Estimates Of Deposition Velocities Research Articles

Evaluation of Deuterium Oxide Deposition Velocity over a Forest Environment.

Field experiments were performed to evaluate the deposition velocity of tritium oxide within a forest environment at the Savannah River Site near Aiken, SC. Field releases were designed to guide selection of deposition velocity values for use in safety-basis modeling. Six releases of deuterium oxide were conducted in 2020 and 2021 with corresponding air samples during and following each release. Samples were analyzed to determine the deuterium-to-hydrogen ratio in water and converted to concentrations of deuterium in the air during the experiment. Measurements were compared to prior model simulations to evaluate model performance and deposition velocity estimates. Field releases demonstrated vertical and horizontal mixing of a plume in a forest. Predicted deposition velocities ranged from 2.4 to 5.4 cm s -1 on average. In all cases, model simulations underpredicted deuterium concentration by 1 to 2 orders of magnitude, indicating the model does not sufficiently mix the plume into the forest. While the model underestimated the transfer of material downward through the forest, it does suggest that the model's estimates are conservative for making downwind dose estimates because of lower plume depletion, leading to higher concentration and dose estimates. While the field releases do not cover all possible meteorological conditions, we conclude it is appropriate to use a non-zero deposition velocity when performing safety-basis modeling of tritium oxide based on conservatism within the model. A recommendation of 1.0 cm s -1 as a deposition velocity is made, which is beyond the 95 th percentile value estimated from the prior modeling study.

Preliminary Experiments Using a Passive Detector for Measuring Indoor 220Rn Progeny Concentrations with an Aerosol Chamber.

This paper describes preliminary experiments using a passive detector for integrating measurements of indoor thoron (²²⁰Rn) progeny concentrations with an aerosol chamber. A solid state nuclear detector (CR-39) covered with a thin aluminum-vaporized polyethylene plate (Mylar film) was used to detect only alpha particles emitted from ²¹²Po due to ²²⁰Rn progeny deposited on the detector surfaces. The initial experiment showed that Mylar film with area density of more than 5 mg cm⁻² was suitable to cut off completely alpha particles of 7.7 MeV from ²¹⁴Po of ²²²Rn progeny decay. In the experiment using the passive detector, it was observed that the net track density increased linearly with an increase of time-integrating ²²⁰Rn progeny concentration. As a result of dividing deposition rates by atom concentrations, the deposition velocity was given as 0.023 cm s⁻¹ for total ²²⁰Rn progeny. The model estimates of deposition velocities were 0.330 cm s⁻¹ for unattached ²²⁰Rn progeny and 0.0011 cm s⁻¹ for aerosol-attached ²²⁰Rn progeny using Lai-Nazaroff formulae. These deposition velocities were in the same range with the results reported in the literature. It was also found that the exposure experiments showed little influence of vertical profiles and surface orientations of the passive detector in the chamber on the detection responses, which was in good agreement with that in the model estimates. Furthermore, it was inferred that the main uncertainty of the passive detector was inhomogeneous deposition of Rn progeny onto its detection surfaces.

Quality Assurance Decisions with Air Models: A Case Study of Imputation of Missing Input Data Using EPA’s Multi-layer Model

Environmental models are frequently used within regulatory and policy frameworks to estimate environmental metrics that are difficult or impossible to physically measure. As important decision tools, the uncertainty associated with the model outputs should impact their use in informing regulatory decisions and scientific inferences. In this paper, we present a case study illustrating a process for dealing with a key issue in the use and application of air quality models, the additional error in annual mean aggregations resulting from imputation of missing data from model data sets. The case study is based on the US Environmental Protection Agency’s Multi-layer Model, which estimates the hourly dry deposition velocity of air pollutants based on hourly measurements of meteorology and site characteristics. A simulation was implemented to evaluate the effect of substituting historical hour-specific average values for missing model deposition velocity predictions on annual mean aggregations. Sensitivity studies were performed to test the effects of different missing data patterns and evaluate the relative impact of the substitution procedure on annual mean SO2 deposition velocity estimates. The substitution procedure was shown to result generally in long-term unbiased estimates of the annual mean and contributed less than 20% additional error to the estimate even when all data were missing. Consequently, it may be possible to use the historical record of deposition velocities to provide reasonably accurate and unbiased annual estimates of deposition velocities for years without meteorological measurements.

