Some Factors that Affect the Deposition Rates of Sulfur Dioxide and Similar Gases on Vegetation

The deposition of sulfur dioxide on growing vegetation is affected by diverse environmental factors, many of which undergo large diurnal and spatial variations. The aerodynamic resistance to vertical transfer in the surface boundary layer can be formulated in terms of the friction velocity, height of observation, vertical heat flux, and surface roughness. Also important are the resistance in the air layer closest to the surface elements and, in dry vegetation, the average stomatal resistance of the plants. The latter variable is among the most difficult to estimate, but over many agricultural field crops like those in the midwestern U.S., a typical minimum value of average stomatal resistance to SO₂ transfer is about 0.7 s cm^-1, as is indicated by various experimental data. The deposition velocity can be estimated as the inverse of the sum of the resistances of the layers, necessarily down to where the concentrations are zero; in the surface boundary layer, any of the various resistances might be dominant. Above the surface layer, the micrometeorological relationships are known with less certainty, but reasonable approximations indicate that during unstable conditions the resistance to transfer is very small at heights of several tens of meters and during stable conditions the aerodynamic resistance is very large aloft.     

A Model for Uptake of Pollutants by Vegetation

A mass transfer approach is used in developing a practical mathematical model of gaseous pollutant uptake by leaves in which a series of resistances is summed across a concentration difference. The body of information presented in this paper is directed to plant pathologists or physiologists in the field of vegetal-pollutant effects and to people interested in the natural removal of air pollutants by vegetation. Correlations are given to calculate the aerodynamic and the stomatal resistances to uptake, while both a qualitative investigation and quantitative estimates are made of the mesophyllic resistance. The factors which control the aerodynamic resistance, r_a, are leaf size and wind speed, while the leaf physiology is the determinant of the stomatal resistance, r_s . It is noted that the chemical reaction rate and pollutant diffusivity in the mesophyll control the mesophyllic resistance, r_m, though the overall gas phase mesophyllic resistance, Hr_m, is strongly a function of pollutant solubility in water. Finally, the overall model is compared to earlier experimental work on vegetal uptake of SO₂.     

Error estimations of dry deposition velocities of air pollutants using bulk sea surface temperature under common assumptions

It is well known that skin sea surface temperature (SSST) is different from bulk sea surface temperature (BSST) by a few tenths of a degree Celsius. However, the extent of the error associated with dry deposition (or uptake) estimation by using BSST is not well known. This study tries to conduct such an evaluation using the on-board observation data over the South China Sea in the summers of 2004 and 2006. It was found that when a warm layer occurred, the deposition velocities using BSST were underestimated within the range of 0.8–4.3%, and the absorbed sea surface heat flux was overestimated by 21 W m^?2. In contrast, under cool skin only conditions, the deposition velocities using BSST were overestimated within the range of 0.5–2.0%, varying with pollutants and the absorbed sea surface heat flux was underestimated also by 21 W m^?2. Scale analysis shows that for a slightly soluble gas (e.g., NO₂, NO and CO), the error in the solubility estimation using BSST is the major source of the error in dry deposition estimation. For a highly soluble gas (e.g., SO₂), the error in the estimation of turbulent heat fluxes and, consequently, aerodynamic resistance and gas-phase film resistance using BSST is the major source of the total error. In contrast, for a medium soluble gas (e.g., O₃ and CO₂) both the errors from the estimations of the solubility and aerodynamic resistance are important. In addition, deposition estimations using various assumptions are discussed. The largest uncertainty is from the parameterizations for chemical enhancement factors. Other important areas of uncertainty include: (1) various parameterizations for gas-transfer velocity; (2) neutral-atmosphere assumption; (3) using BSST as SST, and (4) constant pH value assumption.     

Evaluation of SO2 dry deposition over short vegetation in Portugal

SO₂ dry deposition was studied over short vegetation, in Portugal, by means of the concentration gradient method. The experimental study involved one first phase of long-term measurements carried out in a grassland and, subsequently, a second period of several 1997 intensive field campaigns performed in three places representing different climate and surface conditions. Temporal and spatial patterns of dry deposition parameters show that downward fluxes of SO₂ are by some extent affected by surface processes. Median R_c varied from 140 s cm⁻¹ to values around 200 s cm⁻¹, in a wide range of environmental conditions. Stomatal uptake is an important sink when vegetation is biologically active, but its contribution is effectively low when compared with non-stomatal mechanisms, especially when the surface is wet. Under dry conditions R_c increases by a factor of two, but SO₂ deposition rates then still are significant. The parameterisation of the surface resistance for SO₂ proved to be difficult, but V_d derived with the Erisman parameterisation (Erisman et al., Atmos. Environ. 28 (16) (1994) 2595) compared best with measured values, at low time resolution scale and especially under moisture conditions.     

