltmans et al., 2006; Helmig et al., 2007a) that showed declining annual values through the 1980s and a recovery to values near those at the beginning of the record in the most recent years (Figure 10).
Comparison of the composite seasonal variation for five-year intervals (Figure 11) shows that the long-term pattern of change cannot be attributed to a single season. During the austral summer months ozone has been higher in the most recent 10 years compared to earlier years except for the initial five-year period. However, 1996-2000 has relatively low autumn and winter ozone levels that somewhat offset the higher summer values.
During the overall period of decline in the 1980s the summer months are noticeably lower, but in the lowest period (1991-1995) amounts are lower throughout the year, especially in the spring. This pattern means that during some five-year intervals there are compensating seasonal effects and during other intervals a particular season is responsible for the lower annual value. The recovery seen in the most recent 10 years does appear to primarily result from the larger summertime amounts that are in turn a

13 reflection of the greater frequency of boundary layer ozone enhancement events. This has led to a small increasing trend in the summer months (Helmig et al., 2007a), but no significant change in the year-round record (Figure 10, Helmig et al., 2007a).

14
References
Ayers G.P., Penkett, S.A., Gillett, R.W., Bandy, B., Galbally, I.E., Meyer, C.P., Elsworth,
C.M., Bentley, S.T and Forgan, B.W., 1992. Evidence for photochemical control of ozone concentrations in unpolluted marine air, Nature, 360, 446-449.
Crawford J.H., Davis D.D., Chen G., Buhr M., Shetter R., Lefer B., Hogan A., Arimoto
R., Oltmans S. and Weller R. (2001) Ozone photochemistry at the South Pole as inferred from measurements of OH, NO, and actinic flux during ISCAT. Geophys. Res. Lett. 28 ,
3641-3644.
Davis, D., Nowak, J.B., Chen, G., Buhr, M., Arimoto, R., Hogan, A., Eisele, F., Mauldin,
L., Tanner, D., Shetter, R., Lefer, B. and McMurry, P., 2001. Unexpected high levels of
NO observed at South Pole. Geophys. Res. Lett.
28 , 3625-3628.
Davis, D., Chen, G., Buhr, M., Crawford, J., Lenshow, D., Lefer, B., Shetter, R., Eisele,
F., Mauldin, L., and Hogan, A., 2004. South Pole NOx chemistry: an assessment of factors controlling variability and absolute levels. Atmos. Environ. 38 , 5375-5388.
Dutton, E.G., Stone, R.S., Nelson, D.W. and Mendonca, B.G., 1991. Recent interannual variations in solar radiation, cloudiness, and surface temperature at the South Pole, J.
Climate 4(8), 848-858.
Dutton, E.G., Nelson, D.W., Stone, R.S., Longenecker, D., Carbaugh, G., Harris, J.M., and Wendell, J., 2006. Decadal variations in surface solar irradiance as observed in a globally remote network, J. Geophys. Res.
, 111, D19101, doi:10.1029/2005JD006901.
Frey, M.M., Stewart, R.W., McConnell, J.R., and Bales, R.C., 2005. Atmospheric hydroperoxides in West Antarctica: Links to stratospheric ozone and atmospheric oxidation capacity, J. Geophys. Res., 110 , D23301, doi:10.1029/2005JD006110.
Harris, J.M., Oltmans, S.J., Tans, P.P., Evans, R.D., and Quincy D.L., 2001. A new method for describing long-term changes in total ozone, Geophys. Res. Lett., 38(24),
4535-4538.
Helmig D., Oltmans S.J., Carlson D., Lamarque J.F., Jones A., Labuschagne C., Anlauf
K. and Hayden K., 2007a. Surface ozone in the polar regions. Atmos. Environ ., in press.
Helmig, D. Johnson, B., Oltmans, S. J., Neff, W., Eisele, F., and Davis, D. D., 2007b.
Boundary-layer ozone production at South Pole, Atmos. Environ ., in press.
Helmig D., Johnson B., Warshawsky M., Morse T., Neff W., Eisele F. and Davis D.
2007c. Nitric oxide in the boundary-layer at South Pole during the Antarctic
Tropospheric Chemistry Investigation (ANTCI) Atmos. Environ.
, submitted.

