Responses to ozone pollution of alfalfa exposed to increasing salinity levels

Stomatal closure and biosynthesis of antioxidant molecules are two fundamental components of the physiological machinery that lead to stress adaptation during plant's exposure to salinity. Since high stomatal resistance may also contribute in counteracting O₃ damages, we hypothesized that soil salinization may increase O₃ tolerance of crops. An experiment was performed with alfalfa grown in filtered (AOT40 = 0 in both years) and non-filtered (AOT40 = 9.7 in 2005 and 6.9 ppm h in 2006) open-top chambers. Alfalfa yield was reduced by O₃ (−33%) only in plants irrigated with salt-free water, while the increasing levels of soil salinity until 1.06 dS m⁻¹ reduced both stomatal conductance and plant O₃ uptake, thus linearly reducing O₃ effects on yield. Therefore a reliable flux-based model for assessing the effects of O₃ on crop yield should take into account soil salinity.     

Meta-analysis of the relative sensitivity of semi-natural vegetation species to ozone

This study identified 83 species from existing publications suitable for inclusion in a database of sensitivity of species to ozone (OZOVEG database). An index, the relative sensitivity to ozone, was calculated for each species based on changes in biomass in order to test for species traits associated with ozone sensitivity. Meta-analysis of the ozone sensitivity data showed a wide inter-specific range in response to ozone. Some relationships in comparison to plant physiological and ecological characteristics were identified. Plants of the therophyte lifeform were particularly sensitive to ozone. Species with higher mature leaf N concentration were more sensitive to ozone than those with lower leaf N concentration. Some relationships between relative sensitivity to ozone and Ellenberg habitat requirements were also identified. In contrast, no relationships between relative sensitivity to ozone and mature leaf P concentration, Grime's CSR strategy, leaf longevity, flowering season, stomatal density and maximum altitude were found. The relative sensitivity of species and relationships with plant characteristics identified in this study could be used to predict sensitivity to ozone of untested species and communities.     

Growth-stage dependent crop yield response to ozone exposure

Data from four crop yield-loss field trials were examined to determine if analysis using an imposed phenological weighting function based on seasonal growth stage would provide a more accurate indication of impact of ozone exposure. Alfalfa (Medicago sativa L. cv. Moapa 69), dry bean (Phaseolus vulgaris L. cv. California Dark Red kidney), fresh market and processing tomato (Lycopersicon esculentum Mill. cv. 6718 VF and VF-145-B7879, respectively) were grown at 9-11 ambient field plots within southern California comprising an ambient gradient of ozone. The growing season for each crop was artificially divided into 'quarters' composed of equal numbers of whole days and roughly corresponding to specific growth stages. Ozone exposure was calculated for each of these 'quarters' and regressed against final crop yield using 163 different exposure statistics. Weighting functions were developed using reciprocal residual mean square (1/RMS) or percentage of the best 100 exposure statistics of the 163 tested (TOP100) for each of the quarters. The third quarter of the alfalfa season was clearly most responsive to ozone as measured by both of the weighting functions. Third quarter ozone was also weighted highest by both weighting functions for dry bean. Fresh market and processing tomato were each influenced the greatest by second quartero zone as demonstrated by both weighting functions. The occurrence of ozone during physiologically important events (flowering and initial fruit set in second quarter for tomato; pod development in third quarter for dry bean) appeared to influence the yield of these crops the greatest. Growth-stage-dependent phenological weighting of pollutant exposure may result in more effective predictions of levels of ozone exposure resulting in yield reductions.     

