Interception Fraction Research Articles

INTERCEPTION OF TROPICAL RAIN FOREST: PERFORMANCE OF A CANOPY WATER BALANCE MODEL

The throughfall in dry evergreen forest and mixed forest in central Guyana (South America) was measured over a period of 17 months using both a moving and a fixed stratified random sampling scheme. The throughfall, stemflow and canopy cover fractions of the two forest types were not significantly different. The data set was used to test the canopy water balance model designed by Rutter et al. (1971). Compared with both forest types, the model underestimated the total interception fraction: 13.4% for the Rutter model as opposed to 17.3 and 16.0% of the rainfall for dry evergreen forest and mixed forest. This was caused by a severe underestimation of interception on days with a rainfall of more than approximately 10 mm. Compared with the rainfall frequency distribution of the area, accurate model results were obtained for 20% of the rainfall only. Sensitivity analysis shows that an increase of saturation storage capacity or potential evaporation by 150% will give an acceptable accumulated interception, but that this is caused by large overestimation on days with a low amount of rainfall. The Rutter model was extended with a layered representation of the canopy, with simulation of the microclimate, but using basically the same input data. This resulted in a higher total interception fraction (16.3% of the rainfall), especially in the range 5–20 mm/day, because of a larger storage at the start of each rainstorm and an increased surface available for evaporation.

Comparison of interception and initial retention of wet-deposited contaminants on leaves of different vegetation types

Simulated rain containing both soluble radionuclides and insoluble particles labeled with a radionuclide was manually applied to several kinds of vegetation, including a conifer, a broad-leafed tree, and several herbaceous species. The fraction of each radioactive material intercepted and initially retained by the vegetation was determined for each plant type. This fraction was determined both as the mass interception factor, r Y , and the leaf area interception fraction, LAIF. Mean values of r Y ranged from 0.16 to 2.9 m 2 kg −1 and of the LAIF, from 0.011 to 0.16. There was a greater range in mean retention values among radionuclide types than among plant species; the range among plant types tended to be less with the LAIF than the r Y . Significantly less interception and initial retention was measured for anions than for cations or the insoluble particles.

Preliminary Analysis of Important Site-Specific Dose Assessment Parameters and Exposure Pathways Applicable to a Groundwater Release Scenario at Yucca Mountain

AbstractTo develop capabilities for compliance determination, the Nuclear Regulatory Commission (NRC) conducts total system performance assessment (TSPA) for the proposed repository at Yucca Mountain (YM) in an iterative manner. Because the new Environmental Protection Agency (EPA) standard for YM may set a dose or risk limit, an auxiliary study was conducted to develop estimates of site-specific dose assessment parameters for future TSPAs. YM site-relevant data was obtained for irrigation, agriculture, resuspension, crop interception, and soil. A Monte Carlo based importance analysis was used to identify predominant parameters for the groundwater pathway. In this analysis, the GENII-S code generated individual annual total effective dose equivalents (TEDEs) for 20 nuclides and 43 sampled parameters based upon unit groundwater concentrations. Scatter plots and correlation results indicate the crop interception fraction, food transfer factors, consumption rates, and irrigation rate are correlated with TEDEs for specific nuclides. Influential parameter groups correspond to expected pathway behavior of specific nuclides. Results for nuclides that transfer more readily to plants, such as 99Tc, indicate crop ingestion pathway parameters are most highly correlated with the TEDE, and those that transfer to milk (59Ni) or beef (79Se, 129I, 135Cs, 137Cs) show predominant correlations with animal product ingestion pathway parameters. Such relationships provide useful insight to important parameters and exposure pathways applicable to doses from specific nuclides.