15
Honrath, R.E., Peterson, M.C., Guo, S., Dibb, J.E., Shepson, P.B., and Campbell, 1999.
Evidence of NO
X
production within or upon ice particles in the Greenland snowpack,
Geophys. Res. Lett., 26, 695-698.
Johnson, B.J, Oltmans, S.J., Vömel, H., Smit, H.G.J., Deshler, T., and Kröger C., 2002.
Electrochemical concentration cell (ECC) ozonesonde pump efficiency measurements and tests on the sensitivity to ozone of buffered and unbuffered ECC sensor cathode solution, J. Geophys. Res., 107(D19) , 4393, doi:10.1029/2001JD000557.
Johnson, B.J., Helmig, D., Oltmans, S.J., and Brown, I., 2007. Validation of ozone measurements from a tethered balloon sampling platform at South Pole Station in 2003,
Atmos. Environ ., submitted.
Jones, A.E., Weller, R., Wolff, E.W., and Jacobi, H.W., 2000. Speciation and rate of photochemical NO and NO
2
production in Antarctic snow, Geophys. Res. Lett., 27, 345-
348.
Jones, A.E., Weller, R., Anderson, P.S., Jacobi, H.W., Wolff, E.W., Schrems, O., and
Miller, H., 2001. Measurements of NO
X
emissions from the Antarctic snowpack,
Geophys. Res. Lett., 28, 1499-1502.
Jones, A.E. and Wolff, E.W., 2003. An analysis of oxidation potential of the South Pole boundary layer and influence of stratospheric ozone depletion, J. Geophys. Res.,
108(D18) , 4565, doi:10.1029/2003JD003379.
Komhyr, W.D., 1969. Electrochemical cells for gas analysis, Ann. Geophys., 25 103-110.
Neff, W.D., 1999, Decadal time scale trends and variability in the tropospheric circulation over the South Pole, J. Geophys. Res. 104 , 27,217-27,251.
Neff, W., Helmig, D., Garchev, A., and Davis, D., 2007. A study of boundary layer effects on surface NO concentrations at the South Pole using minisodar, tethered balloon, and surface turbulence measurements. Atmos. Environ ., submitted.
Oltmans, S.J. and Komhyr, W.D., 1976. Surface ozone in Antarctica, J. Geophys. Res.,
81 , 5359-5364.
Oltmans, S.J., 1981. Surface ozone measurements in clean air, J. Geophys. Res., 86 ,
1174-1180.
Oltmans, S. J. and Komhyr, W.D., 1986. Surface ozone distributions and variations from
1973-1984 measurements at the NOAA Geophysical Monitoring for Climatic Change baseline observatories, J. Geophys. Res., 91 , 5229-5236.
Oltmans, S.J. and Levy II, H., 1994. Surface ozone measurements from a global network,
Atmos. Environ., 28(1), 9-24.

16
Oltmans, S.J., Lefohn, A.S., Harris, J.M., Galbally, I., Scheel, H.E., Bodeker, G., Brunke,
E., Claude, H., Tarasick, D., Johnson, B.J., Simmonds, P., Shadwick, D., Anlauf, K.,
Hayden, K., Schmidlin, F., Fujimoto, T., Akagi, K., Meyer, C., Nichol S., Davies, J.,
Redondas, A., Cuevas. E., 2006. Long-term changes in tropospheric ozone, Atmos.
Environ., 40 , 3156-3173.
Schnell, R.C., Liu. S.C., Oltmans, S.J., Stone, R.S., Hofmann, D.J., Dutton, E.G.,
Deshler, T, Sturges, W.T., Harder, J.W., Sewell, S.D., Trainer, M., and Harris, J.M.,
1991. Decrease of summer tropospheric ozone concentrations in Antarctica, Nature, 351 ,
726-729.
Thompson, D. W. J. and Solomon, S., 2002. Interpretation of recent Southern
Hemisphere climate change, Science, 296 , 895-899.

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Figure Captions:
Figure 1: Map of Antarctic station locations.
Figure 2: Comparison of ozone profiles obtained nearly simultaneously from a standard release ozonesonde and an ozonesonde attached to a slow rising tethered balloon at South
Pole on December 18, 2003, 0322 GMT launch time. The altitude of the ozone values is plotted in meters (m) above the surface. This illustrates the influence of the time response of the ozonesonde sensor on the ozone profile shape when the balloon is rising at a relatively rapid rate.
Figure 3: South Pole (a) daily ozone values for the year 2002, (b) monthly average ozone for the 15-year period 1991-2004. The solid dot is the mean, the horizontal bar inside the box is the median, the box is the inner 50 th percentile (25 th and 75 th ), and the whiskers are the inner 90 th
percentile (5 th
and 95 th
), (c) same as a) but for Arrival Heights, and (d) same as b) but for Arrival Heights.
Figure 4: Example of a profile with a strong ozone enhancement in the boundary layer
(upper panel) and a profile with no enhancement (lower panel). The surface altitude at
South Pole is 2830 m above sea level.
Figure 5: Composites of the low altitude portion of ozone, temperature, and frost point temperature for profiles with various boundary layer ozone gradient characteristics based on 215 profiles during the summer months of 1991-2005: strong or moderate ozone gradient (upper panel); no ozone gradient (lower panel).
Figure 6: Examples of hourly surface ozone mixing ratios at South Pole for January 2001 and December 2003.
Figure 7: The normalized frequency for each year of positive and negative deviations of surface ozone at South Pole greater than 1 pppv. The deviations were calculated from a
“projected seasonal variation between October and February” as explained in the text.
The deviations are for November, December, January, and the summer season (Nov.-
Dec.-Jan.) The value for the season is plotted at the January year (e.g., November 2003 +
December 2003 + January 2004 is plotted as 2004).
Figure 8: Examples of hourly surface ozone at South Pole for December 1964 and
January 1968. These examples are from a period before the current continuous time series, which began in 1975.
Figure 9: Monthly averages for the period 1963-1965 and hourly values for December
1964 from Byrd Station, Antarctica illustrating the different characteristics of this inland, but non-plateau site, compared to South Pole.
Figure 10: Long-term variation in surface ozone at South Pole marked by a decline from the beginning of the record into the mid 1990s and a recovery since then. Diamonds are the monthly means, the red (light solid) line is a model fit to the data, the (thick solid)

