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.     

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.     

A mathematical model of drift deposition from a bifurcated cooling tower plume

Cooling tower drift deposition modeling has been extended by including the centrifugal force induced by plume bifurcation as a mechanism for droplet removal from the plume. The model is capable of predicting the trajectory of a single droplet from the stage of strong interaction with the vortex field soon after droplet emission at the tower top through the stage of droplet evaporation after breakaway from the plume. Applications showed the droplet to follow a helical trajectory within the plume, with breakaway and ground impact depending upon plume rise, ambient wind, evaporation and vortex strength.     

Contribution of canopy leaching to sulphate deposition in a Scots pine forest

Radioactive sulphate (35SO4) was applied to the soil below a Scots pine forest on 23 June 1989, and its movement into the canopy and into throughfall and stemflow was measured over 4 months. The specific activity, Bq (mg S)(-1), of the canopy increased monotonically; uptake by current-year (1989) expanding needles was initially twice as fast as by older needles or live twigs. By 10 October the canopy average specific activity was 62 Bq (mg S)(-1). The specific activity of net throughfall (throughfall + stemflow - rain), deduced from measurements from six throughfall collectors, six stemflow collectors and two rain collectors, fell rapidly from 12.6 Bq (mg S)(-1) in late July to <1 Bq (mg S)(-1) in mid-August. The results suggest (assuming rapid equilibration of 35S with sulphate in soil) that root-derived sulphate contributed c. 3% of sulphate in net throughfall and that dry deposition of SO2 and sulphate particles contributed c. 97% of the 0.56 g S m(-2) measured in net throughfall over the period. Simultaneous measurements of SO2 at canopy height and of NH3 above and within the canopy gave mean concentrations of 5.9 and 0.86 microg m(-3), respectively, sufficient to account for the sulphate measured in net throughfall only if codeposition of NH3 and SO2 occurred to canopy surfaces. The large values of specific activity observed in July, however, indicate that throughfall composition may be closely related to recent soil input of sulphate, and that equilibrium cannot be safely assumed. The possibility of a significant contribution of soil-derived sulphate to sulphate deposition in net throughfall cannot be ruled out on the basis of this experiment.     

Evaluation of aquatic environmental fate of 2,4,6-trichlorophenol with a mathematical model

A mathematical model, Simplified Approach for Fate Evaluation of Chemicals in Aquatic Systems (SAFECAS), was applied to evaluate fate of 2,4,6-trichlorophenol(TCP) in three types of aquatic environments; a river, an oligotrophic lake, and an eutrophic lake.Since phenols are dissociative substances, equilibrium constants and rate constants for transfer and transformation processes were measured or estimated as a function of pH.Results suggest that most of TCP is distributed to the water phase in aquatic environments, and is almost removed by the advective outflow in the river and by the photodegradation in lakes.     

Dry deposition of SO2

SO₂ concentration gradient measurements are reported, which have been carried out between May 1975 and September 1976 at the Kernforschungsanlage Jülich. Most of the measurements were made with differential optical absorption spectroscopy.Mirrors at various heights (30, 80, 120m) on the meterological tower were used to reflect the light beam back to the laboratory placed at the ground. In this way absorption paths of about 600m were obtained. In some cases, the optical data were compared with chemical measured concentration profiles (after West & Gaeke. 1976) showing an agreement within ±20%. From simultaneously measured temperature and wind profiles the transfer resistance of the turbulent boundary layer was calculated.Average values of the vertical SO₂-flux obtained from SO₂ concentration gradient and atmospheric transfer resistance varied from about 0.6 μg m⁻²s⁻¹ in winter and 0.1 to 0.4 μg m⁻²s⁻¹ in summer, corresponding to a lifetime for SO₂. if dry deposition is taken as the only sink mechanism, of half a day up to several days, respectively.     

