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A comparison of techniques for automatic aerosol mass concentration measurement
J. Aerosol Sci., 1977, Vol. 8, pp. 73 to 81. Pergamon Press. Printed in Great Britain.
A COMPARISON AEROSOL
OF TECHNIQUES
MASS CONCENTRATION
A. G.
CLARKE,
FOR
AUTOMATIC
MEASUREMENT
M. A. MOGHADASSIand ALAN WILLIAMS
Department of Fuel and Combustion Science, University of Leeds, Leeds, England (Received 21 June 1976; and in revised,]brm 16 August 1976) A b s t r a c t - - M a s s concentrations of ,atmospheric aerosols measured by an integrating nephelometer a n d a piezo electric quartz crystal particle mass monitor have been compared in detail. The results were evaluated relative to a filtration method as standard and the three techniques were found to be equivalent to within + 25/~g/m a over the range 30-200,ug/m 3. The advantages and disadvantages of these instruments for medium term (daily) averages and short term peak concentration measurements are discussed. Differing responses of the methods to small and large particles revealed a sampling error related to wind velocity. A drop in sampled mass concentration by up to a factor of two due to wind speeds up to 9 m/sec was observed.
INTRODUCTION As part of a study of atmospheric aerosols in the Leeds urban area a detailed comparison has been made of three techniques for monitoring aerosol mass concentration. The primary reference measurement was the filtered mass and against this was compared an integrating nephelometer and a particle mass monitor which uses a piezo-electric quartz crystal to weigh collected particles. A unified sampling system was employed so that possible sampling errors could be assessed. The integrating nephelometer is becoming widely used for ambient air monitoring of particulates. This follows extensive development and use in the U.S.A. (Ahlquist and Charlson, 1967; Charlson et al., 1968; Horvath and Noll, 1969) and to a lesser extent in the U.K. (Garland and Rae, 1970, Eggleton and Atkins, 1972). The nephelometer measures the extinction coefficient (b) of atmospheric aerosol due to light scattering. Correlations with mass concentration M have been investigated by a number of workers (Charlson et al., 1968; Ettinger and Royer, 1971; Kretzschmar, 1975) and to a good approximation the relationship b = kM
(1)
is valid. For very clean air it is necessary to allow for the scattering contribution of pure air giving b = A + kM
(2)
With b in units of 10-4m -1 the value of A for pure air is 0.23. This value is based on theoretical calculations for Rayleigh scattering at a wavelength of 500/am (Charlson et al., 1969). Other authors prefer the lower value 0.115 × 10 -4 m -1 which corresponds to a greater visual range in clear air (Davies, 1976bi Middleton, 1958). The difference is negligible for most situations in urban atmospheres since b is dominated by the scattering of light from aerosols. With M in pg/m 3 the range of k values found is quite large. Charlson et al. (1968) in a survey of six different sites obtained values from 0.017 to 0.036 with a mean of 0.026. For low concentrations in a non-urban environment Ettinger and Royer (1971) found k = 0.03 + 0.01 using equation (1). More recently Kretzschmar (1975) fitted his data to equation (2) with A = 0.68 and k = 0.034. At high humidity the relationship breaks down due to the growth of deliquescent aerosol particles by water absorption. The variation of size, mass and extinction coefficient with relative humidity is critically dependent on the chemical composition. 73 A.s. 8/2--A
74
A.G. CLARKE,M. A. MOGHAt)ASSland ALANWILLIAMS
In the mass monitor, particles are electrostatically precipitated on to a quartz crystal surface. (Olin et al., 1971). The frequency shift of the crystal is linearly dependent upon the deposited mass until the crystal becomes overloaded. This point is dependent upon the nature of the aerosol and the sampling conditions such as relative humidity. For ambient aerosol, overloading has been found in this work to begin in the range 30-50 #g accumulated mass. With a mass concentration of 100#g/m a and a flow rate of one litre per minute the instrument can therefore only give reliable results for 5-8 hr before the crystal needs cleaning. One way of overcoming this problem is to arrange a sequential sampling system so that the particle laden air is only sampled for preset intervals with a given frequency e.g. 1 min every 5 rain, 1 hr every 3 hr etc. For the remaining time clean filtered air is sampled. With this modification the mass monitor moves into the class of instruments valuable for long term monitoring, requiring attention only once a day or less depending on the fraction of total time the ambient air is being sampled. Although there have been a number of nephelometer vs filtered mass and mass minitor vs filtered mass comparisons it was felt that a detailed comparison of all three techniques under the same conditions would be valuable. Mass monitor vs nephelometer comparisons can be made at 5 min intervals or less to establish their equivalence as techniques for monitoring short term fluctuations of mass concentration. Longer period averages and comparison with filtration establishes their absolute