Surface Ozone Monitoring and Background Concentration at Zhongshan Station , Antarctica

The background surface O3 concentrations and seasonal changes observed at the Zhongshan Station (69 ̊22'2''S, 76 ̊21'49''E; 18.5 m), east Antarctica from 2008 to 2013 are presented. Irrespective of wind direction, surface O3 concentrations distribute evenly after the removal of polluted air from station operations, accounting for 1.1% of the data. These O3 exhibit the expected lowest in summer, with a peak in winter. The daily range of average O3 in all four seasons is small. The monthly mean O3 is similar to that of other stations in Antarctica, with seasonal CO2 amplitudes in the order of 15 ppb to 35 ppb. Surface O3 significantly negatively correlated with UVB in the spring and autumn, with correlation coefficients of 0.50 and 0.57 under the 0.01 significance test. Furthermore, the surface O3 concentration during polar nights was 1 2 times higher than that during polar days. Thus, the chemical effect of the aurora lights was the dominant cause of ozone destruction, showing that surface O3 observed in Antarctica has a small interferences from human activities in the atmosphere as it moves from the north through the southern hemisphere.


Introduction
The troposphere ozone has two major sources: the photochemical production of the troposphere and transport of the stratosphere.The photochemical source of the troposphere ozone is mainly controlled by the emission of ozone precursors, L. G. Bian  including NO x and volatile organic compounds (VOCs).The majority of surface ozone growth might be related to increased precursor emissions before photochemical reaction.Since the industrial age, the troposphere ozone concentration has at least doubled [1].In polar region, which lack human-induced sources, especially in Antarctic region, the troposphere ozone is mainly determined by natural processes, namely, the meridional transport of airflow and downward transport of the stratosphere.Therefore, surface ozone data in polar region contain important values for evaluating ozone variation trend.The longest ozone observation data of Arctic region is in Barrow station.The annual growth trend was found to be 0.05 ± 0.08 ppbv.By contrast, the longest ozone observation data in Antarctic region is in South Pole station, and annual growth trend was 0.02 ± 0.09 ppbv.The growth trends in both stations are insignificantly different [2].
The surface ozone concentration in Antarctic region shows evident seasonal changes, reaching the highest in winter and the lowest in summer [3].This variation is due to clean atmospheric environment and unique geographic location of the Antarctic region.The ozone is generally accumulated in winter and destroyed by photochemical process in spring and summer [4].In addition, the unique climatic feature and atmospheric boundary structure of Antarctic region render chemical process of ozone complicated [5].In coastal areas of Antarctic region, the surface ozone concentration drops sharply in winter and spring and even occasionally decreases to detection limit of the instrument [6].In 1980s, Alaska Barrow Station [7] and Alert Station in northern Canada appeared a course of event called sharp reduction of ozone concentration as an ozone depletion episode (ODE) [8].In the 1990s, reported ODEs in Antarctic region [9] [10].Given an analysis of historical ozone data in Antarctic region, Roscoe [11] pointed out that an ODE was recorded by Halley Station in 1958.Studies have reported that ODE is mainly driven by halogen chemistry and related to discharge active bromine in sea ice areas [12]).
In Antarctic South Pole Station and Concordia Station, elight-denitrification of snows can influence ozone concentration in atmospheric boundary layer during spring and summer [13] [14].Such denitrification could cause nitrate that has settled in snows to release NO x .In summer, strong solar radiation can convert NO x quickly through photochemical process.Consequently, a small growth of ozone concentration is achieved [15] [16].The local ozone production might be related to environmental pollution.When Antarctic ozone hole (AOH) appears, the amount of UVB that reaches earth's surface increases slightly at the hole.Therefore, photochemical process increases and surface ozone drops significantly.Jones and Wolff [17] proposed that ozone hole in stratosphere in 1980s increased the NOx released by snows at boundary layer and surface ozone photochemical production in South Pole Station.According to this hypothesis, slow recovery of the stratosphere ozone in Antarctic region is expected to lead to a low surface ozone production rate in Antarctic Plateau.Polar region are located far away from other continents and experience smallest interferences from human activities.With support of China's Action Plan in the Fourth International Polar Year (2008/2009), the Antarctic Zhongshan atmospheric monitoring station was established.Both continuous in situ and weekly flask sample measurements of greenhouse gases are part of research program at the station and of continuous meteorological observations [18] [19] [20].In this paper, we analyzed the surface O 3 data from 2008 to 2013 at Zhongshan for the concentrations, seasonality, and trends and their relations to meteorological factors.The variation characteristics of surface O 3 in Zhongshan data were compared with those of other Antarctic Stations obtained through World Data Center for Greenhouse Gases (WDCGG) of WMO.