Dry deposition of aerosol particles within model spruce canopies

A wind tunnel experiment is reported in which the dry deposition of a 1-μm MMAD monodisperse silica aerosol to Norway spruce (Picea abies) was investigated. Model canopies were constructed from individual 2- to 3-year-old saplings so as to allow the full development of a turbulent boundary layer within the wind tunnel above the canopy. Deposition velocities (Vg) and interception fractions (f) are presented for both total canopy and for 14 sub-components of the canopy in four horizontal layers. Total canopy Vg was approximately seven times greater than that of a grass sward in the same wind tunnel. The total canopy f value was also higher than that of grass, with a value for spruce of 98%. Both Vg and f values were attenuated in an approximately exponential fashion with vertical penetration downwards into the canopy. A series of two-way analyses of variance revealed significant differences in deposition between needle and stem tissues, and also between vertically and horizontally distinct canopy components. Furthermore, significant interactions between deposition to needle and stem tissues at different heights within the canopy indicated a complex pattern of deposition. It is suggested that the initial capture of the bulk of aerosol deposits on the upper and outer surfaces of individual spruce trees provides the potential for a substantial prompt redistribution of the deposit by resuspension due to wind action.

Dry deposition of aerosol particles within model spruce canopies

A wind tunnel experiment is reported in which the dry deposition of a 1-μm MMAD monodisperse silica aerosol to Norway spruce ( Picea abies) was investigated. Model canopies were constructed from individual 2- to 3-year-old saplings so as to allow the full development of a turbulent boundary layer within the wind tunnel above the canopy. Deposition velocities (V g) and interception fractions ( f ) are presented for both total canopy and for 14 sub-components of the canopy in four horizontal layers. Total canopy V g was approximately seven times greater than that of a grass sward in the same wind tunnel. The total canopy f value was also higher than that of grass, with a value for spruce of 98%. Both V g and f values were attenuated in an approximately exponential fashion with vertical penetration downwards into the canopy. A series of two-way analyses of variance revealed significant differences in deposition between needle and stem tissues, and also between vertically and horizontally distinct canopy components. Furthermore, significant interactions between deposition to needle and stem tissues at different heights within the canopy indicated a complex pattern of deposition. It is suggested that the initial capture of the bulk of aerosol deposits on the upper and outer surfaces of individual spruce trees provides the potential for a substantial prompt redistribution of the deposit by resuspension due to wind action.

Quantification of the interception and initial retention of radioactive contaminants deposited on pasture grass by simulated rain

Simulated rain containing both soluble radionuclides and insoluble particles labeled with radionuclides was applied to pasture-type vegetation under conditions similar to those found during convective storms. The fraction of material in rain intercepted by vegetation and initially retained was determined for three sizes of insoluble polystyrene microspheres (3, 9 and 25 μm), soluble 7Be 2+ and soluble 131I as periodate or iodide, over a range of rainfall amounts of both moderate- and high-intensity precipitation. Values for the interception and initial retention by vegetation (interception fractions) for soluble forms of 131I in simulated rain are much less than those for insoluble particles and the reactive cation 7Be 2+. The interception fraction for soluble 131I is an inverse function of rain amount, varying from about 0.3 at 1 mm rain to 0.006 at 30 mm. The mass interception factor (the interception fraction normalized for biomass) of 131I is almost solely dependent on the amount of rain, with values from about 2.5 m 2 kg −1 at 1 mm to less than 0.1 m 2 kg −1 at 30 mm; the 131I vegetation-to-rain concentration ratio is relatively constant at approximately 2.6 l kg −1. For 7Be 2+ and the insoluble particles, the interception fractions range from 0.1 to 0.6 with geometric means of approximately 0.3. For these materials there is a greater dependence on biomass than on rain amount; the geometric means of the mass interception factors for these substances range from 0.99 to 2.4 m 2 kg −1, with no single variable being a major controlling factor. These results indicate that anionic 131I is essentially removed with the water once the vegetation surface becomes saturated and that the 7Be cation and the in s oluble particles are adsorbed to or settle out on the plant surface. The discrepancy between the behavior of the anionic and the cationic species is consistent with a negative charge on the plant surface.

