Measurement of dry deposition of ammonia on a forest

Ammonia concentration gradients above a Douglas fir canopy were measured from 16 August to 31 December 1989 by two automated high-precision thermodenuders at the location Speulderbos in The Netherlands. Concentration gradients were used to calculate the dry deposition flux of ammonia via flux-gradient theory. Meteorological data were obtained from a nearby tower. Ammonia concentrations were highly variable with highest values during the night. Concentration gradients were very small during daytime and quite large at night. Median values of the calculated deposition flux and deposition velocity were 0.1 microg m(-2) s(-1) and 3.2 cm s(-1) respectively (N=1624).     

A laboratory investigation of the routes of HNO3 dry deposition to coniferous seedlings

White pine, Norway spruce and red spruce seedlings were exposed to nitric acid vapor concentrations of 10 to 120 ppb in constant stirred tank reactors. Nitric acid dry deposition rates were determined from both the change in nitric acid concentration in the reactor flow stream and from the amount of nitrogen recovered from the seedlings. Nitric acid labeled with 15N was used to distinguish dry-deposited nitrogen in the plant from the nitrogen that was already present. It was found that dry deposition occurs via three routes: surface deposition, trans-cuticular deposition, and stomatal deposition. Resistance to surface deposition is very low (< 4.8 m2-s mol(-1)) for a freshly washed surface, but increases as the surface adsorption sites are occupied. Resistance to trans-cuticular uptake averaged 206 m2-s mol(-1). Stomatal resistance can be calculated from the rate of water diffusion out of the plant. Eighty per cent of the nitric acid deposited via the trans-cuticular and stomatal routes was assimilated by the plant. However, none of the nitric acid deposited on the surface was assimilated. In rural areas with coniferous forests, the combination of low ambient nitric acid concentrations and low initial surface resistance means that most nitric acid will be dry deposited on the tree surface, and thus will not be directly assimilated.     

Relation between estimated dry deposition and throughfall in a coniferous forest exposed to controlled levels of SO2 and NO2

Throughfall was collected in a Scots pine forest exposed to about 14 microg m(-3) of both SO2 and NO2, and in a control forest with 1 microg m(-3) SO2 and < 1 microg m(-3) NO2. Precipitation was collected in a nearby open field. Collection was performed on an event basis during the whole vegetation period. Exposure was made by an open-air release system during the vegetation period, except during rain and at night. Additional sulfate deposition in the exposed forest (compared to control forest) was nearly equal to dry deposition of sulfur dioxide, as estimated with a stomatal conductance model adapted for the particular forest. It is thus concluded that essentially all of the dry deposited sulfur dioxide is eventually extracted and appears in throughfall-including the fraction that has been deposited through stomata. Attempts to relate net throughfall deposition to dry deposition of sulfate in the control forest were inconclusive, since a minor (10%) uncertainty in the water balance had a major influence on calculated deposition velocity for particulate sulfate. Nitrate throughfall deposition is about half of the open field wet deposition, both for the exposed and control forest. Thus, a long-term exposure with about 14 microg m(-3) NO2 decreased nitrate throughfall deposition.     

Soil solution nitrogen and cations influenced by (NH4)2SO4 deposition in a coniferous forest

The effects of chronically enhanced (NH(4))(2)SO(4) deposition on ion concentrations in soil solution and ionic fluxes were investigated in a Picea abies plot at Grizedale forest, NW England. Soil cores closed at the base and containing a ceramic suction cup sampler were 'roofed' and watered every 2 weeks with bulk throughfall collected in the field. Treatments consisted of the inclusion of living roots from mature trees in the lysimeters and increasing (NH(4))(2)SO(4) deposition (NS treatment) to ambient + 75 kg N ha(-1) a(-1). Rainfall, throughfall and soil solutions were collected every 2 weeks during 18 months, and analysed for major cations and anions. NO(3)(-) fluxes significantly increased following NS treatment, and were balanced by increased Al(3+) losses. Increased SO(4)(2-) concentrations played a minor role in controlling soil solution cation concentrations. The soil exchange complex was dominated by Al and, during the experimental period, cores of all treatments 'switched' from Ca(2+) to Al(3+) leaching, leading to mean [Formula: see text] molar ratios in soil solution of NS treated cores of 0.24. The experiment confirmed that the most sensitive soils to acidification (through deposition or changing environmental conditions) are those with low base saturation, and with a pH in the lower Ca, or Al buffer ranges.     