Parameterization of surface resistances to gaseous dry deposition in regional-scale numerical models

Methods for estimating the dry deposition velocities of atmospheric gases in the U.S. and surrounding areas have been improved and incorporated into a revised computer code module for use in numerical models of atmospheric transport and deposition of pollutants over regional scales. The key improvement is the computation of bulk surface resistances along three distinct pathways of mass transfer to sites of deposition at the upper portions of vegetative canopies or structures, the lower portions, and the ground (or water surface). This approach replaces the previous technique of providing simple look-up tables of bulk surface resistances. With the surface resistances divided explicitly into distinct pathways, the bulk surface resistances for a large number of gases in addition to those usually addressed in acid deposition models (SO₂, O₃ NO_x, and HNO₃) can be computed, if estimates of the effective Henry's Law constants and appropriate measures of the chemical reactivity of the various substances are known. This has been accomplished successfully for H₂O₂, HCHO₃ CH₃CHO (to represent other aldehvdes), CH₃O₂H (to represent organic peroxides), CH₃C(O)O₂H, HCOOH (to represent organic acids), NH₃, CH₃C(O)O₂NO₂ and HNO₂. Other factors considered include surface temperature, stomata1 response to environmental parameters, the wetting of surfaces by dew and rain, and the covering of surfaces by snow. Surface emission of gases and variations of uptake characteristics by individual plant species within the landuse types are not considered explicitly.     

The aging process of naturally emitted aerosol (sea-salt and mineral aerosol) during long range transport

The aging processes of two representative natural aerosol, sea-salt and mineral aerosol, are investigated by using a box model equipped with a thermodynamic module (SCAPE). The model is shown to successfully describe the aging processes between the gas-phase anthropogenic pollutants (SO₂, NO_x, and NH₃) and primary aerosol particles, including self-neutralization process/chlorine depletion in the sea-salt aerosol; formation/dissipation of carbonate and bicarbonate ions in the mineral aerosol; irreversible dynamic deposition of SO₂ and H₂SO₄; and reversible thermodynamic distribution of inorganic volatile species. It is found that SO₂ and H₂SO₄ tend to deposit onto the mode with the largest surface area, and that ammonia deposition is controlled by preceding SO₂/H₂SO₄ deposition. During the SO₂/H₂SO₄ deposition, chloride and carbonate are continuously released from the sea-salt and mineral dust particles, respectively. The findings by the model predictions are consistent with field and observational studies.     

预涂动态膜在乳化油废水处理中的阻力分布及其抗污染特性

为讨论预涂动态膜的抗污染特性,测定了陶瓷膜、高岭土预涂动态膜和高岭土/MnO2复合预涂动态膜分别在乳化油废水处理中的阻力分布,并用SEM扫描电镜对污染预涂动态膜形貌进行了表征。结果表明,乳化油污染阻力明显高于基膜(或陶瓷膜)固有阻力及涂膜粒子形成的阻力,乳化油对预涂动态膜或陶瓷膜的污染是导致膜通量衰减的主要原因;高岭土和复合预涂动态膜所用基膜污染阻力低于陶瓷膜的内部污染阻力,其中复合预涂动态膜所用基膜的污染阻力最小;复合动态膜层比高岭土动态膜层的污染程度低;污染预涂动态膜表面呈现明显的凝胶态物质,其基膜外壁面未呈现凝胶态物质。处理乳化油废水中,动态膜层可有效保护基膜,表现出了较强的抗污染性能。     

Influence of sulfur dioxide-related interactions on PM2.5 formation in iron ore sintering

The formation of PM_2.5 (aerosol particulate matter less than 2.5 µm in aerodynamic diameter) in association with SO₂ emission during sintering process has been studied by dividing the whole sintering process into six typical sampling stages. A low-pressure cascade impactor was used to collect PM_2.5 by automatically segregating particulates into six sizes. It was found that strong correlation existed between the emission properties of PM_2.5 and SO₂. Wet mixture layer (overwetted layer and raw mixture layer) had the function to simultaneously capture SO₂ and PM_2.5 during the early sintering stages, and released them back into flue gas mainly in the flue gas temperature-rising period. CaSO₄ crystals constituted the main SO₂-related PM_2.5 during the disappearing process of overwetted layer, which was able to form perfect individual crystals or to form particles with complex chemical compositions. Besides the existence of individual CaSO₄ crystals, mixed crystals of K₂SO₄-CaSO₄ in PM_2.5 were also found during the first half of the temperature-rising period of flue gas. The interaction between fine-grained Ca-based fluxes, potassium vapors, and SO₂ was the potential source of SO₂-related PM_2.5.

