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Li X.,State Key Laboratory of Remote Sensing Science | Li X.,Beijing Normal University | Li X.,Beijing Key Laboralory for Remote Sensing of Environment and Digital Cities | Sun R.,State Key Laboratory of Remote Sensing Science | And 13 more authors.
Shengtai Xuebao/ Acta Ecologica Sinica | Year: 2010

As an important component of urban ecosystem, urban green space has many eco-functions such as releasing oxygen, sequestrating carbon, lowering temperature, increasing humidity, absorbing toxic gases, reducing the noise and so on. To evaluate the effect of urban green space on urban ecosystem, the CO2 flux and meteorological variables including air temperature and incident solar radiation were observed from May, 2006 to March, 2007 in Haidian Park, Beijing. The CO2 flux tower is located in the central of the park, and the vegetation in Haidian Park is mainly woodland. Abnormal data omission and data gap-filling based on the data quality assessment, and the seasonal and diurnal variability of CO2 fluxes were analyzed in this paper. Because the factors impacting CO2 fluxes are different, the CO2 flux data for the daytime and nighttime were processed separately. The data quality assessments for the nighttime measurements include elimination of rain-day data, data below zero, abnormal data determined by the threshold method, and data observed under low turbulence intensities using a threshold value of friction velocity determined by an average values test method. The gap-filling of the nighttime data were carried out by establishing the relationship between valid CO2 flux and air temperature. The invalid daytime CO2 flux data were filtered according to the precipitation observation data and CO2 flux threshold value. The gap-filling was conducted by the look-up table method using measurements of CO2 flux, air temperature, and incident solar radiation. An empirical relationship of daily CO2 flux versus daily averaged air temperature and daily incident solar radiation was established, and then applied to missing or invalid CO2 flux observations to produce daily CO2 flux data for the whole year. The results show that CO2 fluxes in Haidian Park vary with season, with a net absorption of atmosphere CO2 from March to October, while a net emission from November to February. The daily CO2 flux ranges from - 15.0 gCO2/m2d to 10.0 gCO2/m2d, and the annual total CO2 flux is -8.7554 tCO2/hm2a, corresponding to an annual absorption of 8.7554 t/hm2 carbon dioxide in the study area. The diurnal change of the CO2 flux ranges from - 1 mg/m2 s to 1 mg/m2 s, with negative values in the daytime and positive during the nighttime. The CO2 flux is lowest during the mid-day and highest at about 8 pm. The impact factors and the spatial representativeness of observed CO2 flux data were also analyzed. The results show CO2 fluxes decrease as increasing air temperature and incident solar radiation. The main source contribution area for the CO2 flux is located in the northeast and southwest direction of the CO2 flux tower. The woodland and grassland in the northeast area are the major source area in winter. However, the source is mainly from southwest and secondly from northeast in summer.

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