Isidro A. Pérez

Soil CO2 fluxes in cereal land use of the Spanish plateau: influence of conventional and reduced tillage practices

From November 1998 to October 2000, measurements of soil respiration were performed on the Spanish plateau for two patches of non-irrigated barley, one managed with conventional tillage (CT) and the other with reduced tillage (RT). Soil CO2 flux showed seasonal variation on both patches, with an increase from March to October, peaking in May, and a decrease during the winter period by a factor of around 2. The mean value for both combined years was 2.03 and 1.70 micromol m(-2) S(-1), in the CT and RT patches, respectively. In order to analyse the influence of RT on soil CO2 flux, two tests were performed. The first one was the Kruskal-Wallis test to compare whether the differences between the medians in both patches were statistically significant. The results obtained revealed statistically significant differences during the second year, at a 85% and 95% significance level, use being made of annual data and that recorded during the period of maximum interest, March-October, respectively. The decrease in soil respiration in the RT patch was around 24%. The second test was aimed at describing and comparing the influence of soil temperature on soil CO2 flux. By using the data of both patches recorded during the first year, an empirical equation on 10-cm soil temperature was fitted and tested on the data corresponding to the second year in each of the patches. Then, a comparison between the medians of the differences between the estimated and observed values was again performed by means of the Kruskal-Wallis test. The over-prediction of the model in the RT patch, statistically significant at a 90% significance level, was roughly 23%, confirming again the decrease in soil respiration one year after this agricultural management practice had been implemented.

Continuous carbon dioxide measurements in a rural area in the upper Spanish plateau.

Abstract 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 up-take 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·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.

CO2 transport by urban plumes in the upper Spanish plateau

CO(2) transport from two cities, Valladolid, over 20 km away and Palencia, over 40 km away from a rural site is analysed through three years of detrended CO(2) concentrations obtained near the surface. Meteorological data were obtained from a RASS sodar. Directional analysis by histogram of concentrations above the 95th percentile revealed three differing sectors, one associated to a rural origin and two linked to both cities. Modes indicated anticyclonic turning during plume travel, confirmed by the daily evolution of the wind direction. At night, the Valladolid concentration median was 6 ppm above the Palencia median, which was 2 ppm higher than the rural sector median. Monthly evolution of daily maxima evidenced the Valladolid plume influence in spring and September, whereas the Palencia plume was noticeable in October and November. Skewness analysis showed almost symmetric distributions in the Valladolid plume and right skewed distributions in the Palencia and rural sectors. This result was attributed to the different mixing of both plumes. Vertical gradients of wind speed, direction and potential temperature were also calculated, and evidenced a stratified structure of the lower atmosphere at night and an almost uniform layer during the day. Finally, the median gradient Richardson number showed the highest values, occasionally above 0.8 for the Valladolid sector, implying lower mixing with the environment in the Valladolid plume than in the Palencia plume.

Synoptic weather patterns associated with carbon dioxide levels in Northern Spain.

Measurements of atmospheric carbon dioxide, CO(2), were continuously carried out in the upper Spanish plateau over a three-year campaign, 2003-2005. Temporal CO(2) variations were examined. The results allow identification of the average data representative of background conditions, 382.9 ppm, with values ranging from 346.2 to 502.5 ppm. The weekly cycle evidences a difference of 0.2 ppm between weekday and weekend residuals, with levels increasing during the week. Seasonal variation in monthly means was inferred, the largest peak in appearing in spring, about 388 ppm. High values were also recorded in autumn, particularly in 2005 with an additional 5 ppm. By contrast, minimum values were obtained in July, between 374 and 379 ppm. A link between CO(2) concentrations and meteorological variables is explored. Analysis of surface wind speed intervals shows that low winds are the most frequent and are linked to the highest concentrations, around 395 ppm at night and in spring. CO(2) concentrations drop significantly for the 3.1-5.3 ms(-1) interval from which steady levels, around 378 ppm except in autumn, were observed. If different temperature intervals are considered, the 10-15 degrees C interval establishes the boundary between the extreme mean CO(2) levels, except for winter, 5-10 degrees C. The mean associated values ranged between 376.0 and 390.4 ppm, with a greater contrast in spring, 12.8 ppm. Finally, the relation between synoptic-scale atmospheric transport patterns and maximum CO(2) concentrations was also examined. The highest values occur in spring with some quite frequent synoptic situations: continental ridges, troughs to the west, interactions of the two and Atlantic ridges.

Analysis of CO2 daily cycle in the low atmosphere at a rural site.

Directional analysis procedures were extended to study the CO(2) daily cycle in the low atmosphere at a rural site. Weighted average, asymmetric circular functions, and conditional probability curves were applied to concentrations recorded at three levels near surface. Wind speed and potential temperature gradient were the meteorological variables used to establish a link to CO(2). The highest weighted concentration was obtained for wind speed below 4 m s(-1) and for potential temperature gradient above 0.05 °C m(-1). Asymmetric circular functions were fitted to selected percentiles, the generalized von Mises function providing the lowest RMSE. Slight differences among levels were observed in the transition between day and night, and higher gradients were observed during the night. Four intervals were proposed when concentrations were binned by intervals of meteorological variables. Wind speed was only relevant to separate intermediate concentrations. Finally, conditional probability curves provided differences between the highest and lowest levels in up to 20% of cases at 20-22 GMT.