Measurement and modeling of the dry deposition of peroxides

easurements of the dry deposition velocity (V_d) of hydrogen peroxide (H₂O₂) and total organic peroxides (ROOH) were made during four experiments at three forested sites. Details and uncertainties associated with the measurement of peroxide fluxes by the flux-gradient method are discussed. The results are compared to those predicted using a bulk-resistance model of the type commonly used in regional photochemical models. Good agreement between the H₂O₂ V_d measurements and a bulk resistance model is obtained when the model contains a zero surface resistance (R_c) and a common form for the laminar leaf-layer resistance (R_b) based on Schmidt and Prandtl numbers. In this case, a near-zero (<5 s m^-1) surface resistance is confirmed for H₂O₂ within experimental uncertainties. Surface resistances for ROOH were determined to be about 10–15 s m^-1 over a coniferous forest and 20–40 s m^-1 over a deciduous forest. Higher uncertainties for ROOH prevent a detailed analysis of the differences in R_c among forest types. However, the ratio of deposition velocities (ROOH/H₂O₂), computed from normalized concentration gradients, ranged from 0.28 to 0.61 (geometric mean) at the three sites. Differences in molecular diffusivities between H₂O₂ and ROOH can only account for an estimated 16% difference in V_d. Thus, the major constituent of ROOH must also be less soluble and/or less reactive than H₂O₂, which is consistent with the characteristics of methylhydroperoxide (MHP).     

Modelling changes in forest soil chemistry at 16 Swedish coniferous forest sites following deposition reduction

The dynamic forest ecosystem model ForSAFE was applied at 16 coniferous forest sites in Sweden to investigate past and future changes in soil chemistry following changes in atmospheric deposition. The simulation shows a considerable historical soil acidification. Acidification in the southwest, where deposition has been greatest, was more expressed in the deepest soil layers, while it was more evenly distributed through the soil profile in central Sweden, and was greater in the upper soil layers in the north. The simulation also shows that a slight recovery took place after the reduction in emissions, but was counteracted by the effect of harvesting. The simulation predicts an increase in the number of acidified sites in the future. The results also suggest that future acidification will be mainly due to the enhanced tree growth resulting from the chronic high deposition of nitrogen and the removal of soil base cations through harvesting.     

Measurement of dry deposition to surfaces in deciduous and pine canopies

The importance of dry deposition was assessed at perimeter and interior locations in two vegetative canopies. Dry deposition was measured directly by washing particles from leaves. Ambient particles and gases were also collected at both locations within the canopies. Ambient concentrations on the canopy interior were decreased relative to perimeter concentrations due to dry deposition scavenging by the canopy. The least scavenging was found for SO(4)(2-) and NH(4)(+) and the highest scavenging was found for HNO(3). Dry deposition of all species was higher to perimeter vegetative and surrogate surfaces than to interior surfaces, due both to the lower concentrations and the lower wind speeds in the sheltered interior. Deposition velocities compared well with other experimental and theoretical values.     

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.     

