Atmospheric Environment (v.38, #15)

Introduction for ozone deposition special issue by Mike Ashmore; Lisa Emberson; Per Erik Karlsson; Hakan Pleijel (2211-2212).

New Directions: A new generation of ozone critical levels for the protection of vegetation in Europe by Mike Ashmore; Lisa Emberson; Per Erik Karlsson; Håkan Pleijel (2213-2214).

Estimates of diurnal and seasonal variation of ozone deposition to a temperate coniferous forest over 3 years in Southern Norway are presented. The estimates are based on the gradient method. Ozone deposition velocities are also produced by the EMEP deposition module for the same site with measured local meteorology as input. There is relatively good correspondence for diurnal cycles for all four seasons, suggesting that the module is able to provide characteristic results for this site. However, there are some discrepancies in the diurnal peak velocities, particularly in summer. Annual average deposition velocity estimated by the gradient method is 1.8 mms−1 while the corresponding value calculated by the deposition module is 2.1 mms−1.
Keywords: Ozone; Deposition; EMEP,Modelling; Forest;

Growth of Norway spruce (Picea abies) in relation to different ozone exposure indices: a synthesis by L Skärby; S Ottosson; P.E Karlsson; G Wallin; G Selldén; E.L Medin; H Pleijel (2225-2236).
In the Göteborg Ozone-Spruce Project (GOSP), two independent open-top chamber experiments were conducted during four growing seasons, using one clone of Norway spruce (Picea abies). The experiments tested the impact of ozone, alone and in combination with low phosphorus supply and in combination with drought stress, respectively, on biomass accumulation. In this paper, the results from both experiments were combined for the first time in order to analyse the relationship between relative biomass accumulation and different exposure indices (accumulated exposure over a threshold (AOT) with different cut-off concentrations, and the sum of ozone concentrations above 60 nl l−1, referred to as SUM06). In addition, a pooled analysis was made on several European studies of Norway spruce as a first effort to synthesize independent data and test the relative growth in relation to the AOT40 index. Significant negative relationships between the relative biomass of the GOSP-clone and the different indices were obtained. AOT20 and AOT30 resulted in the highest correlations. Based on the regression model, ozone is predicted to reduce the biomass of the GOSP-clone by 1% at the critical level for forest trees in Europe, a seasonal AOT40 of 10 μl l−1  h. A significant negative relationship between relative growth and AOT40 was obtained also with the European data set. At the present ozone critical level, the model predicted a 6% reduction in growth for the most sensitive Norway spruce trees in this data set.
Keywords: Biomass; Relative growth; AOT; SUM; Critical level;

The current short-term critical levels for acute ozone injury on plants were evaluated based on 32 datasets from eastern Austria, Belgium and southern Sweden with subterranean clover (Trifolium subterraneum L., cv. Geraldton). Potential improvements using an exposure index related to ozone uptake (AFst, Accumulated Stomatal Flux), a modified accumulated exposure over the threshold (mAOT) exposure index and the introduction of an effect threshold in the short-term critical level were investigated. The existing short-term critical levels did not accurately describe the effects in terms of observed visible injury. Using a mAOT based on solar radiation and vapour pressure deficit (VPD) improved the explanation of observed visible injury. However, using a simple stomatal conductance model, driven by solar radiation, air temperature, VPD and ozone uptake, the correlation between modelled and observed effects were considerably improved. The best performance was obtained when an ozone uptake rate threshold of 10 nmol m−2  s−1 (AFst10, per unit total leaf area) was used. The results suggested the use of an effect threshold of 10% leaf injury in order to minimise the risk of erroneously recorded visible injury due to observation technique or other injuries hard to distinguish from ozone injury. A new, AFst based exposure index was suggested, an ozone exposure of AFst10=75 μmol m−2 during an exposure period of eight days was estimated to prevent more than 10% visible injury of the leaves. This study strongly suggests that a simple model for ozone uptake much better explains observed effects, compared to the currently used exposure index AOT40. However, if a lower degree of complexity, data requirements and also a lower extent of explanation of observed effects are to be considered a new short-term critical level, based on a mAOT may be suggested: a mAOT30 of 160 ppb h during an exposure period of 8 days is estimated to protect the leaves from visible injury on more than 10% of the leaves.
Keywords: AOT40; Clover; Trifolium subterraneum; Visible injury; Ozone effects; Accumulative stomatal flux (AFst); Modified AOT (mAOT);