Wheat-kernel growth characteristics during exposure to chronic ozone pollution

Chronic exposure to ozone (O(3)) air pollution can reduce yield in wheat; however, little is known concerning the effects of O(3) stress on kernel development. A field study was conducted to investigate the effects of chronic O(3) exposure on kernel-growth components of two soft red winter-wheat genotypes (Seven and MD5518308). Five air-quality treatments, including charcoal-filtered air (CF), non-filtered air (NF), NF + 20, and NF + 40 and NF + 80 nl O(3) liter(-1) air were applied 4 h d(-1), 5 d wk(-1) through maturity. In the case of the NF + treatments, O(3) was added to existing ambient O(3) levels. Spike samples were collected 16, 20, 24, 28, and 32 days after anthesis (DAA). Linear and quadratic equations were fitted to kernel-weight data to estimate kernel-growth rate (KGR) and kernel-fill duration (KFD). Effective filling period (EFP) and assimilate utilization (AU) were also determined. Rates of growth for individual kernels were 0.74 mg d(-1) and 1.07 mg d(-1) for the NF + 80 and CF treatments, respectively. The NF + 80 nL litter(-1) O(3) treatment significantly reduced KGR and AU compared with the CF treatment. Severn had a significantly loger KFD than MD5518308, but O(3) had no significant effect on KFD of either genotype. Each genotype had similar EFP values, and O(3) had no significant effect on EFP. Linear relationships between O(3) exposure and kernel weight suggests that O(3) effects on kernel weight begin soon after anthesis in MD5518308, but, in Severn, O(3) has a greater effect on kernel weight during the later stages of kernel development. These data suggest that decreased economic yield associated with chronic O(3) exposure is primarily the result of decreased KGR.     

Estimating long-term population exposure to ozone in urban areas of Europe

Tropospheric ozone concentrations regarded as harmful for human health are frequently encountered in Central Europe in summertime. Although ozone formation generally results from precursors transported over long distances, in urban areas local effects, such as reactions due to nearby emission sources, play a major role in determining ozone concentrations. Europe-wide mapping and modeling of population exposure to high ozone concentrations is subject to many uncertainties, because small-scale phenomena in urban areas can significantly change ozone levels from those of the surroundings. Currently the integrated assessment modeling of European ozone control strategies is done utilizing the results of large-scale models intended for estimating the rural background ozone levels. This paper presents an initial study on how much local nitrogen oxide (NOx) concentrations can explain variations between large-scale ozone model results and urban ozone measurements, on one hand, and between urban and nearby rural measurements, on the other. The impact of urban NOx concentrations on ozone levels was derived from chemical equations describing the ozone balance. The study investigated the applicability of the method for improving the accuracy of modeled population exposure, which is needed for efficient control strategy development. The method was tested with NOx and ozone measurements from both urban and rural areas in Switzerland and with the ozone predictions of the large-scale photochemical model currently used in designing Europe-wide control strategies for ground-level ozone. The results suggest that urban NOx levels are a significant explanatory factor in differences between urban and nearby rural ozone concentrations and that the phenomenon could be satisfactorily represented with this kind of method. Further research efforts should comprise testing of the method in more locations and analyzing the performance of more widely applicable ways of deriving the initial parameters.     

Influence of the atmospheric conductivity on the ozone exposure of plants under ambient conditions: Considerations for establishing ozone standards to protect vegetation

This paper provides results of ozone flux density measurements above a permanent grassland ecosystem as they relate to an establishment of air quality guidelines or standards. Using a resistance analogue, the product of zone concentration measured at a standard measurement height and the conductivity of the atmosphere reflect the maximum possible ozone flux density towards the envelope of the plants. In other words, this product can be regarded as the ozone exposure potential of the atmosphere for plants. It could be shown that ozone concentrations between 100 and 180 microg m(-3) are likely to have a great phytotoxic potential and are more important than concentrations greater than 180 microg m(-3). From the results presented one can deduce that the application of dose-response relationships based on chamber experiments to ambient conditions results in an overestimation of, for example, yield loses. Any guideline or standard has to take into account the influence of the atmospheric conductivity on the absorbed dose of ozone.     

Evaluation of ozone exposure indices in exposure-response modeling

In exposure-response modeling, a major concern is the numerical definition of exposure in relating crop loss to O3, yet few indices have been considered. This paper addresses research in which plant growth was regressed for soybean, wheat, cotton, corn, and sorghum against 613 numerical exposure indices using the Box-Tidwell model. When the minimum sum of squared errors criterion was used, optimum performance was not attained for any single index; however, near optimum performances were achieved by two censored cumulative indices and from a class of indices called the generalized, phenologically weighted, cumulative impact indices (GPWCIs). The top-performing GPWCIs accumulated concentrations, used sigmoid weighting schemes emphasizing O3 concentrations of 0.06 ppm (118 microg m(-3)) or higher, and had phenological weighting schemes with greatest weight occurring 20 to 40 days prior to crop maturity. These findings indicate that (1) peak concentrations are important, but lower concentrations should be included in the calculations, (2) increased plant sensitivity occurs between flowering and maturity, and (3) plants respond to cumulative exposure impact.     

