THE DEPOSITIONAL
FLUXES OF Pb-210, Po-210 AND Be-7
IN DETROIT,
MICHIGAN
Daphne McNeary1 and M. Baskaran
Department of Geology
Wayne State University
Detroit, Michigan 48202
ABSTRACT:
Short-lived cosmogenic isotopes (such as Beryllium-7, half-life = 53.3 days) and daughter products of radon-222 (such as Lead-210, half-life = 22.1 years and Polonium-210, half-life = 138 days) have been widely used as tracers and chronometers in the environment. These isotopes have also been utilized to determine the aerosol residence time as well as removal rates of aerosols. The behavior of 210Pb and 7Be in the atmosphere provides knowledge on the behavior of other chemical species in the atmosphere.
There is limited data on the depositional fluxes of 7Be, 210Pb and 210Po in the precipitation and air samples from the Midwestern United States. We determined the depositional fluxes of 7Be, 210Pb, and 210Po in the bulk and dry fallout, by deploying a rain collector (dry and bulk) over a 7-month period. We also collected air samples and determined the concentrations of these nuclides. The bulk depositional flux of 210Pb ranges from 0.45 to 2.73 dpm cm-2 y-1, with a mean of 1.37 dpm cm-2 y-1; the 210Po flux in the bulk deposition ranges from 0 to 0.10 dpm cm-2 y-1, with a mean of 0.04 dpm cm-2 y-1; the 7Be flux ranges from 3.98 to 20.46 dpm cm-2 y-1, with a mean of 11.1 dpm cm-2 y-1. The dry depositional flux accounted for about 12% and 4.8% of the bulk depositional flux for 210Pb and 7Be, respectively. The 210Po/210Pb ratios in the bulk deposition range between 0 and 0.20, with a mean of 0.05. This range is comparable to the values reported in other literature. The calculated depositional velocity of aerosols for 9 air samples based on the atmospheric flux and concentration of 210Pb in air ranges from 0.20 to 1.23 cm/s with a mean of 0.77 cm/s. The bulk depositional flux of 210Pb and 7Be, the fraction of the dry fallout for these radionuclides, and the depositional velocity of aerosols based on 210Pb are comparable to the range of values reported in literature for other regions.
INTRODUCTION:
Short-lived radionuclides have provided useful information on the sources and fate of pollutants in the atmosphere. Beryllium-7, 210Pb, and 210Po are all naturally occurring radionuclides. Beryllium-7 (half-life = 53.3 days) is produced in the stratosphere as a result of cosmic ray spallation reactions with oxygen and nitrogen atoms. Lead-210 (half-life = 22.1 years) and 210Po are produced from the decay of 222Rn (half-life = 3.84 days), which primarily emanates from the continents. Polonium-210 (half-life = 138 days) originates from the decay of 210Pb. Previous studies have shown that 7Be and 210Pb are highly particle reactive and they get attached to aerosols in the atmosphere soon after their production and are then scavenged from the atmosphere by precipitation. The behavior of 210Pb and 7Be provides knowledge on the behavior of other chemical species in atmospheric aerosols (Turekian et al., 1983). These radionuclides have also been utilized as tracers of particle-reactive pollutants in the coastal, marine, and lake environments (e.g., Baskaran and Santschi, 1993: Baskaran et al., 1997).
There is limited data on the depositional fluxes of 7Be, 210Pb, and 210Po in the precipitation and air samples from the Midwestern United States. Thus, this investigation will provide the needed baseline data for the bulk depositional fluxes of 7Be, and 210Pb. In addition, the relative importance of dry deposition in the bulk deposition as well as deposition velocities of aerosols will be determined for the Midwestern United States.