Ecophysiological and biochemical strategies of response to ozone in Mediterranean evergreen broadleaf species by C Nali; E Paoletti; R Marabottini; G Della Rocca; G Lorenzini; A.R Paolacci; M Ciaffi; M Badiani (2247-2257).
Three Mediterranean shrubs, Phillyrea latifolia L. (phillyrea), Arbutus unedo L. (strawberry tree), and Laurus nobilis L. (laurel), differing in their morphological and ecological response to water shortage, were exposed for 90 days to 0 or 110 ppb of ozone (O3), 5 h each day. This yielded an accumulated exposure over of a threshold of 40 ppb (AOT40) of 31.5 ppm h over the 3 months experiment. These species showed differing responses to O3: laurel and phillyrea developed foliar chlorotic mottles on the adaxial surface of leaves, whereas strawberry tree leaves showed reddish interveinal stipple-like necrotic lesions. In all cases, however, foliar injury did not exceed 8% of the sampled leaf area. At the end of the exposure period, O3-induced stomatal limitation caused significant decreases of net photosynthesis in strawberry tree and laurel, but not in phillyrea. The relative water content of the leaves was significantly decreased by O3, especially in laurel and strawberry tree, suggesting the occurrence of drought stress. Electrical conductivity of leachates from foliar discs increased in response to the treatment, much more strongly in laurel and in strawberry tree than in phillyrea, suggesting an O3-dependent alteration of the membrane retention capacity. At the end of the experimental period, the activity of superoxide dismutase and the content of reduced glutathione, but not that of reduced ascorbate, were significantly increased in the ozonated leaves of strawberry tree and phillyrea, but not in laurel. The evergreen broadleaves studied here maybe relatively tolerant to realistic O3 levels, at least in terms of visible injury and gas exchange. Such tolerance might overlap with their level of tolerance to drought stress. High constitutive levels, and/or O3-induced increases in antioxidants, might contribute to O3 tolerance in these Mediterranean evergreen broadleaf species.
Keywords: Air pollution; Mediterranean forest; Ozone flux;

Relationships between ozone exposure and yield loss in European wheat and potato—a comparison of concentration- and flux-based exposure indices by H Pleijel; H Danielsson; K Ojanperä; L.De Temmerman; P Högy; M Badiani; P.E Karlsson (2259-2269).
Data from open-top chamber experiments with field grown crops, performed in Sweden, Finland, Belgium, Italy and Germany, were combined to derive relationships between yield and ozone exposure for wheat (Triticum aestivum L.) and potato (Solanum tuberosum L.). Three different exposure indices were compared: AOT40 (accumulated exposure over a threshold ozone concentration of 40 nmol mol−1), CUO t (cumulative stomatal uptake of ozone, using a constant ozone uptake rate threshold of t  nmol m−2  s−1) and mAOTc 0 (conductance modified AOT using a threshold concentration for ozone of c 0 nmol mol−1). The latter is essentially a combination of AOT and CUO. Ozone uptake was estimated using a Jarvis-type multiplicative model for stomatal conductance. In terms of correlation between relative yield (RY) and ozone exposure, CUO5, the CUO index with an ozone uptake rate threshold of 5 nmol m−2  s−1, performed best for both wheat and potato, resulting in r 2 values of 0.77 and 0.64, respectively. CUO5 performed considerably better in terms of the correlation between RY and ozone exposure, than AOT40 for wheat, while mAOT10, the best performing mAOT index in this case, was intermediate in performance for this crop. For potato, the differences between the different ozone exposure indices AOT40, CUO5 and mAOT20 (the mAOT index performing best for potato) in the correlation between RY and ozone exposure were relatively small. To test the assumption that the non-stomatal deposition of ozone was negligible for the uppermost, sunlit leaves, measurements of ozone uptake in relation to leaf conductance for water vapor of wheat leaves in a cuvette system were used. The non-stomatal deposition of ozone to the leaves turned out to be comparatively small. Based on the non-stomatal conductance (g ns=15 mmol m−2  s−1) estimated for the wheat leaves in the cuvette system, it was concluded that the consequence of omitting the non-stomatal conductance is small. In conclusion the study indicated that the ozone uptake based approach showed a high degree of fitting along a north-south European transect of pedoclimatic conditions, and represents a better and more relevant approach to the quantification of ozone effects on crops growth than the use of ozone exposure indices purely based on ozone concentrations.
Keywords: Dose-response; Non-stomatal deposition; Ozone; Solanum; Stomatal conductance; Triticum;

