The Intergovernmental Panel on Climate Change (IPCC) [1] states that the CH₄ content of the atmosphere ranks as the second most important greenhouse gas following CO₂. The residence time of CH₄ in the atmosphere is about 12 years with a strong infrared adsorption band at 7.66 µm that allows it to effectively adsorb long-wave radiation from the earth [2] [3] . The contribution of a single CH₄ molecule to the greenhouse effect is approximately 25 times greater than that of a molecule of CO₂, and its contribution to total global radiative forcing is 18.1% [4] -[6] .

Approximately 40% of the CH₄ discharged into air comes from natural sources such as wetlands whereas 60% is from anthropogenic influenced sources such as ruminants, paddies, fossil fuel production, landfills and biomass burning. The CH₄ reaction with the hydroxyl radical (OH) is its major sink [7] [8] . Given the probable increasing discharges from manmade sources, the current CH₄ content in the global atmosphere (global average of ~1837 ppb in August 2015) is ~265% of the pre-industrial level of ~700 ppb [9] [10] . The growth rate of CH₄ reduced to ~0 from 1999-2006 from 13 ppb∙yr⁻¹ in the early 1980s. CH₄ concentrations then began growing again in 2007. The seasonal CH₄ content of the atmosphere is cyclical with CH₄ concentrations higher in winter in the northern hemisphere with seasonal changes in the southern hemisphere lagging by 8 - 9 months. During the years 1986-1994, the average minimum and maximum CH₄ levels were recorded at the end of June and at the beginning of February, respectively at the NOAA Barrow, Alaska Baseline Observatory (71.32 N; 156.61 W) [11] . At the Xinglong Station in China, the minimum and maximum levels occurred during May-September and October-December respectively [12] .

In the southern hemisphere, the minimum and maximum values were observed in early March and late September at Syowa Station [13] and the South Pole [14] - [16] . The Antarctic region is exposed to the least influence of human activities; hence this region is regarded as being the background of the global atmosphere [17] [18] . With the support of China’s Action Plan in the Fourth International Polar Year (2008/2009), the Antarctic Zhongshan Station atmospheric background monitoring station was established. Both continuous in situ and weekly flask sample measurements of CH₄ are part of the research program at Zhongshan station as well as continuous meteorology measurements.

In the current research, we analyzed CH₄ data from 2008 to 2013 at Zhongshan for concentrations, seasonality and trends in relation to meteorology. The Zhongshan data were compared with other Antarctic stations and these inter comparisons form an important segment of this report.

Data gathered under abnormal conditions such as instrument failure, maintenance of facilities and power failure as well as data during zero-gas measurement periods are deleted from the data record. Original data are then adjusted according to daily zero-gas measurement values. The standard deviations of the daily zero-gas measurements were concentrated within the range of 0 - 0.5 ppb as shown in Figure 2.

All data were processed as averages after the abnormal values from the original data were removed using a variance test, where x_i is the observational data, is the average value and σ is the standard deviation. The integrity rate of online CH₄ observational data was 95.9%.

Above data processing cannot completely exclude the impact of emissions from the station area. Wind is an important factor influencing observational data [22] [23] . Winds measured at 10 m over Zhongshan Station were

binned into 16 intervals of 22.5˚ each. The average seasonal CH₄ concentration was binned with the average wind speed then subjected to further statistical analyses. East wind (45˚ - 135˚) prevails all year round with a frequency of 83.1%, indicating that airflow reaching the station comes mainly from the Antarctic ice sheet and western oceans. Wind frequency (WF), average wind speed (WS), and average CH₄ content in 16 directions during the four seasons from 2010 to 2013 are presented in Figure 3. From Figure 3 it may be observed that there is a large annual cycle in CH₄ with largest concentrations in the Antarctic winter-spring and lowest in summer-autumn. Westerly winds in all seasons exhibit the highest average CH₄ content with the highest annual concentrations in summer-autumn.

Power generation for the station is northwest (downwind) of the CH₄ sampling location because of the prevailing easterly winds. Note that the highest frequency of winds from that sector was only 2.4% thus minimizing

the potential for contamination from the power plant and incinerator.

