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The Surface Air Sampling Program
Since January 1963, the Environmental Measurements Laboratory has conducted the Surface Air Sampling Program (SASP). This study is essentially a continuation of a program that was initiated by the U.S. Naval Research Laboratory (NRL) in 1957, and was continued by them until the end of 1962. The primary objective of this program is to study the spatial and temporal distribution of specific natural and anthropogenic radionuclides in the surface air. SASP provides data that can be used to test the accuracy of model predictions of the trajectories that are followed by natural or artificial aerosols, such as radioactive debris from nuclear weapons test or other nuclear events, which can serve as tracers for point source injections into the atmosphere. The SASP data also provided some of the earliest evidence on the extent of the large scale lateral distribution of pollutants injected into the troposphere; for example, that the tropical troposphere often serves as a barrier to the interhemispheric exchange of pollutants emitted into the troposphere at mid or high latitudes. The occurrence of seasonal cycles of 7Be concentrations in the surface air at many sites in the sampling network was also observed from the data. Some of the factors that cause these seasonal variations, such as atmospheric transport and removal processes, were identified by Feely et al. (1989). Larsen (1993) used data from the program to indicate that a global decrease in the production rate of cosmic-ray products, such as 7Be, had accompanied the recent increase in solar activity. The extensive 7Be and 210Pb database continues to provide the scientific community with tracer data which is used to verify global climate model simulations (Brost et al., 1991; Feichter et al., 1991; Rehfeld and Heimann, 1994). It was suggested by Brost et al. (1991) that the simulation of 7Be and 210Pb might establish the standards for how well a model can represent the concentration and deposition of an aerosol species. As part of this research, EML initiated the simulation of the global distributions of 222Rn and 210Pb using EML's three-dimensional global transport model (Lee et al., 1993a; Lee and Feichter, 1995). Comparing model simulations against measurements of 210Pb and other natural tracers will provide useful information necessary to validate and add, remove or modify the existing model modules that describe the various physical processes of the atmosphere. Following the Chernobyl accident, the data were used to characterize the Chernobyl debris which was transported across North America (Feely et al., 1988; Larsen et al., 1986; Larsen and Juzdan, 1986). Larsen et al. (1989) also used the data in a study of the trasport processes associated with the initial elevated concentrations of the Chernobyl debris in the suface air in the United States. In 1993, the program was used to detect minute quantities of debris from the April 6th accidental release of radioactivity from the Tomsk-7 nuclear complex in Russia, demonstrating the long-range dispersion of radioactivity in the stratosphere from this accident, and the capability of the SASP network to detect it (Larsen et al., 1994; Lee et al., 1993b). Data from SASP are periodically reported in EML reports (Larsen and Sanderson, 1991). The data resulting from this program constitute one of the most extensive and detailed records on atmospheric radioactivity in the world. The resulting data are distributed to scientific organizations throughout the world and have been used by such groups as the United Nations Scientific Committee on the Effects of Atomic Radiation (1993), the United Kingdom's Monitoring and Assessment Research Center (1987), the Max Plank Institute for Chemistry, and more recently the data has been selected for incorporation into the National Implementation Plan for American Participation in the International Arctic Monitoring and Assessment Program. In 1995, EML was selected to assist the World Meteorological Organization's Global Atmospheric Watch program by serving as the "GAW-Global Calibration Facility for Radioactivity". EML was chosen because of its established leadership over the past two decades in the global measurements of natural and anthropogenic radionuclides. Many of the original NRL sites, which were located roughly along the 80th Meridian (west), have been continued in the sampling network of the current program. Since 1963, a number of other sites have been added to this network, on a permanent or temporary basis, to investigate the possible effects of longitude, elevation, proximity to coastlines, over-ocean sampling, and localized contamination. Sampling sites are