METHODS:
In September 1999, a bulk rain collector was deployed at a site in southwest Detroit approximately 1m above ground. In October 1999, a dry collector was deployed on the roof of a building at the same site approximately 3m above ground. A polyethylene drum with a 200-L capacity and a surface area of 2800 cm2 was used as the bulk rain collector and the lid of the drum was used as the dry collector. Prior to the beginning of the collection of each sample, 100 ml of concentrated HCl with 1-ml spike of 209Po (10.61 ± 0.11 dpm), 1 ml of stable Pb (º 1 mg) and Be (º 1 mg) was added. The dry collector was deployed with the same spikes but the lid was filled halfway (1.5 cm) with distilled water since 7Be and 210Pb are primarily used as tracers in aquatic systems where deposition occurs onto the water surface. The bulk rain samples were collected after each significant rainfall and after approximately ten days of dry weather for the dry collector. Immediately after collection, the drum and/or the lid was cleaned with repeated rinsing of 6M HCl to remove adsorbed Be, Pb, and Po isotopes from the surface of the drum and/or the lid. The samples were then taken to the lab for radiochemical processing. Beryllium-7 and 210Pb concentrations were determined using gamma-ray spectrometry; polonium-210 concentration was determined by using alpha-ray spectrometry. The stable Be and Pb spikes were measured by atomic absorption spectrometry for determining the chemical yield.
RESULTS AND DISCUSSION:
Over the past seven months, from September 1999 to April 2000, we have collected ten bulk precipitation samples, five dry deposition samples and nine air samples to determine the concentrations of 210Pb, 210Po, and 7Be. The variations of bulk depositional fluxes of 7Be, and 210Pb and the amount of rainfall are given in Figure 1. The bulk 210Pb flux ranges from 0.45 to 2.73 dpm cm-2 y-1, with a mean of 1.37 dpm cm-2 y-1 with the highest flux occurring in early fall (late September to early October) and the lowest flux occurring in mid-fall (mid-October to early November). The bulk 7Be flux ranges from 3.98 to 20.5 dpm cm-2 y-1, with a mean of 10.3 dpm cm-2 y-1, with the highest and lowest flux occurring during the same time period as the highest and lowest flux of 210Pb. The measured depositional fluxes of 7Be and 210Pb in our present study falls within the range of values reported by other researchers (Baskaran et al., 1993, Turekian et al., 1983, Dibb, et al., 1989, Todd, et al., 1989). The bulk 210Po flux ranges from 0 to 0.10 dpm cm-2 y-1, with a mean of 0.04 dpm cm-2 y-1 with the highest fluxes in late fall and early winter. The dry 210Pb flux ranges from 0.07 to 0.22 dpm cm-2 y-1 with a mean of 0.15 dpm cm-2 y-1; the dry 7Be flux ranges from 0.16 to 1.62 dpm cm-2 y-1, with a mean of 0.63 dpm cm-2 y-1; the dry 210Po flux ranges from 0.002 to 0.096 dpm cm-2 y-1 with a mean of 0.059 dpm cm-2 y-1.
Figure 1: Depositional fluxes of 210Pb and 7Be and amount of rainfall.
The specific
concentrations of 7Be ranged from 90 to 358 dpm/L, with a mean of
177 dpm/L. This value is slightly higher than values reported by other
researchers (Baskaran et al.,1993 and Turekian et al., 1983). The specific concentrations of 7Be
decreased with the amount of precipitation indicating that there is a weak
correlation between specific concentrations of 7Be and the amount of
precipitation, suggesting that with more dilution, there is less 7Be
concentration to be scavenged from the atmosphere. However, the concentration of 7Be in rain may be
higher during drier months and periods characterized by short-duration
precipitation events.
Figure 2: Bulk depositional fluxes of 7Be against 210Pb fluxes.
The specific concentration of 210Pb ranges from 6 to 37 dpm/L, with a mean of 24 dpm/L. The correlation between the specific concentration of 210Pb and the amount of precipitation is also weak, similar to 7Be. However, it has been shown that the annual flux of 7Be and 210Pb is primarily controlled by the amount of rainfall. This lack of correlation could also be attributed to such factors as sources, pathways, the altitude of clouds, and transit times of aerosols that contain 210Pb.
ACKNOWLEDGMENT:
We thank Dr. Peter Warner and Mark Baron of the Wayne County Air Quality Management Division for their support to deploy the rain collector at their site and their generosity in allowing us to use their aerosol sampling equipment for collecting our samples. We thank the Wayne State University for a Graduate Research Assistantship to the first author.