Origin of atmospheric deposition and canopy buffering capacity in stands of radiata pine and pedunculate oak in the Basque Country

Total atmospheric deposition to four mature forest sites in the Basque Country was estimated using the canopy flux method and a generalised deposition model (average deposition velocities multiplied by average concentration of gases in the atmosphere). Potential acidity under aerobic conditions and acidity flux from atmospheric deposition entering the forest ecosystems were also estimated. The sites were close to a number of opencast limestone quarries and were located at different distances from a livestock farm, which was considered to be a source of emission of ammonia. Two forest stands, a Pinus radiata plantation and a semi-natural Quercus robur woodland (“Dur pine” and “Dur oak”, respectively), were selected in the vicinity of the livestock farm where approximately 300 head of cattle are grazed. Another radiata pine plantation and a pedunculate oak plantation at a distance of approximately 15 km from the farm were also selected for study: “Urk pine” and “Urk oak”, respectively. Bulk precipitation and throughfall were sampled and the concentration of different constituents analysed. Concentrations of gases in the atmosphere close to the field stations were provided by Basque Government's Environmental Protection Agency and best estimates of deposition velocities were based on data in the available literature. Calcium was the most abundant chemical species in bulk precipitation and throughfall at every site, whereas sulphate was the most abundant anion in these precipitation fractions. Precipitation was not acidic and the pH values at the “Dur” and the “Urk” site were 5.8 and 5.5, respectively. The results suggest that deposition of calcium carbonate from nearby quarries may be an important factor at both sites. A high proportion of the sodium collected in bulk precipitation at both sites was found to be of non-marine origin. Ammonium deposition in bulk precipitation and throughfall was higher at the “Dur” than at the “Urk” site. Statistical analyses showed that: (a) there exists a strong relationship among base cations (including sodium) at both sites indicating a possible common source for them; (b) co-deposition of SO x and NO y along with NH x occurs at both sites, but chloride was not co-deposited with ammonium, and (c) buffering of total deposition and ammonia uptake in forest canopies may be brought about by cation exchange. Total deposition of NH x and the acidity flux of total deposition were higher at the “Dur” than at the “Urk” site (approximately 30 kg ha −1 year −1 of N.NH x and from 270 to 290 mmolc m −2 year −1, respectively, at the former site and less than 10 kg ha −1 year −1 of N.NH x and from 65 to 85 mmolc m −2 year −1, respectively, at the latter site). At both sites the acidity flux entering the forest soil along with throughfall was higher under pine canopies than under oak (approximately 115% at the “Dur” site and almost 200% at the “Urk” site). Nevertheless, acidity entering the forest ecosystems along with the total deposition was very similar, regardless of the tree dominant species. The estimated nitrogen loads (NO y + NH x ) to the sites were approximately 35 kg ha −1 year −1 at the “Urk” site and 55 kg ha −1 year −1 at the “Dur” site. These values are higher than the critical threshold values, suggesting potential nitrogen saturation of these forest ecosystems.

Ammonia and hydrogen sulphide flux and dry deposition velocity estimates using vertical gradient method at a commercial beef cattle feedlot

Ammonia and hydrogen sulphide flux and dry deposition velocity were estimated using micrometeorological vertical gradient flux method at a commercial cattle feedyard of approximately 50,000 head of beef cattle and average 14.4 m²/head (150 ft²/head) stocking density. Ammonia-N and H2S-S loss had general diurnal patterns with the highest fluxes in daytime and lowest fluxes in nighttime that correlated to temperature changes and active evaporation process during daytime. The highest average deposition velocities also occurred during daytime with unstable atmospheric conditions and the lowest during nighttime with very stable conditions. There are exponential relationship between NH3-N flux and ambient temperature with R² = 0.57 for NH3 (NH3-N flux = – 1.46 + 7.96e0.077*Temperature) and R² = 0.22 for H2S-S (H2S-S flux = – 0.75 + 0.8e-0.013*Temperature).

