Radionuclides in Surface Air at BRW, MLO, SMO, and SPO During 1994 and 1995 

JOHN KADA AND COLIN G. SANDERSON

Environmental Measurements Laboratory, U.S. Department of Energy, New York, 10014­3621

INTRODUCTION

High volume air filter samples are routinely collected by CMDL personnel at BRW, MLO, SMO, and SPO for the Surface Air Sampling Program (SASP), a global network of aerosol sampling sites operated by the Environmental Measurements Laboratory. On a global scale the SASP network provides the capability to track atmospheric releases of artificial radionuclides due to nuclear weapons tests or nuclear accidents. At BRW this was most recently demonstrated by the detection of anthropogenic radionuclides released into the atmosphere by a chemical explosion and fire at the Tomsk-7 nuclear complex in Russia, 5000 km from BRW [Larsen et al., 1994]. The SASP network also produces data on temporal and spatial trends in the worldwide distribution of the natural radionuclides ⁷Be and ²¹⁰Pb. The SASP data has proven to be a valuable check on the transport and aerosol scavenging components of global climate models [Rehfeld and Heimann, 1994] and provides the bulk of the data available for this purpose.
 

MATERIAL AND METHODS

Air samplers drawing ~1700 m³ of air per day through polypropylene air filters are in continuous operation at all four sites. Air filters are changed four times per month by local CMDL personnel and sent back to our laboratory on a monthly basis. Monthly composite samples are formed using half samples from each air filter, and these are analyzed for gamma emitting radionuclides using an n­type coaxial high­purity germanium (HPGe) detector. Under special circumstances, as discussed below for SMO samples collected in the autumn of 1995, monthly analyses are supplemented by individual filter sample analyses; subsamples representing about 15% of the active area of each filter are analyzed using a 1.5-cm diameter HPGe well detector. More detailed information on preparation and analysis of samples is available in reports containing data for the full SASP network [Larsen et. al., 1995].
 

RESULTS

In Table 1 monthly ²¹⁰Pb, ⁷Be, ⁹⁵Zr, ¹³⁷Cs, and ¹⁴⁴Ce concentrations in surface air at BRW, MLO, SMO, and SPO during 1994 and 1995 are decay corrected to the midpoint of the sample collection month. Concentrations reported as "less than" values reflect the lower limits of detection for the individual nuclides, which we take to be three times the one sigma uncertainty in the background counts in the region of the gamma spectrum used to quantitate each nuclide. The gap in ⁷Be and ²¹⁰Pb data for the SPO site in 1994 is due to the loss of samples during shipment back to EML; occasional gaps in ⁷Be data for other sites are the result of unavoidably long delays between collection and analysis of samples allowing the relatively short-lived ⁷Be (t_1/2 = 53 days) to radioactive decay below detection levels.
 

TABLE 1. Average Surface Air Concentrations of Radionuclides

  7Be

  210Pb

95Zr

137Cs

144Ce

Date

(mBq m-3)

(mBq m-3)

(mBq m-3)

(mBq m-3)

(mBq m-3)

BRW

1994

 Jan.

       n.d.

 .74 ± .09

 <87

 <3

 <10

 Feb.

2.1 ± 0.3

 .83 ± .09

 <9

 <2

 <8

 March

2.4 ± 0.3

1.12 ± .12

 <6

 <2

 <6

 April

2.3 ± 0.2

 .48 ± .05

 <4

 <1

 <5

 May

2.2 ± 0.2

 .24 ± .03

 <6

 <1

 <6

 June

0.5 ± 0.1

 .04 ± .01

 <25

 <1

 <7

 July

      n.d.

 .05 ± .02

 <31

 <2

 <11

 Aug.

0.5 ± 0.1

 .07 ± .01

 <30

 <2

 <11

 Sept.

0.9 ± 0.2

 .10 ± .02

 <56

 <3

 <5

 Oct.

1.1 ± 0.2

 .22 ± .03

 <13

 <2

 <9

 Nov.

1.6 ± 0.2

 .48 ± .05

 <20

 <2

 <3

 Dec.

1.8 ± 0.2

 .69 ± .07

 <4

 <1

 <4

1995

 Jan.

2.0 ± 0.2

 .84 ± .09

 <4

 <1

 <6

 Feb.

1.6 ± 0.2

 .71 ± .08

 <20

 <1

 <7

 March

1.7 ± 0.2

 .53 ± .06

 <13

 <1

 <6

 April

1.6 ± 0.2

 .35 ± .04

 <7

 <1

 <8

 May

1.1 ± 0.2

 .12 ± .02

 <15

 <2

 <9

 June

1.3 ± 0.2

 .09 ± .01

 <6

 <2

 <5

 July

0.7 ± 0.1

 .07 ± .01

 <2

 <1

 <2

 Aug.