accuracy. The results that follow were obtained over a period of 28 days in September/October 1975, sampling for about 8 hr per day. Since instrument comparison was the primary objective no attempt was made in these runs to measure 24 hr averages for the Leeds atmosphere and no discussion of the wider significance of the levels obtained is included. EXPERIMENTAL DETAILS Atmospheric aerosol was sampled from a room on the roof of the Houldsworth School of Applied Science, University of Leeds. The sampling system is shown in Fig. 1. Air was drawn at a flow rate of 400 1/min through a vertical 1.4 m pipe, 50 mm i.d. reaching i m above the store room roof or 4-5 m above the general roof level. The pipe passed through a window to cylindrical chamber (500mm x 150mmdia.) with a conical inlet section from which air streams to the various devices were drawn. The mass monitor inlet, drawing 1 l/min, was designed to give isokinetic sampling. The air then passed via the sequential sampling system to the monitor. (Thermo Systems
'X
monitoIr
\
Mass
]
~ ~ Nephelomet erC~J~ I m01
. I Recorder
Fig. 1. Schematic arrangement of the experimental sampling system. S.S.--sequential sampling; t.h.--thermohygrometer.
Automatic aerosol mass concentrationmeasurement
75
Inc. 3205 A with Model 10320 sequential sampling system). The open faced filter holder was mounted directly in the sampling chamber and employed 7 cm dia. Whatman glass fibre filters, G F C grade, with a flow rate of 2501/min. The nephelometer air stream (1501/min) was drawn through an electrical air heater before entering the instrument. (Meteorology Research Inc., Model 1550). Since the objective was to measure mass concentrations of solid aerosols, but not the visibility, it was necessary to dry out aqueous droplets which occur at high relative humidity. The heater achieved a temperature rise above ambient air of about 20°C. With a residence time of the order of 0.5 sec. before the particles enter the nephelometer this should cause evaporation to dryness of all except very large droplets and those such as H2SO4 which cannot by their nature be dried out. During the sampling period there were no heavy mists or fogs which might contain very large droplets and it is assumed that the nephelometer reading was for dry particles. Provision was made in the sampling chamber for a ThermoHygrometer (Wallac EP-400) and the analogue outputs representing the relative humidity, the mass concentration and the extinction coefficient were connected to a three pen chart recorder. The design sampling periods for the mass monitor are 0.6 sec., 1 min or 10 min. As well as continuous sampling, the instrument can be set to sample at intervals of 5, 10, 30 or 60 min. The use of other intervals requires an external timing system. To try to obtain short term and long term information on mass concentration variations the system was set to sample one minute in every five. The next option giving a 24 hr capability without overloading the crystal would have been 10 min in each hour. This would have given no information about short term variations. The nephelometer was used with a flashing rate of one per 4 sec and a time constant of 100 sec. The nephelometer output therefore represents a running average for b over a period of about 2 min. Faster flashing rates and shorter time constants are available if required. In practice 5 min averages of the readings were taken for comparison with the mass monitor representative value for that period. These data were used to compute hourly and "daily" (8 hr) averages for statistical analysis and comparison with the daily average filtration results. RESULTS (a) Daily mean mass concentrations The daily (8 hr) mass concentration averages for the mass monitor (Mm.m) are plotted against those obtained by filtration (MF) in Fig. 2. The regression line is: M,,.,, = 13.3 + 0.77 MF,
(3)
with correlation coefficient 0.95 and 95% confidence limits on M,~.m of _+23 #g/m 3. Mm.m = M r at 58 pg/m a. For the nephelometer the daily average extinction coefficients were converted to mass concentrations using the manufacturer's recommended factor: Mn(#g/m 3) = 38b(10 -4 m - 1)
(4)
and the results are plotted against the filtration results in Fig. 3. The regression line is: Mn = 27 + 0.81 MF,
(5)
with correlation coefficient 0.90 and 95% confidence limits on M~ of _+36 #g/m a. At low concentrations the points tend to lie above the M~ = Me line. The main factor appeared to be a sampling error which occurred at high wind speeds and this phenomenon is discussed in detail in section (c) below. If the days on which the daily average wind speed was greater than 6 m.sec -1 are excluded an improved correlation is obtained. The suspect data are those marked by crosses in Fig. 3. The preferred regression line is: M n = 15.4 + 0.85 MF,
(6)
76
A, G. CLARKE, M. A. MOGHADASSI and ALAN WILLIAMS 200
/
I 7 .=
15C
125 =." o
I00
E
75
:~
50
.=
z5
I 25
50
75
i00
Filter,
125
150
175
200
/.¢g.m -3
Fig. 2. Correlation between mass concentrations obtained with the mass monitor M . . filtration Mr. Regression line equation (3).
and
with correlation coefficient 0.92 and 95% confidence limits on M. of + 24 pg/m 3. This equation corresponds to: b = 0.405 + 0.022 M r.