Comparison between AOT40 and ozone uptake in forest trees of different species, age and site conditions by R. Matyssek; G. Wieser; A.J. Nunn; A.R. Kozovits; I.M. Reiter; C. Heerdt; J.B. Winkler; M. Baumgarten; K.-H. Häberle; T.E.E. Grams; H. Werner; P. Fabian; W.M. Havranek (2271-2281).
The current AOT40 concept for inferring risks in forest trees by ozone (O3) injury is based on an accumulated external O3 exposure rather than an internal O3 dose or uptake rate. AOT40 assumes O3 concentrations below 40 nl l−1 and night-time exposure to be negligible. Hence, this concept is rather inconsistent with observed forest conditions. In contrast, the flux concept of cumulative O3 uptake (CU) into the leaves has the potential of reflecting a physiologically meaningful internal O3 dose experienced by trees. In this paper, we relate AOT40 to cumulative O3 uptake into European beech (Fagus sylvatica), Norway spruce (Picea abies), European larch (Larix decidua) and cembran pine (Pinus cembra) trees differing in size, age and site conditions. We demonstrate that the flux concept can be extended to the tree and the stand level, making use of sap flow measurements through tree trunks. Although in both seedlings and adult trees AOT40 may show some linearity in correlations with average CU, the latter varies, at given AOT40, by 25±11% within and between species. This is because O3 flux is primarily influenced by stomatal aperture, the latter being affected by climate, canopy position, leaf and tree age while varying between species. In particular, if weighed by detoxification capacity, we suggest, therefore, O3 uptake related air quality indices to be promoted towards ecologically meaningful standards in forest protection, overcoming the shortcomings of exposure concepts. As O3 injury results from the balance between O3 uptake and detoxification in the leaf mesophyll, we conclude the flux concept in combination with measures of biochemical defence to have the capacity for predicting tree response to O3 stress.
Keywords: Fagus sylvatica; Picea abies; Larix deciduas; Pinus cembra; Ozone; AOT40,O3 flux; O3 uptake; Stomatal conductance; Sap flow;

New critical levels for ozone effects on young trees based on AOT40 and simulated cumulative leaf uptake of ozone by P.E. Karlsson; J. Uddling; S. Braun; M. Broadmeadow; S. Elvira; B.S. Gimeno; D. Le Thiec; E. Oksanen; K. Vandermeiren; M. Wilkinson; L. Emberson (2283-2294).
Leaf or needle ozone uptake was estimated for young trees at seven experimental sites across Europe using a stomatal conductance simulation model. Dose–response relationships based on cumulative leaf uptake of ozone (CUO) were calculated using different hourly ozone flux thresholds and these were compared to dose–response relationships based on daylight AOT40, which is currently used within the UNECE Convention on Long-Range Transboundary Air Pollution (CLRTAP). Regression analysis showed that the CUO–biomass response relationships were highly significant for both coniferous and broadleaf trees, and independent of which ozone flux threshold was applied. On the basis of this regressions analysis, an hourly flux threshold of 1.6 nmol m−2  s−1 (CUO>1.6) is proposed as the most appropriate for all species categories in deriving dose–response relationships. The analysis indicated that the current critical level for ozone impacts on European forests of AOT40 10 ppm h may not protect the most sensitive receptors and that critical levels for AOT40 and CUO>1.6 of 5 ppm h and 4 mmol m−2, respectively, are more appropriate. The research identified weaker dose–response relationships for the CUO exposure index compared with AOT40. Distinguishing between sensitive and less sensitive species substantially improved the CUO–biomass response relationships although, still, to a lesser extent than when exposure was expressed as AOT40.
Keywords: Critical levels; Ozone uptake; Ozone flux; AOT40; Biomass reduction;

