The effect of sampling rate on the efficiency of the Warren Spring sampler in the determination of ambient concentrations of airborne particles

Atmospheric Environment Vol. 24A, No. 8, pp. 2267 2270, 1990

0004 6981/90 $3.00 + 0.00 Pergamon Press plc

Printed in Great Britain.

TECHNICAL

NOTE

THE EFFECT OF SAMPLING RATE ON THE EFFICIENCY OF THE WARREN SPRING SAMPLER IN THE D E T E R M I N A T I O N OF AMBIENT C O N C E N T R A T I O N S OF AIRBORNE PARTICLES C. E. MILLER Environmental Health and Housing Division, University of Salford, Salford M5 4WT, U.K.

and R. LEwis Environmental Health Department, Manchester City Council, Manchester, U.K. (First received 13 March 1989 and receivedjbr publication 6 March 1990)

Al~tract--Field measurements of total suspended particulates and airborne lead concentrations are presented showing evidence of a relationship between the sampling efficiency of the Warren Spring M-type sampler and the pumping rate employed. K e y word index: Aerosols, samplers, systematic, experimental error, lead, EEC directive.

INTRODUCTION Data from measurements of urban concentrations of atmospheric lead and total suspended particulates suggest that the efficiency of an aerosol sampler with a downward facing filter may be dependent upon the pumping rate under certain circumstances. In an attempt to test the relative level of systematic and experimental error inherent in a low volume sampling technique for atmospheric particulates, an identical pair (see Fig. 1) of the Warren Spring samplers (Mclnnes, 1976) 1 m apart were used in a series of experiments in the centre of Manchester from July 1981 until June 1983. The Warren Spring M-type sampler was chosen since it is representative of a number of designs of low volume samplers employing downward facing filters and also because of its widespread use as an aerosol sampler in a variety of different monitoring exercises (see for example Davis (1981), Clayton and Wallin (1981), Turner et al. (1980)). The samplers' inlets were situated 3.5 m above ground level and were remote from point sources of pollution; their location could be taken as subject to typical urban background (rather than kerbside) concentrations. In the gravimetric analysis of total suspended particulate errors arising from variable humidity were addressed by the use of pairs of membrane filters matched in weight to within 10 gm. Following the method laid down by NIOSH (1977), the exposed filters were then digested in concentrated nitric acid and flame atomic absorption spectroscopy used to give the lead level, The duration of the sampling periods was the same for both sets of apparatus (either 1 week or two); but after operating both sets at National Survey recommended (WSL, 1966) pumping rate of c. 1.5 / min- 1, a series of measurements were then taken with the pumping rate of one sampler (B) increased by a factor of approximately x 2, and in a third series by approximately x 3, over that of the other sampler (A) 1 m distant. The mean concentrations of lead and particulate given by each set of apparatus over these three series (IIII) ofmeasurements are given in columns (2) and (3) of Table 1. For lead and particulate, and for all three series of readings,

the standard error of the group means are such that the differences in group means shown between sets A and B could be explained superficially by experimental error alone (at most particulates in Series I l l - - t h e difference is c. 13% of the mean). However, in columns (5) and (6), the mean and standard error of the difference ( B - A ) between concurrent pairs of values of concentration given by the two sets of apparatus reveal (Fig. 2) a steady and significant departure from zero for both lead and total particulate as the sampling rate of B is increased over that of A. In statistical terms, the 'method of pairs' rejects the hypothesis that the difference between the samplers is zero. Physical checks (using the method prescribed in British Standard 416 t) on the calibration of the two airflow meters at the beginning and at the conclusion of the sampling campaign gave no indication of a systematic error in airflow measurement. According to Barrett et al. (1985), such an error should produce differences in concentration propor° tional to the mean concentration: scattergrams of pairwise differences (B-A) vs mean 1/2 (A+ B) concentrations are given in Figs 3 and 4 for total particulate and lead, respectively. Considerable scatter with no obvious trend is apparent in both diagrams: linear regression on the particulate data gives a line with a slope consistent with zero well in excess of the 10% level of confidence. Whilst the lead data might appear to be indicative of a proportional error in the airflow measurement of one of the sets of apparatus, the apparently significant negative slope of the regression line disappears when the scattergrams of the three series (differentiated by pumping rate of B) are analyzed separately: all three slopes are then found to be consistent with zero with confidence exceeding the 10% level. Wind tunnel tests of this type of sampler by Ralph et al. (1982) failed to show a similar dependency on sampling rate. However only the lower of those authors' sampling rates (4.4 and 13.2 E min- t) is comparable with the highest of those shown in column (1) of Table 1; and they readily concede the possibility of a 'gravitational elutriation' effect by which coarser particles, with a terminal falling speed greater than the speed of the

2267

2268

Technical Note S E C T I O N A L VIEW Dimensions in mm TUBING TO PUMP AND METER

CONNECTOR

.