(7)
The pure air extinction coefficient (0.23) contributes over half the intercept. The slope is 157/o less than the average slope of 0.026 found by Charlson et al. (1968) but it is well within their range of values (0.0174).036). For a mass concentration of 100 #g/m 3 the two parameter equation (7) (our results) gives the same b value as the one parameter equation (4) (Charlson's results). The overall mean mass concentrations for the 28 days and the means with the 9 wind affected days excluded are shown in Table 1. The nephelometer response to particles depends on the product Er2n where n is the number of particles per unit volume of radius r and extinction efficiency E. The
5!
200
150 125
; g
10(3
x
75
z
2~
25
50
I
75
Filter,
I
I00
I
125
I
150
I
175
200
/~g.m -3
Fig. 3. Correlation between mass concentrations obtained with the nephelometer M. and filtration M p Data believed to be affected by sampling errors are shown as crosses and were excluded when calculating the regression line equation (6). M , calculated from b using equation (4).
Automatic aerosol mass concentration measurement
77
Table 1. Mean mass concentrations ( p g . m -3) for the total sampling period
Filter M a s s monitor Nephelometer*
All days (28)
Low wind speed days (19)
74 70 86
91 8l 94
* Using M . (~tgm -3) = 0 . 0 2 6 b ( 1 0 - 4 m
1).
mass depends on rZn. For a size distribution typical of Los Angeles smog Davies (1975) has shown that the extinction coefficient is largely determined by particles in the range 0.1-0.7/~m with the maximum contribution at 0.25 ttm. The nephelometer does not "see" particles less than 0.1/tm or greater than about 2.5 pm although these contribute significantly to the mass concentration. The good correlation between extinction coefficient and mass concentration therefore demonstrates that the size distributions of aerosols in an urban area do not vary greatly from day to day despite wide variations in concentration. The fairly good agreement between the b vs M equations derived by workers in different locations point to the same conclusion. This is the assumption on which the use of the nephelometer as an instrument for mass concentration measurement is based. One might anticipate a closer agreement between the extinction coefficient and the respirable dust mass concentration. Such a comparison would be possible using the version of the particle mass monitor which employs a cyclone before the crystal collector to remove large particles. (Sem and Tsurubayashi, 1975). (b) D e t a i l e d statistical analysis The statistical significance of any differences between the methods was assessed using t-tests. The usual t-test for the comparison of the means of two sets of data involves the standard deviations of the daily values about the period mean for each method. These standard deviations are largely determined-by day to day variations common to both methods and any systematic instrumental errors tend to be masked. The test was therefore carried out using the "method of paired comparisons" (Davies, 1961) which uses the daily difference between methods, dr -= M~j - Mzj. The expression for t is: (8)
t = dNl/E/sd,
where S2d = Igj(dj - d)2/(N -
1).
(9)
N is the total number of days and the number of degrees of freedom is (N - 1). The hypothesis is tested that d is significantly different from zero. The results are shown in Table 2 both for the 28 days and the 19 days believed to be unaffected by wind. For 95~o confidence that d # 0, t should be greater than 2.05 or less than -2.05. (N = 28). For N = 19 the corresponding figures are +2.09. From Table 2 it can be seen that for the nephelometer and filter methods d is significantly different to zero for the 28 days but not for the 19 days. This confirms the Table 2. t-tests for comparison of methods of measurement of the mass concentration t values (equation 8) N = 28 N = 19 Nephelometer---Filter Mass M o n i t o r - - F i l t e r
2.91 - 1.14
0.73 - 2.66
For 95~o confidence that the differences between methods are insignificant Itl < 2.05 (N = 28) or 2.09 (N = 19).