Observation of SO2 dry deposition velocity at a high elevation flux tower over an evergreen broadleaf forest in Central Taiwan

A 60-m flux tower was built on a 2100 m mountain for the measurement of the air pollutant concentration and the evaluation of dry deposition velocity in Central Taiwan. The tower was constructed in an evergreen broadleaf forest, which is the dominant species of forest in the world. Multiple-level SO₂ concentrations and meteorological variables at the site were measured from February to April 2008. The results showed that the mean dry deposition velocities of SO₂ were 0.61 cm s^?1 during daytime and 0.27 cm s^?1 during nighttime. From the comparison of the monthly data, a tendency was observed that the dry deposition velocity increases with LAI and solar radiation. Furthermore, it was observed that the deposition velocity was larger over wet canopy than over dry canopy, and that higher deposition velocities in the wet season were mainly caused by non-stomatal uptake of wet canopy. Over wet canopy, the mean dry deposition velocities of SO₂ were estimated to be 0.83 cm s^?1 during daytime and 0.47 cm s^?1 during nighttime; and 0.44 cm s^?1 during daytime and 0.19 cm s^?1 during nighttime over dry canopy. There is good agreement between the results of this study and those in other studies and the predictions of Zhang et al. (2003a). The medians (geometric means) of derived r_c during daytime are 233 (266) m s^?1 over dry canopy and 147 (146) m s^?1 over wet canopy. It was found that solar radiation is the critical important meteorological variable determining stomatal resistance during daytime. For non-stomatal resistance, clear dependencies were observed on the friction velocity and relative humidity.     

Input of atmospheric sulfur by dry and wet deposition to two central European forest ecosystems

The total input of atmospheric sulfur to a beech and a spruce forest in the Federal Republic of Germany has been measured over a period of 6 years. The contribution of dry deposition to the total input was determined indirectly by comparing seasonal changes in the sulfur flux coupled with precipitation beneath the canopy of the deciduous beech forest. As a result of these investigations seasonal and annual sulfur fluxes are reported corresponding to removal rates of atmospheric sulfur. The experimental data show clearly that the removal rates depend upon the quality of the atmosphere/ land interface, in forested areas from the tree species forming the canopy. The 6-years average of total deposition on bare soil is 23kg S ha⁻¹ y⁻¹, on a beech forest 47–51 kg S ha⁻¹y⁻¹, on a spruce forest 80–86 kg S ha⁻¹ y⁻¹. Based upon the experimental results the role of the forest vegetation in the removal of sulfur from the atmosphere in the area of the Federal Republic of Germany is considered. The figures indicate, that at least 50% of the total sulfur deposition takes place on forested areas which cover only 28% of the total land surface.     

Estimation of NH3 bi-directional flux from managed agricultural soils

The Community Multi-Scale Air Quality model (CMAQ) is used to assess regional air quality conditions for a wide range of chemical species throughout the United States (U.S.). CMAQ representation of the regional nitrogen budget is limited by its treatment of ammonia (NH₃) soil emission from, and deposition to underlying surfaces as independent rather than tightly coupled processes, and by its reliance on soil emission estimates that do not respond to variable meteorology and ambient chemical conditions. The present study identifies an approach that addresses these limitations, lends itself to regional application, and will better position CMAQ to meet future assessment challenges. These goals were met through the integration of the resistance-based flux model of Nemitz et al. (2001) with elements of the United States Department of Agriculture EPIC (Environmental Policy Integrated Climate) model. Model integration centers on the estimation of ammonium and hydrogen ion concentrations in the soil required to estimate soil NH₃ flux. The EPIC model was calibrated using data collected during an intensive 2007 field study in Lillington, North Carolina. A simplified process model based on the nitrification portion of EPIC was developed and evaluated. It was then combined with the Nemitz et al. (2001) model and measurements of near-surface NH₃ concentrations to simulate soil NH₃ flux at the field site. Finally, the integrated flux (emission) results were scaled upward and compared to recent national ammonia emission inventory estimates. The integrated model results are shown to be more temporally resolved (daily), while maintaining good agreement with established soil emission estimates at longer time-scales (monthly). Although results are presented for a single field study, the process-based nature of this approach and NEI comparison suggest that inclusion of this flux model in a regional application should produce useful assessment results if nationally consistent sources of driving soil and agricultural management information are identified.     