Ozone (O3) phytototoxicity has been reported on a wide range of crops and wild Central European plant species, however no information has been provided regarding the sensitivity of plant species from dehesa Mediterranean therophytic grasslands in spite of their great plant species richness and the high O3 levels that are recorded in this area. A study was carried out in open-top chambers (OTCs) to assess the effects of O3 and competition on the reproductive ability of three clover species: Trifolium cherleri, Trifolium subterraneum and Trifolium striatum. A phytometer approach was followed, therefore plants of these species were grown in mesoscosms composed of monocultures of four plants of each species, of three plants of each species competing against a Briza maxima individual or of a single plant of each clover species competing with three B. maxima plants. Three O3 treatments were adopted: charcoal filtered air (CFA), non-filtered air (NFA) and non-filtered air supplemented with 40 nl l−1 of O3 (NFA+). The different mesocosms were exposed to the different O3 treatments for 45 days and then they remained in the open. Ozone exposure caused reductions in the flower biomass of the three clover species assessed. In the case of T. cherleri and T. subterraneum this effect was found following their exposure to the different O3 treatments during their vegetative period. An attenuation of these effects was found when the plants remained in the open. Ozone-induced detrimental effects on the seed output of T. striatum were also observed. The flower biomass of the clover plants grown in monocultures was greater than when competing with one or three B. maxima individuals. An increased flower biomass was found in the CFA monoculture mesocosms of T. cherleri when compared with the remaining mesocosms, once the plants were exposed in the open for 60 days. The implications of these effects on the performance of dehesa acid grasslands and for the definition of O3 critical levels is discussed.
Keywords: Seed production; Flower biomass; Mediterranean grasslands; Dehesa; Critical levels;

On the response of two populations of Quercus coccifera to ozone and its relationship with ozone uptake by Susana Elvira; Victoria Bermejo; Esteban Manrique; Benjamin S Gimeno (2305-2311).
Ozone (O3) effects on the growth and gas exchange-related parameters of two ecotypes of Quercus coccifera L. presenting different traits in the field were assessed during a 2-years exposure experiment. One-year-old seedlings grown from the acorns collected at a rock (rock ecotype) or at a well-developed soil (garrigue ecotype) were continuously exposed in open-top chambers to three different O3 treatments: charcoal filtered air, non-filtered air and non-filtered air supplemented with 40 nl l−1. Above-ground relative growth rate (RGR) and gas-exchange-related parameters were measured during each growth season. Above- and below-ground biomass were also measured at the beginning and the end of the experiment. Ozone exposure did not induce any significant effect on RGR or any biomass-related parameters. However, the two populations differed in their sensitivity to O3 regarding gas-exchange-related parameters. The observed responses were related with accumulated O3 exposure or cumulative O3 uptake. Although the re-parameterisation of the existing European Monitoring and Evaluation Programme O3 uptake model improved its performance by 30%, O3 accumulated exposure was a better predictor of the observed responses than O3 uptake. The results of the present experiment indicate the need of taking into account the intraspecific variations in plant sensitivity to O3 when assessing the potential risks or impacts of this pollutant.
Keywords: Stomatal conductance; Net photosynthesis; Biomass; Ecotype; Critical levels;