Observation Site
Zhongshan Station is located at the Larsemann Hills of Prydz Bay of the east Antarctic continent.Meanwhile, the atmospheric composition monitoring site is situated on highest hills at northwest edge of the station, specifically, flat bare rock at top of Tiane Range (69˚'22'12''S, 76˚21'49''E, 18.5 m).This site is next to ocean in west and north directions and next to ice-covered areas in east and south (Figure 1).There are 54 polar days and 58 polar nights at the station.Northern east wind prevails throughout the year with an average wind speed of 7.5 m/s.Gale wind (>8 scale) lasts for 174 days, and the extreme wind speed has reached 43.6 m/s.The annual mean temperature is below −10˚C.The minimum and the maximum temperature were −44˚C and 10˚C, respectively, at the station from 1989 to 2013.

Observation Instrument
An EC9810A ozone analyzer and an EC 9811 ozone calibrator (ECO tech) have Figure 1.Study area (a) and locations of the monitoring site for surface ozone concentrations at the Zhongshan Station (b).Atmospheric and Climate Sciences been used for surface ozone measurement with data storage system from January 2008 to December 2013.The sampling frequency was 3 min and calibration every 3 months by five standard ozone samples.In data processing, the data during machine failure and instrument maintenance were deleted from the duty records.UVB data were derived from the observation data of the Brewer ozone spectrophotometer on the total ozone and UVB levels at Zhongshan Station.The measured UVB refers to the UV radiation that reaches ground level.The wavelength, scanning interval, and integral time were set as 295 -325 nm, 0.5 nm, and 0.224 s, respectively.

Data Treatment
Peak and uncommon data were deleted by variance test on the basis of the criteria of 3 i x x σ − > , where i x is measured data, x is time series mean, and σ is standard deviation [18] [19].While calculating hourly mean, the hourly means smaller than two data groups were not included and viewed as missing.The processed monthly sample size of hourly average surface ozone concentrations in Zhongshan Station is shown in Figure 2. Except for relatively small data size in January and February 2010 due to missing data, overall data integrity was relatively high and reached 91%.All statistical analyses were performed with SPSS statistics 17.0 and Microsoft Excel for Windows 2007 [20].The surface ozone data from other Antarctic stations were collected from WDCGG of WMO (http://gaw.kishou.go.jp/cgi-bin/wdcgg/).

Wind Influence on Surface Ozone Concentration
The influences of emissions from Station are not being eliminated completely through above-mentioned data processing.Surface wind is an important factor influencing observed data [21].Extracting observation data of atmosphere background that has not been influenced by local factors is basis for discussing background characteristics of surface ozone levels and source influences.To eliminate possible effects on surface ozone concentration exerted by local sources operations from Zhongshan Station and Russia Station, we drawn a rose diagram on basis of 16 wind directions data at Zhongshan Station.We also counted wind    The variation amplitudes of surface ozone concentration with wind speed in four seasons are basically similar in Figure 6.When WS is small (<3 m•s −1 ) in winter, variation range of surface ozone concentration is relatively larger than those under other wind speed levels.This event might be related that the ODE in Antarctic coastal areas is often accompanied by lower wind speed.In addition, when the wind speed is >20 m•s −1 in summer, variation range of surface ozone concentration is significantly higher than those under the other wind speed levels, this effect might be caused by limited samples.Hence, processed data of surface ozone without affect of local pollution from Station can represent background concentration.

