Features of the annual evolution of CO2 and CH4 in the atmosphere of a Mediterranean climate site studied using a nonparametric and a harmonic function

Özet

Concentrations of CO2 and CH4 measured over 3 years at a rural site in the Spanish northern plateau wereinvestigated together with vegetation and meteorological variables. Two procedures were implementedto study the annual evolution. Kernel estimation provided a detailed time description, and the harmonicmodel may be fitted easily. The site was characterised by grass from autumn to spring. However, vigorousgrowth was observed during the latter season due to the biological cycle of plants under favourablemeteorological conditions. A CO2 peak was observed a fortnight before the time of maximum NDVI, andwas attributed to the prevalence of respiration over photosynthesis. A pronounced trough was apparentin summer and was explained by the death of vegetation and active dispersion in a highly developedboundary layer. CH4 evolution was characterised by a deficit period from May to October, indicating thatmeteorological evolution played a key role. The harmonic model showed that annual and half-annualcycles evidenced a similar contribution for CO2, whereas said weight for the half-annual cycle wasconsiderably smaller for CH4