eliminated from the network as specific studies are completed, as data appear to become redundant, or as sampling at a particular site ceases to be possible. It is expected, however, that the continuity of sampling will be maintained at most of the current sites. Beginning in late 1982, our sampling in the Southern and Eastern Hemispheres was increased significantly by the establishment of new sites in New Zealand, Australia, the Falkland Islands, and Antarctica. Some of these new sampling sites were established for EML's Remote Atmospheric Measurements Program (RAMP), an extension and modification of SASP. The objective of RAMP is to perform field measurements of gamma-ray emitting radionuclides having short half-lives, such as 7Be, on air filter samples directly after collection at remote or weathered-in stations. The logistical problems associated with remote sites and the relatively short half-life of the predominant radionuclide being collected on the air filters are two important factors that led to the development of RAMP. To accomplish this objective, RAMP sites are equipped with remote atmospheric measurement systems (RAMS) which measure the gamma-ray activity in air filter samples at the field locations using either a sodium iodide or germanium detector and the resulting spectra are transmitted to EML using the ARGOS communcation system flown aboard National Oceanic and Atmospheric Administration satellites (Sanderson, et al., 1994). Air filters collected at RAMP sites are also analyzed at EML using germanium deteectors, primarily for the detection of 210Pb. In addition to gamma-ray spectrometric analysis, many of the air filter samples collected are also analyzed for the major cations and anions by the University of Miami's (UOM) Division of Marine and Atmospheric Chemistry. Special emphasis is placed on studies of sulfur and nitrogen species that play an important role in aerosol chemistry and may also impact on aerosol-related climate processes (Prospero et al., 1991; Savoie et al., 1992, 1993). The co-related chemical and radionuclide data are especially useful for the development of global atmospheric transport models. During 1993, air filter samples were collected at 41 sites, 16 in the Northern Hemisphere and 25 in the Southern Hemisphere, extending in latitude from about 76° north to 90° south. The sampling sites used in the program are listed below. In 1991, the second generation of RAMP hardware and software developed at EML were installed at five remote locations: Base Marsh, King George Island; Palmer Station, Anvers Island; Mawson, Antarctica; St. Denis, Reunion Island, Indian Ocean; and Pelindaba (Pretoria), South Africa. In 1992, EML, in collaboration with the UOM, established RAMP sites at Cape Point, South Africa, and at Marion Island in the southern Indian Ocean. During 1993, the network was expanded by the incorporation of four sites (Tromso, Norway; Gibraltar; Hong Kong; and Singapore) from the United Kingdom's Atomic Energy Authority's (UKAEA) former global network. The UKAEA will continue to maintain the environmental sampling capability at the four sites, but the samples will be analyzed at EML. Both UKAEA and EML will share the data and interact on future research activities. In February 1993, a SASP site was established at Durazno, Uruguay with the assistance of the Governor, and it is maintained by the Institute of Science and Technology. EML and the Institute of Science and Technology will share the data and interact on future research activities. In December 1993, a third-generatio, state-of-the-art radiation detection system, designed, developed, constructed and tested at EML, was successfully installed at Geyokcha, Turkmenistan as part of an upgraded seismic site supported by the Advanced Research Project Agency (ARPA). The data from this site will be shared by EML and ARPA. Sampling Systems Three air sampling systems are currently used in SASP: a Roots 24-AF or 24-URAI blower connected to a 1HP electric motor by a fan belt and a Fuji ring compressor directly connected to either a 0.5 or 1 HP electric motor. the Roots sampler accomodates a 20.3 cm diameter filter, which has an effective exposure area (the area of a filter exposed to the airstream) of about 266 cm2, while the Fuji sampler accomodates a 20.3 cm by 25.4 cm rectangular filter with an effective exposure area of about 407 cm2. In general, the samples are collected at weekly intervals. The typical range in the flow rates through an air filter using the Fuji and Roots samplers are about 0.8-1.0 (Fuji 0.5HP), 1.5-1.8 (Fuji 1HP), and 0.9-1.5(Roots 1HP) m3/min, respectively. Microsorban filter material was primarily used in SASP until 1988 when our stock was depleted following the termination