Dry deposition to forests—On the use of data from clearings

To quantify dry deposition rates to forest canopies normally requires the use of a tower erected so that measurements can be made above the treetops, as well as often within the canopy. However, wet deposition protocols call for deployment of instruments in forest clearings. The question arises as to whether forest clearing measurements can be used to estimate dry deposition rates to the surrounding forest canopy. A trial program was implemented in the 1980s, to explore this possibility. Two nearby sites were set up at Oak Ridge, Tennessee, as part of the Dry Deposition Inferential Method network, now a component of the Atmospheric Integrated Research Monitoring Network (AIRMoN). One site was a tower extending to about 35 m above the floor of a mixed deciduous forest, with trees about 27 m tall. The second site was in the forest clearing used for measurements under the protocols of the National Atmospheric Deposition Program (NADP). The data indicate that clearing concentrations of SO 2, SO 4 2−, and HNO 3 are consistently and significantly different from those above the forest, as measured at the nearby tower. In addition, turbulence data obtained in the clearing do not lead to estimates of deposition velocities like those derived from the tower instrumentation. The data indicate that the ratio F f/ F c (where F f is the dry deposition rate to the forest canopy and F c is that of the same chemical to the clearing) varies with the chemical species of interest, and with the time of year in a manner that suggests a direct linkage with the leaf area index of the canopy. On the average, F f/ F c appears to be slightly above 2.0 for SO 2 and HNO 3, and slightly below 2.0 for SO 4 2−.

Interpretation of aerosol particle fluxes over a pine forest: Dry deposition and random errors

Aerosol particle fluxes were measured above a Scots pine forest in southern Finland by the eddy covariance (EC) technique. Condensational particle counter (CPC) was used in the EC system to detect the total particle number concentration in the size interval from about 10 nm to 500 nm. Turbulent records of velocity components and concentrations as well as fluxes were analysed for non‐stationary behaviour by using quality tests. Aerosol particle concentration revealed often non‐stationary behaviour, more frequently than other scalars like temperature, water vapour and carbon dioxide. During stationary episodes most of the flux estimates indicated deposition of particles into forest. Size‐dependent deposition velocities were estimated by using auxiliary measurements of particle size distribution. A semi‐empirical deposition model explained most of the variance observed in particle fluxes when size spectrum was dominated by below 100 nm particles. However, deposition velocity estimates for above 100 nm particles remained very uncertain.

Analysis of measurement techniques to determine dry deposition velocities of aerosol particles with diameters less than 100 nm

Modeled aerosol dry deposition velocities to natural surfaces have not been verified against experimental data because data is not available. However due to recent instrumentation developments measuring vertical number fluxes of aerosols with sizes down to 10 nm became possible. Additional instrument modifications which are discussed here allows measuring size dependent fluxes of 10– 100 nm aerosols and would fulfill the data gap. This study analyzes uncertainties in such measurements. Low particle count is identified as a major contributor to the measurement uncertainty, limiting the accurate estimation of size-resolved dry deposition velocities in remote environment to particles with diameters less than 50 nm using current instrumentation. Even in this size range high concentrations are required. In the case of two particle counters with different detection limits dry deposition velocity estimates can be also systematically biased, depending on the aerosol size distribution. A difference in detection limits of counters about 10 nm is narrow enough to avoid bias in size-dependent dry deposition velocities in most circumstances.

A Photosynthesis-Based Dry Deposition Modeling Approach

We present a dry deposition modeling approach that includes vegetation-atmosphere interactions through photosynthesis/carbon assimilation relationships. Gas deposition velocity (Vd) is calculated using an electrical resistance-analog approach in a coupled soil-vegetation-atmosphere transfer (SVAT) model. For this, a photosynthesis-based surface evapotranspiration and gas exchange model is dynamically coupled to an atmospheric model with prognostic soil hydrology and surface energy balance. The effective surface resistance (composed of aerodynamic, boundary layer, and canopy-based resistances) is calculated for a realistic and fully interactive estimation of gaseous deposition velocity over natural surfaces. Based on this coupled framework, the photosynthesis- based gas deposition approach is evaluated using observed deposition velocity estimates for ozone over a soybean field (C3 photosynthesis pathway) and a corn field (C4 photosynthesis pathway). Overall, observed Vd and modeled Vd show good qualitative and quantitative agreement. Results suggest that photosynthesis-based physiological approaches can be adopted to efficiently develop deposition velocity estimates over natural surfaces. Such a physiological approach can also be used for generalizing results from field measurements and for investigating the controlling relationships among various atmospheric and surface variables in estimating deposition velocity.

