A canopy stomatal resistance model for gaseous deposition to vegetated surfaces

A gaseous deposition model, based on a realistic canopy stomatal resistance submodel, is described, analyzed and tested. This model is designed as one of a hierarchy of simulations, leading up to a “big-leaf” model of the processes contributing to the exchange of trace gases between the atmosphere and vegetated surfaces. Computations show that differences in plant species and environmental and physiological conditions can affect the canopy stomatal resistance by a factor of four. Canopy stomatal resistances to water vapor transfer computed with the present model are compared against values measured with a porometer and computed with the Penman-Monteith equation. Computed stomatal resistances from a soybean canopy in both well-watered and water-stressed conditions yield good agreement with test data. The stomatal resistance submodel responds well to changing environmental and physiological conditions. Model predictions of deposition velocities are evaluated for the case of ozone, transferred to maize. Calculated deposition velocities of O₃ overestimate measured values on the average by about 30%, probably largely as a consequence of uncertainties in leaf area index, soil and cuticle resistances, and other modeling parameters, but also partially due to imperfect measurement of O₃ deposition velocities.     

Ozone fluxes above and within a pine forest canopy in dry and wet conditions

The physiological and physical processes controlling ozone dry deposition to vegetated surfaces are still not fully understood. In particular, the role of the understorey and the possible action of dew on ozone deposition have not received much attention so far. This paper presents the results of an experiment aimed at quantifying ozone dry deposition to a maritime pine forest in the “Les Landes” area in France. Ozone deposition fluxes were measured using the eddy-covariance technique above and within the canopy. We investigate the factors acting on ozone deposition in both dew-wetted and dry conditions. The values obtained for the ozone deposition velocity are well in the range of previously published measurements over coniferous forests. For the present forest, ozone uptake by the understorey is a significant portion of ozone deposition to the whole pine stand. The understorey contributes more to the overall ozone flux than to the other measured scalar fluxes (sensible heat and water vapour). During dry nights the surface conductance for ozone and the friction velocity are strongly correlated, showing that ozone deposition is largely controlled by dynamical processes. During the day, in dry conditions, the canopy stomatal conductance is the major parameter controlling ozone deposition. However, in winter, when the stomatal conductance is low, the influence of dynamical processes persists during day-time. It is also found that surface wetness associated with dew significantly enhances ozone deposition, during the night as well as in the morning.     

Ozone dynamics and deposition processes at a deforested site in the Amazon basin

The goal of this study is to investigate the impact of deforestation on ozone dynamics and deposition in the Brazilian Amazon basin. This goal is accomplished through i) analyses of ozone levels and deposition rates at a deforested site during the rainy season; and ii) comparisons of these data with similar information derived at a forest. At the pasture site maximum ozone mixing ratios reach 20 parts per billion on a volume basis (ppbv) but about 6 ppbv prevail over the forest. Maximum ozone deposition velocities for pastures can reach 0.7 cm s-1, which is about threefold lower than values derived for forests. Combining ozone abundance and deposition velocities, pasture maximum ozone fluxes reach approximately 0.2 microgram (ozone) m-2 s-1. This flux represents approximately 70% of the deposition rates measured over the forest. Hence, this study suggests that conversion of rainforests to pastures could lead to a net reduction (30%) in the ozone sink in the Amazon.     

Modelling stomatal ozone flux across Europe

A model has been developed to estimate stomatal ozone flux across Europe for a number of important species. An initial application of this model is illustrated for two species, wheat and beech. The model calculates ozone flux using European Monitoring and Evaluation Programme (EMEP) model ozone concentrations in combination with estimates of the atmospheric, boundary layer and stomatal resistances to ozone transfer. The model simulates the effect of phenology, irradiance, temperature, vapour pressure deficit and soil moisture deficit on stomatal conductance. These species-specific microclimatic parameters are derived from meteorological data provided by the Norwegian Meteorological Institute (DNMI), together with detailed land-use and soil type maps assembled at the Stockholm Environment Institute (SEI). Modelled fluxes are presented as mean monthly flux maps and compared with maps describing equivalent values of AOT40 (accumulated exposure over threshold of 40 ppb or nl l(-1)), highlighting the spatial differences between these two indices. In many cases high ozone fluxes were modelled in association with only moderate AOT40 values. The factors most important in limiting ozone uptake under the model assumptions were vapour pressure deficit (VPD), soil moisture deficit (for Mediterranean regions in particular) and phenology. The limiting effect of VPD on ozone uptake was especially apparent, since high VPDs resulting in stomatal closure tended to co-occur with high ozone concentrations. Although further work is needed to link the ozone uptake and deposition model components, and to validate the model with field measurements, the present results give a clear indication of the possible implications of adopting a flux-based approach for future policy evaluation.     

