Analysis of CO2 and ch4 temporal patterns and the valladolid urban plume influence over the upper spanish plateau

Résumé

Since the Industrial Revolution, the atmospheric mole fractions of greenhouse gases have changed due to human activities, leading to an increase in the average temperature of the planet. Determining greenhouse gases at regional background sites is crucial vis-à-vis assessing what impact anthropogenic emissions have on the atmospheric environment. However, observational studies are still scarce at such background sites. The current thesis first seeks to improve existing knowledge concerning the evolution of the two major greenhouse gases (CO2 and CH4) in terms of trends, growth rate and seasonal variations in the lower atmosphere. Secondly, the effect of the Valladolid urban plume on the final CO2 and CH4 mixing ratios recorded at the Low Atmosphere Research Centre (CIBA) was analysed in order to better understand its impact on the final mixing ratios recorded at the station. To achieve this goal, dry continuous CO2 and CH4 mixing ratios were carried out over five and a half years (from 15 October 2010 to 29 February 2016) at 8.3 m. height using a Picarro G1301 analyser at a remote rural site at the CIBA station on the upper Spanish plateau. This is the first study conducted at the CIBA station with such lengthy records, employing a database collected with a high precision instrument and differentiating between diurnal and nocturnal records. Firstly, in order to analyse temporal patterns, the time series was conveniently detrended and deseasonalised from the observed values so as to capture the intrinsic dynamics of the time series associated to different phenomena. In order to accurately describe CO2 and CH4 temporal evolution over time, three scientific works employing different mathematical functions were performed. The use of different mathematical functions enabled detection of possible bias caused by the method applied and provided a comparison among the mathematical functions employed in terms of ease of use, computational cost involved in the calculations and final data fit. The first paper used a harmonic equation comprising a third-degree polynomial (trend) plus a series of four harmonics (seasonal cycle), each made up of a constant and a variable part along the time series. The second paper applied an Epanechnikov, a Gaussian, a biweight, a triangular, a tricubic and a rectangular kernel function to extract the salient features of the CO2 and CH4 temporal patterns. Moreover, a novel method for simultaneously determining the optimal bandwidths of kernel functions for the long and short-term based on experimental contour plots of R2 values was proposed. The third paper was grounded on the hypothesis that local linear regressions were able to capture CO2 and CH4 temporal evolution equally as well as quadratic linear regressions. The results derived from the temporal analysis point to different behaviour between day and night CO2 and CH4 measurements, with the highest mixing ratios during the night-time when atmospheric mixing and turbulent processes are low. A seasonal pattern was also inferred for the study period for both gases, revealing summer minima, partially due to greater summertime photosynthesis as regards CO2 and to maximum OH concentration during the summer as regards CH4. A simpler cycle was found for CH4, showing only a maximum for diurnal and nocturnal data in winter, partially due to the low presence of OH radicals. However, a different behaviour between diurnal and nocturnal data was revealed by CO2 observations. As regards nocturnal CO2 data, two maxima, one in spring and another in autumn, were reported. These two maxima were linked to an increase in rainfall which corresponded to a period of maximum vegetation growth, thus increasing respiration rates. For CO2 daytime records, only the spring peak was detected. The mixing ratios of the abovementioned gases at the CIBA station were comparable to those at other background sites around the world. Increasing growth rates were obtained for both gases over the whole study period. The slight differences among mixing ratios at different sites may be mainly attributable to the impacts of anthropogenic emissions near the background sites and to regional atmospheric transport. Secondly, it should be considered that assessing the link between atmospheric mixing ratios and wind direction data can help to identify possible pollution sources, thereby giving additional information to the previous temporal patterns described. Unfortunately, the influence of urban plumes on the final measurements recorded at rural stations is an issue which has rarely been touched upon in detail. Thus, in order to provide a full understanding of the temporal patterns at the CIBA station, a fourth scientific paper considering CO2 and CH4 mixing ratio data measured at CIBA, surface wind direction data and back-trajectories at 500 m a.g.l. computed with the METEX model, was produced to analyse the impact of the Valladolid urban plume on the final mixing ratio measured at the station. The final south-westerly component of the urban mean back-trajectory, its longer time spent travelling over the Iberian Peninsula and its recirculation in the final 24 h before impacting the CIBA station, might result in more pollutants being dragged to the station. As a result, the highest CO2 and CH4 values were detected for southern sectors, showing what effect the Valladolid plume has on final CO2 and CH4 measurements. Finally, the results derived from the current thesis prove crucial in terms of understanding the processes that govern CO2 and CH4 trend and cycle evolution. Describing the gases in this way would enable more effective mitigation policies to be planned in order to achieve the goal of reducing the amount of greenhouse gases in the atmosphere.