The CH₄ content in west winds during autumn is slightly elevated which may be real, but also be an artifact of the small sample size of 0.6% of the total wind observations. Apart from these few autumn data, the CH₄ concentrations over a single season do not significantly change with wind direction suggesting that wind direction at Zhongshan Station exerts little influence on CH₄ concentrations.

To analyze the effects of wind speed on CH₄ measured at the station, the wind speed data from 2010 to 2013 were divided in seven groups: ≤0.5 m∙s⁻¹, 0.5 - 3 m∙s⁻¹, 3 - 6 m∙s⁻¹, 6 - 10 m∙s⁻¹, 10 - 15 m∙s⁻¹, 15 - 20 m∙s⁻¹, and >20 m∙s⁻¹ and plotted along with corresponding CH₄ concentrations as shown in Figure 4. Numerically, up to 94.7% of the wind speeds at Zhongshan Station fall in the range of 0.5 - 3 m∙s⁻¹. The wind frequency in the group of >20 m∙s⁻¹ is 1%. The wind in the group of ≤0.5 m∙s⁻¹, which can be seen as calm, demonstrates a frequency of 0.3% and is associated with occasional fluctuations in CH₄ concentrations indicating a possible local influence on the measurements, or the small sample size has biased the data slightly. In other wind speed bands, CH₄ content in each season does not change with changes in wind speed implying an even distribution of upwind CH₄ concentrations. This also suggests that the influence of local pollution on CH₄ concentrations at the Zhongshan Station is minimal.

To know the reliability of the in situ CH₄ measurements, Figure 5 shows the relationship between air samples analyzed CH₄ content and on line observed data from 2010-2013. There was excellent agreement with a correlation coefficient of 0.975. Based on these observations, it is felt that it is reasonable to use flask measurements

from the years 2008 and 2013 prior to the initiation of in situ measurements to extend the CH₄ record back in time.

The average CH₄ concentration per hour of the day and the standard deviation for the years (2010-2013) is presented in Figure 6 for January (summer), April (autumn), July (winter), and October (spring). The daily standard deviation shows peak values within the range 0.3 ~ 0.6 ppb with over 90% of data are between 0.3 and 1.5 ppb. The standard deviation shows that at the same moment in different seasons, the standard deviation is highest in spring and the lowest in summer. The average daily variation is 0.9, 0.5, 0.4, and 0.8 ppb for summer, autumn, winter, and spring, respectively. This indicates the absence of strong local sources and sinks around Zhongshan Station which in turn suggests that the CH₄ concentrations observed at Zhongshan are probably representative of the CH₄ background of a larger region of Antarctica.

The hourly average continuous in situ CH₄ data record for Zhongshan for 2010-2013 is presented in Figure 7 where it may be observed that the lowest CH₄ concentrations were measured in February (summer) and the largest in late September-early October (spring). A steady annual increase in CH₄ concentrations is evident in the four year record. Occasionally, there are large, three to five day regime changes in CH₄ concentrations at Zhongshan Station related to maritime air mass intrusions exemplified by a decrease in air pressure, increase in air temperature and an increase in vapor pressure. One such event is presented in Figure 8 for August 3rd through 5th, 2011 during the season when CH₄ concentrations were reaching their annual peak. Over this 3 day

period, hourly average CH₄ concentrations decreased from 1771.3 ppb to 1762.8 ppb as air pressure decreased from 998 hPa to 965 hPa. Over the same period, air temperature rose by 15˚C (−17.3˚C to −1.7˚C) while vapor pressure increased from 1 hPa to 5 hPa. Over the next two days as the air cooled and water vapor dropped, CH₄ concentrations recovered about 50% of their value of two days earlier and then stabilized at what was to be the new norm until another air mass change occurred. These step function air mass related shifts in CH₄ concentrations (decreases and increases) occur in all seasons as may be observed.

To evaluate the representativeness of in situ CH₄ measurements at Zhongshan Station, CH₄ data from Casey Station (66.28˚S, 110.53˚E) and Syowa Station (69˚S, 39.6˚E) obtained from the World Meteorological World Data Center for Greenhouse Gases (http://ds.data.jma.go.jp/gmd/wdcgg/cgi-bin/wdcgg/catalogue.cgi) were plotted together. Figure 9 displays the time series of average monthly CH₄ concentrations at Zhongshan, Casey, and Syowa. For Zhongshan, flask data from 2008 through 2009 are used and in situ data for 2010 through 2013. For Casey and Syowa, monthly flask data were used. The accuracy of flask data is discussed in References [24] [25] . From the data presented in Figure 9, it may be seen that there is close agreement between the CH₄ measurements at the three stations across the six year overlapping records. Over this period, it may be observed that monthly CH₄ concentrations ranged from a low of 1722 to a high of 1782 ppb with a fairly steady growth rate over a six year period.

