Differences between carbon dioxide levels over suburban and rural sites in Northern Spain

Abstract

A comparison between suburban and rural atmospheric carbon dioxide concentrations in Northern Spain is made. Measurements were carried out from April 2006 to November 2007 using MIR 9000 continuous analyzers based on the infrared technique. Two locations were established: a suburban site in Valladolid, a medium-sized city, and a rural location at CIBA, approximately 24 km northwest of Valladolid. Mean suburban CO₂ concentrations are 4.5 ppm higher than rural levels. Frequencies of suburban concentrations from 400 to 450 ppm were around twice the rural frequencies. The suburban daily evolution presented two maxima in the predawn hours and at 1900–2000 GMT, mainly associated to anthropogenic emissions which also affected maximum concentrations recorded in autumn–winter and minimum levels in summer. The rural variation with minimum values during the day and high levels at night is mainly related to vegetation activity which also impacts the highest CO₂ concentrations obtained in spring, coinciding with maximum vegetation growth. Boundary layer processes also affected variability of concentrations recorded at both sites. Air quality at the rural site was also influenced by air mass transport from the urban plume (S, SSE, and ESE), which had a mean CO₂ value of 402 ppm. By contrast, concentrations were low when SW–W and NW winds prevailed and brought cleaner air. The relationship between rural CO₂ concentrations and wind speed using a mathematical fit provides a valuable estimation of the background level at the site, 384.8 ppm.