0.6 ± 0.1

 .06 ± .01

 <6

 <1

 <6

 Sept.

1.9 ± 0.3

 .22 ± .03

 <9

 <2

 <10

 Oct.

1.3 ± 0.1

 .18 ± .02

 <8

 <1

 <5

 Nov.

1.6 ± 0.2

 .41 ± .05

 <8

 <1

 <8

 Dec.

1.9 ± 0.2

1.21 ± .13

 <13

 <2 

 <1

MLO

1994

 Jan.

6.3 ± 0.7

.26 ± .04

 <17

 <2

 <11

 Feb.

4.4 ± 0.5

.13 ± .02

 <12

 <3

 <14

 March

5.8 ± 0.6

.33 ± .05

 <11

 <3

 <14

 April

8.9 ± 0.9

.47 ± .06

 <9

 <3

 <16

 May

5.4 ± 0.6

.32 ± .04

 <6

 <2

 <9

 June

5.6 ± 0.6

.21 ± .03

 <11

 <3

 <14

 July

4.4 ± 0.5

.18 ± .03

 <12

 <3

 <15

 Aug.

4.1 ± 0.5

.15 ± .04

 <17

 <4

 <17

 Sept.

4.6 ± 0.5

.18 ± .04

 <11

 <3

 <15

 Oct.

6.0 ± 0.6

.30 ± .04

 <13

 <2

 <10

 Nov.

6.1 ± 0.6

.18 ± .02

 <5

 <1

 <5

 Dec.

8.0 ± 0.8

.30 ± .04

 <28

 <3

 <6

1995

 Jan.

0.3 ± 0.1

.03 ±.01

 <7

 <2

 <8

 Feb.

5.6 ± 0.6

.32 ± .04

 <4

 <1

 <5

 March

7.0 ± 0.7

.43 ± .04

 <13

 <1

 <6

 April

6.8 ± 0.7

.45 ± .07

 <14

 <4

 <18

 May

7.1 ± 0.8

.45 ± .05

 <11

 <2

 <10

 June

7.8 ± 0.9

.41 ± .05

 <22

 <3

 <18

 July

5.4 ± 0.6

.20 ± .03

 <45

 <5

 <8

 Aug.

6.9 ± 0.8

.24 ± .05

 <14

 <3

 <16

 Sept.

5.8 ± 0.6

.25 ± .05

 <14

 <4

 <16

 Oct.

7.0 ± 0.8

.31 ± .05

 <16

 <5

 <23

 Nov.

3.6 ± 0.4

.18 ± .03

 <11

 <2

 <9

 Dec.

5.8 ± 0.6

.20 ± .03

 <29

 <4 

 <8

SMO

1994

 Jan.

2.3 ± 1.2

 55 ± 12

 <504

 <2

 <6

 Feb.

2.0 ± 0.2

 64 ± 13

 <13

 <2

 <11

 March

1.6 ± 0.2

 34 ± 12

 <5

 <1

 <5

 April

1.1 ± 0.1

 27 ± 19

 <6

 <2

 <9

 May

1.7 ± 0.2

 44 ± 20

 <10

 <2

 <10

 June

2.7 ± 0.3

 52 ± 11

 <12

 <2

 <3

 July

2.3 ± 0.2

 46 ± 11

 <3

 <1

 <4

 Aug.

3.1 ± 0.3

 56 ± 21

 <5

 <2

 <10

 Sept.

2.5 ± 0.4

 105 ± 20

 <74

 <3

 <6

 Oct.

1.5 ± 0.2

 63 ± 15

 <6

 <1

 <6

 Nov.

2.0 ± 0.2

 55 ± 11

 <15

 <2

 <3

 Dec.

0.9 ± 0.1

 39 ± 10

 <8

 <2

 <8

1995

2.2 ± 0.2

 36 ± 15

 <18

 <4

 <5

 Jan.

1.7 ± 0.2

 51 ±   8

 <8

 <1

 <2

 Feb.

1.7 ± 0.2

 47 ± 15

 <5

 <2

 <7

 March

2.2 ± 0.2

 53 ± 13

 <11

 <1

 <6

 April

1.3 ± 0.2

 60 ± 14

 <50

 <3

 <5

 May

2.9 ± 0.4

 69 ± 14

 <10

 <2

 <10

 June

2.5 ± 0.3

 65 ± 20

 <9

 <2

 <10

 July

2.4 ± 0.3

 55 ± 12

 <6

 <1

 <8

 Aug.

3.2 ± 0.4

 86 ± 15

 <5

 <1

 <8

 Sept.

2.7 ± 0.3

 84 ± 22

 <8

 <2

 <11

 Oct.