Promoting the O3 flux concept for European forest trees

Tropospheric ozone (O3) levels are predicted to stay high, being a factor within "global change" with potential effects on the carbon sink strength of forest trees. Hence, new approaches to O3 risk assessment and their validation are required, although appropriate databases for adult trees are scant. Approaches based on external O3 exposure are presently being evaluated against the ones on O3 flux into leaves, as the cumulative uptake has the capacity for deriving O3 risk from cause-effect relationships. The effective dose, however, needs to account for the trees' O3 defence and tolerance in addition to O3 uptake. The current status of promoting the preferable mechanistic O3 flux concept is highlighted for major regions of Europe, addressing refinements and simplifications needed for routine use. At the pan-European scale, however, the flux-based concept is ready for use in O3 risk assessment and has the potential of meso-scale application at the forest ecosystem level.     

Concentration-dependent NH3 deposition processes for mixed moorland semi-natural vegetation

Dry deposition modelling typically assumes that canopy resistance (R_c) is independent of ammonia (NH₃) concentration. An innovative flux chamber system was used to provide accurate continuous measurements of NH₃ deposition to a moorland composed of a mixture of Calluna vulgaris (L.) Hull, Eriophorum vaginatum L. and Sphagnum spp. Ammonia was applied at a wide range of concentrations (1–100 μg m⁻³). The physical and environmental properties and the testing of the chamber are described, as well as results for the moorland vegetation using the ‘canopy resistance’ and ‘canopy compensation point’ interpretations of the data.Results for moorland plant species demonstrate that NH₃ concentration directly affects the rate of NH₃ deposition to the vegetation canopy, with R_c and cuticular resistance (R_w) increasing with increasing NH₃ concentrations. Differences in R_c were found between night and day: during the night R_c increases from 17 s m⁻¹ at 10 μg m⁻³ to 95 s m⁻¹ at 80 μg m⁻³, whereas during the day R_c increases from 17 s m⁻¹ at 10 μg m⁻³ to 48 s m⁻¹ at 80 μg m⁻³. The lower resistance during the day is caused by the stomata being open and available as a deposition route to the plant. R_w increased with increasing NH₃ concentrations and was not significantly different between day and night (at 80 μg m⁻³ NH₃ day R_w=88 s m⁻¹ and night R_w=95 s m⁻¹). The results demonstrate that assessments using fixed R_c will over-estimate NH₃ deposition at high concentrations (over ∼15 μg m⁻³).     

A theoretical evaluation on the HNO3 artifact of the annular denuder system due to evaporation and diffusional deposition of NH4NO3-containing aerosols

A mathematical model was developed to evaluate HNO₃ artifact of the annular denuder system due to evaporation and diffusional deposition of nitrate-containing aerosols. The model performance was validated by comparing its numerical solutions with laboratory and numerical data available in the literature for evaporation and diffusional deposition of monodisperse and polydisperse NH₄NO₃ aerosols. Measurement artifacts were evaluated by varying typical sampling ranges of ambient temperature, HNO₃ gas concentration, aerosol number concentration, aerosol mass median diameter, and nitrate mass fraction of <2.5 μm aerosols to see their respective effects. Potential application of the present model on estimating HNO₃ artifacts was demonstrated using literature data sampled in USA, Taiwan, Netherlands, Korea and Japan. Significant measurement artifact could be found in Taiwan and Netherlands due either to low HNO₃ gas concentration and high nitrate concentration in <2.5 μm aerosols or to high ambient temperature.     

The sensitivity of a 3-dimensional Eulerian model to uncertainties in emissions and dry deposition velocities

A 3-dimensional STEM-I Eulerian, regional scale transport/chemistry model for SO₂ and sulfate is used to investigate the relationships between model predictions and the resolution and uncertainty in the emissions and dry deposition velocities. Model results for conditions of 4 July 1974 where the emissions are time modulated, where the emissions are increased and decreased, and where the dry deposition velocities are increased and decreased are presented and discussed.     