Assessing the relative importance of surface ozone influential variables in regional-scale analysis

The multilayer perceptron (MLP) model is frequently used to assess the relative importance (RI) of surface ozone influential variables when they are used to investigate ozone variation mechanisms. Previous studies, however, suffer from two limitations: 1) indentifying or searching the optimal MLP topology to avoid a biased RI assessment inevitably incurs a heavy computational burden, and 2) the model is suitable only for local-scale analysis of ozone variation mechanisms. To tackle both problems, we selected three typical air-quality monitoring sites in Hong Kong as our study targets, as the ozone variations at these sites are inhomogeneously affected by regional and local factors. An MLP model trained by automatic relevance determination (referred to as MLP-ARD), a Bayesian MLP approach, was employed to assess the RI for both regional and local ozone influential variables. The results indicated the following remarks: 1) The MLP-ARD model, due to its high degree of resistance to both the over-fitting and the under-fitting problems, is exempt from identifying or searching the optimal MLP topology when used to obtain an unbiased RI assessment and thus avoid the heavy computational burden. Furthermore, the RI assessment results obtained with the MLP-ARD method are comparable to those of the best assessment method in the literature. Based on these findings, decision-makers can scientifically promulgate a site-specific air pollution control strategy site; 2) Regional-scale analysis of ozone variation is indispensable, as taking the key regional ozone influential variables into account significantly improves the prediction accuracy of the MLP-ARD model, especially on peak ozone days.     

Vegetation exposure to ozone over the continental United States: Assessment of exposure indices by the Eta-CMAQ air quality forecast model

The main use of air quality forecast (AQF) models is to predict ozone (O₃) exceedances of the primary O₃ standard for informing the public of potential health concerns. This study presents the first evaluation of the performance of the Eta-CMAQ air quality forecast model to predict a variety of widely used seasonal mean and cumulative O₃ exposure indices associated with vegetation using the U.S. AIRNow O₃ observations. These exposure indices include two concentration-based O₃ indices, M7 and M12 (the seasonal means of daytime 7-h and 12-h O₃ concentrations, respectively), and three cumulative exposure-based indices, SUM06 (the sum of all hourly O₃ concentrations ≥ 0.06 ppm), W126 (hourly concentrations weighed by a sigmoidal weighting function), and AOT40 (O₃ concentrations accumulated over a threshold of 40 ppb during daylight hours). During a three-month simulation (July–September 2005), the model over predicted the M7 and M12 values by 8–9 ppb, or a NMB value of 19% and a NME value of 21%. The model predicts a central belt of high O₃ extending from Southern California to Middle Atlantic where the seasonal means, M7 and M12 (the seasonal means of daytime 7-h and 12-h O₃ concentrations), are higher than 50 ppbv. In contrast, the model is less capable of reproducing the observed cumulative indices. For AOT40, SUM06 and W126, the NMB and NME values are two- to three-fold of that for M7, M12 or peak 8-h O₃ concentrations. The AOT40 values range from 2 to 33 ppm h by the model and from 1 to 40 ppm h by the monitors. There is a significantly higher AOT40 value experienced in the United States in comparison to Europe. The domain-wide mean SUM06 value is 14.4 ppm h, which is about 30% higher than W126, and 40% higher than AOT40 calculated from the same 3-month hourly O₃ data. This suggests that SUM06 and W126 represent a more stringent standard than AOT40 if either the SUM06 or the W126 was used as a secondary O₃ standard. Although CMAQ considerably over predicts SUM06 and W126 values at the low end, the model under predicts the extreme high exposure values (>50 ppm h). Most of these extreme high values are found at inland California sites. Based on our analysis, further improvement of the model is needed to better capture cumulative exposure indices.     

