Study of CO2 variability at different temporal scales recorded in a rural Spanish site

From February 2000 to December 2008 systematic measurements of CO2 ambient concentrations were taken using a MIR 9000 continuous infrared analyser in a rural area of the upper Spanish plateau. The wide database obtained has enabled us to quantify CO2 levels at the measuring site and describe daily and seasonal cycles as well as inter-annual variability. The daily cycle and seasonal variation were dominated by strong temperature inversions, TI, prevailing at the measuring site, up to 7.5 °C per 100 m, which largely contributed to trapping CO2 during night-time, particularly during the growing season. This paper presents the influence of TI on CO2 and describes the results using available TI values below 3 °C as a filtering criterion addressed to minimise local disturbances. The filtered monthly means exhibited a seasonal variation characterised by a primary peak in May, a secondary weak peak in October and a minimum in July–August. Seasonal amplitude yielded 8.7 ppm. Over the whole study period, the annual trend, estimated by means of a second order harmonic model applied to the monthly means (R2 = 95.6%), yielded a positive linear increase of 2.98 and a 0.64 ppm year−1 rise in the amplitude of the seasonal cycle.