RESIDENCE TIMES OF ARCTIC HAZE AEROSOLS USING SHORT-LIVED RADIONUCLIDES

M. Baskaran¹ and G. E. Shaw²

1: Department of Geology, Wayne State University, Detroit, MI 48202 (E-Mail: Baskaran@chem.wayne.edu)

2: Geophysical Research Institute, University of Alaska, Fairbanks, AK 99775

ABSTRACT:

Pollutants from mid latitudes (mainly from Eurasia) reach the polar regions through long-range atmospheric transport during winter. Due to very low precipitation in the arctic region, the pollutant-laden aerosols in the stable, and dark polar atmosphere are believed to have relatively longer residence times. The disequilibrium between the daughter products of radon-222, in conjunction with the concentrations of ⁷Be in arctic aerosols can be utilized to obtain information on the residence times and sources of the arctic haze.

We have analyzed 8 aerosol samples from two stations, Poker Flat and Eagle, Alaska for the concentrations and activity ratios of ⁷Be, ²¹⁰Po, and ²¹⁰Pb. The activity ratios of ²¹⁰Po/²¹⁰Pb varied between 0 and 0.18. The corresponding residence time obtained using the disequilibrium between the ²¹⁰Po and ²¹⁰Pb, varied between 0 and 39 days. It appears that there is no extraneous source of ²¹⁰Po to the arctic and most of the ²¹⁰Po is derived from the decay of ²¹⁰Pb locally. This is the first time an estimate on the residence time of aerosols from the arctic haze is determined. The ⁷Be/²¹⁰Pb activity ratios varied between 0.6 and 14.0, possibly suggesting varying inputs of ⁷Be to the arctic atmosphere. Since long-range atmospheric transport is one of the major pathways of arctic pollution, this study has potential significance on the fate and transport of inorganic and organic atmospheric pollutants in the arctic region.

INTRODUCTION:

Two classes of radionuclides have been used for the determination of the residence time of aerosols as well as other atmospheric processes. They include: (i) radioactive decay products of radon which emanates from continental surfaces into the atmosphere, and (ii) radionuclides in the atmosphere produced by cosmic-ray interactions. Of these two classes, beryllium-7 (half-life = 53.3 d) and lead-210 (half-life = 22.1 y) are two radionuclides, which have been widely used as tracers in atmospheric systems [e.g., Martell and Moore, 1974; Poet et al., 1972; Turekian et al., 1977; Baskaran, 1995]. Even though the sources of these two nuclides to the atmosphere are distinctly different, both are highly particle-reactive and thus get attached to aerosols in the atmosphere.

The Arctic haze is believed to be derived largely from long-range transport of midlatitude pollution products (Rahn, 1982). Earlier studies indicated that the concentrations of ²¹⁰Pb in Arctic aerosols are the highest so far documented in the world, suggesting that most of the ²¹⁰Pb are likely continent-derived. Information obtained from the satellite measurements and weather records from ships and islands have revealed that the amount of precipitation in the oceans adjoining the Arctic regions, such as North Atlantic and North Pacific, are considerably higher during winter than those in the continents. This will result in efficient removal of aerosols in the oceans. Thus, the continental pathway is a much more efficient transporter of aerosol to the Arctic than the Atlantic or Pacific marine pathway (Rao et al., 1976; Rahn, 1982). The marine air should be highly scavenged, while that of the stagnant Arctic air (polar air mass) should be relatively old (10-100 days, Shaw, 1991).

During the long-range atmospheric transport of air masses from sub-polar regions, if all the particle-reactive radionuclides produced from the decay of ²²²Rn during long-range atmospheric transport are scavenged from the atmosphere before reaching the arctic area, then, almost all of ²¹⁰Pb and its daughter products (²¹⁰Bi, half-life =5.01 days and ²¹⁰Po, half-life = 138 days) in the arctic aerosols would be produced from the decay of ²²²Rn. In such a case, the disequilibrium between ²¹⁰Pb and ²¹⁰Po can be utilized to obtain the residence time of aerosols under the assumption that these nuclides are instantaneously removed on to aerosols after their production. It is pertinent to point out that the time span of the existence of arctic haze is only 4-5 months and is comparable to the mean-life of ²¹⁰Po and hence the ²¹⁰Po/²¹⁰Pb ratios can be utilized to obtain information on the residence time of aerosols from the arctic haze. This is the first attempt to determine the residence time of aerosols collected from the arctic haze.