Implications: The emission property of PM_2.5 and SO₂ throughout the sintering process exhibited well similarity. This phenomenon tightened the relationship between the formation of PM_2.5 and the emission of SO₂. Through revealing the properties of SO₂-related PM_2.5 during sintering process, the potential interaction between fine-grained Ca-based fluxes, potassium vapors, and SO₂ was found to be the source of SO₂-related PM_2.5. This information can serve as the guidance to develop efficient techniques to control the formation and emission of PM_2.5 in practical sintering plants.     

Description and evaluation of a model of deposition velocities for routine estimates of air pollutant dry deposition over North America.: Part I: model development

This paper describes the development of a detailed dry deposition model for routine computation of dry deposition velocities of SO₂, O₃, HNO₃ and fine particle SO₄²⁻ across much of North America. Four different dry deposition/surface exchange sub-models have been combined with the current Canadian weather forecast model (Global Environmental Multiscale model) with a 3 h time resolution and a horizontal spatial resolution of 35 km. The present model uses the US Geological Survey North American Land Cover Characteristics data to obtain fourteen different land use and five seasonal categories. The four sub-models used are a multi-layer model for gaseous species over taller canopy land-use types, a big-leaf model for gaseous species over lower canopies (including bare soil and water) and for HNO₃ under all surface types and, two different models for SO₄²⁻, one for tall canopies and the other for short canopies. All necessary parameters for each sub-model, chemical species, land-use and seasonal categories have been selected from available data libraries or from the values reported in the literature. The purpose for developing this model (referred to as the Routine Deposition Model (RDM)), when coupled with air concentration data, is to provide estimates of seasonal dry deposition, which can be combined with wet deposition to produce total deposition estimates. Model theory is discussed in this paper and model sensitivity tests and results will be presented in a companion paper.     

Laboratory measurement of the dry deposition of sulfur dioxide onto northern Chinese soil samples

We investigated soil surface resistance R_c to dry deposition of sulfur dioxide (SO₂) onto different types of soils in laboratory experiments, using samples collected from the arid loess plateau and deserts of northern China. We evaluated the factors that affect R_c, which depends on the physical and chemical interaction between a trace constituent and the deposition surface. We observed that the values of R_c for SO₂ decreased with increase of soil weight and increased with SO₂ concentration, although surface coverage had little effect on R_c. The SO₂ uptake rate by all the northern Chinese soil samples seemed to be, on the whole, dependent on relative humidity (RH). In all of the northern Chinese soil samples, R_c was in the range 0.028–0.65 s mm⁻¹, and was exponentially related to the effective surface area of each soil sample, regardless of RH. Wet chemical analysis of sulfur deposited onto the soil samples showed that oxidation ratio of sulfur(IV) to sulfur(VI) was related to RH, which might be related to complex interactions among the amount of water on the soil sample, the pH, and the metallic ions in the liquid phase.     

Using illumination and shadow to model aerodynamic resistance and flow separation: An isotropic study