Sensitivity analysis of an ozone deposition model

In this study, sophisticated sensitivity analyses of a detailed ozone dry deposition model were performed for five soil types (sand, sandy loam, loam, clay loam, clay) and four land use categories (agricultural land, grass, coniferous and deciduous forests). Deposition velocity and ozone flux depend on the weather situation, physiological state of the plants and numerous surface-, vegetation-, and soil-dependent parameters. The input data and the parameters of deposition-related calculations all have higher or lower spatial and temporal variability. We have investigated the effect of the variability of the meteorological data (cloudiness, relative humidity and air temperature), plant-dependent (leaf area index and maximum stomatal conductance) and soil-dependent (soil moisture) parameters on ozone deposition velocity. To evaluate this effect, two global methods, the Morris method and the Monte Carlo analysis with Latin hypercube sampling were applied. Additionally, local sensitivity analyses were performed to estimate the contribution of non-stomatal resistances to deposition velocity. Using the Monte Carlo simulations, the ensemble effect of several nonlinear processes can be recognised and described. Based on the results of the Morris method, the individual effects on deposition velocity are found to be significant in the case of soil moisture and maximum stomatal conductance. Temperature and leaf area index are also important factors; the former is primarily in the case of agricultural land, while the latter is for grass and coniferous forest. The results of local sensitivity analyses reveal the importance of non-stomatal resistances.     

Comparison of seasonal variations of ozone exposure and fluxes in a Mediterranean Holm oak forest between the exceptionally dry 2003 and the following year

Ozone and energy fluxes have been measured using the eddy covariance technique, from June to December 2004 in Castelporziano near Rome (Italy), and compared to similar measurements made in the previous year. The studied ecosystem consisted in a typical Mediterranean Holm oak forest. Stomatal fluxes have been calculated using the resistance analogy and by inverting the Penmann-Monteith equation. Results showed that the average stomatal contribution accounts for 42.6% of the total fluxes. Non-stomatal deposition proved to be enhanced by increasing leaf wetness and air humidity during the autumnal months. From a comparison of the two years, it can be inferred that water supply is the most important limiting factor for ozone uptake and that prolonged droughts alter significantly the stomatal conductance, even 2 months after the soil water content is replenished. Ozone exposure, expressed as AOT40, behaves similarly to the cumulated stomatal flux in dry conditions whereas a different behaviour for the two indices appears in wet autumnal conditions. A difference also occurs between the two years.     

Ozone flux over a Norway spruce forest and correlation with net ecosystem production

Daily ozone deposition flux to a Norway spruce forest in Czech Republic was measured using the gradient method in July and August 2008. Results were in good agreement with a deposition flux model. The mean daily stomatal uptake of ozone was around 47% of total deposition. Average deposition velocity was 0.39 cm s⁻¹ and 0.36 cm s⁻¹ by the gradient method and the deposition model, respectively. Measured and modelled non-stomatal uptake was around 0.2 cm s⁻¹. In addition, net ecosystem production (NEP) was measured by using Eddy Covariance and correlations with O₃ concentrations at 15 m a.g.l., total deposition and stomatal uptake were tested. Total deposition and stomatal uptake of ozone significantly decreased NEP, especially by high intensities of solar radiation.     

The interfacial mass transfer resistances of the ozone dry deposition over the field soil of Taichung County in Taiwan

This study aims to design a dry deposition chamber and to measure ozone depletion over the Taichung field soil. This study seeks to verify the phenomena by an experimental and mathematical model. It is demonstrated that interfacial mass transfer resistances of ozone dry deposition involve reactive resistance (R(sr)) and kinetic resistance (R(sk)). It reveals the chemical reaction (O3 + NO --> NO2) to produce the reactive resistance, and verifies that the interfacial mass transfer resistances depend on nitrogen oxide emission and soil temperature. It shows that the interfacial mass transfer resistances are reduced with increasing soil temperature (T(S)). The model profiles are smaller than the observed data within a relative error of 15%. The reactive resistance decreases exponentially with increasing soil temperature; R(sr)(-1) (cm x sec(-1)) = 0.0001 exp (0.1455T(S)). The kinetic resistance decreases linearly with increasing soil temperature; R(sk)(-1)(cm x sec(-1)) = 0.0108T(S) + 1.4012. This model is more accurate with higher soil temperature and larger ozone concentration. Results are consistent with thermodynamics and reaction kinetics. Ozone dry deposition over agricultural soil causes conversion of nitrogen oxide (NO) to nitrogen dioxide (NO2).     

