| Longfengshan regional background station in Heilongjiang province is one of the WMO/GAW stations, and is also a national field scientific observation station. Continuous observation of the greenhouse gases (CO2 and CH4) and related tracers (CO and H2) can help us to understand the variation of their concentration and estimate the strength of source and sink, which can provide basic data and support for making strategy of low carbon economy. Based on the in-situ observation results of atmospheric CO2, CH4 and CO mole fractions from two levels (10 m and 80 m above the ground) at Longfengshan regional background station in Heilongjiang province during January 2009 to December 2011, we studied the data quality assurance, the effects of local source/sink, the influence of long-distance transport, filtering methods for background and non-background, and trend analysis. Based on the self-assembly gas chromatography (GC) system with a pulsed discharge helium ionization detector (PDHID), atmospheric H2 was preliminary studied in Longfengshan station. The main results were obtained as follows:Atmospheric CO2 and CH4 showed similar diurnal variation with low value in daytime and high value in nighttime. In summer, the diurnal amplitude was 41.2±4.1×10-6 (mole fractions, the same as below) for CO2 and 131±37×10-9 for CH4, which was larger than amplitude in other seasons and other WMO/GAW stations in China. Diurnal variation of CO was strongly influenced by anthropogenic activities with two peaks at 08:00-10:00 and 17:00-20:00 in spring, autumn and winter. In summer, diurnal variation of CO was quite different, with peak at 11:00-12:00 and valley at 20:00-21:00. The diurnal amplitude of CO was largest in winter and lowest in summer with values of 114±118×10-9 and 37±42×10-9, respectively. The observed CO2 and CH4 at 10 m were strongly influcenced by the local sources/sinks. The differences between the 10 m and 80 m results were relatively small during the daytime (08:00-17:00). In spring, summer and winter, higher CO2 mole fractions were observed when surface winds came from the E-ESE-SE-SSE sectors, while, in winter, surface winds from the N-NNW sectors significantly enhanced atmospheric CO2, CH4 and CO. Generally, lower CO2, CH4 and CO mole fractions were accompanied with higher wind speed in the four seasons and these phenomenons were most significant in winter. The correlation analysis among CO2, CH4 and CO revealed that there were significant positive correlation between two of the three gases in winter, and the determination coefficient R2 between CO2 and CO was 0.86. This indicated that CO2, CH4 and CO were from similar sources, mainly fossil fuel emissions. But in May to September, the determination coefficients R2 of any two gases were less than 0.1. Cluster and CWT (concentration weighted trajectory) analysis indicated that there were two main potential sources at Longfengshan station. One was from the northwest of the station where Harbin, Daqing and Qiqihar cities were located, and the other one was from the southwest of the station where Changchun, Jilin, Shenyang, Fushun and Anshan cities were located.Applicability of SWDV (Surface Winds and Diurnal Variations) method and REBS (Robust Extraction of Baseline Signal) technique in atmospheric CO2 flagging were studied. Except summer, both flagging methods could be applied to the observed data at Longfengshan regional station. The background data flagged by both methods could reflect the CO2 variation trend, and the non-background data flagged by both methods could reflect the influence of the local sources/sinks. But REBS was more disadvantageous to be used in summer with heavy vegetation coverage at Longfengshan regional station. About 30.7% and 58.9% valid hourly data were flagged as regional representative by SWDV method and REBS technique, respectively.21.5% and 32.0% of the observed data were commonly classified as background and non-background events, respectively. The difference of background seasonal averaged CO2 mole fractions between the two methods was smallest in spring and largest in summer with value of 0.1±0.3×10-6 and 4.2±1.0×10-6, respectively. Case studies proved that the data during daytime when the airflow was from the southwest direction by SWDV method and the data at calm conditions by REBS method were occasionally mis-flagged as background events. In summer, it was difficult to distinguish whether the CO2 mole fractions were influenced by anthropogenic pollution in the daytime by REBS technique or not, because the photosynthesis was strengthened in that period, which leaded to the large difference of background values between two methods.The SWDV, REBS and TRA methods were used to filter the observed atmospheric CH4, which classified about 24.6%,44.5% and 39.6% of the observations as "background", respectively. As the hourly CH4 mole fraction selected by SWDV method as non-background data from 0:00-7:00 and 18:00-23:00 period, the ratio of background CH4 mole fractions was apparently lower than other two methods. The TRA method apparently induced errors on background CH4 estimations, especially in summer. SWDV and REBS methods could flag most of the hourly CH4 mole fractions with standard deviation lager than 10×109 as non-background data, and the percentage of mis-flaged data was 15%. The percentage of mis-flaged data by TRJ was 32%. The TRA method was not suitable for the background CH4 data selection at Longfengshan station. Considering the filtered results and the ratio of the background data, REBS method was suggested to flag the CH4 data at Longfengshan station.Seasonal variation and trend variation of atmospheric CO2, CH4 and CO in different seasons were obtained. The background CO2 seasonal variation showed a peak in winter and a valley in summer, with seasonal peak to peak amplitude of 36.3±1.4×10-6 which was higher than the values at similar latitude from marine boundary layer references (MBL) and WMO/GAW stations. Seasonal variation of CH4 presented two peaks. One peak appeared in December or January, and the other one appeared in July or August. The seasonal amplitude of CH4 was 53±10-9, which was also higher than the value at similar latitude from MBL. The seasonal variation of CO displayed a peak value in winter and a valley value in summer or early autumn, with the amplitude of 156±44×10-9 which was higher than the values at Ulaan Uul regional stations in Mongolia and Ryori regional stations in Japan. Both the atmospheric CO2 and CH4 mole fractions displayed increasing trends during 2009 to 2011. Annual averaged CH4 and CO2 values were higher than the global values from 2009 to 2011.A high precision GC system with a pulsed discharge helium ionization detector was set up based on the commercial Agilent 7890A gas chromatography. The gas was identified by retention time and the concentration was calculated through the peak height Detection limit of the system was about 1×10-9. The standard deviation of 140 continuous injections with a standard cylinder (mole fraction is roughly 600×10-9) was better than 0.3×10-9. Between 409.3×10-9 and 867.7×10-9, molecular hydrogen mole fractions and peak height had good linear response. By using two standards to quantify the air sample, the precision met the background molecular hydrogen compatibility goal within the World Meteorological Organization/Global Atmosphere Watch (WMO/GAW) program. Atmospheric molecular hydrogen mole fraction was preliminarily measured by this method at Longfengshan station from December 4,2013 to January 8,2014, which varied from 461×10-9～608×10-9. |
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