Long-term ozone exposure and ozone uptake of grapevines in open-top chambers by G. Soja; T.G. Reichenauer; M. Eid; A.-M. Soja; R. Schaber; H. Gangl (2313-2321).
Ozone uptake of pot-grown grapevines, fruit yield and sugar concentrations in juice were studied over the course of 4 years. In three of the study years, plants were exposed to contrasting ozone levels in open-top chambers. Results showed that juice quality was more sensitive to ozone exposure than grape yield. Ozone-induced reductions in grape yield were best described by the level of ozone exposure in the 2 years prior to harvest, whereas sugar content of juice was mainly affected by current and previous year ozone exposure. Regression models were developed to facilitate quantitative assessments of the effects of ozone on yield and quality. The observed effects on grape yield and on carbohydrate accumulation in fruit were slightly better explained when a threshold of 6 nmol m−2  s−1 was applied to cumulative uptake of ozone (CUO6) compared with AOT40 exposure or other CUO thresholds. Curvilinear regression models were developed which explained up to 75% of variance of the datasets. These models indicate a critical level of 1.1 mmol m−2 ozone (CUO6 from June to September) to protect carbohydrate accumulation in fruit with ozone taken up by the plants over consecutive years. For grape yield the CUO6 threshold was 2.2 mmol m−2 ozone. Exceedance of this threshold would be expected to reduce grape yield and sugar translocation to fruit by 10% with substantial degree of uncertainty (95% confidence interval 1–19%). For individual years, the critical level could be increased to 2.3 mmol m−2 for carbohydrate accumulation and 3.5 mmol m−2 CUO6 for grape yield if the CUO6 of 0 in the subsequent year was not exceeded.
Keywords: Vitis vinifera; Grape quality; Ozone uptake; AOT40; OTC;

Present air quality standards to protect vegetation from ozone are based on measured concentrations (i.e., exposure) rather than on plant uptake rates (or dose). Some familiar cumulative exposure-based indices include SUM06, AOT40, and W126. However, plant injury is more closely related to dose, or more appropriately to effective dose, than to exposure. This study develops and applies a simple model for estimating effective ozone dose that combines the plant canopy's rate of stomatal ozone uptake with the plant's defense to ozone uptake. Here the plant defense is explicitly parameterized as a function of gross photosynthesis and the model is applied using eddy covariance (ozone and CO2) flux data obtained at a vineyard site in the San Joaquin Valley during the California Ozone Deposition Experiment (CODE91). With the ultimate intention of applying these concepts using prognostic models and remotely sensed data, the pathways for ozone deposition are parameterized (as much as possible) in terms of canopy LAI and the surface friction velocity. Results indicate that (1) the daily maximum potential for plant injury (based on effective dose) tends to coincide with the daily peak in ozone mixing ratio (ppbV), (2) potentially there are some significant differences between ozone metrics based on dose (no plant defense) and effective dose, and (3) nocturnal conductance can contribute significantly to the potential for plant ozone injury.
Keywords: Air quality standards; Effective dose; Dry deposition resistances; Ozone;

It is widely acknowledged that the possible impacts of ozone on forest trees are more closely related to ozone flux through the stomata than to external ozone exposure. However, the application of the flux approach on a European scale requires the availability of appropriate models, such as the European Monitoring and Evaluation Programme (EMEP) ozone deposition model, for estimating ozone flux and cumulative ozone uptake. Within this model stomatal conductance is the key variable, since it determines the amount of ozone absorbed by the leaves. This paper describes the suitability of the existing EMEP ozone deposition model parameterisation and formulation to represent stomatal behaviour determined from field measurements on adult Norway spruce (Picea abies (L.) Karst.) trees in the Central European Alps. Parameters affecting maximum stomatal conductance (e.g. seasonal phenology, needle position, needle age, nutrient deficiency and ozone itself) and stomatal response functions to temperature, irradiance, vapour pressure deficit, and soil water content are investigated. Finally, current limitations and possible alterations of the EMEP model will be discussed with respect to spatial scales of available input data for future flux modelling.
Keywords: Ozone uptake; EMEP O3 deposition model; Stomata; Season; Temperature; Irradiance; Water Vapour pressure deficit;