Atmospheric deposition to the edge of a spruce forest in Denmark

Atmospheric deposition was measured during 1 year at the forest edge of a Norway spruce stand in Denmark. Inside the forest the deposition of H(+), Ca(2+), Mg(2+), Na(+), K(+), Cl(-), NO(3)(-), NH(4)(2) and SO(4)(2-) with canopy throughfall varies with the distance from the forest edge. The deposition at the edge is found to be 10-20 times as high as deposition to an open field and 2-8 times as high as deposition inside the stand. An exponential decrease in deposition as a function of the distance from the forest edge is found. Increased deposition of K(+) and non-sea salt Mg(2+), which mainly originates as a result of leaching from the needles may be explained by a larger leaf area index (LAI) at the forest edge. Deposition of particulate substances, especially Na(+), Cl(-), Mg(2+) and to some extent SO(4)(2-), NH(4)(+) and NO(3)(-) is increased much more than the LAI, which we believe to be caused by changes in wind movements at the forest edge.     

Numerical modeling of dry deposition coupled to 22 photochemical reactions

Three Eulerian models for the dry deposition of photochemically reactive species were formulated and evaluated: a K-theory model with independent transport and deposition of each species, a K-theory model coupled with 22 gas-phase reactions, and a second-order flux-budget model coupled with 22 reactions. Operator splitting was used to separately solve the reaction and dispersion terms in the models including photochemistry. In an evaluation of numerical method performance, the Adams–Moulton method with a pseudo-steady-state approximation for the free radicals was found to be consistent with, but more computationally efficient than Gear’s method for the solution of the reaction terms. The sensitivity of profiles of vertical concentration and flux to the Eulerian model formulation varied according to the species’ Damköhler number. Although a K-theory model is adequate for weakly depositing species with Damköhler numbers <10^-3, a second-order flux-budget model is required for species with Damköhler numbers that exceed unity, such as nitrogen oxides, free radicals, and those sensitive to net flux production by chemical reactions. Selection of an appropriate model formulation for the entire system depended on the most reactive species. Simple K-theory models may not accurately predict dry deposition fluxes in the urban surface layer.     

Concentration-dependent NH3 deposition processes for moorland plant species with and without stomata

Currently, in operational modelling of NH₃ deposition a fixed value of canopy resistance (R_c) is generally applied, irrespective of the plant species and NH₃ concentration. This study determined the effect of NH₃ concentration on deposition processes to individual moorland species. An innovative flux chamber system was used to provide accurate continuous measurements of NH₃ deposition to Deschampsia cespitosa (L.) Beauv., Calluna vulgaris (L.) Hull, Eriophorum vaginatum L., Cladonia spp., Sphagnum spp., and Pleurozium schreberi (Brid.) Mitt. Measurements were conducted across a wide range of NH₃ concentrations (1–140 μg m⁻³).NH₃ concentration directly affects the deposition processes to the vegetation canopy, with R_c, and cuticular resistance (R_w) increasing with increasing NH₃ concentration, for all the species and vegetation communities tested. For example, the R_c for C. vulgaris increased from 14 s m⁻¹ at 2 μg m⁻³ to 112 s m⁻¹ at 80 μg m⁻³. Diurnal variations in NH₃ uptake were observed for higher plants, due to stomatal uptake; however, no diurnal variations were shown for non-stomatal plants. R_c for C. vulgaris at 80 μg m⁻³ was 66 and 112 s m⁻¹ during day and night, respectively. Differences were found in NH₃ deposition between plant species and vegetation communities: Sphagnum had the lowest R_c (3 s m⁻¹ at 2 μg m⁻³ to 23 at 80 μg m⁻³), and D. cespitosa had the highest nighttime value (18 s m⁻¹ at 2 μg m⁻³ to 197 s m⁻¹ at 80 μg m⁻³).