Estimation of deposition velocity of sulfur dioxide and sulfate aerosol by using a substitute surface technique

Estimation of deposition velocity of sulfur dioxide and sulfate aerosol by using a substitute surface technique

Precision of atmospheric dry deposition data from the Clean Air Status and Trends Network

A collocated, dry deposition sampling program was begun in January 1987 by the US Environmental Protection Agency to provide ongoing estimates of the overall precision of dry deposition and supporting data entering the Clean Air Status and Trends Network (CASTNet) archive. Duplicate sets of dry deposition sampling instruments were installed adjacent to existing instruments and have been operated for various periods at 11 collocated field sites. All sampling and operations were performed using standard CASTNet procedures. The current study documents the bias-corrected precision of CASTNet data based on collocated measurements made at paired sampling sites representative of sites across the network. These precision estimates include the variability for all operations from sampling to data storage in the archive. Precision estimates are provided for hourly, instrumental ozone (O 3) concentration and meteorological measurements, hourly model estimates of deposition velocity ( V d) from collocated measurements of model inputs, hourly O 3 deposition estimates, weekly filter pack determinations of selected atmospheric chemical species, and weekly estimates of V d and deposition for each monitored filter pack chemical species and O 3. Estimates of variability of weekly pollutant concentrations, expressed as coefficients of variation, depend on chemical species: NO 3 −∼8.1%; HNO 3∼6.4%; SO 2∼4.3%; NH 4 +∼3.7%; SO 4 2−∼2.3%; and O 3∼1.3%. Precision of estimates of weekly V d from collocated measurements of model inputs also depends on the chemical species: aerosols ∼2.8%; HNO 3∼2.6%; SO 2∼3.0%; and O 3∼2.0%. Corresponding precision of weekly deposition estimates are: NO 3 −∼8.6%; HNO 3 −∼5.2%; SO 2∼5.6%; NH 4 +∼3.9%; SO 4 2−∼3.5%; and O 3∼3.3%. Precision of weekly concentration, V d estimates, and deposition estimates are comparable in magnitude and slightly smaller than the corresponding hourly values. Annual precision estimates, although uncertain due to their small sample size in the current study, are consistent with the corresponding weekly values.

Atmospheric PAH Deposition: Deposition Velocities and Washout Ratios

The urban snowpack effectively acts as a collection device for atmospheric-deposited PAHs. When these PAHs are flushed out in a short time interval along with springtime snowmelt, these cause shockloading to receiving waters. The assessment of PAH deposition, accumulation, and possibly remediation of PAHs in urban snowpacks requires an understanding of source-receptor linkages. In this research, a mechanistic model based on atmospheric dispersion and the physics of wet and dry deposition is described for the transport of PAHs and time-dependent accumulation of PAHs in the snowpack. Two independent data sets of long-term PAH deposition levels from the study area of Sault Ste. Marie, Ontario, Canada have been utilized in this study. The first data set is used to estimate the model parameters ~dry deposition velocities and washout ratios! for three PAH species ~phenanthrene, fluoranthene, and pyrene ! and the second data set for the validation of the model. The dry deposition velocity for all three PAH species is statistically significant, although the washout ratio is not significant in a statistical sense. The statistical conclusions indicate that dry deposition is a dominant deposition process in the urban environment. The point estimates of deposition velocities and washout ratios compare favorably with previous investigations into deposition parameters.