An Analysis of Vegetation Interception Data Pertaining to Close-in Weapons Test Fallout

This paper reports an analysis of literature data pertaining to the fraction of particles resulting from regional weapons fallout intercepted by vegetation. A simple model for the interception parameter, alpha, was developed that predicted an increasing interception fraction with increasing distance from detonation site. The predicted values of alpha close to the detonation site are much smaller than most accepted values that pertain to aerosols and gases. The apparent dependence of the interception parameter on distance is believed to partially reflect the changes of the particle-size distribution of the fallout cloud with the time aloft and the ability of vegetation surfaces to better retain particles of small size. The validity of a set of predicted values was tested using an assessment model and literature data of radioiodine concentrations in milk following a nuclear test.

The interrelationships among plant biomass, plant surface area and the interception of particulate deposition by grasses.

The interrelationships among plant biomass, plant surface area and interception fraction were determined for the interception by corn of 238Pu-bearing particles released to the atmosphere from the H-Area nuclear fuels chemical separations facility on the U.S. Department of Energy's Savannah River Plant in Barnwell County, South Carolina. The relationship between interception fraction and corn biomass was accurately approximated by a filtration model with an absorption coefficient of 3.60 m2 kg-1. A filtration model with an absorption coefficient of 2.91 m2 kg-1 accurately approximated the relationship between biomass and interception fraction for data compiled from the literature for a variety of grass species. A linear regression model accurately approximated the relationship between interception fraction and surface area, but was not a better predictor of interception fraction than the filtration model for biomass.

Interception and Retention of Technetium by Vegetation and Soil

AbstractField experiments were performed to quantify the initial interception and retention of technetium (Tc) by herbaceous vegetation as a consequence of direct deposition and uptake from soil. A simulated rain containing a solution of 95mTcO4− was applied to plots of bare soil and plots with standing vegetation. Vegetation emerging in the bare‐soil plots obtained Tc predominantly from root uptake. Vegetation standing during the initial application obtained Tc from both direct foliar interception and root uptake. For the plots with standing vegetation, the estimated initial interception fraction (r) ranged from 0.079 to 0.17. The mass interception factor (r/Yv) ranged from 0.65 to 1.1 m2/kg. The retention of Tc by vegetation receiving direct foliar contamination varied with environmental half‐times (Tw) ranging from 15.9 to 18.7 days. Adjusting for the effects of growth dilution increased the values of Tw (range 21.6–28.7 days). For vegetation emerging from the plots of initial bare soil, retention was equivalent to a Tw of 43 days. These data indicated that uptake of Tc from soil diminished with time and that Tc was removed from plant tissue. In soil, a downward movement was evident, because Tc in the top 2 cm decreased with time and concentrations at lower soil depths (6–8 and 14–15 cm) increased with time. The application of these data to predict steady‐state concentrations of Tc in vegetation resulting from continuous deposition does not differ substantially from predictions based on the use of generic‐default parameter values recommended in Regulatory Guide 1.109 of the U.S. Nuclear Regulatory Commission (1977).

A probabalistic theory of aerosol penetration through glass fibre filters

A theoretical model is developed capable of predicting aerosol penetration through glass fibre filters. The filter is described as an assembly of pores which conduct the flow. Some of the aerosol particles, the interception fraction, move along with the tortuous flow; the remainder, the inertial fraction, pursue their paths through the filter along straight lines until they are re-entrained in the flow. Of the interception fraction of particles, a part is captured as a result of pure interception, a part due to an increase of interception by both inertia and diffusion, and the rest by pure diffusion. From the inertial fraction of particles, part is captured due to pure inertia, part due to inertial interception and part due to inertial diffusion. The final expression for the penetration does not contain any adaption parameter, only directly assessable parameters related to flow-, filter-, fibre- and particle properties. The final relation has been tested experimentally, by varying particle diameter, face velocity, porosity fibre diameter, and filter composition. Theory and experiment appear to be in good agreement. A suggestion is made for optimal filter design.