[Atmospheric CO2 data filtering method and characteristics of the molar fractions at the Longfengshan WMO/GAW regional station in China].

Based on the in-situ observation results of atmospheric CO2 molar fractions at two levels (10 m and 80 m above the ground) at Longfengshan (LFS) regional background station in Heilongjiang Province during January 2009 to December 2011, this study mainly focused on the results from 10 m above the ground level (a. g. l.). The results indicated that the observed data from 10 m were strongly affected 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) with values smaller than (0.5 +/- 0.5) x 10(-6). In spring, summer and winter, higher CO2 molar fractions were observed when surface winds came from the E-ESE-SE-SSE sectors, while, in winter, surface winds from the N-NNW-NW-WNW sectors obviously enhanced the observed values. Generally, lower CO2 values were accompanied with higher wind speed in the four seasons. This phenomenon was most obvious in winter. Based on the analysis of the observed diurnal cycles and the local meteorological conditions, the observed data from 10 m were filtered into background/non-background events. About 30.7% valid hourly data were filtered as regional background representative. The background CO2 variation displayed a peak in winter and a valley in summer with a seasonal peak to peak amplitude of (36.3 +/- 1.4) x 10(-6), which was higher than the values at similar latitude from Marine Boundary Layer (MBL) References and WMO/GAW stations. The yearly CO2 increasing rate at LFS was roughly estimated to be 2.4 x 10(-6) a(-1).