Seasonal Variation of Surface Ozone Background Concentration
Average daily cycle of surface ozone concentration at Zhongshan Station in January (summer), April (autumn), July (winter), and October (spring) is illustrated respectively in Figure 7.The daily variation amplitudes of surface ozone concentrations in the four seasons were very litter, with values of 0.72, 0.24, 0.30, and 0.83 ppb, respectively.Observation site is a bare rock region without vegetation in summer and it is covered by snow in other seasons.Hence, the region exerts no local impact on the observed surface ozone levels.The variations in ozone concentration were mainly related to large-scale atmospheric transport.
This result further confirms that the monitored surface ozone concentration at Zhongshan Station can reflect changes in background concentration.The average surface ozone concentrations and standard deviations in the spring (SON), summer (DJF), autumn (MAM), and winter (JJA) every year are listed in Table 1.In the study period, the surface ozone concentration in winter is the highest every year, followed by those in spring and autumn with small differences.By contrast, the surface ozone concentration in the summer is the lowest.Interannual changes in surface ozone concentration exist in each season, but no significant variation trend is observed.The correlation coefficient between the UVB and surface ozone concentration in summer was −0.24 (Figure 12).Although the both were negatively correlated, but the correlation coefficient was lower than those in the autumn and spring.

Comparison with Other Antarctic Stations
This shows the weaker photochemical ozone destruction of UVB in summer than those in spring and autumn.The lowest surface ozone concentration and the strongest UVB occurred in summer and their relationship is relative scatter.
This relation is of complicated reasons, such as increasing human activity, as well as growth in ozone production rate and cloud amount.Further related studies are needed.

Conclusion
The surface ozone observed at Zhongshan Station is only slightly affected by

Figure 2 .
Figure 2. Monthly samples of hourly average surface ozone concentrations at the Zhongshan Station from 2008 to 2013.2008 2009 2010 2011 2012 2013 100 200 300 400 500 600 700 800 Figure 4 show that ENE wind prevails at Zhongshan Station in four seasons, but WF of NE wind increases slightly in summer.This change might be related to seasonal changes of general atmospheric circulation.WS distribution was similar to the annual distribution.The maximum WS occurred at the direction of

Figure 5 .
Figure 5. Occurrence frequency of different wind speed levels and the corresponding surface ozone concentrations from 2008 to 2013 (the black frame indicates the occurrence frequency of different wind speed levels and the vertical line is the variation range of the corresponding surface ozone concentrations).

AFigure 6 .
Figure 6.Occurrence frequency of different wind speed levels in four seasons and the corresponding surface ozone concentrations from 2008 to 2013 (the black frame indicates the frequency of occurrence of different wind speed levels and vertical line denotes variation range of corresponding surface ozone concentrations).

Figure 7 .
Figure 7. Average daily changes in surface ozone concentration at Zhongshan Station in January, April, July, and October.

Figure 8 .
Figure 8. Boxplots of monthly surface ozone concentrations recorded by Zhongshan Station at different years from 2008 to 2013.

AFigure 9 .
Figure 9. Distribution of surface ozone observation sites at South Pole.

presented in Figure 10 .Figure 10 .
Figure 10.Time series of monthly average surface ozone concentrations at the Antarctic stations from 2008 to 2013.

6 .Figure 11 .
Figure 11.Time series of surface ozone (O 3 ) concentrations and UVB at the Zhongshan Station from 2008 to 2013.

Figure 12 .
Figure 12.Relationships between the surface ozone (O 3 ) concentration and UVB at the Zhongshan Station in autumn (MAM), spring (SON), and summer (DJF).
et al.

Table 1 .
Average surface ozone concentrations and standard deviations in four seasons at the Zhongshan Station from 2008 to 2013.

Table 2 .
Annual average surface ozone concentrations at different observation sites from 2008 to 2013.