78
A.G. CLARKE, M. A. MOGHADASSIand ALAN WILLIAMS
qualitative conclusion from Fig. 3 that there is a systematic error present in the 9 excluded results which is believed to be a sampling error. For the mass monitor and filter methods the situation is reversed. For all 28 days the differences are insignificant while for the 19 days they are significant. From Fig. 2 this would appear to be due to the low mass monitor readings relative to the filter at high concentrations. The windy days generally have low concentrations and their exclusion has the effect that the differences at high concentrations become more significant. The mass monitor and filter should be equally affected by any sampling errors at the main air inlet and the results point to a small instrumental error in the mass monitor. (c) Samplin9 errors With a linear velocity of over 3 m. sec-1 in the main sampling tube losses due to diffusion to the walls or sedimentation should be negligible. However the unshielded vertical sampling inlet made possible a reduction in sampling efficiency with increasing horizontal wind speed. The effect on the nephelometer reading would be small since it is mainly determined by the number of 0.1 to 2 particles whose sampling efficiency would be relatively unaffected by wind speed. It is the larger particles which tend to be lost because their higher momentum prevents them following the right-angled bend of the air stream into the sampling inlet. The effect on the collected mass can be severe since a few large particles may contribute significantly to the total. For example Ettinger and Royer (1971) found 30-60~o of the mass lay in particles greater than 5/~m dia. The expected drop in filtered mass Mr relative to the mass M, computed from the nephelometer reading via equation 4 has been observed. Figure 4 shows a plot of the ratio M,/Mv against wind speed. Wind speed data are the means for the period 9 a.m.-6 p.m. at Wilsden Meteorological Station, near Bradford, 20 km west of Leeds. Up to 6 m. sec-1 there is a random scatter of points near the value 1.0. Above this figure there are positive deviations to over 2.0 representing a drop in sampling efficiency on a mass basis to 50~, assuming the b values are unaffected. The mass concentration ratio M(sampled)/M(true) is defined as the aspiration coefficient A. Calculations of A values for vertical sampling tubes in a wind have been made by Davies (1976a) for various particle sizes. Using wind speeds of 10 and 5 m.sec-1, which correspond to wind speed to sampling velocity ratios of 3 and 1.5 respectively, and allowing for the 50 mm inlet dia. the approximate values of the aspiration coefficient shown in Table 3 were calculated by interpolation of Davies' data. For particles 5 #m and below the calculations suggest that the sampling efficiency was greater than 73~o every day and greater than 93~o in all except the 9 wind-affected Wind speed/sampling velocity ratio
I
2
3
2.r. "--+
18 L
~ + / / +
+'-~+ ~ '
+
+
I
+ I.C "
0.f2
i ~.i. l_~I- "I"
++
:
+
-I+
O.E
I
I
J
I
3
,
I
5
Wind speed,
L
I
7
,
I
9
m.s -~
Fig. 4. The effect of wind speed on the mass concentration ratio nephelometer M./filter MF.
Automatic aerosol mass concentration measurement
79
Table 3. Theoretical aspiration coefficients(sampling efficiencies)for the experimental sampling system Aerodynamic particle diameter (#m)
Wind speed 10 m.sec -1 5 m.sec -1
1
5 10 50
1
1
0.73 0:20 0.05
0.93 0.68 0.05
days. The extinction coefficient can therefore be considered to be unaffected by sampling errors to within the accuracy of the instrument. At high wind speeds over half the 10 p m and virtually all the 50 p m particles are lost and Fig. 4 indicates the importance of these particles to the filtered mass. This type of error could probably have been reduced by shielding the inlet so that the air is sampled from a nearly stagnant zone. (May, 1967, Davies, 1976a). (d) Short period results The data for several days were analysed to compare the performance of the nephelometer and mass monitor as instruments for peak concentration measurements. In our experiment five minute values were taken but it should be noted that while these refer to 5 min average values for the nephelometer they refer to one minute's sampling out of five for the mass monitor due to the sequential sampling system. F o r constant concentration conditions the standard deviation of 5 rain values, $5, about the daily mean would show r a n d o m fluctuations due to the instrument. These would be indistinguishable from short term r a n d o m fluctuations in concentration if only one instrument was considered. Over a period of an hour these r a n d o m fluctuations should be averaged out and the standard deviation of hourly means about the daily mean $6o shows the extent of n o n - r a n d o m variations in concentration. This variation also increases S 5. It is interesting to plot $5 against $6o as shown in Fig. 5 using 9 days results. For the nephelometer the points lie on a straight line of unit slope passing nearly through the origin i.e. S 5 g $6o. This indicates that the 5 rain values show little fluctuation about the hourly means. For the mass monitor there is considerably more short term fluctuation giving $5 > $6o Even when $6o tends to zero corresponding to essentially constant mass concentrati6ns during the day there is a standard deviation of 20-30 #g m - 3.
