Forest vegetation as a sink for atmospheric particulates: Quantitative studies in rain and dry deposition

Abstract

Radionuclides in the atmosphere are associated with nonradioactive air particulates and hence serve to trace the fluxes of air particulates to various surfaces. Natural and artificial radioactivities found in the atmosphere have been measured in vegetation for 10 years to elucidate some of the mechanisms of acquirement by forest trees of atmospheric particulates. Whole tree analysis, in conjunction with soil assay, has served to establish the fraction of the flux of radionuclides retained by above-ground tissues of a forest stand. Interpretation is facilitated because most radionuclides in the atmosphere are superficially acquired. Typically 5-20% of the total open field flux is retained by the forest canopy in a moderately rainy climate (120 cm/year). Short-lived daughters of radon give a dry deposition velocity of particulates in the Aitken size range of 0.03-0.05 cm/s, thus permitting an estimate of transient removal by forest canopies by dry deposition of this size fraction.Paper number 80C0916. 0148-0227/81/080C-0916501.00 lated near the tips of plants, where the maximum surface area offers the greatest opportunity for collection. Rain functions to add, to remove, and to transfer pollutants in the tree system.Whole tree analysis provides a method for estimating an average efficiency appropriate to a specified accumulation time for each radionuclide measured. However, we must distinguish between soil-derived and superficially deposited radionuclides. Morphological rather than physiological processes apparently are chiefly responsible for the distribution of most superficially deposited radionuclides in the tree. Nuclides we have found suitable for estimation of superficial efficiency are the rare earths 32.5 day •4•Ce, 285 day •44Ce, 4.93 year •SSEu. Other useful isotopes in the superficial class are 65 day 9SZr, 39.7 day •ø3Ru, 369 day •øSRu, and 2.7 year mSb. The foregoing are all products of nuclear fission, and they encompass a range of physical and chemical properties. Another important radionuclide that follows superficial behavior is 53.5 day ?Be. This isotope is of cosmogenic origin and represents one of the more promising tracers for future work in the absence of nuclear testing in the atmosphere. Long-lived fission product nuelides that simulate macronutrients, such as 30 year •37Cs and 29 year 9øSr have significant soil reservoirs and are found in forest trees largely as a consequence of uptake from the soil. With the exception of vøSr, all the above radionuclides are easily quantified via their gamma ray emissions. Other important gamma-emitting radioactive species encountered in our work include 310 day •4Mn, daughter products of •6Ra and •aSRa, and 4øK. The 10.6 h daughter of •2øRn is particularly useful for study,.ing dry deposition processes. METHODS Beginning in 1968 we have measured radionuclides and on occasion nonradioactive pollutants in vegetation taken from forested and cultivated settings. Sampling was conducted in Newton, Massachusetts, 10 km west of Boston; in Framingham, Massachuset...

Atmospheric Transfer and Deposition Mechanisms of 210Pb Aerosols onto Forest Soils

“…Thus, the bulk precipitation/throughfall ratio obtained here could be interpreted as a mere indication of matter being brought to foliar surfaces by rain, rather than the efficiency with which the Sitka spruce needles capture and retain the 210 Pb-carrier aerosols from bulk precipitation. Previous investigations by Russell et al (1981) have revealed high contamination surface densities of radioactive aerosols ( 7 Be, 144 Ce, 141 Ce and 95 Zr) on twig and limb surfaces of pine, oak and maple trees.…”

“…The interaction of atmospheric radionuclides with forest soils is mainly due to the eluded or eroded aerosol laden radionuclides that had deposited onto plant foliage. The aspect of radionuclide accumulation by plant has been reviewed (Russell 1965;Russell et al 1981). The isotope of 210 Pb in the atmosphere has been shown to be particularly useful as an atmospheric tracer because it is associated mainly with submicronsized aerosols, which contain the bulk of the pollutant sulphur and nitrogen .…”

Using atmospheric fallout to date organic horizon layers and quantify metal dynamics during decomposition

“…Given the limited penetration of 7 Be in the O horizon, the strong partitioning of Pb to organic matter (Kerndorff and Schnitzer, 1980;Sauve et al, 2000), the importance of dry deposition for atmospheric Pb (Miller and Friedland, 1994) and the long residence time of atmospheric Pb reported for O horizons (Wang and Benoit, 1997;Kaste et al, 2003Kaste et al, , 2006aKlaminder et al, 2006), it is unlikely that Pb is transported to significant depths in the soil following deposition. It is more likely that Pb is bound strongly to the initial surfaces that it comes into contact with, that is, live leaves and surface litter (Russell et al, 1981). As leaves containing Pb are buried by subsequent years of litterfall, Pb is quantitatively retained by the material.…”

“…Similar to Pb, these atoms hydrolyze even in acidic soils and thus strongly adsorb to mineral or organic phases (Kim et al, 1997;Artinger et al, 2002). Because the input functions for gasoline-derived Pb and the weapons fallout can be treated essentially as single short-duration events, and the atoms are sequestered by forest vegetation (Russell et al, 1981), the distributions are similar in the soil profiles: a contaminated layer at depth is overlain by fresh "clean" litter, and underlain by even older material that pre-dates the era of peak atmospheric contamination. However, pollutant Pb and weapons-derived nuclides were introduced into the atmosphere over different time intervals, and thus provide independent constraints of forest litter "exposure" age.…”

Potentially Mobile Lead Fractions in Montane Organic-Rich Soil Horizons

“…Qualls et al (1991) found that up to 40% of freshly fallen leaf litter was extractable in water. Since the forest canopy scavenges significant amounts of aerosols from the troposphere (Russell et al, 1981) and the Oi layer scavenges metals from throughfall solutions, litter could be a source of mobile organic-Pb complexes. Tam et al (1991) found that approximately 10% of the total Pb in a litter layer was removed by a single water extraction (passing through a 2.5-µ m filter).…”