Persistence analysis of CO2 concentrations recorded at a rural site in the upper Spanish plateau

Persistence of CO2 daily means recorded at a rural site over three years was analysed using three procedures: the ST method, rescaled range analysis and detrended fluctuation analysis. Four series were previously considered, since the original series was detrended and gaps were filled. The detrending procedure comprised four steps: a linear evolution of about 3 ppm per year, a quadratic function with a maximum yearly range of 11 ppm, and two harmonic functions, the first with yearly and semi-yearly cycles and the second with a weekly cycle. Yearly ranges of both functions increased with time, their maxima being about 5 and 2 ppm, respectively. Gaps were filled with a method based on previous observation and a Gaussian function, whose standard deviation was fitted so as to resemble original data. The ST method proved extremely sensitive to the detrending procedure. A detailed analysis of the Hurst exponent allowed us to establish four regions where persistence gradually decreased. Finally, detrended fluctuation analysis provided a similar result to rescaled range analysis. From these methods, persistence was high in an interval of about three weeks.