of its manufacture. During the fall of 1988 we changed to the use of Microdon LM2020 filter material (Pellon Company, 20 Industrial Ave., Chelmsford, MA 01824). Since production of this filter was also discontinued, Gelman glass fiber material was also used for several months at some of the Chilean sites until we supplied these sites with Dynaweb DW7301L filter material (Web Dynamics, Ironia Road, Flanders, NJ, 07836). We phased in Dynaweb filter material into the program as our stock of Microdon diminished. Dynaweb filter material is currently used at all sites in the program. Dynaweb DW7301L is composed of three layers of 100% polypropylene web sandwiched between two sheets of 100% polyester protective scrim. The collection efficiency of this material as a function of particle size and face velocity has been identified (Larsen, 1990). Intercomparison data on the collection of 7Be and 210Pb using Microdon and Dynaweb indicate no significant differences in the collection efficiency of these two filter materials. Detailed descriptions of these filter materials, the air samplers and the techniques used to calibrate the samplers and determine air flow rates through the filters are presented in the EML Procedures Manual (1992). Sample Collection, Processing, and Analysis At most SASP stations the filters are changed on the 1st, 8th, 15th, and 22nd of the month, or more frequently if the filter becomes clogged. At RAMP stations the filters are changed once a week. The air filter samples that are collected on approximately a weekly basis are referred to as "weekly samples" or "individual samples". The weekly samples collected at all SASP sites are composited to form monthly samples. Monthly samples, which consist of weekly samples that represent a minimum of 14 days of exposure during any given month, are referred to as "monthly" or "composite" samples. Frequent readings of the pressure drop across the filter or, at RAMP stations, across a fixed orifice, and of the temperature are submitted to EML along with the filters to permit the calculation of the volume of air that was sampled. The filters from most sites are returned to EML for analysis at the end of each month. Because of transportation difficulties, the samples collected at the South Pole Station, Mawson, Marion Island, Palmer and Marsh Antarctica during the winter months are retained at the sites until they can be shipped to EML. This adversely affects the detection and the precision of measurements of short-lived radionuclides in these filters. Typically, the weekly samples from most of the sites that use a 20.3 cm diameter filter were cut in half; one half of each filter is included in a monthly composite sample, while the other half was archived. The monthly composite samples are compressed into 45-cm3 plastic planchets and are analyzed for gamma-ray emitting radionuclides using either n-type low-energy coaxial, high-purity germanium (HPGe) detectors or p-type coaxial high-resolution germanium lithium (GeLi) or HPGe detectors. All weekly samples from sites using 20.3 cm by 25.4 cm rectangular filters are treated differently. A section (80.6 cm2 for Microdon, 65.3 cm2 for Dynaweb) of each of these filters is removed and compressed into a 1-2 cm3 cylinder, which is analyzed by gamma-ray spectrometry using a HPGe detector with a 1.5 cm diameter well. The remainder of the filter is archived. These filters are not composited into monthly composite samples. The activities of specific isotopes (7Be, 95Zr, 137Cs, 144Ce, and 210Pb) are determined by computer analysis of the spectral data from both monthly composite and weekly samples. The total gamma-ray activity of each monthly composite sample is determined by summing the total counts obtained with germanium detectors between 100 keV and 2.0 MeV, without any correction for detector efficiency or radioactive decay. Detection Limits The lower limit of detection is defined as three times the statistical variation of the background associated with each nuclide. The LLD's vary with the isotope, sample activity, detector, and count time. The approximate LLD for the detectors used to analyze the monthly composite samples, using a 16 hour count time, are 300, 75, 45, 150 and 500 mBq for 7Be, 95Zr, 137Cs, 144Ce and 210Pb respectively. For the well detectors used to analyze the weekly samples, the LLD for a 16-hour count time are ~45, 10, 6, 20 and 50 mBq for 7Be, 95Zr, 137Cs, 144Ce and 210Pb respectively. The minimum detectable concentration for monthly composite samples using an air sampler having a flow rate of about 1.2 m3 is about 12, 3, 2, 6, and 19 µBq m-3 for 7Be, 95Zr, 137Cs, 144Ce and 210Pb respectively. The minimum detectable concentration for weekly samples collected on a 20.3 by 25.4 