Wintertime wet and dry deposition in northern Michigan

Wet and dry deposition were monitored at the University of Michigan Biological Station in rural northern Michigan for three winters. Dry deposition was measured by both the conventional bucket method and by measuring increases in concentration in exposed, elevated snow samples. Average results of the two methods were in reasonable agreement. The cumulative wet and dry deposition quantities are in good agreement with snowpack accumulations until the first thaw period. Dry deposition to snow accounts for less than 15% of the total H⁺, SO²⁻₄, NO⁻₃ and NH⁺₄ and approximately 25% of the Ca ²⁺, Mg ²⁺, Na⁺, K⁺ and Cl⁻ during an average precipitation year. Snowpack measurements were also made under deciduous and red pine canopies. Decreases in H⁺ and NO⁻₃ were observed under the red pine canopy.     

Determination of PM₁₀ deposition based on antimony flux to selected urban surfaces

Deposition of PM(10) particles to several types of urban surfaces was investigated within this study. Antimony was chosen as a tracer element to calculate dry deposition velocities for PM(10), since antimony proved to be present almost exclusively in PM(10) particles in ambient urban air. During 18 months, eight sampling sites in Berlin and Karlsruhe, two cities in Germany, were operated. PM(10) concentrations and dry deposition were routinely sampled as two week averages. Additionally, leaf-samples were collected at three sites with tall vegetation. The obtained deposition velocities ranged from 0.8 to 1.3 cms(-1) at roadside sites and from 0.4 to 0.5 cms(-1) at the other sites. With reference to the whole canopy, additional deposition velocities of about 0.5 cms(-1) were obtained for leaf surfaces. As a consequence, it can be concluded that vegetation-covered areas beside streets show the highest potential to capture particles in urban areas.     

Preliminary data from the USEPA dry deposition network: 1989

The objective of the National Dry Deposition Network is to determine patterns and trends of dry deposition for various sulfur and nitrogen species at roughly 50 locations throughout the continental USA. Each site is equipped for collection of continuous meteorological and ozone data and weekly average concentrations of SO4(2-), NO3-, SO2 and HNO3, using a three-stage filter pack. Results from 40 eastern US sites operational throughout 1989 show species-dependent variability from site to site, season to season, and day to night. Annual average concentrations of atmospheric SO4(2-), NO3-, SO2 and HNO3 ranged from 2.7 to 7.9, 0.2 to 3.9, 2.4 to 23.2 and 0.7 to 3.6 microg/m(-3), respectively. Seasonal variability was considerable for all constituents. Day/night data indicate that SO2 and HNO3, but not SO4(2-) and NO3-, are typically found at moderately to substantially lower concentrations at night, especially during spring and summer. Estimated dry deposition for SO2 and HNO3 appear to be much greater than for SO4(2-) and NO3-, respectively. Comparison of measured wet deposition and estimated dry deposition at numerous sites suggests that the two are similar in magnitude over much of the eastern USA.     

Apply appropriate models in the study of dry deposition fluxes of particulate matter and polycyclic aromatic hydrocarbons (PAHs) in Tsukuba, Japan

In this study, plates for downward flux and upward flux were used to measure atmospheric dry deposition fluxes for particulate mass and polycyclic aromatic hydrocarbons (PAHs) in TERC (Tsukuba), Japan. Ambient particles concentrations were also collected using a high-volume air sampler, and ambient particle size distributions between 0.01 μm and 13.1 μm were measured using a low-pressure cascade impactor to characterise the PAHs levels and dry deposition. The results indicated that the average cumulative fraction of dry deposition flux for particles and PAHs which attached with them was caused by the particle size of greater than 1.2-6.3 μm (97%).     

Spatial variability of throughfall fluxes in a spruce forest

The spatial variability of throughfall deposition of H(+), Ca(2+), Mg(2+), Na(+), K(+), Cl(-), NO(3)(-), NH(4)(+), O(4)(2-) to a Norway spruce (Picea abies (L.) Karst.) forest was intensively examined during the period October 1986 to October 1987. Large systematic spatial variability of the atmospheric deposition within the forest was observed. The flux of throughfall water was higher away from the trunk compared to the flux close to the trunk. In contrast to this, the deposition of all substances was considerably higher close to the trunk compared to the deposition at the periphery of the canopy. A linear decrease in deposition as a function of the distance from the nearest tree trunk was found. Further, the deposition varied quite dramatically between trees according to their size. The observed spatial variability in throughfall may be due to variabilities in the processes taking part in altering the distribution and composition of the precipitated water as it moves through the canopy. The influence of these processes of precipitation, wash-off, dry deposition and canopy exchange is discussed, and it is found that both increased dry deposition and canopy exchange in the tree tops contribute to the higher solute fluxes found close to the tree trunk.     