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.     

Relations between crown condition and ozone and its dependence on environmental factors

It is now widely accepted that stomatal ozone uptake provides a more reliable indicator for the assessment of risk of ozone damage to forest trees than ozone exposure. Thus, environmental factors influencing stomatal opening strongly affect the sensitivity of forest trees towards high ozone concentrations. In this study the hydrological model WAWAHAMO is used to simulate stomatal opening dependent on air temperature, solar radiation, vapor pressure deficit and soil water potential. Then, a statistical analysis was carried out to investigate relations between crown defoliation and ozone concentrations dependent on stomatal opening. The results support the hypothesis that tropospheric ozone contributes to the observed crown defoliation. However, environmental factors strongly affect tree response to high ozone concentrations.     

Measurements of ammonia concentrations,fluxes and dry deposition velocities to a spruce forest 1991–1995

The dry deposition velocities and fluxes of ammonia have been estimated from measurements of the vertical gradient of ammonia and micrometeorology above a spruce forest in western Jutland, Denmark. Measurements have been made in seven periods, each lasting about one week and covering all seasons and different meteorological situations. Different deposition characteristics were observed, depending on the ammonia concentration and the relative humidity. At conditions with westerly winds, the wind brings air masses from the North Sea with low concentration levels of ammonia to the site, while at conditions with easterly winds, the air have passed central Jutland with large emission areas. Some of the relatively low deposition velocities or emissions were observed during conditions with low ammonia concentration and westerly winds. These observations might relate to a compensation point of the forest, i.e. an ammonia concentration below which the trees and/or the surface emit ammonia due to an equilibrium with the ammonia inside the needles or on the surface. Emission of ammonia was also observed at relatively high ammonia concentration levels (above 2 μg NH₃–N m^-3), mainly during one measuring period characterized by easterly winds with dry conditions and high ammonia concentrations, and the emissions might relate to evaporation from ammonia saturated surfaces or emission from mineralization in the forest soil. In general, relatively high net deposition velocities were observed during conditions with relative humidity above 80% or at ammonia concentrations moderate higher than a given (temperature dependent) compensation point. During stable conditions some observations revealed that the gradient above the canopy not necessarily represents the exchange with the canopy.     

Visible leaf injury in young trees of Fagus sylvatica L. and Quercus robur L. in relation to ozone uptake and ozone exposure. An Open-Top Chambers experiment in South Alpine environmental conditions

An Open-Top Chambers experiment on Fagus sylvatica and Quercus robur seedlings was conducted in order to compare the performance of an exposure-based (AOT40) and a flux-based approaches in predicting the appearance of ozone visible injuries on leaves. Three different ozone treatments (charcoal-filtered; non-filtered; and open plots) and two soil moisture treatments (watered and non-watered plots) were performed. A Jarvisian stomatal conductance model was drawn up and parameterised for both species and typical South Alpine environmental conditions, thus allowing the calculation of ozone stomatal fluxes for every treatment. A critical ozone flux level for the onset of leaf visible injury in beech was clearly identified between 32.6 and 33.6 mmolO3 m(-2). In contrast, it was not possible to identify an exposure critical level using the AOT40 index. Water stress delayed the onset of the leaf visible injuries, but the flux-based approach was able to take it into account accurately.     

Summer and autumn ozone fluxes to a forest in the Czech Republic Brdy Mountains

In an effort to examine ozone (O3) deposition over a forest site in the Czech Republic, a low cost eddy flux experiment using slow response ozone and temperature sensors was implemented in July 1993 within the Brdy Mountains. Half-hour 2-Hz ozone and sensible heat measurements made at the Brdy Mountains for 98 days during the period 7 July 1994-20 October 1994 are analyzed and reported. While the Czech Brdy Mountains AOT40 level for the overall 104 day period was 7.6 ppm h (15.1 ppm h for the full 24-h summation), indicating a slight potential for 03 injury, the 1994 summer to autumn'measured forest O3 uptake was 2.4 (+/- 0.9) g m(-2), not unusually high compared to other studies. Average summer midday 03 fluxes and depositidn velocities were -1.0 (+/- 0.6) microg m(-2) s(-1) and 1.1 (+/- 0.7) cm s(-1). and autumn values were -0.36 (+/- 0.4) microg m(-2) s(-1) and 0.7 (+/- 0.5) cm s(-1) respectively. A unique contribution of this study is the first time demonstrated use of slow responding sensors for eddy covariance flux measurements at heights of 20 m above a forest.     