MATERIALS AND METHODS:

Large volume air samples from Poker Flat (65.1 N; 147.5 W) and Eagle (65.9 N; 141.2 W) were filtered through an air filtration assembly at a typical flow rate of 1 m³/minute. Whatman 41 cellulose fiber filter was utilized for the collection of aerosols. All the samples analyzed in this study were collected between January and March 1996. The time of collection varied between 2 and 8 days (Table 1). One half-section of the filter was digested (with 12 ml 6M HCl + 2ml Conc. HNO₃ + 0.8 ml conc. HF) in a microwave for 2 hours. After the digestion was complete, the solution was dried and taken in dilute HCl and then transferred into a gamma counting vial. After gamma counting the vial for ⁷Be and ²¹⁰Pb, the solution was further processed for polonium plating after adding ²⁰⁹Po spike. Polonium was electroplated onto a silver planchet and then the planchet was counted in an alpha spectrometer. The activities of ²¹⁰Po, ²¹⁰Pb and ⁷Be were corrected for decay and ingrowth from the mid-time of collection to the time of counting. The errors reported are propagated errors arising from the one sigma counting uncertainty due to detector calibration and background correction.

RESULTS AND DISCUSSION:

The concentrations of ⁷Be, ²¹⁰Pb and ²¹⁰Po on the 10 samples are given in Table 1. The concentrations of ⁷Be varied between 99 to 339 dpm m^-3, with a mean value of 158 dpm m^-3. Concentrations of ²¹⁰Pb and ²¹⁰Po varied between 13.4 and 155 dpm m^-3 (mean value = 42.7 dpm m^-3) and below detection limit and 9.11 dpm m^-3 (mean value = 2.51 dpm m^-3), respectively. The activity ratios of ²¹⁰Po/²¹⁰Pb and ⁷Be/²¹⁰Pb varied between 0 to 0.177 (mean value = 0.085) and 0.55 and 14.0 (mean value = 5.8), respectively.

The residence time of aerosols based on the disequilibrium between ²¹⁰Po-²¹⁰Pb is given in Table 1. This residence time is based on the following assumptions:

i) The initial concentration of ²¹⁰Po is zero at the time when the aerosols reach the arctic (²¹⁰Pb concentration need not be zero for the determination of residence time using ²¹⁰Po/²¹⁰Pb disequilibrium); and

ii) The entire Po is derived from the decay of ²¹⁰Pb and there is no extraneous source(s) of ²¹⁰Po to the aerosols collected for this study.