Comparison of dry deposition velocities for SO2, HNO3 and SO4 2? estimated with two inferential models

Dry deposition velocity estimates of SO, HNO3 and SO42− were computed for six locations in eastern North America using two different inferential models; a Big-Leaf model utilized by the U. S. National Dry Deposition Network (NDDN) and, a land-use based model (LUM) that has been used in the past to estimate the relative importance of dry versus wet deposition over selected Canadian regions. There were consistent differences between models that were related to the surface type, chemical species and time of year. Mean monthly dry deposition velocities based upon the 1990–91 time period were compared at two locations. The seasonal cycles in deposition velocity were similar between models, but there were considerable differences in the amplitude of the cycles. The LUM predicted about a 400% increase in S042- deposition velocity from the winter to the summer months, while there was a 50 to 100% increase in the NDDN model estimates, depending upon location. According to the LUM, HN03 deposition to crop land increased by about a factor of 6 from winter to summer, while the big leaf model predicted a 50% increase. Overall, there was better agreement for SO2. Averaged over 12 months, the differences in deposition velocity between models were smaller and generally within the range of uncertainty associated with inferential models. For all six sites, the mean percent difference between models in deposition velocity for SO2, HNO3 and SO42− were 13, 35 and 79, respectively. These differences highlight the effect of using different methods for estimating dry deposition and the importance of applying the same model when examining regional patterns in dry/total deposition rates.

The contributions of snow, fog, and dry deposition to the summer flux of anions and cations at Summit, Greenland

Experiments were performed during the period May–July of 1993 at Summit, Greenland. Aerosol mass size distributions as well as daily average concentrations of several anionic and cationic species were measured. Dry deposition velocities for SO42− were estimated using surrogate surfaces (symmetric airfoils) as well as impactor data. Real‐time concentrations of particles greater than 0.5 μm and greater than 0.01 μm were measured. Snow and fog samples from nearly all of the events occurring during the field season were collected. Filter sampler results indicate that SO42− is the dominant aerosol anion species, with Na+, NH4+, and Ca2+ being the dominant cations. Impactor results indicate that MSA and SO42− have similar mass size distributions. Furthermore, MSA and SO42− have mass in both the accumulation and coarse modes. A limited number of samples for NH4+ indicate that it exists in the accumulation mode. Na, K, Mg, and Ca exist primarily in the coarse mode. Dry deposition velocities estimated from impactor samples and a theory for dry deposition to snow range from 0.017 cm/s +/− 0.011 cm/s for NH4+ to 0.110 cm/s +/− 0.021 cm/s for Ca. SO42− dry deposition velocity estimates using airfoils are in the range 0.023 cm/s to 0.062 cm/s, as much as 60% greater than values calculated using the airborne size distribution data. The rough agreement between the airfoil and impactor‐estimated dry deposition velocities suggests that the airfoils may be used to approximate the dry deposition to the snow surface. Laser particle counter (LPC) results show that particles > 0.5 μm in diameter efficiently serve as nuclei to form fog droplets. Condensation nuclei (CN) measurements indicate that particles < 0.5 μm are not as greatly affected by fog. Furthermore, impactor measurements suggest that from 50% to 80% of the aerosol SO42− serves as nuclei for fog droplets. Snow deposition is the dominant mechanism transporting chemicals to the ice sheet. For NO3−, a species that apparently exists primarily in the gas phase as HNO3(g), 93% of the seasonal inventory (mass of a deposited chemical species per unit area during the season) is due to snow deposition, which suggests efficient scavenging of HNO3(g) by snowflakes. The contribution of snow deposition to the seasonal inventories of aerosols ranges from 45% for MSA to 76% for NH4+. The contribution of fog to the seasonal inventories ranges from 13% for Na+ and Ca2+ to 26% and 32% for SO42− and MSA. The dry deposition contribution to the seasonal inventories of the aerosol species is as low as 5% for NH4+ and as high as 23% for MSA. The seasonal inventory estimations do not take into consideration the spatial variability caused by blowing and drifting snow. Overall, results indicate that snow deposition of chemical species is the dominant flux mechanism during the summer at Summit and that all three deposition processes should be considered when estimating atmospheric concentrations based on ice core chemical signals.

A Spatial Model of Atmospheric Deposition for the Northeastern U.S.

