Average Surface Air Concentrations of 210Pb and 7Be at BRW, MLO, SMO, and SPO
John Kada
Environmental Measurements Laboratory, U.S. Department of Energy, New York, New York 10014-3621
Introduction
High volume bulk aerosol samples have been collected for many years by CMDL personnel at the Barrow Observatory, Alaska (BRW), the Mauna Loa Observatory, Hawaii (MLO), the Samoa Observatory, American Samoa (SMO), and the South Pole Observatory, Antarctica (SPO) for the Surface Air Sampling Program (SASP), a global network of aerosol sampling sites created to determine the global distribution of artificial radionuclides released into the atmosphere by nuclear weapons tests or nuclear accidents. This network has also produced the bulk of the available data on temporal and spatial trends in the worldwide distribution of the natural radionuclides 7Be and 210Pb. The atmospheric production rate of 7Be, a cosmogenic nuclide, increases with altitude, and thus 7Be is generally considered to be a tracer of upper tropospheric and stratospheric airmasses; 210Pb is a decay product of 222Rn gas emanating from continental soils and thus is generally considered to be a tracer of continental airmasses. In this report we summarize data on seasonal trends in surface air 7Be and 210Pb concentrations at BRW, MLO, SMP, and SPO.
Materials and Methods
Aerosol samplers drawing about 1700 standard cubic meters of air per day through polypropylene filters are in continuous operation at each site. Four samples are collected each month and mailed to our laboratory for analysis. A portion is cut from each filter and assembled into a monthly composite sample which is then subjected to gamma spectroscopic analysis. Gamma spectrometers used at our laboratory in the 1970s and 1980s were not capable of measuring 210Pb, thus a considerably larger data set exists for surface air concentrations of 7Be than for 210Pb. Analysis for 210Pb began between 1981 and 1983 at BRW, MLO, and SPO, and in 1989 at SMO; 7Be analyses began in 1971 at MLO and SPO, in 1976 at BRW and in 1977 at SMO.
Results
Figure 1 shows average monthly 7Be and 210Pb surface air concentrations at BRW, MLO, SMO, and SPO. Seasonal variations in one or both nuclides are apparent at all four sites reflecting a combination of seasonal variations in vertical mixing rates, lateral transport, and aerosol scavenging rates specific to each site. At BRW, concentrations of both 7Be and 210Pb exhibit clear seasonal variations. The timing of these variations suggests that the processes responsible for the arctic haze phenomenon, i.e., seasonal variations in transport from Eurasia [Harris and Kahl, 1994] and increased aerosol scavenging rates during the arctic summer [Barrie et al., 1981], play an important role in the seasonal cycles of 210Pb and 7Be.
Fig. 1. Box plots of 7Be and 210Pb concentrations in surface air at Barrow, Mauna Loa, Samoa and the South Pole grouped by month. Each box shows the range in which the central 50% of the data fall. The box borders show the first and third quartiles and the crossbar shows the median; values exceeding the quartile values by more than factor of 1.5, the interquartile distance, are marked as circles.
At MLO there is no clear seasonal variation in 7Be concentration, however, 210Pb concentrations exhibit a seasonal high from March through June coinciding with the Asian dust season [Parrington et al., 1983]. The increase in 210Pb concentration at MLO most likely results from the same process responsible for the influx of Asian dust: the lofting of surface air to high altitudes over Asia followed by rapid transport through the free troposphere by strong westerlies prevalent at this time of the year.
At SMO 7Be and 210Pb concentrations both exhibit seasonal cycles. Concentrations of both nuclides are relatively low from January through April, but the season of high 210Pb concentration tends to be shorter and tends to be centered later in the year compared to the season of high 7Be concentration. Seasonal variations in transport of air originating at high altitudes explain some of the seasonal variability in surface ozone concentrations [Harris and Oltmans, 1997] and could likewise explain some of the seasonal variability in 7Be. It has also been suggested that seasonal variations in precipitation scavenging plays a role in the seasonal cycle of 7Be at SMO [Feeley, 1989]; such variations could cause seasonal variations in 210Pb at SMO as well.
At SPO 7Be and 210Pb concentrations exhibit similar seasonal cycles with higher concentrations of both nuclides occurring during the austral summer months. This matches the observed seasonality in crustal element concentrations at the South Pole which are thought to arise from enhanced seasonal transport of lower latitude airmasses through the midtroposphere [Tuncel et al., 1989].
Acknowledgment. We wish to thank the NOAA CMDL staff at BRW, MLO, SMO, and SPO for the collection of air filter samples for SASP.
References
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