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.     

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.     

Elevational patterns of sulfur deposition at a site in the Catskill Mountains,New York

In this paper we report measurements of SO^2-₄ fluxes in throughfall and bulk deposition across an elevational transect from 800 to 1275 m on Slide Mountain in the Catskill Mountains of southeastern New York State. The net throughfall flux of SO^2-₄ (throughfall-bulk deposition), which we attribute to cloud and dry deposition, increased by roughly a factor of 13 across this elevational range. Part of the observed increase results from the year-round exposure of evergreen foliage at the high-elevation sites, compared to the lack of foliage in the dormant season in the deciduous canopies at low elevations. Comparison of the net throughfall flux with estimates of cloud deposition suggests that both cloud deposition and dry deposition increased with elevation. Dry deposition estimates from a nearby monitoring site fall within the measured range of net throughfall flux for SO^2-₄. The between-site variation in net throughfall flux was very high at the high-elevation sites, and less so at the lower sites, suggesting that studies of atmospheric deposition at high-elevations will be complicated by extreme spatial variability in deposition rates. Studies of atmospheric deposition in mountainous areas of the eastern U.S. have often emphasized cloud water deposition, but these results suggest that elevational increases in dry deposition may also be important.     

Atmospheric dry deposition to leaf surfaces at a rural site of India

Dry deposition flux of major ions (Na+, K+, Ca2+, Mg2+, NH4+, F-, Cl-, NO3- and SO4(2-) to natural surfaces [guava (Psidium guyava) and peepal (Ficus religiosa) leaves] are determined at Rampur, a rural site of semi-arid region of India. Dry deposition flux is the highest for Ca2+ on guava leaves and for NH4+ on peepal leaves. Overall dry deposition flux is higher on guava leaves than of peepal leaves. The variation in deposition flux may be due to surface characteristics (surface roughness) and arrangement of leaves. Peepal leaves are arranged along the axis of the stem, whereas guava leaves are at right angles to the stem. The deposition flux of cations contributes 66% and 76% of dry deposition of all major ions on guava and peepal leaves, respectively as soil is major contributor towards dry deposition flux in tropical regions. ANOVA revealed no significant seasonal difference in deposition, although there is a trend for higher in winter. Deposition velocities of NH4+, NO3- and SO4(2-) are greater on guava leaves than peepal leaves, which can be attributed to the rougher surface of the guava leaf.     

Particle-bound PAH content in ambient air

Ambient air samples from a traffic intersection, an urban site and a petrochemical-industrial site (PCI) were collected by using several dry deposition plates, two Microorifice uniform deposited impactors (MOUDIs), one Noll Rotary Impactor (NRI) and several PS-1 (General Metal Work) samplers from March 1994 to June 1995 in southern Taiwan, to characterize the atmospheric particle-bound PAH content of these three areas. Twenty-one individual polycyclic aromatic hydrocarbons (PAHs) were analyzed primarily by using a gas chromatograph/mass spectrometer (GC/MS). In general, the sub-micron particles have a higher PAH content. This is due to the fact that soot from combustion sources consists primarily of fine particles and has a high PAH content. In addition, a smaller particle has a higher specific surface area and therefore may contain more organic carbon, which allows for more PAH adsorption. For a particle size range between 0.31 and 3.2 microm, both Urban/Traffic and PCI/Traffic ratios of particle-bound total-PAH content have the lowest values, ranging from 0.25 to 0.28 (mean = 0.26) and from 0.07 to 0.13 (mean = 0.10), respectively. This indicates that, during the accumulation process, the PAH mass shifted from a particle phase to a gas phase, or the particles aggregated with lower PAH-content particles, resulting in a reduction in particle-bound PAH content. By using the particle size distribution data, the dry deposition model in this study can provide a good prediction for the PAH content of dry deposition materials. In general, lower molecular weight PAHs had a larger fraction of dry deposition flux contributed by the gas phase; for 2-ring PAH (50.4, 46.3 and 28.4%), 3-ring PAHs (15.2, 15.4 and 11.7%) and 4-ring PAHs (13.0, 3.60 and 5.01%) for the traffic intersection, urban and PCI sites, respectively. For higher molecular weight PAHs-5-ring, 6-ring and 7-ring PAHs-their cumulation fraction (F%) of dry deposition flux contributed by the gas phase was lower than 3.26%. At the traffic intersection, urban and PCI sites, the mass median diameter of dry deposition materials (MMD(F)) of individual PAHs was between 25.3 and 49.6 microm, between 27.6 and 43.9 microm, and between 19.1 and 41.9 microm, respectively. This is due to the fact that PAH dry-deposition primarily resulted from gravitational settling of the coarse particulates (> 10 microm).     