cm Microdon filter using an air sampler having a flow rate of about 0.9 m3/min is about 25, 6, 3, 11 and 28 µBq m-3 for 7Be, 95Zr, 137Cs, 144Ce and 210Pb respectively. For Dynaweb material, we estimate minimum detectable concentrations of about 32, 7, 4, 14, and 35 µBq m-3 at sampler flow rates of about 0.9 m3/min for 7Be, 95Zr, 137Cs, 144Ce and 210Pb respectively. The sensitivity of the well detectors, which are used to analyze weekly samples, is about 10 times higher than the detectors used to analyze the monthyl composite samples. If a single 20.3 cm by 25.4 cm rectangular Dynaweb filter is analyzed for radionuclides, only about 16% of the exposed surface area can be used in the well detectors. This restriction in sample size is necessary to form the required 1-2cm3 pellet. This sample size limitation does not apply for the coaxial detectors used to analyze the monthyl composite samples. Thus, for our application, the apparent sensitivity of the well detectors is about 1.6 times higher than the coaxial detectors. Data Uncertainty and Reporting The overall uncertainty of the data results primarily from uncertainty in volume determination, sample processing, and analysis. We estimate this uncertainty from the square root of the sum of the squares of these individual uncertainties. During flow calibrations of Roots air samplers at EML, sampler flow rates may vary by about 0-5% at a fixed sampler setting. We estimate that these variations will be greater for samplers located at field sites. We have measured changes with time ranging from 0-16% in the flow rates of Roots samplers located at field sites. In general, these changes averaged about 7% and occurred over periods of several years. We feel that these changes primarily result from variations with time in sampler efficiency and sampler gauge operations. The components of the Fuji air sampler are protected from the environment; therefore, we expect that the flow rate changes with time will be less than those measured for the Roots air sampler. We assume an average uncertainty in sample flow determinations of 7% for the Roots and 5% for the Fuji air sampler. It is possible, however, for larger systematic errors to be present in the volume determination. Currently, we try to calibrate the air samplers as frequently as possible, and also try to perform other tests that are designed to monitor the performance of the air samplers. These measures will help to minimize the occurrence of large systematic errors. The uncertainty associated with sample preparation comes about primarily from the handling and cutting of samples during processing. The nonhomogeneous distribution of activity in some samples and variations in the sample geometry, such as differences in sample volume following filter compression, also contribute to the overall sample preparation uncertainty. We attribute an average uncertainty of 3% to sample preparation. The uncertainty from counting includes the standard deviation (SD) calculated using the Poisson assumption, as well as other sources of uncertainty such as positioning of the sample in the detector. We estimate the total error from analysis for each isotope by using twice the SD, which is calculated from the activity measured during each gamma-ray spectrometric analysis. The minimum reported concentration for gross gamma-ray activity is 0.01 counts min-1 m-3 of sampled air. Previous SASP data indicated that when low concentration of fission products are present in the samples, the gross gamma-ray activities are often at or above 0.01 counts min-1 m-3. Gross gamma-ray activities below 0.01 counts min-1 m-3 seem to vary with hte activieis of the natural radioisotopes in the sample and do not provide additional significiant information, especially since 7Be and 210Pb are analyzed separately. To monitor the quality of the data from this program, four types of quality control samples (reference, duplicate, replicate, blank are regularly submitted to the analysts together with routine monthly composite and weekly samples. These quality control samples are submitted "blind" (i.e., in such a way as to be indistinguishable from the routine samples by the analyst) insofar as this is possible. Reference samples, spiked with known amounts of radionuclides, are used to test the accuracy of the gamma-ray spectrometric analysis. For monthly composite samples, weighed portions of reference solutions are added to halves of four clean filters (to duplicate as closely as possible the characteristics of monthly composite samples). For weekly samples, the reference solutions are added to filter sections which are then compressed into 1-2 cm3 cylinders. If reference solutions are available, 7Be, 95Zr, 137Cs, 144Ce and 210Pb are routinely