Applications of air mass trajectories

Air trajectory calculations are commonly found in a variety of atmospheric analyses. However, most of reported research usually focuses upon the transport of pollutants via trajectory routes and not on the trajectory itself. This paper explores the major areas of research in which air trajectory analyses are applied with an effort to gain deeper insights into the key points which highlight the necessity of such analyses. Ranging from meteorological applications to their links with living beings, air trajectory calculations become important tool especially when alternative procedures do not seem possible. This review covers the reports published during last few years illustrating the geographical distribution of trajectory applications and highlighting the regions where trajectory application research proves most active and useful. As a result, relatively unexplored areas such as microorganism transport are also included, suggesting the possible ways in which successful use of air trajectory research should be extended.

Spatial analysis of CO2 concentration in an unpolluted environment in northern Spain

CO(2) transport in the low atmosphere near the surface at night was analysed using spatial procedures applied to back trajectories. Concentration and meteorological data were obtained at a rural site during a seven and a half month campaign. Daily evolution of CO(2) skewness showed positive values during the night and negative values during the day. One concentration and one recirculation factor, an indicator of local recirculation, were associated with each back trajectory calculated during the night to study source and meteorological influences on concentrations recorded. Moreover, four procedures were used to analyse the trajectories, and their strengths and weaknesses were also investigated. (1) The nonparametric trajectory analysis applied with two weight functions successfully marked the most contributing region. (2) Hexagonal cells were used to account for radial distribution of trajectories. The potential source contribution function calculated in these cells highlighted the influence of the source against meteorology, this procedure therefore proving the best to mark the source direction. (3) Trajectory sector analysis revealed the most contributing wind sector and emphasised the role of recirculation in the E-S sectors. (4) Cluster analysis grouped neighbouring trajectories and was the most flexible procedure to classify them, providing a contrast of around 12 ppm between medians obtained in the SE cluster and the least contributing group.

Autocorrelation Analysis of Meteorological Data from a RASS Sodar

Abstract Autocorrelation analysis is necessary in persistence studies and identification of cyclical processes. In this paper, autocorrelations of available wind speed and temperature data from a radio acoustic sounding system (RASS) sodar were calculated. This device was placed on flat terrain, and the measuring campaign extended over April 2001. Ten-minute averages were considered from 40 to 500 m in 20-m levels. The direction frequency rose indicated clear, prevailing directions in the east-northeast–west-southwest axis. Analysis of median temperatures revealed that east-northeast advections were 5°C colder than those from the west-southwest. A defined pattern was obtained for both autocorrelations, comprising deterministic and random parts. Noise became more relevant at the higher levels. The deterministic part could be considered as an initial fast-decaying term with the addition of two harmonic functions. The initial decay, linked to fast changes, increased with height for wind speed and decreased f...

Carbon dioxide at an unpolluted site analysed with the smoothing kernel method and skewed distributions.

CO₂ concentrations recorded for two years using a Picarro G1301 analyser at a rural site were studied applying two procedures. Firstly, the smoothing kernel method, which to date has been used with one linear and another circular variable, was used with pairs of circular variables: wind direction, time of day, and time of year, providing that the daily cycle was the prevailing cyclical evolution and that the highest concentrations were justified by the influence of one nearby city source, which was only revealed by directional analysis. Secondly, histograms were obtained, and these revealed most observations to be located between 380 and 410 ppm, and that there was a sharp contrast during the year. Finally, histograms were fitted to 14 distributions, the best known using analytical procedures, and the remainder using numerical procedures. RMSE was used as the goodness of fit indicator to compare and select distributions. Most functions provided similar RMSE values. However, the best fits were obtained using numerical procedures due to their greater flexibility, the triangular distribution being the simplest function of this kind. This distribution allowed us to identify directions and months of noticeable CO₂ input (SSE and April-May, respectively) as well as the daily cycle of the distribution symmetry. Among the functions whose parameters were calculated using an analytical expression, Erlang distributions provided satisfactory fits for monthly analysis, and gamma for the rest. By contrast, the Rayleigh and Weibull distributions gave the worst RMSE values.

Analysis of wind data in the low atmosphere from a RASS sodar

This paper focuses on the ability of a sodar to describe some characteristics of the atmospheric vertical structure and presents some techniques for meteorological data evaluation. The measuring campaign took place in April 2001 and consisted of 10-min averages covering the lower atmosphere from 40 to 500 m at 20-m levels. Three methods were considered, the first of which was a scalar analysis performed using a combination of wind and temperature median profiles. A noticeable contrast between day and night was obtained. Flat wind profiles during the day were a consequence of prevailing convective conditions that determined thermal turbulence. A stable layer above 260 m capped the unstable layer situated below and guaranteed the stability of the boundary layer. During the night, the presence of a low level jet was the most significant feature. The height of the core was 340 m and the higher vertical winds defined it clearly. The second method focused on the wind vector. In this analysis, the anti-cyclonic rotation of hourly averages was considered in the lower levels where it was observed. After a translation of the origin, an empirical, robust model with two parts was then proposed for the resulting vector. The angle was described linearly and the module by a second order model for cylindrical data. Finally, as a third method, three regression analyses were investigated: vectorial, taking every wind component separately and scalar. The two first seemed to be more complete due to their description of anti-cyclonic wind rotation when height increased. Correlation coefficients also proved to be more satisfactory. As a consequence, these techniques, although less frequently used, are more suitable to study wind in the low atmosphere.