Ambient concentrations and dry deposition fluxes of trace elements in Izmir,Turkey

Dry deposition samples were collected using a smooth surrogate surface at the Kaynaklar Campus of the Dokuz Eylul University in Izmir, Turkey. Concurrently ambient aerosol samples were collected. All samples were analyzed for anthropogenic and crustal trace elements. The average trace element concentrations and fluxes measured in this study were generally higher than those reported previously for urban and rural areas. The contribution of local terrestrial and anthropogenic sources were also investigated using enrichment factors (EFs) calculated relative to the local soil. Relatively lower EFs for ambient samples and high ambient concentrations indicated that the local soil was polluted and contributed significantly to ambient trace element concentrations. Deposition samples had higher EFs than the air samples. The EF sequences of trace elements were also different for deposition and ambient samples, probably due to the fact they have different mass median diameters and deposition velocities. The overall dry deposition velocities for trace elements calculated by dividing the particulate fluxes measured with the surrogate surfaces by ambient concentrations ranged from 0.6 (Al) to 6.2 cm s⁻¹ (Fe). The agreement between the experimental dry deposition velocities determined in this study and the previously reported ones using similar techniques for trace elements was good.     

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 theoretical assessment of pollutant deposition to individual land types during a regional-scale acid deposition episode

A mesoscale model of pollutant transport, transformation and deposition was used to perform a detailed analysis of acidic deposition to the states of New York and Ohio during a 3-day springtime deposition episode. This model can be used to assess the roles of wet and dry deposition to individual land types in the removal of pollutants from the atmosphere. Over two-thirds (67 %, Ohio; 78 %, New York) of the acidic deposition during this rainy period fell as wet deposition, primarily in the form of H₂SO₄. Dry deposition of SO₂ accounted for 70–75 % of the total dry acidic deposition in both areas, and most of the remaining dry deposition occurred as HNO₃. Over both deposition areas, particulate sulfate deposition accounted for <1 % of the total acid deposition. Due to the highly surface-specific nature of the dry deposition process, individual land types displayed unique patterns of pollutant uptake. Water surfaces absorbed primarily SO₂, while rougher forested areas absorbed a larger proportion of HNO₃ vapor. Urban areas, with their associated material surfaces, were found to absorb significantly less acid in the dry form, and during dry periods most of this deposition may occur as HNO₃ vapor, although considerable uncertainty exists regarding the treatment of rainfall-wetted surfaces. These model results suggest that dry pollutant fluxes to individual surface types will show significant variability from any ‘averaged’ flux estimates over larger areas encompassing numerous land types.     

Urban scale modeling of particle number concentration in Stockholm

A three-dimensional dispersion model has been implemented over the urban area of Stockholm (35×35 km) to assess the spatial distribution of number concentrations of particles in the diameter range 3–400 nm. Typical number concentrations in the urban background of Stockholm is 10 000 cm⁻³, while they are three times higher close to a major highway outside the city and seven times higher within a densely trafficked street canyon site in the city center. The model, which includes an aerosol module for calculating the particle number losses due to coagulation and dry deposition, has been run for a 10-day period. Model results compare well with measured data, both in levels and in temporal variability. Coagulation was found to be of little importance in terms of time averaged concentrations, contributing to losses of only a few percent as compared to inert particles, while dry deposition yield particle number losses of up to 25% in certain locations. Episodic losses of up to 10% due to coagulation and 50% due to deposition, are found some kilometers downwind of major roads, rising in connection with low wind speed and suppressed turbulent mixing. Removal due to coagulation and deposition will thus be more significant for the simulation of extreme particle number concentrations during peak episodes.The study shows that dispersion models with proper aerosol dynamics included may be used to assess particle number concentrations in Stockholm, where ultrafine particles principally originate from traffic emissions. Emission factors may be determined from roadside measurements, but ambient temperature must be considered, as it has a strong influence on particle number emissions from vehicles.     