added to the reference samples. The % deviations reported for these quality control samples are influenced by a number of factors besides the accuracy of the gamma-ray spectrometric analysis. Errors in the calibration of the reference solution, weighing errors during the application of the standard solution to blank filters, and a nonhomogeneous distribution of the reference solution in the sample all contribute to the overall reported deviation. Therefore we believe that these quality control results represent the minimum accuracy obtained in the program. Reference samples with % deviations less than 20% are thus considered acceptable. In addition, the accuracy of the data is considered acceptable if the mean of the % deviations obtained over long time periods (one year or longer) is less than 10%. Some samples are split into two sections, each submitted under a different sample number, to determine the precision of the analyses. The monthly composite duplicate sample is obtained by cutting in half each of the weekly filters in a monthly composite sample, and combining the halves to form two equivalent monthly composite samples that are then submitted to the analyst under different sample numbers. To prepare duplicates from a weekly sample, duplicate sections are cut from a weekly filter to form two equivalent samples. When the routine environmental samples are counted, the filters are folded and compressed into a planchet. It is assumed that this process produces a close approximation to a homogeneous source. Part of the deviation between the duplicate samples results from the failure of this assumption. Any inhomogeneities in the filter material that allow for higher flow rates, and thus higher collection efficiencies, for some sections of the filter than for others also contribute to deviations between duplicate samples and to errors in the data for routine samples. Smaller deviations are to be expected from weekly duplicate samples compared to monthly duplicate samples. When weekly duplicate samples are counted, a section of each filter is cut and compressed into a pellet. It is assumed that this process produces a closer approximation to a homogeneous source than the process used to count a composite sample. Duplicate samples with % deviations less than 15% are thus considered acceptable. In addition, the accuracy of the data is considered acceptable if the mean of the % deviations obtained over long time periods (one year or longer) is less than 10%. A single sample spiked with 137Cs and 210Pb is analyzed from time to time to monitor the operation of the gamma-ray spectrometers over long time periods. The monthly composite sample was prepared by compressing two filters spiked with 137Cs and 210Pb into a planchet. The weekly sample was prepared by compressing a filter section spiked with 137Cs and 210Pb into a 1 cm3 pellet. These samples are repeatedly submitted for analysis with different identification numbers. these replicate samples are not intended to be 'blind' to the analyst. These samples provide the best estimate of the long-term precision of analysis since, unlike duplicate samples, the sample homogeneity is remains constant during each analysis. We expect the % deviations to be similar in magnitude to those expected due to random error. Replicate samples with % deviations less than 10% are thus considered acceptable. In addition, the accuracy of the data is considered acceptable if the mean of the % deviations obtained over lng time periods (one year or longer) is less than 5%. Blank filters are submitted to test for possible contamination of the samples during handling and analysis, and to monitor the sensitivity of the analytical technique. A monthly composite blank sample consists of one or more clean filters while w weekly blank sample consists of a clean section of a filter. blank values are acceptable if they are less than the lower limit of detection for the analytical techniques that we currently use. If the quality control samples indicate trends of unacceptable results, we attempt to identify and correct the source of the error.
For more information about the Surface Air Sampling Program, contact: webmaster@eml.st.dhs.gov
The EML Sample Archives makes available environmental radiological data collected for programs funded through the U.S. Atomic Energy Commission, the U. S. Energy Research and Development Administration and the U. S. Department of Energy. All of these programs have been terminated. The databases were last updated in 1999. No additional data will be added to these databases. Any inquiries about these programs should be made to webmaster@eml.st.dhs.gov.
Contact: webmaster@eml.st.dhs.gov
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