Comparison of different stomatal conductance algorithms for ozone flux modelling

A multiplicative and a semi-mechanistic, BWB-type [Ball, J.T., Woodrow, I.E., Berry, J.A., 1987. A model predicting stomatal conductance and its contribution to the control of photosynthesis under different environmental conditions. In: Biggens, J. (Ed.), Progress in Photosynthesis Research, vol. IV. Martinus Nijhoff, Dordrecht, pp. 221-224.] algorithm for calculating stomatal conductance (g(s)) at the leaf level have been parameterised for two crop and two tree species to test their use in regional scale ozone deposition modelling. The algorithms were tested against measured, site-specific data for durum wheat, grapevine, beech and birch of different European provenances. A direct comparison of both algorithms showed a similar performance in predicting hourly means and daily time-courses of g(s), whereas the multiplicative algorithm outperformed the BWB-type algorithm in modelling seasonal time-courses due to the inclusion of a phenology function. The re-parameterisation of the algorithms for local conditions in order to validate ozone deposition modelling on a European scale reveals the higher input requirements of the BWB-type algorithm as compared to the multiplicative algorithm because of the need of the former to model net photosynthesis (A(n)).     

A comparison of two different approaches for mapping potential ozone damage to vegetation. A model study

Two very different types of approaches are currently in use today for indicating risk of ozone damage to vegetation in Europe. One approach is the so-called AOTX (accumulated exposure over threshold of Xppb) index, which is based upon ozone concentrations only. The second type of approach entails an estimate of the amount of ozone entering via the stomates of vegetation, the AFstY approach (accumulated stomatal flux over threshold of Y nmol m(-2) s(-1)). The EMEP chemical transport model is used to map these different indicators of ozone damage across Europe, for two illustrative vegetation types, wheat and beech forests. The results show that exceedences of critical levels for either type of indicator are widespread, but that the indicators give very different spatial patterns across Europe. Model simulations for year 2020 scenarios suggest reductions in risks of vegetation damage whichever indicator is used, but suggest that AOT40 is much more sensitive to emission control than AFstY values.     

A critical evaluation of the implications for risk based land management of the environmental chemistry of Sulphur Mustard

Sulphur Mustard, or "Mustard Gas" is in fact an oily liquid which was used as a chemical weapon primarily for its vesicant action which necessitates whole body protection. It is also now recognised as a carcinogenic agent upon chronic exposure. Soil contaminated with Sulphur Mustard continues to present both acute and chronic human health risks and risks to groundwater, surface water and the wider ecology at a number of sites globally and, in some cases, has done for many decades. This is at odds with the simple aqueous chemistry of the compound which would suggest that it should be short lived in the environment, especially in the presence of water. A number of studies have examined the possible factors for this longevity and, though the causes are generally assumed to be understood, the precise reasons have not yet been definitively determined and the evidence in support of the existing theories is at best circumstantial. At present, the prevailing view is that Sulphur Mustard is somehow protected by oligomeric or polymeric sulphonium species produced during incomplete hydrolysis reactions. The following review discusses the pertinent degradation mechanisms in the environment; hydrolysis and thermal degradation and the reasons put forward for the longevity of Sulphur Mustard in the literature. Other factors, such as the role of polymeric species in Sulphur Mustard droplets in modifying the mobility of the agent are also examined. Ultimately, without a thorough understanding of the abiotic fate of the Sulphur Mustard, uncertainties will remain in the application of risk assessment and remediation strategies to such sites, potentially compromising the validity or effectiveness of such actions.     

The effects of tropospheric ozone on the species dynamics of calcareous grassland

Although ozone has been shown to reduce the growth of individual species and to alter the composition of simple species mixtures, there is little understanding of its long-term effects on species dynamics and composition in real communities. Intact turfs of calcareous grassland were exposed to four different ozone regimes in open-top chambers over three consecutive summers. Treatments provided a mean seasonal AOT40 ranging from approximately zero to 15 ppm h. Cumulative ozone exposure was a significant factor in compositional change, but only explained 4.6% of the variation. The dominant grass species (Festuca rubra) showed a consistent decline in cover in the high ozone treatment over time and the forb Campanula rotundifolia was lost from all three ozone treatments. The frequency of some species (Galium verum and Plantago lanceolata) increased with ozone exposure. Long-term effects of ozone on species composition in chalk grassland may be a function of both the sensitivity of individual species and the response of the dominant species.     

Effects of Traffic Management and Transport Mode on the Exposure of Schoolchildren to Carbon Monoxide

The exposure to CO of schoolchildren was assessed in the town of Northampton, UK, both by direct measurement and by GIS-based activity modelling. Personal measurement of CO showed that exposures when travelling by car were significantly greater than those when walking, although journey times by car were shorter. However, journey exposures had little effect on maximum 8h mean CO exposures. CO concentration fields in the study area were modelled from current traffic flows, and those expected under different traffic management scenarios. These fields were then used, in combination with children's home and school location, and their activity profiles, to simulate frequency distributions of exposure for different transport modes and traffic management scenarios. The results show a large variability in the effect of traffic management interventions, depending on the child's home and school location.     