1.8 ± 0.2

 51 ± 11

 <6

 <2

 <8

 Nov.

3.2 ± 0.3

 77 ±   9

 <6

 <1 

 <2

 Dec.

SPO

1994

 Jan.

6.1 ± 0.7

 36 ± 12

 <10

 <2

 <9

 Feb.

      n.d.

 28 ±1 2

 <399

 <2

 <43

 March

5.9 ± 1.0

 30 ±   6

 <141

 <1

 <12

 April

4.2 ± 1.3

 39 ± 18

 <427

 <4

 <11

 May

2.2 ± 0.6

 34 ± 15

 <72

 <2

 <12

 June

4.1 ± 0.7

 34 ± 10

 <86

 <3

 <16

 July

  n.s.

  n.s.

  n.s.

     n.s.

   n.s.

 Aug.

  n.s.

  n.s.

  n.s.

     n.s.

   n.s.

 Sept.

  n.s.

  n.s.

  n.s.

     n.s.

   n.s.

 Oct.

  n.s.

  n.s.

  n.s.

     n.s.

   n.s.

 Nov.

  n.s.

   n.s.

  n.s.

     n.s.

   n.s.

 Dec.

  n.s.

  n.s.

  n.s.

     n.s.

   n.s.

1995

 Jan.

  n.s.

    n.s.

  n.s.

     n.s.

   n.s.

 Feb.

11.2 ± 1.4

 51 ±   9

 <41

 <1

 <9

 March

11.3 ± 1.6

 54 ± 17

 <166

 <5

 <32

 April

5.9 ± 0.8

 29 ±   9

 <36

 <1

 <12

 May

5.0 ± 0.8

 41 ± 19

 <52

 <3

 <23

 June

4.9 ± 0.7

 48 ± 40

 <56

 <4

 <26

 July

3.8 ± 0.5

 33 ± 10

 <11

 <1

 <10

 Aug.

4.6 ± 0.6

 40 ±   8

 <20

 <1

 <8

 Sept.

4.3 ± 0.7

 44 ± 35

 <56

 <5

 <25

 Oct.

7.1 ± 0.8

 47 ± 15

 <77

 <4

 <7

 Nov.

10.9 ± 1.2

 83 ± 24

 <37

 <5

 <26

 Dec.

10.8 ± 1.1

 70 ± 14

 <16

 <3

 <14

        n.d. = not detected

        n.s. = no sample

Seasonal cycles in the concentrations of ⁷Be and ²¹⁰Pb at the four sites are consistent with previous observations [Larsen et al., 1995].  Concentrations of artificial radionuclides were below detection limits in all analyzed samples from the four sites.  There were no known large scale releases of anthropogenic radionuclides into the atmosphere in regions likely to influence surface air concentrations at BRW, MLO, and SPO, thus the anthropogenic radionuclide data for these three sites are as expected.  A potential source of anthropogenic radioactivity at SMO was a series of subsurface detonations of low-yield nuclear devices conducted by the French government at Muraroa and Fangataufa atolls, ~3000 km ESE of SMO between September 1995 and February 1996.  In spite of reassurances on the part of the French government, there was widespread public concern regarding the possibility of environmental releases of artificial radioactivity from these tests.  Surface air concentrations of ⁹⁵Zr, ¹³⁷Cs, and ¹⁴⁴Ce in monthly composite samples from SMO (Table 1) remained below detection limits in the last 3 months of 1995 supporting the contention that no release of anthropogenic radioactivity into the atmosphere occurred.  During the period of testing at Muraroa and Fangataufa, air filters collected at SMO were returned to our laboratory by express courier service, and monthly composite analyses were supplemented by analyses of individual air filters; artificial radionuclide activities were below detection limits in all of these samples as well.

Acknowledgment.   We wish to thank the CMDL staff at BRW, MLO, SMO, and SPO for the collection of air filter samples for SASP.
 

REFERENCES

Rehfeld, S. and M. Heimann, Three dimensional atmospheric transport simulation of the radioactive tracers ²¹⁰Pb, ⁷Be, ¹⁰Be, and ⁹⁰Sr, Max-Planck-Institut fur Meteorologie Rep. No. 144, Hamburg, 1994.

Larsen, R.J., C.G. Sanderson and J. Kada, EML surface air sampling program, 1990-1993 data, U.S. DOE Rep. EML­572, U.S. Dept. of Energy, Environmental Measurements Laboratory, New York, 1995.

Larsen, R.J., C.G. Sanderson, H.N. Lee, K.M. Decker and H.L. Beck, Fission products detected in Alaska following the Tomsk-7 accident, J. Environ. Radioact., 23, 205-209, 1994.
 

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