Spatial patterns of atmospheric deposition across the northeastern United States were evaluated and summarized in a simple model as a function of elevation and geographic position within the region. For wet deposition, 3-11 yr of annual concentration data for the major ions in precipitation were obtained from the National Atmospheric Deposition Program/National Trend Network (NADP/NTN) for 26 sites within the region. Concentration trends were evaluated by regression of annual mean concentrations against latitude and longitude. For nitrate, sulfate, and ammonium concentrations, a more than twofold linear decrease occurs from western New York and Pennsylvania to eastern Maine. These trends were combined with regional and elevational trends of precipitation amount, obtained from 30-yr records of annual precipitation at >300 weather stations, to provide long-term patterns of wet deposition. Regional trends of dry deposition of N and S compounds were determined using 2-3 yr of particle and gas concentration data collected by the National Dry Deposition Network (NDDN) and several other sources, in combination with estimates of deposition velocities. Contrary to wet deposition trends, the dominant air concentration trends were steep decreases from south to north, creating regional decreases in total deposition (wet + dry) from the southwest to the northeast. This contrast between wet and dry deposition trends suggests that within the northeast the two deposition forms are received in different proportions from different source areas, wet deposited materials primarily from areas to the west and dry deposited materials primarily from urban areas along the southern edge of the region. The equations generated describing spatial patterns of wet and dry deposition within the region were entered into a geographic information system (GIS) containing a digital elevation model (DEM) in order to develop spatially explicit predictions of atmospheric deposition for the region.

Dry deposition velocity estimates of SO2 from models and measurements over a deciduous forest in winter

Measurements were made of concentrations and vertical flux densities (using the eddy correlation technique) for SO2 over a deciduous forest during March and April 1990. These were compared with estimates of dry deposition velocities, obtained from resistance analogue models that employ measured meteorological data as input. The models include: (1) the dry deposition module that forms part of a larger Eulerian air quality model, known as ADOM (Acid Deposition and Oxidant Model), (2) a modified version of ADOM and (3) a new parameterization for Rc. The observations yielded Vdvalues of about 0.5 cm s−1 for the daytime and 0.1 cm s−1 at night. ADOM's estimates were much larger, averaging about 3.0 cm s−1 during the daytime and about 2.0 cm s−1 at night. When the ADOM canopy resistance was increased by modifying its formulation, the modified dry deposition estimates were slightly larger during the day than at night, averaging about 0.5 cm s−1. The new parameterization for Rcyielded smaller Vdestimates with no diurnal variation. An attempt is made to relate the observed diurnal cycle of the dry deposition velocity to meteorological parameters.

Atmospheric dry deposition flux of metallic species to the North Sea

Air sampling on a series of 10 research cruises on the North Sea (south of 56°N) has yielded detailed spatial distributions of atmospheric metal concentrations, Al, Ca, Cd, Cu, Fe, Mg, Na, Pb and Zn which closely parallel the results of earlier published models. Air mass back trajectory analysis demonstrates the strong influence which source region may have upon the elemental composition of the North Sea atmosphere. A cascade impactor designed to collect efficiently large as well as small aerosol has produced detailed size distributions from which mass weighted deposition velocity estimates have been produced (Al, 0.33; Cd, 0.24; Cu, 0.44; Fe, 0.30; Pb, 0.13; Zn, 0.30 cm s −1) enabling estimates for the dry deposition flux to the study area to be made. Extrapolation of these data to the whole of the North Sea yields dry deposition flux estimates (Cd, 33; Cu, 350; Pb, 370; Zn, 2640 tonnes yr −1) which are in some instances substantially lower than those previously reported, but nevertheless represent a significant pathway for metallic species to enter this marine environment. The size distributions show the clear dominance that large aerosol has upon the overall dry deposition flux. Flux estimates are thus highly sensitive to the sampling of this large aerosol component, and to assumptions made regarding the sea surface as a source of giant trace metal-enriched particles which act only as a means of recycling marine metals.

Semi-Empirical Estimation of Deposition Velocity of Unattached Radon Daughters in a Room

Deposition velocity of unattached radon daughters was estimated semi-empirically based on a mass balance model. Measurements were made of concentrations of radon and its daughters (attached and unattached), as well as of air ventilation rates, in a laboratory and a typical Japanese house. Analyses of these data showed that f-F relation, where «f» is a fraction of unattached radon daughters and «F» is an equilibrium ratio, was affected significantly only by deposition velocity of the unattached daughters while it is insensitive to ventilation rate, deposition rate of attached daughters and outdoor radon daughters. Based on these, fittings of measured and calculated f-F curves were done to identify the best fitted deposition velocity