Earlier studies based on ²¹⁰Bi/²¹⁰Pb activity ratios on the residence times of aerosols for the continental United States yielded a few days while that of ²¹⁰Po/²¹⁰Pb resulted in a month or more (summarized in Moore et al., 1976 and Robbins, 1978). This discrepancy of residence times between ²¹⁰Bi/²¹⁰Pb and ²¹⁰Po/²¹⁰Pb was attributed to the additional sources of ²¹⁰Po to the atmosphere, including dust storms, coal-burning power plants, forest fires, and plant exudates (Moore et al., 1976). Unlike the earlier studies in the continental United States and other places (e.g., Moore et al., 1976), additional sources of ²¹⁰Po to the study area are likely insignificant. From an evaluation of various sources of ²¹⁰Po to the atmosphere in the continental United States, Moore et al. (1976) estimated that the suspended soil particles could account for 53% of the additional sources of ²¹⁰Po, 37% due to release from plant exudates, 3.2 % from stratospheric injection, 0.8% from forest fires, and the remaining 5.8% from anthropogenic sources (including phosphate fertilizer dispersion, by-product gypsum, lead production, cement and other metal production, and fossil fuel burning). The relative importance of each of these sources to the winter arctic air is unknown; however, certain input terms during winter such as anthropogenic (in the form of phosphate fertilizer, fossil fuel burning, etc.), suspended soil particles, release from plant exudates (ability of leaf surfaces to collect and retain ²¹⁰Pb and ²¹⁰Po aerosols were reported by Moore et al., 1976) are likely negligible. During the long-range transport of aerosols from Europe and Asia through the upper atmosphere, it is likely that most of the daughter products of ²²²Rn (²¹⁰Po, ²¹⁰Bi, and ²¹⁰Pb) are removed and only ²²²Rn reaches the arctic air. Recent study by Kim et al. (2000) on the atmospheric fallout of ⁷Be, ²¹⁰Pb and stable Pb in the Chesapeake Bay area indicates that stable Pb is mainly transported from remote areas through the upper atmosphere while ²¹⁰Pb transport is primarily through lower atmosphere. In addition, information obtained from the satellite measurements and weather records from ships and islands have indicated that the amount of precipitation in the oceans adjoining the Arctic regions, such as North Atlantic and North Pacific, are considerably higher during winter than those in the continents. This will result in efficient removal of aerosols in the oceans (Rao et al., 1976). However, the pollutants that reach the arctic are likely transported from remote areas through the upper troposphere without much removal. When ²²²Rn alone are transported to the arctic and there is no additional source of ²¹⁰Po other than from the decay of ²¹⁰Pb, the residence time calculated using the ²¹⁰Po/²¹⁰Pb ratio will be a realistic value of the residence time of aerosols.

The residence times varied between 0 to 39±3 days (Table 1). As we discussed before, earlier studies using ²¹⁰Po-²¹⁰Pb disequilibrium for the continental United States resulted in residence times of >1 month and our values of ~ 0 days on two samples (Eagle -1 and 2) likely indicate that there are no additional sources of ²¹⁰Po to the arctic air. The arctic haze typically lasts for about 3-4 months during winter and if the aerosols in the haze stay for that long, then, the residence time of individual aerosol particles could vary from few days to 3-4 months. In this context, the range obtained for the residence time seems quite reasonable. It must be pointed out the reliability of this residence time critically depends on the initial concentration of ²¹⁰Po in the incoming aerosols to the arctic region. If there is some amount of ²¹⁰Pb (but negligible amount of ²¹⁰Po) present when the aerosols reach the arctic region, then, that would not affect the values of the residence time. Since we do not have any other radionuclide data on the incoming aerosols prior to coming into the arctic region, we cannot rigorously address this issue.

Table 1: Sample location, concentrations of ⁷Be and ²¹⁰Pb and activity ratios of ²¹⁰Po/²¹⁰Pb and⁷Be/²¹⁰Pb of the aerosols from Arctic and non-Arctic haze.

Station	Date of Collection (Days collected)	^7Be dpm/ 10^3 m^3	^210Po dpm/ 10^3 m^3	^210Pb dpm/ 10^3 m^3	^210Po/^210Pb Activity Ratio	^7Be/^210Pb Activity Ratio	Residence Time (days)
Poker Flat - 1	8 Jan. 1996 (8.0 d)	339.3	9.11±1.00	61.3±0.9	0.149±0.025	5.50	32±5
Poker Flat - 2	26 Feb. 1996 (2.0 d)	182.4	2.87±0.49	49.3±1.7	0.058±0.012	3.69	12 ±2
Eagle - 3	4 March 1996 (1.6 d)	109.5	1.05±0.49	16.5±2.0	0.063±0.037	6.60	13±7
Eagle -4	6 March 1996 (2.0 d)	253.9	3.20±0.23	18.1±0.6	0.177±0.016	14.0	39±3
Eagle- 5	8 March 1996 (3.5 d)	159.1	3.55±0.36	26.7±1.4	0.133±0.017	5.95	29±4
Eagle - 6	11 March 1996 (1.5 d)	126.8	1.55±0.45	33.2±2.2	0.047±0.016	3.81	10±3
Eagle - 7	13 March 1996 (2.3 d)	98.8	1.90±0.27	22.1±1.5	0.086±0.015	4.45	18±3
Eagle - 8	15 March 1996 (3.0 d)	152.3	1.90±0.23	13.4±1.5	0.141±0.025	11.3	30±5

Similar studies in the continental United States by Marenco and Fontan (1972) and Nevissi et al. (1974) showed that the ²¹⁰Po/²¹⁰Pb ratio in rain samples were high as compared to the aerosols indicating longer tropospheric aerosol residence times which also could be due to stratospheric air intrusions into the stratosphere during the passage of frontal systems (Moore et al., 1976).