Ozone uptake by citrus trees exposed to a range of ozone concentrations

The Citrus genus includes a large number of species and varieties widely cultivated in the Central Valley of California and in many other countries having similar Mediterranean climates. In the summer, orchards in California experience high levels of tropospheric ozone, formed by reactions of volatile organic compounds (VOC) with oxides of nitrogen (NO_x). Citrus trees may improve air quality in the orchard environment by taking up ozone through stomatal and non-stomatal mechanisms, but they may ultimately be detrimental to regional air quality by emitting biogenic VOC (BVOC) that oxidize to form ozone and secondary organic aerosol downwind of the site of emission. BVOC also play a key role in removing ozone through gas-phase chemical reactions in the intercellular spaces of the leaves and in ambient air outside the plants. Ozone is known to oxidize leaf tissues after entering stomata, resulting in decreased carbon assimilation and crop yield. To characterize ozone deposition and BVOC emissions for lemon (Citrus limon), mandarin (Citrus reticulata), and orange (Citrus sinensis), we designed branch enclosures that allowed direct measurement of fluxes under different physiological conditions in a controlled greenhouse environment. Average ozone uptake was up to 11 nmol s^?1 m^?2 of leaf. At low concentrations of ozone (40 ppb), measured ozone deposition was higher than expected ozone deposition modeled on the basis of stomatal aperture and ozone concentration. Our results were in better agreement with modeled values when we included non-stomatal ozone loss by reaction with gas-phase BVOC emitted from the citrus plants. At high ozone concentrations (160 ppb), the measured ozone deposition was lower than modeled, and we speculate that this indicates ozone accumulation in the leaf mesophyll.     

Ozone reactions with indoor materials during building disinfection

There is scant information related to heterogeneous indoor chemistry at ozone concentrations necessary for the effective disinfection of buildings, i.e., hundreds to thousands of ppm. In the present study, 24 materials were exposed for 16 h to ozone concentrations of 1000–1200 ppm in the inlet streams of test chambers. Initial ozone deposition velocities were similar to those reported in the published literature for much lower ozone concentrations, but decayed rapidly as reaction sites on material surfaces were consumed. For every material, deposition velocities converged to a relatively constant, and typically low, value after approximately 11 h. The four materials with the highest sustained deposition velocities were ceiling tile, office partition, medium density fiberboard and gypsum wallboard backing. Analysis of ozone reaction probabilities indicated that throughout each experiment, and particularly after several hours of disinfection, surface reaction resistance dominated the overall resistance to ozone deposition for nearly all materials. Total building disinfection by-products (all carbonyls) were quantified per unit area of each material for the experimental period. Paper, office partition, and medium density fiberboard each released greater than 38 mg m⁻² of by-products.     

Interfacial transfer velocities of ozone dry deposition over agricultural soils: experimental and theoretical analysis

A mathematical dry deposition model was developed and an experiment performed to verify that the interfacial transfer velocity (V(S)) of ozone dry deposition includes the interfacial reactive velocity (V(Sr)) and interfacial kinetic velocity (V(Sk)), as determined by measuring the ozone depletion over agricultural field soils in a dry deposition chamber. Experimental results indicate that the chemical reaction (O3 + NO --> NO2 + O2) produces the reactive velocity. Observed interfacial transfer velocities depend on nitrogen oxide emission (NO) and soil temperature (T(S)). Additionally, observed kinetic velocities of conditioned field soils increased linearly with soil temperature. Moreover, observed reactive velocities of field soils increased exponentially with soil temperature, and depend on the emission rate of nitrogen oxide. Results in this study demonstrate that interfacial transfer velocities are variable velocities for long-term transportation, that influenced factors are chemical kinetics, thermodynamics and biochemical mechanisms.     