2,°

.

Fig. 1. Warren Spring M-type sampler

details of sampling head.

Table 1. Effect of pumping rate on measurement of concentrations of total particulate and lead (l-Ill separate sampling heads: IV-V shared sampling head) (1) Series I

Pumping rate* (m 3 wk - 1)

(2) Concentration of total particulate (l~g m - 3)

(3) Concentration of lead (#g m - 3 x 10- 2)

A:

14.40+0.14

53.52+4.45

64.75+5.2 ~

B:

14.72+0.18

53.08+4.34

64.70+5.2

II

A:

14.48+0.05

50.52+3.97

55.49+5.2 ~

B:

30.55-t-0.60

52.98+4.31

56.89+5.1

III

A:

15.94+0.46

41.69+3.05

43.17+2.9 "~

B:

48.96+0.43

47.63+3.47

49.49+2.8 J

C:

14.47+0.46

47.48+4.28

46.08+3.4 ~

IV

V

(4) Number of readings

(5) (6) Difference in Difference in conc. total part conc. lead (#g m - 3) (~g m - 3 × 10 - 2)

20

-0.44+0.81

17

2.46+0.80

1.40+0.8

18

5.93+0.99

6.32+0.6

16

1.50+0.64

0.29+0.3

0.01+0.34

~ 0 ~1 9 ~ 0~ 1

) )

)

D:

13.86+0.34

45.97+4.13

45.794-3.6

C:

57.94+0.88

47.29+5.54

17.87+1.3 ~

D:

56.31+0.65

47.28+5.48

18.06+1.3

)

16

"10.08 m3wk -1 -= 1 Fmin -1 .

updraught created by the sampling pump, would fail to be collected on the open face filter in conditions of lighter winds, thereby decreasing the efficiency of sampling. Wind speeds in the range 0-2 m s - 1 prevailed for 73% of the time during the experiments, This point is of more than academic interest since the EEC lead directive (82/884/EEC) specifies minimum acceptable sampling efficiencies for ranges of wind speed and aerodynamic diameters. However, it does not specify a minimum pumping rate--it stipulates only that fluctuations must not vary from 5% of the 'nominal value'. It is conceivable that the attainment of the 95 % sampling efficiency specified for 5 #m particles in wind speeds of 2 m s - 1 could be prejudiced simply by an inadequate pumping rate. Thus claims in the literature that the 'precision and accuracy' of different sampiing procedures, involving sampling rates of 1.7, 3.0, 4.9 and 6.0fmin -1, 'are sufficiently good for the results to be con-

sidered directly comparable' (Mclnnes, 1988) require further substantiation. In an attempt to examine the replicability of experiments using the M-type sampler, further series of measurements were taken at the same site as those earlier. In Series V (Table 1) two filter holders (C and D) were placed, with their centres about 5 cm apart, in one single sampling head but they were connected to separate pumps, each operating at about four times the National Survey rate. The mean pairwise difference in concentration of particulate is very small (0.01) compared with Series IV (1.50) in which the conditions were identical but for a lower pumping rate of 1.5 ~ min- 1. (Comparison of the lead figures is complicated by the reduction of lead in petrol which occurred between Series IV and V.) Since our sample consists of only two simultaneous observations, a little algebraic manipulation shows that the sample standard deviation is given by I C - D I and therefore the

Technical Note

10

20

30

40

50

I

I

,I

I

I

.....

7-

E ~

10

20

30

40

I

I

I

I

50 ,I

7

7

6

6

5-

5

5

5

4.

4

4-

4

3-

-3

6-

?×

2269

~

?~

7

t

6

~ 3-

3

g

&

°

~r~

21

}

o

21

I

-1 -2

0

1;

~ 30

2'0

410

5J0

~n"

t

2-1.

o

o.

"1

-1-

-2

-2

P u m p i n g r a t e of s a m p l e r B m3 w k "1

-21

i

o --1

1'0

2'0

~ 40

310

50

-2

P u m p i n g r a t e of s a m p l e r B m a w k "1

Fig. 2. Mean pairwise differences in concentrations as a function of pumping rate.