CONCENTRATIONS OF ⁷Be AND USE OF ⁷Be/²¹⁰Pb ACTIVITY RATIOS AS ATMOSPHERIC TRACER:

The ⁷Be concentration in 6 aerosol samples from Eagle station collected between a period of two weeks varied between 99 and 254 dpm m^-3, with a mean value of 150 dpm m^-3. The concentrations of ⁷Be in aerosols are primarily controlled by three factors: (1) variations in the amount of precipitation which causes the change in the deposition fluxes as well as air concentrations of ⁷Be; (2) changes in the stratosphere-troposphere exchange during late winter and early spring seasons; and (3) variations in the amount of ⁷Be brought by long-range transport. Since the production source functions of ²¹⁰Pb and ⁷Be are distinctly different, a relatively high correlation between the concentrations of ⁷Be and ²¹⁰Pb will provide information on the atmospheric removal behavior of these nuclides as well as information whether these two radionuclides can be used as independent atmospheric tracers (Todd et al., 1989; Baskaran et al., 1993; Baskaran, 1995). The ⁷Be/²¹⁰Pb activity ratios varied over a factor of 25 in the arctic aerosol samples while a factor of 15 has been reported in the precipitation samples from Galveston (Baskaran et al., 1993).

The concentrations of ⁷Be plotted against ²¹⁰Pb in the aerosol filter samples (Figure 2) indicate that there is a positive correlation between the two (r = 0.63, P < 0.1). A strong positive correlation between the depositional fluxes of ⁷Be and ²¹⁰Pb (in rainfall) was observed for several places suggesting that the atmospheric removal behavior of ⁷Be and ²¹⁰Pb are relatively similar and that these two nuclides cannot be used as two independent atmospheric tracers (Todd et al., 1989; Baskaran et al., 1993). From a synthesis of all the earlier published data on the depositional fluxes of ⁷Be and ²¹⁰Pb, Baskaran et al. (1993) concluded that in most of the continental and coastal stations, ⁷Be and ²¹⁰Pb cannot be used as two independent atmospheric tracers and only in oceanic and a few coastal stations, do ⁷Be and ²¹⁰Pb fluxes seem to vary independently and in such places these nuclides can be used as independent air mass tracers. Our data is limited and hence we are unable to address this issue from our pilot data set if these tracers can be used independent atmospheric tracers. More extensive database from several stations in the Arctic will enable us to address the issues on the sources and transport of arctic aerosols.

Conclusions:

From a limited number of samples, we can draw the following conclusions:

i) The activity ratios of ²¹⁰Po/²¹⁰Pb in arctic aerosol samples varied between 0 and 0.177 corresponding to a range of residence times between ~ 0 and 39 days. This range of values is in contrast with earlier studies for the continental US where additional sources of ²¹⁰Po were attributed to the longer residence time. Thus it appears that there may not be any additional sources of ²¹⁰Po to the arctic and most of the ²¹⁰Po to the study area is derived from the decay of ²¹⁰Pb present in the arctic air.

ii) There appears to be correlation between ⁷Be and ²¹⁰Pb, possibly suggesting that these tracers cannot be used as independent atmospheric tracers.

More extensive study from the beginning till the disappearance of the arctic haze will enable us to determine the usefulness of ²¹⁰Po/²¹⁰Pb and ²¹⁰Bi/²¹⁰Pb disequilibria for the residence times of arctic haze. In addition, such studies will also help to elucidate various sources of air to the arctic region.

ACKNOWLEDGEMENTS:

This work was supported in part by NSF grant (OCE-9732536).

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