The use of kriging to estimate monthly ozone exposure parameters for the Southeastern United States

This paper explores the feasibility of (1) using kriging to predict the monthly mean of daily 7-h mean (0900-1559) O3 concentrations, (2) using kriging to estimate the per cent of hourly mean O3 concentrations equal to or greater than 0.07 ppm (137 microg m(-3)) for a specific month, and (3) developing a quantitative relationship between the monthly mean of the daily 7-h (0900-1559) average O3 concentration and the monthly number of hourly concentrations > or = 0.08p ppm (157 microg m(-3)). We found that kriging can be used to estimate the (1) monthly mean of daily 7-h mean O3 concentrations and (2) the percentage of hourly concentrations for a given month > or = 0.07 ppm when sufficient spatial coverage was available. However, the per cent > or = 0.07 ppm parameter exhibited much greater relative variability than the monthly 7-h exposure index. A strong statistical association was found between the monthly number of occurrences > or = 0.08 ppm and monthly 7-h mean concentrations above 0.05 ppm (98 microg m(-3)). Because of the variability that cumulative indices, such as the monthly percentage of hourly concentrations > or = 0.07 ppm , exhibit from site to site, it appears that whether kriging techniques or mathematical regressions are used to estimate the number of elevated O3 hourly concentrations above selected thresholds, large uncertainties associated with the predicted values will exist. These large uncertainties will make it difficult to accurately estimate vegetation effects caused by ambient levels of O3. However, if a generalized quantitative relationship between repeated occurrences of hourly mean concentrations > or = 0.07 ppm or > or = 0.08 and vegetation effects can be developed, it may be possible, using kriged monthly values accompanied with confidence intervals, to identify those areas where vegetation may be at risk. However, before it will be possible to implement such an approach, researchers will have to better quantify the relationship between realistic O3 exposures and vegetation effects.     

Chronic ozone exposure increases the susceptibility of hybrid Populus to disease caused by Septoria Musiva

Rooted cuttings of hybrid Populus (DN34, Populus deltoides X nigra) were grown outdoors in pots in open-top chambers at Ithaca, NY (74.5 degrees W, 42.5 degrees N), during 1988 and 1989 (Experiment 1) and during 1989 and 1990 (Experiment 2). Ambient air was passed through charcoal filters to produce a 0.5 times ambient ozone treatment, and ozone generated from oxygen was added to produce one and two times ambient ozone treatments. In Experiment 1, treatments were applied for 8-12 h each day for 112 days of the 1988 growing season; then the plants were grown outdoors with ambient ozone in 1989. In Experiment 2, treatments were applied for 9 h each day for 98 days of the 1989 growing season; then the plants were grown outdoors with ambient ozone in 1990. Shallow wounds were made into the bark tissue and inoculated with either an aqueous suspension of conidia of Mycosphaerella populorum or sterile water on 1 and 2 September 1988 (Experiment 1) or 16 and 17 August 1989 (Experiment 2). In Experiment 1, wounds were inoculated either 0, 7, or 14 days after wounding. In Experiment 2, wounds were inoculated either 0, 3, or 6 days after wounding. Canker development was measured after harvest on 16 and 17 July 1989 (Experiment 1) and 28 May 1990 (Experiment 2). In both experiments, chronic exposure to ozone significantly increased the incidence of canker formation in inoculated wounds, and no cankers formed in wounds that received only sterile water. In Experiment 1, cankers formed only on plants inoculated the same day as wounding. No cankers formed on plants inoculated either 7 or 14 days after wounding. In Experiment 2, cankers formed on plants inoculated on the same day as wounding, and on a few plants inoculated 3 days after wounding. No cankers formed on plants inoculated 6 days after wounding. Additionally, in Experiment 2, exposure to increased concentrations of ozone caused a significantly higher number of plants to die during the subsequent winter. Analysis of partial correlation coefficients among plant growth and plant disease variables suggested that the observed ozone-induced increase in the susceptibility of the plants to disease was not mediated by alterations in plant growth.     

Temporal processes that contribute to nonlinearity in vegetation responses to ozone exposure and dose