A comparison of North American and Asian exposure–response data for ozone effects on crop yields

Modelling-based studies to assess the extent and magnitude of ozone (O₃) risk to agriculture in Asia suggest that yield losses of 5–20% for important crops may be common in areas experiencing elevated O₃ concentrations. These assessments have relied on European and North American dose–response relationships and hence assumed an equivalent Asian crop response to O₃ for local cultivars, pollutant conditions and climate. To test this assumption we collated comparable dose–response data derived from fumigation, filtration and EDU experiments conducted in Asia on wheat, rice and leguminous crop species. These data are pooled and compared with equivalent North American dose–response relationships. The Asian data show that at ambient O₃ concentrations found at the study sites (which vary between ～35–75 ppb 4–8 h growing season mean), yield losses for wheat, rice and legumes range between 5–48, 3–47 and 10–65%, respectively. The results indicate that Asian grown wheat and rice cultivars are more sensitive to O₃ than the North American dose–response relationships would suggest. For legumes the scatter in the data makes it difficult to reach any equivalent conclusion in relative sensitivities. As such, existing modelling-based risk assessments may have substantially underestimated the scale of the problem in Asia through use of North American derived dose–response relationships.     

Robustness of modelled ozone exposures and doses

This study evaluates the robustness of the AOTX and AF(st)Y indices for assessing the ozone-induced risk to vegetation. These indices represent the accumulated concentration and stomatal flux, respectively, above a threshold value. The robustness is expressed as the sensitivity to changes in inputs and the uncertainty due to input errors. The input data are taken from a regional-scale chemical transport model. Both indices show increasing sensitivity with increasing threshold values. The sensitivity depends on the threshold and the characteristics of the frequency distribution for concentrations and stomatal fluxes. AF(st)Y appears less sensitive than AOTX for the thresholds adopted for critical levels. The couplings between concentration gradients and deposition algorithms complicate the assessment of the total uncertainty. For AF(st)Y, the uncertainty due to the modelled stomatal conductance may sometimes increase, but sometimes decrease, the overall uncertainty significantly. In particular, the maximum stomatal conductance plays an important role in determining the uncertainty.     

Modelling stomatal ozone flux and deposition to grassland communities across Europe

Regional scale modelling of both ozone deposition and the risk of ozone impacts is poorly developed for grassland communities. This paper presents new predictions of stomatal ozone flux to grasslands at five different locations in Europe, using a mechanistic model of canopy development for productive grasslands to generate time series of leaf area index and soil water potential as inputs to the stomatal component of the DO(3)SE ozone deposition model. The parameterisation of both models was based on Lolium perenne, a dominant species of productive pasture in Europe. The modelled seasonal time course of stomatal ozone flux to both the whole canopy and to upper leaves showed large differences between climatic zones, which depended on the timing of the start of the growing season, the effect of soil water potential, and the frequency of hay cuts. Values of modelled accumulated flux indices and the AOT40 index showed a five-fold difference between locations, but the locations with the highest flux differed depending on the index used; the period contributing to the accumulation of AOT40 did not always coincide with the modelled period of active ozone canopy uptake. Use of a fixed seasonal profile of leaf area index in the flux model produced very different estimates of annual accumulated total canopy and leaf ozone flux when compared with the flux model linked to a simulation of canopy growth. Regional scale model estimates of both the risks of ozone impacts and of total ozone deposition will be inaccurate unless the effects of climate and management in modifying grass canopy growth are incorporated.     

Assessing the risk caused by ground level ozone to European forest trees: a case study in pine, beech and oak across different climate regions

Two different indices have been proposed for estimation of the risk caused to forest trees across Europe by ground-level ozone, (i) the concentration based AOT40 index (Accumulated Over a Threshold of 40 ppb) and (ii) the recently developed flux based AFstY index (Accumulated stomatal Flux above a flux threshold Y). This paper compares the AOT40 and AFstY indices for three forest trees species at different locations in Europe. The AFstY index is estimated using the DO(3)SE (Deposition of Ozone and Stomatal Exchange) model parameterized for Scots pine (Pinus sylvestris), beech (Fagus sylvatica) and holm oak (Quercus ilex). The results show a large difference in the perceived O(3) risk when using AOT40 and AFstY indices both between species and regions. The AOT40 index shows a strong north-south gradient across Europe, whereas there is little difference between regions in the modelled values of AFstY. There are significant differences in modelled AFstY between species, which are predominantly determined by differences in the timing and length of the growing season, the periods during which soil moisture deficit limits stomatal conductance, and adaptation to soil moisture stress. This emphasizes the importance of defining species-specific flux response variables to obtain a more accurate quantification of O(3) risk.     

Reduced nitrogen has a greater effect than oxidised nitrogen on dry heathland vegetation

We investigated the effects of different ratios of reduced (NH4+) versus oxidised (NO3(-)) nitrogen in deposition on heathland and species-rich grassland vegetation at high nitrogen deposition levels in large mesocosms filled with nutrient-poor soils to which different NH4+/NO3(-) ratios were applied. The response of the forbs, Antennaria dioica, Arnica montana, Gentiana pneumonanthe, Thymus serpyllum, the grasses Danthonia decumbens, Deschampsia flexuosa, Nardus stricta and the shrub Calluna vulgaris was recorded. The forb A. dioica and the grass D.decumbens preferred low NH4+/NO3(-) ratios and were characterised by a negative correlation between NH4+/NO3(-) ratios and biomass and survival, whereas the grasses N. stricta and D. flexuosa showed no correlation with NH4+/NO3(-) ratios. Lime addition eliminated the negative effects of high NH4+ concentrations in deposition for A. dioica and the grass D. decumbens. The implications of these findings for heathland vegetations are discussed.