Vertical gradients of ozone and carbon dioxide within a deciduous forest in Central Pennsylvania

Ambient concentrations of ozone (O(3)) and carbon dioxide (CO(2)) were measured at locations from the forest floor to the top of the canopy in a deciduous forest at the Moshannon State Forest in northcentral Pennsylvania. O(3) concentrations were measured from May-September for three years (1993-1995) while CO(2) concentrations were measured only during July and August of 1994. O(3) concentrations increased steadily during the day at all locations, peaking during the middle to late afternoon hours. O(3) concentrations then steadily declined to their lowest point, just before dawn. Vertical O(3) concentration gradients varied seasonally and among years. However, O(3) concentrations were highest within the forest canopy and lowest at the forest floor, with an average difference of approximately 13%. Differences in O(3) concentrations between the canopy and forest floor were greatest at night. O(3) concentrations were slightly higher at locations within the canopy than above the canopy. CO(2) concentrations were consistenly higher near the forest floor and were higher above the canopy than within the canopy. CO(2) concentrations were higher at night than during the day at all locations, especially near the forest floor.     

Dry deposition of 4–50 μm dolomite particles on vegetation,flat surfaces and deposition gauges

Dry deposition of dolomite particles from a small plume onto various kinds of surfaces was studied. The concentration was measured in the air via isokinetic sampling on filters. The deposition and size distribution of the deposited particles were determined with a sedimentation scale. The results show significant variations in size distribution of deposited material and deposition velocity with wind velocity, particle size and type of surface. Clear indications of resuspension and ‘bounce-off’ were found.     

Dry deposition of O3, SO2, and HNO3 to different vegetation in the same exposure environment

Agricultural meteorological modeling techniques are used to investigate the relative and absolute dry deposition fluxes of SO2 (as sulfur), HNO3 (as nitrogen) and O3 to large fields of maize, soybeans, and alfalfa exposed in conditions as measured in northern Illinois, central Pennsylvania, and eastern Tennessee. For HNO3, the differences in seasonal deposition rates among the three types of plant species are small. Within the same environment, the soybean canopy has the potential to receive substantially more gaseous dry deposition of SO2 and O3 than the maize and alfalfa (which are about the same), as a result of lower stomatal resistance and consequently higher deposition velocities. Deposition differences among the sites are small except for the case of SO2, for which deposition rates estimated for northern Illinois are nearly double those at the other locations. The high SO2 deposition at the northern Illinois location is a consequence of the higher air concentrations observed there.     

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.     

Atmospheric deposition and canopy exchange processes in heathland ecosystems

The aims of the present study were to determine canopy exchange processes and to quantify total atmospheric deposition of sulphur and nitrogen in heathland. The study was carried out in dry inland heath vegetation, dominated by Calluna vulgaris, in two nature reserves in the eastern part of the Netherlands. Atmospheric deposition was determined with throughfall-stemflow measurements, adapted for low vegetation. Throughflow measurements (sum of throughfall and stemflow) in artificial Calluna canopies showed co-deposition of SOx and NHy upon heathland vegetation. In the real Calluna canopy, a significant part of the deposited ammonia/ammonium was directly assimilated by the Calluna shoots, especially in wet periods. The concentrations of potassium, calcium and magnesium in throughflow, after passage through the Calluna canopy, increased significantly compared with bulk precipitation. The amount of cations lost from the canopy were in good agreement with the observed ammonium uptake by the Calluna. A field experiment demonstrated that losses of the above-mentioned cations can be doubled by application of ammonium sulphate. It was shown that interception deposition is an important component of the atmospheric deposition of sulphur and nitrogen upon Calluna heathland; bulk precipitation amounted to only c. 35-40% of total atmospheric input. Total atmospheric deposition of sulphur and nitrogen in the investigated heathlands was 1.5-2.1 (27-33 kg S ha(-1) yr(-1)) and 2.1-3.1 kmolc ha(-1) yr(-1) (30-45 kg N ha(-1) yr(-1)), respectively. It is concluded that the present atmospheric nitrogen deposition is a continuous threat for the existence of heathlands in Western Europe.     

Deposition to forests in Europe: most important factors influencing dry deposition and models used for generalisation

Dry deposition of gases and particles to forests is influenced by factors influencing the turbulent transport, such as wind speed, tree height, canopy closure, LAI, etc. as well as by factors influencing surface condition, such as precipitation, relative humidity, global radiation, etc. In this paper, an overview of these factors is given and it is shown which are the most important determining temporal and spatial variation of dry deposition of sodium and sulphur. Furthermore, it is evaluated how well current deposition models are able to describe the temporal and spatial variation in dry deposition. It is concluded that the temporal variation is not modelled well enough, because of limited surface-wetness exchange parameterisations. The influence of forest characteristics are modelled reasonably well, provided enough data describing the forests and the spatial variation in concentration is available. For Europe these data are not available. The means to decrease the atmospheric deposition through forest management is discussed.