Modeling seasonal ozone fluxes to grassland and wheat: model improvement, testing, and application by Seraina Bassin; Pierluigi Calanca; Tamas Weidinger; Giacomo Gerosa; Jürg Fuhrer (2349-2359).
The Ozone DEposition Model (ODEM) was developed for national scale modeling of total and fractional ozone fluxes to grasslands and wheat using high-resolution gridded input data. The aim of this study was to improve the preliminary version 1.1 to enable season-long simulations by introducing a new parameterization of leaf area index (LAI), and the effect of soil moisture on soil surface resistance, and by refining the descriptions of friction velocity, u∗ , stability correction function, and all resistances depending on these parameters. Version 1.2 was tested by comparing simulation results with field measurements over grassland at Braunschweig, Germany, and wheat at Comun Nuovo, Lombardy, Italy. These comparisons showed that the simulation of ozone flux and u * for grassland was greatly improved by the modifications, yielding a root mean square error for model vs. simulation of 0.056 ppb m s−1, as compared to 0.106 ppb m s−1 obtained with the original model. Atmospheric, boundary layer, and in-canopy resistances, which all depend on u *, increased as a result of the combined modifications. For wheat, the agreement between simulated and measured ozone flux was good under wet conditions, but less during a phase characterized by drying soils. It is suggested that in-canopy processes, which are uncertain in the model, become more important under these conditions. The application of the model using 6-year ozone and meteorological data from a site in southern Switzerland (Cadenazzo) revealed that the monthly cumulative stomatal ozone flux for grassland remained more stable across the growing season than for wheat because of less variation in green LAI. Highest stomatal ozone uptake in wheat occurred during anthesis and declined later due to decreasing green LAI. In both grassland and wheat, cumulative seasonal ozone uptake was less variable between years than was the AOT40 index. The results demonstrate the improvements in ODEM 1.2, and they underline the importance of species-specific parameterizations of plant development. But it is also concluded that for high-resolution modeling of ozone fluxes the overall quality of model outputs may critically depend on the quality of an extensive set of site-specific biological, climatic, and edaphic input data.
Keywords: Ozone; Ozone flux; Ozone deposition model; Leaf area index; Grassland; Wheat;

Five-year measurements of ozone fluxes to a Danish Norway spruce canopy by T.N. Mikkelsen; H. Ro-Poulsen; M.F. Hovmand; N.O. Jensen; K. Pilegaard; A.H. Egeløv (2361-2371).
Ozone concentrations and fluxes have been measured continuously during 5 years (1996–2000) by the gradient method in a Norway spruce dominated forest stand in West Jutland, Denmark, planted in 1965. The method has been validated against other methodologies and a relatively good relationship was found. The data are analysed to quantify diurnal, seasonal and yearly fluxes, and non-stomatal and stomatal removal are estimated. Monthly means of climatic data are shown, and day and night values of the aerodynamic resistance, r a, viscous sub-layer resistance, r b, and the surface or canopy resistance, r c, are presented. The yearly ozone deposition is approximately 126 kg ha−1. The canopy ozone uptake is highest during the day and during the summer. This is interpreted as increased stomatal uptake and physical and chemical reactions. The daily means of ozone concentration and fluxes averaged over 5 years correlate, but the correlation is primarily based on two different uncoupled processes outside and inside the stomates: (1) The ozone destruction in the canopy occurring outside the stomates is much influenced by temperature, light and humidity, e.g. surface reactions, NO- and VOC-emissions. (2) The same factors have a strong influence on the stomatal opening, e.g. midday and night closure. Thus, looking at diurnal variations, the diurnal ozone concentration and ozone flux do not correlate at all during the growing season. The maximum diurnal difference for the ozone concentration is a factor 1.3 and the maximum diurnal difference for the ozone flux is a factor 3. From dawn to ca. 8:00 the ozone deposition increases and the ozone concentration decreases.The yearly stomatal uptake of ozone is estimated to minimum 21% of the total deposition, being highest in May–August (30–33%) and lowest in November–February (4–9%). The physiological ozone uptake per leaf area is estimated to 0.33 g ozone m−2  y−1.
Keywords: Canopy resistance; Deposition; Deposition velocity; O3; Stomatal uptake; Picea abies;