The momentum extracted from fluid flow by the underlying surface roughness is important for understanding processes of entrainment, transportation and deposition of sediments. The parameter z₀ is a length scale that characterizes the loss of wind momentum attributable to the roughness elements. However, it is very difficult to estimate accurately and precisely even under carefully controlled conditions in wind tunnels. This limits the use of the parameter over large areas and in particular across scales of roughness, e.g., grain to form scale. This is problematic for studies of wind erosion and dust dispersion which require estimates of aerodynamic resistance over very large areas.A new concept is proposed with the potential to unify the estimates of fluid flow resistance along the continuum of sparse to tightly packed object spacing and across multiple scales. It is based on the creation of shadows by the illumination of roughness elements and the assumption that flow separation is created behind roughness elements on a plane surface as a function of free-stream wind velocity and obstacle height. The concept was implemented using a computer program and validated against a wind tunnel study that estimated z₀ for configurations of spheroids. Various spheroid coverages used in the wind tunnel study were reconstructed using a digital elevation model of the surface simulated by the computer. A strong relationship was established (R²=0.91) over two orders of magnitude between the shadow area ratio (SAR) and z₀.Fluid drag was shown to be dependent on the arrangement of roughness elements at the surface. The configurations of spheroids were replaced by cylinders of the same basal area and computer simulations of shadow area were repeated. Object shape was evidently important to the overlap of shadow with downstream adjacent obstacles and hence aerodynamic resistance was dependent on object shape. These findings appear to contradict empirical evidence of previous studies.Illumination and shadow of objects on a plane surface appears to adequately represent z₀. Shadow appears to approximate the flow separation behind an obstacle and to represent a wake. The overlap on to downstream adjacent objects of the shadow cast from an upstream object appears to mimic the interference of wakes caused by fluid flow moving around stationary objects with close spacing. There is a compelling argument for the use of SAR as a unifying measure of aerodynamic resistance over the continuum between isolated and tightly packed objects. Furthermore, given elevation data of objects on a plane surface the results show that shadow length is a point-based measure that may be integrated for all points evaluated to provide SAR. The demonstrated angular relationship between illumination and drag (shadow and flow separation) has considerable potential for estimating aerodynamic resistance over multiple scales and for significant investigations of (i) the anisotropic nature of aerodynamic resistance and (ii) its estimation using directional measurements of reflectance and bidirectional reflectance models.     

Pore Structure Effects on Ca-Based Sorbent Sulfation Capacity at Medium Temperatures: Activated Carbon as Sorbent/Catalyst Support

Abstract

The reaction between three different Ca-based sorbents and SO₂ were studied in a medium temperature range (473–773 K). The largest SO₂ capture was found with Ca(OH)₂ at 773 K, 126.31 mg SO₂?g Ca(OH)₂ ^?1, and the influence of SO₂ concentration on the sorbent utilization was observed. Investigations of the internal porous structure of Ca-based sorbents showed that the initial reaction rate was controlled by the surface area, and once the sul-fated products were produced, pore structure dominated. To increase the surface area of Ca-based sorbents available to interact with and retain SO₂, one kind of CaO/activated carbon (AC) sorbent/catalyst was prepared to study the effect of AC on the dispersion of Ca-based materials. The results indicated that the Ca-based material dispersed on high-surface-area AC had more capacities for SO₂ than unsupported Ca-based sorbents. The initial reaction rates of the reaction between SO₂ and Ca-based sorbents and the prepared CaO/AC sorbents/cata-lysts were measured. Results showed that the reaction rate apparently increased with the presence of AC. It was concluded that CaO/AC was the active material in the des-ulfurization reaction. AC acting as the support can play a role to supply O₂ to increase the affinity to SO₂. Moreover, when AC is acting as a support, the surface oxygen functional group formed on the surface of AC can serve as a new site for SO₂ adsorption.     

Toxicologic Appraisal of Particulate Matter,Oxides of Sulfur,and Sulfuric Acid

An examination of the available toxicological literature indicates that sulfur dioxide itself would be properly classified as a mild respiratory irritant, the main portion of which is absorbed in the upper respiratory tract. The reported industrial experience of symptoms of mild chronic respiratory irritation from exposures at or above 5 ppm is compatible with what would have been predicted on the basis of available toxicological data. The basic physiological response to inhalation of pure SO₂ appears to be a mild degree of bronchoconstriction reflected in a measurable increase in flow resistance. Although the response is highly variable, most individuals tested have responded to 5 ppm and levels of 5 to 10 ppm have upon occasion produced severe bronchospasm in sensitive individuals. This serves to point up the fact that experience with the industrial Threshold Limit Value (5 ppm) is not applicable as a guide for the general population. Although the majority of individuals tested have shown no detectable response to levels of 1 ppm, there are again sensitive individuals who have responded. It is not known whether these individuals would have responded to concentrations lower than this. The response of these more sensitive individuals to 1 ppm would be classified as detectable response, not as severe bronchospasm. An examination of the available toxicological literature also indicates that sulfuric acid and irritant sulfates, to the extent that the latter have been examined, are more potent irritants than sulfur dioxide. This has been demonstrated in studies using morality and lung pathology as criteria as well as in studies using alterations in pulmonary function in experimental animals and human subjects. The irritant potency of these substances is affected by particle size and by relative humidity, which factors are probably interrelated. It is unfortunate that these substances have not been as yet studied in as great detail as has the less irritant sulfur dioxide. There is evidence which cannot be ignored, even though it is based entirely on animal experiments of one investigator, indicating that the presence of particulate material capable of oxidizing sulfur dioxide to sulfuric acid caused a three to fourfold potentiation of the irritant response. The aerosols causing this potentiation were soluble salts of ferrous iron, manganese and vanadium all of which would become droplets upon inhalation. Insoluble aerosols such as carbon, iron oxide fume, triphenylphosphate or fly ash did not cause a potentiation of the irritant action of SO₂ even when used at higher concentrations. The concentrations of SO₂ used in these various experiments were in some cases as low as 0.16 ppm. The catalytic aerosols were used at concentrations of 0.7 to 1 mg/m³ which is above any reported levels of these metals in urban air. If the SO₂ present as an air pollutant remained unaltered until removed by dilution, there would be no evidence in the toxicological literature suggesting that it would be likely to have any effects on man at prevailing levels. Studies of atmospheric chemistry have shown that SO₂ does not remain unaltered in the atmosphere, especially under onditions of high humidity and in the presence of particulate material, but is converted to H₂SO₄. Such a conversion increases its irritant potency. On this basis the toxicological literature combined with the literature of atmospheric chemistry suggest that sulfur dioxide levels be controlled in terms of the potential formation of irritant particles. This means that control measures as far as feasible should be aimed at both SO₂ and particulate material and not against either alone.     