Ozone interacts with plant tissue through distinct temporal processes. Sequentially, plants are exposed to ambient O₃ that (1) moves through the leaf boundary layer, (2) is taken up into plant tissue primarily through stomata, and (3) undergoes chemical interaction within plant tissue, first by initiating alterations and then as part of plant detoxification and repair. In this paper, we discuss the linkage of the temporal variability of apoplastic ascorbate with the diurnal variability of defense mechanisms in plants and compare this variability with daily maximum O₃ concentration and diurnal uptake and entry of O₃ into the plant through stomata. We describe the quantitative evidence on temporal variability in concentration and uptake and find that the time incidence for maximum defense does not necessarily match diurnal patterns for maximum O₃ concentration or maximum uptake. We suggest that the observed out-of-phase association of the diurnal patterns for the above three processes produces a nonlinear relationship that results in a greater response from the higher hourly average O₃ concentrations than from the lower or mid-level values. The fact that these out-of-phase processes affect the relationship between O₃ exposure/dose and vegetation effects ultimately impact the ability of flux-based indices to predict vegetation effects accurately for purposes of standard setting and critical levels. Based on the quantitative aspect of temporal variability identified in this paper, we suggest that the inclusion of a diurnal pattern for detoxification in effective flux-based models would improve the predictive characteristics of the models. While much of the current information has been obtained using high O₃ exposures, future research results derived from laboratory biochemical experiments that use short but elevated O₃ exposures should be combined with experimental results that use ambient-type exposures over longer periods of time. It is anticipated that improved understanding will come from future research focused on diurnal variability in plant defense mechanisms and their relationship to the diurnal variability in ambient O₃ concentration and stomatal conductance. This should result in more reliable O₃ exposure standards and critical levels.     

A statistical model for FEV1 response to arbitrary dynamic ozone exposure conditions

Lung function response to inhaled ozone at ambient air pollution levels is known to be a function of ozone concentration, exposure duration, and minute ventilation. Most data-driven exposure-response models address exposures under static condition (i.e., with a constant ozone concentration and exercise pattern). Such models are simplifications, as both ambient ozone concentrations and normal human activity patterns change with time. The purpose of this study was to develop a dynamic model of response with the advantages of a statistical model (a relatively simple structure with few parameters). A previously proposed mechanistic model for changes in specific airways resistance was adapted to describe the percent change in forced expiratory volume in one second (FEV1). This model was then reduced using the fit to three existing exposure-response data sets as criterion. The resulting model consists of a single linear differential equation together with an algebraic logistic equation. Under restricted static conditions the model reduces to a logistic model presented earlier by the authors.     

Assessing vegetation exposure to ozone: properties of the AOT40 index and modifications by deposition modelling

A discussion is presented on the application of micrometeorological deposition modelling principles to improve the characterisation of vegetation exposure to ozone and thus the use of critical levels as the basis of targeted emission control. The AOT40 (accumulated exposure over a threshold of 40 ppb or nl l(-1)) ozone exposure index is shown to impose a differential weighting that results in a high sensitivity, by a factor of two to 10 depending on the pollution climate, with respect to concentration. This makes it necessary to correct for systematic effects, such as the concentration profile below the measurement height, in order to justify a comparison with the biological data obtained from well-mixed exposure chambers. Available studies indicate a 50-70% lower AOT40 at the vegetation height. The resistance method for estimating the profile is extended to allow for stomatal effects that potentially bias the plant response predicted with an exposure index. This integrated profile-uptake correction refines the current approach and serves as a transitional step towards a real flux-based approach. For the latter, a new deposition parameterisation is tested against field observations.     

Estimated crop yield losses due to surface ozone exposure and economic damage in India

In this study, we estimate yield losses and economic damage of two major crops (winter wheat and rabi rice) due to surface ozone (O₃) exposure using hourly O₃ concentrations for the period 2002–2007 in India. This study estimates crop yield losses according to two indices of O₃ exposure: 7-h seasonal daytime (0900–1600 hours) mean measured O₃ concentration (M7) and AOT40 (accumulation exposure of O₃ concentration over a threshold of 40 parts per billion by volume during daylight hours (0700–1800 hours), established by field studies. Our results indicate that relative yield loss from 5 to 11 % (6–30 %) for winter wheat and 3–6 % (9–16 %) for rabi rice using M7 (AOT40) index of the mean total winter wheat 81 million metric tons (Mt) and rabi rice 12 Mt production per year for the period 2002–2007. The estimated mean crop production loss (CPL) for winter wheat are from 9 to 29 Mt, account for economic cost loss was from 1,222 to 4,091 million US$ annually. Similarly, the mean CPL for rabi rice are from 0.64 to 2.1 Mt, worth 86–276 million US$. Our calculated winter wheat and rabi rice losses agree well with previous results, providing the further evidence that large crop yield losses occurring in India due to current O₃ concentration and further elevated O₃ concentration in future may pose threat to food security.     