Testing and improving the EMEP ozone deposition module by J.-P Tuovinen; M.R Ashmore; L.D Emberson; D Simpson (2373-2385).
This study evaluates the ozone deposition module developed for the European Monitoring and Evaluation Programme photochemical transport model. The module allows an estimation of both total and stomatal ozone flux to different vegetation types, taking into account plant phenology and environmental conditions. Based on a series of model tests against micrometeorological measurements of ozone fluxes, including data on stomatal conductances, the performance of the land-cover specific parameterisations was assessed and improvements implemented. Tests were carried out for coniferous forest, moorland and wetland ecosystems in Northern Europe, and for a wheat field and a grassland site located in Southern Europe. The improvements consisted of a needle age factor for coniferous forests, revised ground surface conductances, and a new leaf-to-canopy up-scaling method for wheat. These resulted in better agreement with observed surface conductances and deposition velocities, although a number of discrepancies are noted. In general, the parameterisation of non-stomatal deposition and the performance of the module under high soil moisture deficits were identified as key uncertainties requiring further investigation.
Keywords: Dry deposition; Ozone flux; Deposition velocity; Surface resistance; Stomatal conductance; Model evaluation;

Measurements of ozone removal by Scots pine shoots: calibration of a stomatal uptake model including the non-stomatal component by Nuria Altimir; Juha-Pekka Tuovinen; Timo Vesala; Markku Kulmala; Pertti Hari (2387-2398).
The calculation of ozone uptake to vegetation requires the determination of both the stomatal uptake and the non-stomatal sinks. This paper analyses the relative contribution of stomatal and non-stomatal sinks at the shoot level based on direct measurements of ozone flux that included both components. Ozone deposition was estimated for Scots pine shoots enclosed in chambers, connected to a gas-exchange system, at the SMEAR II field station, in southern Finland. For the analysis of stomatal conductance, we utilised the formulation within the dry deposition module of the photochemical EMEP model, for which we revised the parameterisation for Scots pine. In addition, we introduced modifications in the treatment of spring phenology and parameter estimation. Provided these modifications, the parameter values estimated from our data fell close to the default values, although presenting a less sensitive VPD response during the summer. A major proportion of the measured removal was due to the non-stomatal component. We introduced a non-stomatal term dependent on the ambient relative humidity, which significantly improved the agreement between the calculated and measured values of total shoot conductance. The non-stomatal contribution to total conductance was in the order of 50%.
Keywords: Stomatal uptake; Surface removal; Deposition velocity; Scots pine;

Carbohydrate concentrations in different plant parts of young beech and spruce along a gradient of ozone pollution by Sabine Braun; Ulrike Zugmaier; Vera Thomas; Walter Flückiger (2399-2407).
Young beech and spruce were grown in pots along a gradient of ozone pollution in Switzerland. Spruce was harvested after one and beech after two seasons and carbohydrate concentrations were measured in different plant fractions. Ozone uptake was calculated as cumulative flux, cumulative flux with thresholds of 1.6 or 3.2 nmol m−2  s−1, or as AOT40. In beech, the monosaccharide concentration in fine roots showed a decreasing trend with increasing ozone, with similar results for all methods used to quantify ozone. In spruce, the concentration of starch decreased in both the thicker root fractions (∅ 1–5 and >5 mm) and stems with increasing ozone, whereas starch concentrations in needles increased with increasing ozone. In the needles, the increase in starch concentration showed the best correlation with cumulative ozone uptake. The defining of the length of the growing season proved to be a crucial parameter in the flux calculations. The results suggest that carbohydrate concentrations may be used as an indicator for ozone impact in spruce.
Keywords: Ozone; Picea abies; Fagus sylvatica; Carbohydrates; Carbon allocation; Ozone flux; AOT40;