Aerosol Aerodynamic Size Conventions For Inertia! Sampler Calibration

The body of information presented in this paper is directed to investigators using inertial samplers for precise and accurate studies of respirable aerosols. The conventions commonly used for aerodynamic size for aerosol particles are discussed including the definition popularized by the "Task Group on Lung Dynamics" of the ICRP, and the "Lovelace" definition. To emphasize the distinction, the Task Group definition (unit density sphere equivalent) is called the aerodynamic equivalent diameter, D_ae, and the Lovelace definition (characteristic expression based upon viscous resistance) is called the aerodynamic resistance diameter, D_ar. The implications and efficacy of these conventions are related to procedures for calibration of cascade impactors, cyclones, and spiral centrifuges. The calibration of a spiral centrifuge at different altitudes is used as an example of the potential problems associated with the use of the different conventions for describing aerodynamic size. The aerodynamic resistance diameter is recommended for calibration of inertial samplers to be used to collect aerosols in the respirable size range.     

Dry Deposition of Ammonia at Environmental Concentrations on Selected Plant Species

The deposition velocity of NH₃ on six plant species at environmental concentrations has been studied in a dynamic plant gas exchange reactor. The total resistance to the transport of NH₃ was studied. The aerodynamic resistance was determined directly by NH₃ gas absorption in aqueous solutions at environmental concentrations in a two-phase gradientless reactor modeling the transfer processes through the stomata in a leaf. The concentration of NH₃ in the gas phase ranged from 50 to 1000 ppb and the temperature varied from 25 to 30°C. The results for the deposition velocity for NH₃, during the day, varied from 0.3 to 1.3 cm/s. The deposition velocities at night were about one order of magnitude smaller. These results are compared with estimates from the Frdssling equation which consistently yields higher values of the same order of magnitude. To determine accurate atmospheric transport models or global budget models, a variable deposition velocity should be used to account for the diurnal and seasonal variations in the surface resistance.     

Description and evaluation of a model of deposition velocities for routine estimates of dry deposition over North America. Part II: review of past measurements and model results

Numerical sensitivity tests and four months of complete model runs have been conducted for the Routine Deposition Model (RDM). The influence of individual model inputs on dry deposition velocity as a function of land-use category (LUC) and pollutant (SO₂, O₃, SO²⁻₄ and HNO₃) were examined over a realistic range of values for solar radiation, stability and wind speed. Spatial and temporal variations in RDM deposition velocity (V_d) during June – September 1996 time period generated using meteorological input from a mesoscale model run at 35 km resolution over north-eastern North America were also examined. Comparison of RDM V_d values to a variety of measurements of dry deposition velocities of SO₂, O₃, SO²⁻₄ and NHO₃ that have been reported in the literature demonstrated that RDM produces realistic results. Over northeastern NA RDM monthly averaged dry deposition velocities for SO₂ vary from 0.2 to 3.0 cm s⁻¹ with the highest deposition velocities over water surfaces. For O₃, the monthly averaged dry deposition velocities are from 0.05 to 1.0 cm s⁻¹ with the lowest values over water surfaces and the highest over forested areas. For HNO₃, the monthly averaged dry deposition velocities have the range of 0.5 to 6 cm s⁻¹, with the highest values for forested areas. For SO²⁻₄, they range from 0.05–1.5 cm s⁻¹, with the lowest values over water and the highest over forest. The monthly averaged dry deposition velocities for SO₂ and O₃ are higher in the growing season compared to the fall, but this behaviour is not apparent for HNO₃ and sulphate. In the daytime, the hourly averaged dry deposition velocities for SO₂, O₃, SO²⁻₄ and HNO₃ are higher than that in the nighttime over most of the vegetated area. The diurnal variation is most evident for surfaces with large values for leaf area index (LAI), such as forests. Based on the results presented in this paper, it is concluded that RDM V_d values can be combined with measured air concentrations over hourly, daily or weekly periods to determine dry deposition amounts and with wet deposition measurements to provide seasonal estimates of total deposition and estimates of the relative importance of dry deposition.     