18 blue line is a fit to the residuals (difference between the observations and the model), and the dashed blue lines are the 95% confidence interval for the fit to the residuals (Harris et al., 2001). The overall trend is –0.61

0.66 % decade
-1
. The trend is the average growth rate, which is found by differentiating the tendency curve (thick solid blue line) over the full record.
Figure 11: Five-year average monthly mean surface ozone mixing ratios at South Pole covering the period 1976-2005 (six pentads).

Figure 1: Map of Antarctica showing location of stations.
19

20
Figure 2: Comparison of ozone profiles obtained nearly simultaneously from a standard release ozonesonde and an ozonesonde attached to a slow rising tethered balloon at South
Pole on December 18, 2003, 0322 GMT launch time. The altitude of the ozone values is plotted in meters (m) above the surface. This illustrates the influence of the time response of the ozonesonde sensor on the ozone profile shape when the balloon is rising at a relatively rapid rate.

21
Figure 3: South Pole (a) daily ozone values for the year 2002, (b) monthly average ozone for the 15-year period 1991-2004. The solid dot is the mean, the horizontal bar inside the box is the median, the box is the inner 50 th percentile (25 th and 75 th ), and the whiskers are the inner 90 th
percentile (5 th
and 95 th
), (c) same as a) but for Arrival Heights, and (d) same as b) but for Arrival Heights.

22
Figure 4: Example of a profile with a strong ozone enhancement in the boundary layer
(upper panel) and a profile with no enhancement (lower panel). The surface altitude at
South Pole is 2830 m above sea level.

23
Figure 5: Composites of the low altitude portion of ozone, temperature, and frost point temperature for profiles with various boundary layer ozone gradient characteristics based on 215 profiles during the summer months of 1991-2005: strong or moderate ozone gradient (upper panel); no ozone gradient (lower panel).

24
Figure 6: Examples of hourly surface ozone mixing ratios at South Pole for January 2001 and December 2003.

25
Figure 7: The normalized frequency for each year of positive and negative deviations of surface ozone at South Pole greater than 1 ppbv. The deviations were calculated from a
“projected seasonal variation between October and February” as explained in the text.
The deviations are for (a) November, (b) December, (c) January, and the (d) summer season (Nov.-Dec.-Jan.) The value for the season is plotted at the January year (e.g.,
November 2003 + December 2003 + January 2004 is plotted as 2004).

26
Figure 7: The normalized frequency for each year of positive and negative deviations of surface ozone at South Pole greater than 1 ppbv. The deviations are from a “projected seasonal variation between October and February” as explained in the text. The deviations are for (a) November, (b) December, (c) January, and the (d) summer season
(Nov.-Dec.-Jan.) The value for the season is plotted at the January year (e.g., November
2003 + December 2003 + January 2004 is plotted as 2004).

27
Figure 8: Examples of hourly surface ozone at South Pole for December 1964 and
January 1968. These examples are from a period before the current continuous time series, which began in 1975.

28
Figure 9: Monthly averages for the period 1963-1965 and hourly values for December
1964 from Byrd Station, Antarctica illustrating the different characteristics of this inland, but non-plateau site, compared to South Pole.

29
Figure 10: Long-term variation in surface ozone at South Pole marked by a decline from the beginning of the record into the mid 1990s and a recovery since then. Diamonds are the monthly means, the red (light solid) line is a model fit to the data, the (thick solid) blue line is a fit to the residuals (difference between the observations and the model), and the dashed blue lines are the 95% confidence interval for the fit to the residuals (Harris et al., 2001). The overall trend is –0.61

0.66 % decade
-1
. The trend is the average growth rate, which is found by differentiating the tendency curve (thick solid blue line) over the full record.

30
Figure 11: Five-year average monthly mean surface ozone mixing ratios at South Pole covering the period 1976-2005 (six pentads).