Dry deposition of particles to wave surfaces: I. Mathematical modeling

Previous estimates of dry deposition to water surfaces were generally based on deposition to flat, solid surfaces. This paper examines the effects of waves on dry deposition rates by numerically simulating particle trajectories over wave surfaces. Airflows over two-dimensional sine waves with height-to-length ratios 2a/λ=0.1, 0.07, and 0.03 were calculated with a commercial computational fluid dynamics model. Results from the airflow simulations (velocity, kinetic energy, energy dissipation rate, and shear stress) provided inputs for a stochastic particle trajectory model. Particles were released from a height of 300 non-dimensional wall units at different locations along the wave. For those between 1 and 20 μm, deposition was found to be greatest for particles released to the upslope portion of the wave, followed by the trough, crest and downslope. Overall deposition rates were enhanced due to the presence of waves. Increases ranged from 5% (d_p=80 μm) to 100% (d_p=1 μm) for waves with 2a/λ=0.07 and 0.1 and were approximately 50% greater (d_p=1−80 μm) for 2a/λ=0.03. Deposition rates were enhanced due to increases in impaction and turbulent transport, both of which increase with increasing wave slope. However, an increased slope also produced regions of low or reversed flow in the trough and downslope, which decreased deposition rates. Due to these competing effects with respect to wave slope, deposition rates did not increase monotonically with wave slope.     

Dry deposition of airborne sulfate and nitrate to soybeans

Dry deposition contributes a substantial part of the total deposition of acidic pollutants and acid precursors to agricultural systems. However, because of the relative intractability of measurement of dry deposition fluxes, little work has been done to directly quantify dry inputs of pollutants to crops. In this research, foliar surface sampling ('leaf-washing') methods were developed and shown to be a practical and fairly precise means of monitoring the accumulation of dry-deposited SO4(2-) and NO3- on plant surfaces. Leaching of these ions from plant tissues was shown to be negligible; however, uptake by plants (e.g. stomatal gas exchange of SO2 or HNO3 and/or assimilation of surface accumulations of materials) is not accounted for by the sampling method. The significance of dry deposition to modification of the chemical microenvironment of leaf surfaces appears to be a factor of 3 to 20 or more greater than that of wet deposition alone. This is due to the cyclic reactivation of accumulated materials by dew and light rains, which may dissolve and mobilize, but not remove, the pollutant surface deposit. Therefore, while dry deposition of SO2 and SO4(2-) containing particles may contribute only part of the total mass of sulfur inputs to crop systems, the exposure of plant surface tissue to pollutants can be dominated by the dry-deposited material. The alteration of leaf surface chemistry may contribute to possible stress-producing mechanisms such as reduction of cuticular integrity, cellular injury and death, enhanced leaching of primary and secondary metabolites, and changes in pathogen infection efficiency.     

Dry deposition of particles to wave surfaces: II. Wind tunnel experiments

Wind tunnel measurements of particle dry deposition to wavy and flat surfaces were made to estimate the enhancement of deposition rates due to waves on water surfaces. Measurements were made of 4.0 and 6.7 μm uranine particles at wind speeds of 5 and 10 m s⁻¹ to sinusoidal waves with height to length ratios 2a/λ=0.1 and 0.03 and to flat surfaces. Results showed that deposition was greatest to the upslope portion of the wave, accounting for 40–45% of the total mass, followed by the trough (30%), downslope (15%), and crest (10–15%). These results generally agreed within experimental variability with modeling predictions (Zufall et al., 1999). Deposition was enhanced at the upslope due to the effects of particle interception and impaction on the wave. Total deposition to the wave surfaces was greater than deposition to the flat surface for a large majority of the cases. The average increase in deposition to both wave surfaces for both particle sizes and wind speeds over deposition to the flat surface was 80%.     

Dry deposition of NOx to grass in rural east Anglia

A single set of analytical instruments was utilised, with a two-level sampling system, to determine the dry deposition of NO_x by the gradient method over a cut-grass field during day-time in the spring. The range of deposition velocities is around O.1–0.6 cm s⁻¹, and the bulk surface resistance range is about O.8–5.6 s cm⁻¹.     

