Study on the processing method of nighttime CO2 eddy covariance flux data in ChinaFLUX

At present, using Eddy Covariance (EC) method to estimate the “true value” of carbon sequestration in terrestrial ecosystem arrests more attention. However, one issue is how to solve the uncertainty of observations (especially the nighttime CO₂ flux data) appearing in post-processing CO₂ flux data. The ratio of effective and reliable nighttime EC CO₂ flux data to all nighttime data is relatively low (commonly, less than 50%) for all the long-term and continuous observation stations in the world. Thus, the processing method of nighttime CO₂ flux data and its effect analysis on estimating CO₂ flux annual sums are very important. In this paper, the authors analyze and discuss the reasons for underestimating nighttime CO₂ flux using EC method, and introduce the general theory and method for processing nighttime CO₂ flux data. By analyzing the relationship between nighttime CO₂ flux and air fraction velocity u*, we present an alternate method, Average Values Test (AVT), to determine the thresholds of fraction velocity (u*_c) for screening the effective nighttime CO₂ flux data. Meanwhile, taking the data observed in Yucheng and Changbai Mountains stations for an example, we analyze and discuss the effects of different methods or parameters on nighttime CO₂ flux estimations. Finally, based on the data of part ChinaFLUX stations and related literatures, empirical models of nighttime respiration at different sites in ChinaFLUX are summarized.