Within season and carry-over effects following exposure of grassland species mixtures to increasing background ozone

Few studies have investigated effects of increased background ozone in the absence of episodic peaks, despite a predicted increase throughout the northern hemisphere over the coming decades. In this study Leontodon hispidus was grown with Anthoxanthum odoratum or Dactylis glomerata and exposed in the UK to one of eight background ozone concentrations for 20 weeks in solardomes. Seasonal mean ozone concentrations ranged from 21.4 to 102.5 ppb. Ozone-induced senescence of L. hispidus was enhanced when grown with the more open canopy of A. odoratum compared to the denser growing D. glomerata. There was increased cover with increasing ozone exposure for both A. odoratum and D. glomerata, which resulted in an increase in the grass:Leontodon cover ratio in both community types. Carry-over effects of the ozone exposure were observed, including delayed winter die-back of L. hispidus and acceleration in the progression from flowers to seed-heads in the year following ozone exposure.     

Nonparametric functional data estimation applied to ozone data: prediction and extreme value analysis

The study of extreme values and prediction of ozone data is an important topic of research when dealing with environmental problems. Classical extreme value theory is usually used in air-pollution studies. It consists in fitting a parametric generalised extreme value (GEV) distribution to a data set of extreme values, and using the estimated distribution to compute return levels and other quantities of interest. Here, we propose to estimate these values using nonparametric functional data methods. Functional data analysis is a relatively new statistical methodology that generally deals with data consisting of curves or multi-dimensional variables. In this paper, we use this technique, jointly with nonparametric curve estimation, to provide alternatives to the usual parametric statistical tools. The nonparametric estimators are applied to real samples of maximum ozone values obtained from several monitoring stations belonging to the Automatic Urban and Rural Network (AURN) in the UK. The results show that nonparametric estimators work satisfactorily, outperforming the behaviour of classical parametric estimators. Functional data analysis is also used to predict stratospheric ozone concentrations. We show an application, using the data set of mean monthly ozone concentrations in Arosa, Switzerland, and the results are compared with those obtained by classical time series (ARIMA) analysis.     

Assessment of a regulatory model's performance relative to large spatial heterogeneity in observed ozone in Houston, Texas

In Houston, some of the highest measured 8-hr ozone (O3) peaks are characterized by sudden increases in observed concentrations of at least 40 ppb in 1 hr or 60 ppb in 2 hr. Measurements show that these large hourly changes appear at only a few monitors and span a narrow geographic area, suggesting a spatially heterogeneous field of O3 concentrations. This study assessed whether a regulatory air quality model (AQM) can simulate this observed behavior. The AQM did not reproduce the magnitude or location of some of the highest observed hourly O3 changes, and it also failed to capture the limited spatial extent. On days with measured large hourly changes in O3 concentrations, the AQM predicted high O3 over large regions of Houston, resulting in overpredictions at several monitors. This analysis shows that the model can make high O3, but on these days the predicted spatial field suggests that the model had a different cause. Some observed large hourly changes in O3 concentrations have been linked to random releases of industrial volatile organic compounds (VOCs). In the AQM emission inventory, there are several emission events when an industrial point source increases VOC emissions in excess of 10,000 mol/hr. One instance increased predicted downwind O3 concentrations up to 25 ppb. These results show that the modeling system is responsive to a large VOC release, but the timing and location of the release, and meteorological conditions, are critical requirements. Attainment of the O3 standard requires the use of observational data and AQM predictions. If the large observed hourly changes are indicative of a separate cause of high O3, then the model may not include that cause, which might result in regulators enacting control strategies that could be ineffective.     

A chamberless field exposure system for ozone enrichment of short vegetation

Only few studies have been conducted as yet which focus on the effects of rising tropospheric ozone levels on semi-natural vegetation under free-air conditions. A new technical approach was used to examine the response of calcareous grassland to ozone employing a chamberless fumigation system. Four different ozone regimes were applied (1-, 1.33-, 1.66- and 2-fold ambient air levels) with five replicates each. Ozone enrichment was carried out on circular plots of 2 m in diameter by a computer controlled exposure system. Transparent windscreens encircling each plot accelerated the mixing of ambient air and ozone released. Thus, the use of blowers could be avoided. The exposure system presented here is regarded as an appropriate technique for free-air trace gas enrichment on short vegetation avoiding microclimatic alterations known to affect plant growth and pollutant uptake. Furthermore, the chosen technical set-up was rather cost-effective. Hence, it enabled the establishment of a larger number of replications providing the basis for results of higher statistical power.