Dry deposition and internal circulation of nitrogen,sulphur and base cations to a coniferous forest

The dry deposition of sulphur, nitrogen and base cations to a spruce stand was estimated during a five year period using a surrogate surface resembling needles, throughfall and bulk deposition measurements. The deposition was calculated from the ratio between the deposition of an ion and sodium on the surrogate surface and the net throughfall of sodium to the forest. The dry deposition represented a large fraction of the total atmospheric input of base cations. For Na⁺, Mg²⁺, Ca²⁺, and K⁺ they were 66, 67, 53 and 42%, respectively. The internal circulation was 95% of non-marine net throughfall fro K⁺ and 76% for Ca²⁺. The dry deposition of SO₂ to the canopies regulates the internal circulation of Ca²⁺. The dry deposition of SO₂ to the canopies regulates the internal circulation of Ca²⁺. The dry depositions of ammonium and nitrate are close to the net throughfall of Kjeldahl-N and nitrate, respectively. The obtained deposition velocities are comparable to other studies. The calculated dry deposition of ammonium was compared to the net throughfall of ammonium at three nearby forest stands receiving different ammonium amounts on the soils. No correlation to nitrogen level was found, but most ammonium was lost and converted to organic nitrogen in the canopies of the wettest forest stand.     

Atmospheric deposition and ionic interactions within a beech canopy in the Karkonosze Mountains

A method, of artificial foliage placed above rain collectors, which enables to estimate total atmospheric inputs of elements, was employed to study processes related to ionic flow through a tree canopy. The investigations were conducted within a beech forest in the Karkonosze Mountains, in 1996-1997. An analysis of net canopy exchange (throughfall flux-atmospheric input) revealed that NH4+, NO3-, H+ had been retained in the canopy, Ca2+, Na+, Cl-, SO(2-)4, PO(3-)4 flowed passively, whereas K+ and Mg2- had been removed from the tree foliage. Ammonium absorption was more efficient than that of NO3- ions, because NH4+ was taken up from rain-waters and aerosol-gaseous fraction of the atmospheric input, whereas NO3- ions were captured primarily from the latter source. Retention of H+ was also related exclusively to the aerosol-gaseous input. Leaching losses of K+ and Mg2+ did not result from exchange reactions of these ions with retained H+, but from ionic exchange between NH4+ and K+ + Mg2+. At the same time, neutralisation of H+ in the canopy has been attributed to NO3- absorption, resulting most likely from HNO3 vapour penetration into the plants.     

Dry deposition of ozone: some measurements ofdeposition velocity and of vertical profiles to 100 metres

Deposition velocities for O₃ to grassland have been measured by the gradient method. Values range from 0.08 to 0.91 cms⁻¹ and are shown to be influenced most substantially by changes in surface conditions. O₃ deposition is inhibited by the presence of snow or moisture at the surface and tends to decrease as the afternoon progresses. Vertical profiles of O₃ and temperature to 100 m height are indicative of surface depletion of O₃ by dry deposition and a diminution of concentration at ground-level due to inhibition of vertical mixing from aloft during periods of atmospheric stability. Dry deposition fluxes are shown to be adequate to account for the diurnal variation of O₃ observed at ground-level in rural areas during summer anticyclonic conditions.     

Dry deposition of so2 on agricultural crops

A micrometeorological method has been used to estimate dry deposition rates of sulphur dioxide on agricultural crops from vertical gradients of SO₂ concentration, windspeed and air temperature above the crop surface. Field measurements in a wide range of atmospheric and surface conditions enabled analysis of the results to separate the various atmospheric and surface processes controlling the flux. For wheat deposition velocity (Vg) 1 m above the surface varied between 0.1 and 1.5cm s⁻¹ and was controlled primarily by surface processes, surface resistance generally contributing 70% of total resistance (rt). Values of surface resistance are determined essentially by deposition at two sinks, the sub-stomatal cavity and leaf cuticle acting in parallel. With stomata open 1/3 of the total SO₂ flux was to the leaf surface and 2/3 to the sub-stomatal cavity. When foliage was wet with rain or dew, provided the pH of the liquid was >3.5 surface resistance is negligible and Vg then controlled by atmospheric resistance may exceed 1 cm s⁻¹. For agricultural areas of Britain (14 × 10⁶ha) dry deposition has been estimated at 72 kg SO₂ ha⁻¹ annually, 60% of which is deposited during the winter (October–March, inclusive), equivalent to a deposition velocity of 0.6cm s⁻¹ for the area considered.     