The contribution of dry deposited ammonia and sulphur dioxide to the composition of precipitation from continuously open gauges

A series of experiments using bulk precipitation collectors of the type used in the UK precipitation chemistry network measured the amounts of NH₄⁺, SO₄²⁻ and other ions that could be washed from funnels (diameter 15 cm) exposed to a wide range of NH₃ and SO₂ concentrations over periods from hours to days. In dry conditions, the average deposition flux of NH₃ was between 50 and 120 nmol NH₄⁺ funnel⁻¹ d⁻¹ (0.1–0.3 kg N ha⁻¹ yr⁻¹), and was independent of the concentration of NH₃. Dry deposition of NH₃ to wet funnels at small NH₃ concentrations was almost 5 times that to dry funnels under the same conditions (average 240 nmol funnel⁻¹ d⁻¹; 0.7 kg ha⁻¹ yr⁻¹), and increased with increasing NH₃ concentrations. The amount of NH₄⁺ ions remaining on the funnel surface was inversely proportional to the vapour pressure deficit during the experiment. This result was interpreted as a dependence on the duration of surface wetness, with greater deposition of NH₄⁺ when evaporation rates of surface water were small.The amount of SO₂ deposited on funnel surfaces was closely related to the amount of NH₃ deposited, in both wet and dry conditions, but was not strongly correlated with the SO₂ concentration. At low NH₃ and SO₂ concentrations the average deposition to dry funnels was 70 nmol SO₄²⁻ funnel⁻¹ d⁻¹ (0.5 kg ha⁻¹ yr⁻¹), and to wet funnels was approximately 2.5 times larger. The results are interpreted in terms of the balance between the rate of evaporation of surface water, and the rate of oxidation of SO₂, which leads to the ‘fixing’ of NH₄⁺ ions on the surface as involatile salts.It is predicted that dry deposition of NH₃ to funnel surfaces across the UK Secondary Network could account for as much as one-half of the measured bulk wet deposition at sites where wet deposition of NH₄–N is small. The amount of dry deposition depends on how long and how often funnel surfaces are wetted by rain or dew, and on the air concentrations of NH₃. These predictions are based on funnels being wetted only once per day. More frequent wetting would increase the contribution from dry deposition, and the consequent overestimate of wet deposition of NH₄–N across the UK by using data obtained from bulk collectors. To some extent this overestimate may be offset by microbial degradation and loss of NH₄–N in weekly bulk precipitation samples during collection and storage.     

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.     

Severity of SO2-lnduced Leaf Necrosis on Caribbean,Scots, and Virginia Pine Seedlings

Caribbean pine, an economically important tree of tropical lowlands, is at risk of SO₂ exposure in certain locales. Twenty-week old seedlings of Caribbean, Scots, and Virginia pine were exposed to 0.5, 1.0, and 2.0 ppm SO₂ (1300, 2600, and 5200 μm^?3, respectively) for 1, 2, 4, and 8 h in modified controlled-environment chambers. Severity of SO₂-induced leaf necrosis for each species was related to SO₂ concentration and exposure duration using a regression model. The three dose-response relationships differed in detail, but Caribbean pine seedlings were generally as sensitive to SO₂ as seedlings of the two highly sensitive temperate species. In addition, 173 4-wk-old Caribbean pine seedlings were exposed to 0.5 ppm SO₂ for 4 h. Over one-half of these seedlings exhibited some necrosis and over one-sixth had more than 5 percent of leaf surface necrotic. It is concluded that Caribbean pine seedlings are highly sensitive to acute doses of SO₂.