The results obtained during several campaigns conducted at various places in Southern Europe and focused on ozone deposition on vegetation are analyzed and interpreted with the main scope of determining the relative contribution of stomatal uptake to the total ozone flux. The stomatal ozone fluxes are calculated using the analogy with water vapour fluxes inside the stomata. By integrating stomatal fluxes over time, ozone doses received by the plants are calculated. At most sites, the stomatal ozone flux accounts <50% of the total flux, except for those sites located at the northern edge of the geographical domain studied. Sites characterized by a rather dry Mediterranean climate generally show reduced stomatal fluxes, even in the case of high ozone concentrations. The role of water supply, influencing stomatal activity, appears essential.This approach, using time integration of ozone stomatal fluxes, yielding ozone doses to vegetation, is then compared with the use of the exposure index AOT40, widely used in ozone risk assessment work. A fairly good correlation exists between both approaches, but the latter gives no indication about the physiological uptake processes.
Keywords: Ozone deposition; Stomatal uptake; Mediterranean ecosystems;

Stomatal ozone fluxes over a barley field in Italy. “Effective exposure” as a possible link between exposure- and flux-based approaches by Giacomo Gerosa; Riccardo Marzuoli; Stanislaw Cieslik; Antonio Ballarin-Denti (2421-2432).
Fluxes of ozone as well as of sensible and latent heat were measured over a barley field in Northern Italy from April to June 2002 with the eddy-correlation technique in order to determine the dose of ozone taken up by plants during the whole grain filling and maturation period. Stomatal ozone fluxes were then calculated by using the similarity between gaseous exchange processes occurring inside the stomata involving ozone and water vapour, whose flux was available from the measurements. The dose was then obtained by integrating the stomatal flux over time. On the average, the stomatal flux was found to be approximately 50% of the total flux. This approach was compared to the currently used ozone risk assessment procedure based on the use of the exposure index AOT40, calculated from routine ozone concentration records. Important differences between these two approaches appeared.In order to assess ozone risk to vegetation in a realistic way, a new concept of “effective exposure”, which combines both exposure and uptake approaches, is developed. A new risk assessment index is obtained by weighting exposure by a factor which includes stomatal conductance, obtained by a modelling procedure, which uses observational data available from routine monitoring stations. This new index, formally similar to AOT40, uses a lower ozone concentration threshold (<20 ppb instead of 40 ppb) and follows much more closely the stomatal ozone dose evolution than the original AOT40 index. This approach permits to calculate ozone exposure taking physiological aspects into account, for situations where only ozone concentrations and meteorological observations are available, as direct flux data are difficult to obtain on a routine basis.
Keywords: Ozone fluxes; Barley fields; Stomatal uptake; Stomatal conductance modelling; Flux modified exposure;

The current European critical levels for ozone (O3) to protect crops, natural and semi-natural vegetation and forest trees (Level I) are based on the accumulated exposure over a threshold of 40 ppb exposure index. For the revision of the 1999 UN-ECE protocol (Protocol to the 1979 Convention on Long-Range Transboundary Air Pollution to Abate Acidification, Eutrophication and Ground-Level Ozone. Gothenburg, 30 November), critical levels are partly replaced by critical cumulative stomatal uptake (critical absorbed dose or CAD). Here, we discuss possible improvements in deriving CAD. At present, the main problems with the flux approach can be attributed to uncertainties in the parameterisation of, (1) stomatal conductance, (2) non-stomatal deposition, and (3) the representativeness of species used in flux-effect studies. Although in general, the flux approach is well accepted by plant effects scientists in North America, concerted research efforts have not taken place in that direction. Furthermore, because of the differences in the approach to setting ambient air quality standards, it appears very doubtful that policy makers and air quality regulators in the US and Canada will readily accept the overall philosophy.
Keywords: Ozone; Stomatal and non-stomatal deposition; Critical level; Flux-based limiting values; Spatial scales;