Dry deposition of sulphur over eastern South Africa

Dry deposition of sulphur is estimated in three climatic regions of Mpumalanga, South Africa, using the inferential method. Data from June 1996 to May 1997 are used at Elandsfontein and Palmer on the industrialised highveld, as well as data from two-week monitoring campaigns in the late-winter and in summer at Blyde on the eastern escarpment and at Skukuza in the lowveld. Total dry deposition rates for sulphur range across the Mpumalanga highveld from 13.1 kg ha^-1 a^-1 at Elandsfontein to 3.1 kg ha^-1 a^-1 at Palmer, are associated with the strong SO₂ gradient between the two stations and are attributed mostly to dry deposition of sulphur from SO₂. The deposition flux varies less from Palmer eastward over the escarpment and the lowveld and ranges from 3.9 kg S ha^-1 a^-1 at Blyde to 3.3 kg S ha^-1 a^-1 at Skukuza. A weak seasonal variation in sulphur dry deposition flux occurs on the central highveld with the maximum in summer and the minimum in winter. Conversely, the maximum sulphur dry deposition on the periphery of the highveld, the escarpment and in the lowveld occurs in winter with the minimum in summer. More than 80% of the dry deposition of sulphur in Mpumalanga occurs during daytime in all seasons.     

Testing a comprehensive acid deposition model

This paper describes the development and evaluation of a comprehensive acid deposition model referred to as ADOM (Acid Deposition and Oxidant Model). The first part of the paper describes the general approach to the formulation of the model. The second part presents the evaluation of the model against observations collected during the OSCAR (April 1981) field study. Although the model performs well in explaining the magnitudes and variations of the observations, the discrepancies between model predictions and observations are not small. Because of the uncertainties in the data used to run and evaluate the model, these discrepancies do not necessarily suggest the need for modifications in the model. Therefore, we have supplemented the direct comparison of model predictions with observations with indirect methods that establish the correspondence between the model and reality. This paper describes one such technique that relies on the washout ratio, which is sometimes used to characterize scavenging of pollutants.     

Dry deposition fluxes and deposition velocities of PAHs at an urban site in Turkey

Even though dry deposition and air–water exchange of semivolatile organic compounds (SOCs) are important for surfaces in and around the urban areas, there is still no generally accepted direct measurement technique for dry deposition. In this study, a modified water surface sampler (WSS) configuration, including a filter holder and an XAD-2 resin column, was employed to investigate the polycyclic aromatic hydrocarbon (PAH) dry deposition in an urban area. The measured total (particle+dissolved) PAH fluxes to the WSS averaged to be 34 960±16 540 ng m⁻² d⁻¹. Average particulate PAH flux, determined by analyzing the filter in the WSS, was about 8% of the total PAH flux. Temporal flux variations indicated that colder months (October–April) had the highest PAH fluxes. This increase could be attributed to the residential heating as well as meteorological effects including lower mixing height. A high volume air sampler was concurrently employed to collect ambient air concentrations. The average total (gas+particle) atmospheric PAH concentration (456±524 ng m⁻³) was within the range of previously measured values at different urban locations. PAH concentrations in urban areas are more than two orders of magnitude higher than those measured in pristine areas and this result may indicate that urban areas have major source sectors and greater deposition rates are expected near to these areas. The average contribution of particle phase was about 10% in total concentration. Simultaneous particulate phase dry deposition and ambient air samples were collected in this study. Then, particulate phase apparent dry deposition velocities were calculated using the fluxes and concentrations for each PAH compound and they ranged from 0.1 to 1.2 cm s⁻¹. These values are in good agreement with previously reported values.