Average monthly concentrations of CH₄ at the three stations are shown in Figure 10 where as mentioned before, CH₄ content along the East Antarctic continent is lowest in February-March and reaches a peak in September-October. This annual cycle is related to the seasonal variations in CH₄ sources and sinks: predominately transport from lower and middle latitudes in both the southern and northern hemispheres, and a possible small effect from Antarctic marine coastal biological processes [26] [27] . As such, the seasonal variation in CH₄ is mainly caused by transport from the northern hemisphere in the general circulation of the global atmosphere.

Global annual contents and growth rates of CH₄ at six Antarctic stations (locations shown in Figure 1) from 2008 through 2013 are listed in Table 1. Little significant differences can be observed in the CH₄ concentrations measured at the six stations which are about 50 ppb lower than the global average. From 2008 through 2013, the average global CH₄ concentrations increased at a rate of ~5.2 ppb∙yr⁻¹ whereas in Antarctica it increased between 4.5 - 5.2 ppb∙yr⁻¹. As such observations of background CH₄ concentrations in Antarctica faithfully reflect the changing background content of CH₄ globally.

One of the goals of this research was to determine if in situ CH₄ measurements at Zhongshan Station were of high quality, consistent and stable over time and that error detection and data processing algorithms were adequate to produce an end product that would be accepted by the larger scientific community. Through regular measurements of calibration and zero gases, and the excellent agreement between in situ CH₄ measurements and CH₄ in air collected in flasks and analyzed in Beijing, we are confident that the in situ analyzer is producing acceptable data. By studying CH₄ concentration in winds with different directions and speeds at Zhongshan Station, it was found that ~1% of the CH₄ measurements were suspected of possibly being contaminated by station effluents. These data were removed from the record. Overall, the Zhongshan station data suggest that there are no strong natural local sources and sinks of CH₄ in the proximity of the station.

Since there was excellent agreement between the in situ and flask measurements that overlapped from 2010 through 2013, we suggested that the flask measurements from 2008 to 2013, prior to the installation of the in situ analyzer, were also of high enough quality to be added to the Zhongshan record. Comparing this six year record with similar measurements from Casey and Syowa stations produced overlapping long term records that were essentially indistinguishable. These records follow the well-known annual cycle in CH₄ concentrations in Antarctica with peaks in September-early October and troughs in February-March.

The in situ CH₄ measurements at Zhongshan exhibit occasional, fairly rapid changes in concentration that generally signal a change to a new norm which may last for a few day to a few weeks. The study of one of these events showed that a decrease of ~10 ppb in CH₄ in a day was associated with the arrival of a low pressure marine air mass that brought lower latitude air of lesser content CH₄ to the station.

As noted earlier, average global CH₄ is growing at 0.29%.yr⁻¹ and recent inter-annual CH₄ growth ranges from 5 ppb to 6 ppb. The CH₄ content in the Antarctic as reported in this study was found to be 0.28%.yr⁻¹, and the inter-annual growth ranged from 4.5 ppb to 5.2 ppb. Since the CH₄ data from Zhongshan Station are in excellent agreement with other stations on the East Antarctic coast, we suggest that the in situ measurements from Zhongshan may be well suited to study the finer details of the behavior of CH₄ in the Antarctic than weekly average measurements from flask samples.

This work was supported by the Program of China Polar Environment Investigation and Assessment (Project No. CHINARE2015-2016), the authors appreciate the assistance of all staff wintered in Zhongshan station during data collection.

LingenBian,ZhiqiuGao,YulongSun,MinghuDing,JieTang,Russell C.Schnell, (2016) CH₄ Monitoring and Background Concentration at Zhongshan Station, Antarctica. Atmospheric and Climate Sciences,06,135-144. doi: 10.4236/acs.2016.61012