I/2[B+A) #gm "3 25

50

75

I

I

100 .......

l

10.0

~ 10.0

eo

•o

5.0

•

• O

::L

0.0

•

SLOPE

•

•

•

5.0

•

•

•

•

•

-- ( 0 . 5 8 " 1 " 3 , 6 3 ) x

1 0 -~

•

ee • 0

0.0

•

~

-5.0

•

-10.0

-5.0

-10.0

25

|

I

50

75

100

1/2(B+A) /xg m"3 Fig. 3. Scattergram of difference vs mean concentration of total sampled particulate. relative sample standard deviation (RSSD) (see Benarie, 1974) is proportional to I C - D I / ( C + D). The mean value of this quantity for total particulates and for lead in Series IV and V is given in Table 2. For the total particulate, the reduction which results from increasing the pumping rate is quite marked but explicable as a reflection of the decreasing influence of residual humidity effects as the total weight of

AE(AI

24:8-2

sampled particulate rises. Since absorbed water and other weighing errors do not affect the measurement of lead concentrations, the constancy of the equivalent figures for lead is not surprising. Nevertheless the reduced relative sample variance for particulates is further (if any were needed) justification for adhering to the higher pumping rate.

2270

Technical Note I / 2 { A + B ) ~ g m-3 0.50

0.75

I

1.00

I

I

0.10"

0.10

005 -

0.05

'E

'E

~,

47 +2.35)

d~

• 0.00

"

°

o. •

•°•

x 10"

.~

,

•

•

~'~ •

0.00

° •

•

°

o

-0.05-

-0.05

i

,

0:50

i

0.75

1.00

1 / 2 [ A + B ~ p,g rn"3

Fig. 4. Scattergram of difference vs mean concentration of airborne lead.

Table 2. Relative sample standard deviations Pumping rate (~ min- 1)

RSSD (total particulates)

RSSD (lead)

IV

c.1.5

0.021

0.010

V

c.6.0

0.013

0.009

Series

REFERENCES Barrett C. F. et al. (1985) A wind tunnel study and field comparison of three samplers of suspended particulate matter, p. 79. Report LR 544 (AP)M, Warren Spring Laboratory, Stevenage. Benarie M. (1974) The effect of the sample variance on the field evaluation of air pollution monitoring instruments. Atmospheric Environment 8, 1203-1204. British Standard 4161 (1968) Specification for gas meters. Part 3: Unit construction meter of 6 cubic meters ( 2 1 2 cubic feet) per hour rating. Appendix H. Accuracy test for meters, HMSO. Clayton P, and Wallin S. C. (1981) A survey of quartz concentrations in the atmosphere in the vicinity of a granite quarry. Report LR 384 (AP), Warren Spring laboratory, Stevenage.

Davis B. J. (1981) The measurement of particulate and selected metal concentrations in the neighbourhood of a steelworks, Stevenage. Warren Spring Laboratory. Report LR 415 (AP), Warren Spring Laboratory, Stevenage. EEC Directive 82/884/EEC (1982) Limit value for lead in the air, OJ No. L378, 31 December 1982, p. 15. Mclnnes G. (1976) Multi-element and sulphate in particulate surveys: monitoring locations, sampling and analytical methods, and preliminary result reporting system. Report LR 247 (AP), Warren Spring Laboratory, Stevenage. Mclnnes G. (1986) Airborne lead concentrations and the effect of reductions in the lead content of petrol, p. 6. Report LR 587 (AP) M, Warren Spring Laboratory, Stevenage. National Institute for Occupational Safety and Health (1977) Manual of Analytical Methods, Vol. 2, Cincinnati, Ohio. Ralph M. O., Barrett C. F. and Upton S, L. (1982) A wind tunnel study of the inlet effiency of Warren Spring Labor~/toty suspended particle sampler, p. 27. Report LR 420 (AP), Warren Spring Laboratory, Stevenage. Turner A. C., Carroll J. D. and Barrett C. F. (1980) The determination of environmental lead near works and roads in conjunction with the EEC blood lead survey 1978-1979. Report LR 344 (AP), Warren Spring Laboratory, Stevenage. Warren Spring Laboratory (1966)National survey of smoke and sulphur dioxide instruction manual, p. 45. Warren Spring Laboratory, Stevenage.