Physiological and foliar symptom response in the crowns of Prunus serotina, Fraxinus americana and Acer rubrum canopy trees to ambient ozone under forest conditions

The crowns of five canopy dominant black cherry (Prunus serotina Ehrh.), five white ash (Fraxinus americana L.), and six red maple (Acer rubrum L.) trees on naturally differing environmental conditions were accessed with scaffold towers within a mixed hardwood forest stand in central Pennsylvania. Ambient ozone concentrations, meteorological parameters, leaf gas exchange and leaf water potential were measured at the sites during the growing seasons of 1998 and 1999. Visible ozone-induced foliar injury was assessed on leaves within the upper and lower crown branches of each tree. Ambient ozone exposures were sufficient to induce typical symptoms on cherry (0-5% total affected leaf area, LAA), whereas foliar injury was not observed on ash or maple. There was a positive correlation between increasing cumulative ozone uptake (U) and increasing percent of LAA for cherry grown under drier site conditions. The lower crown leaves of cherry showed more severe foliar injury than the upper crown leaves. No significant differences in predawn leaf water potential (psi(L)) were detected for all three species indicating no differing soil moisture conditions across the sites. Significant variation in stomatal conductance for water vapor (g(wv)) was found among species, soil moisture, time of day and sample date. When comparing cumulative ozone uptake and decreased photosynthetic activity (P(n)), red maple was the only species to show higher gas exchange under mesic vs. drier soil conditions (P < 0.05). The inconsistent differences in gas exchange response within the same crowns of ash and the uncoupling relationship between g(wv) and P(n) demonstrate the strong influence of heterogeneous environmental conditions within forest canopies.     

Dry deposition of particles and canopy exchange: Comparison of wet,bulk and throughfall deposition at five forest sites in Italy

The contribution of dry deposition to the total atmospheric input of acidifying compounds and base cations is of overwhelming importance. Throughfall measurements provide an estimate of the total deposition to forest soils, including dry deposition, but some uncertainties, related to the canopy interaction processes, affect this approach. We compared the concentrations and the fluxes of the main ions determined in wet-only, bulk and throughfall samples collected at five forest sites in Italy. The contribution of coarse particles deposited onto the bulk samplers was of prime importance for base cations, representing on average from 16% to 46% of the bulk deposition. The extent of this dry deposition depended on some geographical features of the sites, such as the distance from the sea and the annual rainfall. The possibility of applying specific bulk/wet ratios to estimate the wet deposition proved to be limited by the temporal variability of these ratios, which must be considered together with the spatial variability. A direct comparison of the dry contribution deriving from the bulk–wet and the throughfall–wet demonstrated that an extensive natural surface (forest canopy) performs better than a small synthetic surface (funnel of the bulk sampler) in collecting dry deposition of SO₄²⁻, NO₃⁻ and Na⁺. The canopy exchange model was applied to both bulk and wet data to estimate the contribution of dry deposition to the total input of base cations, and the uncertainty associated to the model discussed. The exclusive use of bulk data led to a considerable underestimation of base cation dry deposition, which varies among the study sites.     

An evaluation of ozone exposure metrics for a seasonally drought-stressed ponderosa pine ecosystem

Ozone stress has become an increasingly significant factor in cases of forest decline reported throughout the world. Current metrics to estimate ozone exposure for forest trees are derived from atmospheric concentrations and assume that the forest is physiologically active at all times of the growing season. This may be inaccurate in regions with a Mediterranean climate, such as California and the Pacific Northwest, where peak physiological activity occurs early in the season to take advantage of high soil moisture and does not correspond to peak ozone concentrations. It may also misrepresent ecosystems experiencing non-average climate conditions such as drought years. We compared direct measurements of ozone flux into a ponderosa pine canopy with a suite of the most common ozone exposure metrics to determine which best correlated with actual ozone uptake by the forest. Of the metrics we assessed, SUM0 (the sum of all daytime ozone concentrations > 0) best corresponded to ozone uptake by ponderosa pine, however the correlation was only strong at times when the stomata were unconstrained by site moisture conditions. In the early growing season (May and June). SUM0 was an adequate metric for forest ozone exposure. Later in the season, when stomatal conductance was limited by drought. SUM0 overestimated ozone uptake. A better metric for seasonally drought-stressed forests would be one that incorporates forest physiological activity, either through mechanistic modeling, by weighting ozone concentrations by stomatal conductance, or by weighting concentrations by site moisture conditions.