Continuous carbon dioxide (CO2) measurements over the period 2004-2005 for a rural area in the upper Spanish plateau were examined to characterize the influence of sources and sinks. The diurnal pattern and the annual cycle are presented. The baseline CO2 levels over the time frames researched are determined so as to achieve a more accurate verification of the ambient conditions when uptake is deployed at the site. The results reveal a mean concentration of 384.2 ppm, with 9.8-ppm variability. The mean maximum concentration levels at night, 4:00 a.m. Greenwich Mean Time (GMT), are 390.7 ppm, mainly when atmospheric stability increased. Moreover, mean CO2 levels increase in spring, peaking in May at 388.5 ppm. Concentrations then decline in summer and again increase in autumn, reaching a similar mean value in December. The results also show consistency with vegetation and crop growth, as well as the influence of meteorological conditions, soil features, and human activity in the area. Minimum and maximum CO2 concentrations present a similar but opposite variation, 4.4 ppm x yr(-1), with values decreasing in the latter. Diurnal variation is more pronounced during the growing season and higher in 2004, partly because of abundant rainfall. The lower daily amplitudes in the remaining months are attributed to the reduction in plant and soil respiration processes. The influence of wind on CO2 concentrations has enabled us to identify the contribution of emissions from the cities of Valladolid and Palencia. An increase in mean CO2 concentrations was observed in the, east-southeast, southeast, south-southeast, and south sectors for the former city, and north and east for the latter. The ratio of CO2 increase in the wind sectors influenced by these sources yielded a factor of 1.2 with respect to the relationship between the populations of the two cities.