Unidad de excelencia María de Maeztu MdM-2015-0552

In order to improve the understanding of the carbon cycle in the Pyrenean region, two atmospheric monitoring mountain stations were set up within the Long-Term Ecological Research node of Aigüestortes i Estany de Sant Maurici at Central Pyrenees, Spain. The atmospheric concentration of carbon dioxide (CO2) was measured over 2008-2014 and 2010-2014 at Estany Llong (ELL) site and Centre de Recerca d'Alta Muntanya (CRAM), respectively. Measurements were carried out fortnightly off-line with high precision instrumentation at ELL and every minute online with a lower precision sensor at CRAM in conjunction with meteorological variables. The two datasets were analyzed in this study, quantifying whenever possible annual growth rates (AGR), seasonal variability, and diurnal amplitudes. Results were also compared with the NOAA Marine Boundary Layer (MBL) reference product and CO2 data from other background monitoring stations. Four-harmonics adjusted CO2 data from ELL showed a high correlation with the NOAA MBL reference product for the same latitude (Spearman's rho ρ = 0.96). In addition, AGRs of CO2 at ELL correlated well with those observed at Mace Head (MHD) station (ρ = 0.94), suggesting that ELL can be considered a background station. Winter CRAM CO2 data was not statistically different from ELL data, while in summer, it was 5.5 ppm lower on average, suggesting a higher photosynthesis uptake. The amplitude of the CO2 diurnal cycle at CRAM was found to be exponentially related to the local mean daily temperature and dependent on forthcoming wind sector (N-NW or E-SE-S-SW). An increase in CRAM CO2 concentrations was observed under N-NW winds during daytime, which could be related to traffic emissions. This study demonstrates that the use of CO2 sensors with low precision but continuously corrected and periodically calibrated can be used for the study of local and regional CO2 sources and sinks.