Received 15 October 2015; accepted 26 January 2016; published 29 January 2016
The Intergovernmental Panel on Climate Change (IPCC)  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   . 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%  - .
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   . 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   . 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)  . At the Xinglong Station in China, the minimum and maximum levels occurred during May-September and October-December respectively  .
In the southern hemisphere, the minimum and maximum values were observed in early March and late September at Syowa Station  and the South Pole  -  . 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   . 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.
2. Observation Point and Monitoring Instruments
2.1. Site Location
Located along the coast of East Antarctica, Zhongshan Station (Figure 1) is China’s second year-round Antarctic research station. Factors such as power generation, traffic, and human activities in the region were considered when placing gas, aerosol and meteorological observation sites around the station site. The main facility for the CH4 monitoring systems is located in a small sampling building on a flat, bare rock in the west side of the so called Tian’e Range at the northwest end of the station (69˚22'2''S, 76˚21'49''E, at an elevation of 18.5 m). These observation facilities are 400 m upwind of the power generation building and garbage incinerator.
3. Data Treatment
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 xi 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%.
4. Winds Influencing on CH4 Concentration
Above data processing cannot completely exclude the impact of emissions from the station area. Wind is an important factor influencing observational data   . Winds measured at 10 m over Zhongshan Station were
Figure 1. Locations of monitoring stations in Antarctica relative to Zhongshan Station.
Figure 2. Standard deviations of daily zero-gas measurements at Zhongshan Station from 2010 to 2013.
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
Figure 3. Wind frequency (WF) and average wind speed (WS) in 16 directions and average CH4 content during spring (September-November), summer (December-February), autumn (March-May), and winter (June-August) at Zhongshan Station from 2010 to 2013.
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
Figure 4. Wind frequencies in four seasons and corresponding CH4 content at Zhongshan Station.
Figure 5. Correlation between in situ observational CH4 concentrations and corresponding flask sampling and analysis conducted in Beijing at a later date.
from the years 2008 and 2013 prior to the initiation of in situ measurements to extend the CH4 record back in time.
5. Variation Characteristics of CH4 Concentration
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
Figure 6. Average hourly CH4 content and daily variations of maximum and minimum at Zhongshan Station in January, April, July, and October.
Figure 7. Time array of average daily CH4 content at Zhongshan Station observed in situ from March 2010 to September 2013.
Figure 8. CH4 concentrations changes and related temperature (t), vapor pressure (e) and air pressure (p) from August 3rd to August 7th, 2011 at Zhongshan Station.
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.
6. Seasonal Variation and Trend of CH4 Background Concentration
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   . 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.
Figure 9. Time series of monthly mean CH4 concentrations at Zhongshan, Casey and Syowa Stations from 2008 to 2013.
Figure 10. Seasonal variations of monthly mean CH4 concentration at Zhongshan, Casey and Syowa Stations for monthly data from 2008 through 2013.
Table 1. Annual mean CH4 concentrations and growth rates measured in Antarctica compared to the global average.
7. Discussion and Conclusions
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.