6C
5c ,_~ 4C
•
,
~4 .~ 3c 2(?
y, I
I0
I
20
60min
I
30
S D.,
I
40
I
50
I
60
~g m - 3
Fig. 5. Plot of the standard deviations of 5 min values against the standard deviations of the hourly mean values for both mass monitor (full line) and nephelometer (dashed line) for 9 days. Both standard deviations relative to the daily mean.
80
A.G. CLARKE,M. A. MOGHADASSIand ALAN WILLIAMS
This short term fluctuation is partly due to instrument limitations and partly to variations in the sampled mass. The mass monitor sensitivity is specified in terms of the frequency shift of the crystals and is quoted as 180 Hz//ag with a measurement error of 1 Hz corresponding to 0.006/ag. At a mass concentration of 50/ag/m 3 the total sampled mass in one minute is 0.05 #g so that the instrumental error is of the order of 11~o. Since the observed standard deviation on the hourly mean can be about 30/ag/m 3 (60~o) a substantial part of this variation must be due to the actual sampled mass, unless the instrument is performing below specification. A variation in mass concentration could be due to changing numbers of particles over the whole size spectrum or to fluctuations mainly in the number of large particles. Since the nephelometer output is relatively constant the latter explanation is most likely. One 40#m dia. spherical particle or eight 20 #m particles of density 2 have a mass of 0.06/ag--more than the figure for the total sampled mass in l min quoted above. So there is a fundamental limitation to this type of measurement. The aerosol mass concentration is not well defined in a volume of 1 litre, however well mixed, because of the random fluctuations in the very small number of giant particles. Reliable measurements with the mass monitor in ambient air do require at least a 10min sampling period i.e. a 101 sampled volume. For shorter sampling periods to give accurate results would require the development of an instrument with a higher flow rate. CONCLUSIONS
1. Comparison of the mass concentrations obtained by the particle mass monitor, nephelometer and filtration indicate that they can give the same results for daily means to within +25 #g/m 3 with 95~o confidence in the range 30-200/ag/m 3. The differences found were not entirely random for the mass monitor which gave slightly lower results than filtration at high concentrations. 2. For filtration and the mass monitor particular care is required in the design of the sampling system inlet if accurate results are to be obtained. High wind speeds ( > 6 m . s e c -1) are believed to have led to the loss of 50~ of the mass through the failure of large particles to enter the sampling tube. For the nephelometer the sampling system requirements are less stringent since the instrument responds largely to the sub. 2/am particle size range which are efficiently sampled for most combinations of wind speed, sampling tube diameter and flow velocity likely to be employed. 3. The mass monitor can be used for 24 hr continuous sampling if fitted with a sequential sampling system giving for example a 1 min in 5 or 10min per hour capability. This arrangement is necessary to avoid overloading the crystal. Because of the considerable random fluctuations in the one minute values the extension of the overall sampling period is made at the cost of a loss in accuracy for the monitoring of peak concentrations of less than one hour's duration. 4. The correlation between the light extinction coefficient and the mass concentration is good, a correlation coefficient of 0.92 being found in this work. However the range of the slopes of the regression lines between b and the filtered mass found by different workers is wide (___50~o). These differences may be partly due to experimental factors and partly to variations in size distribution and chemical composition of the measured aerosol. Consequently the nephelometer cannot be regarded as a primary instrument for the measurement of total mass concentration. Its advantages are that it can combine short term measurements of peak concentrations with long term measurements in one instrument requiring minimal maintenance and infrequent calibration. Acknowledyement This research was supported by a grant from the Science Research Council whose assistance is gratefully acknowledged.
REFERENCES Ahlquist, N. C. and Charlson, R. J. (1967) J. Air Pollut. Control Ass. 17, 467. Charlson, R. J., Ahlquist, N. C. and Horvath, H. 0968) Atmos. Env. 2, 455.
Automatic aerosol mass concentration measurement
81
Charlson, R. J., Ahlquist, N. C., Selvidge, H. and MacCready, P. B. (1969) J. Air Pollut. Control Ass. 19, 937. Davies, C. N. (1976a) Physics of Sampling Particles in Flowing Gases, p. 624. Chemistry and Industry. Davies, C. N. (1975) J. Aerosol Sci. 6, 335. Davies, C. N. (1976b) Private communication. Davies, O. L. (Editor) (1961) Statistical Methods in Research and Production, p. 56. Oliver and Boyd, London. Eggleton, A. E. J. and Atkins, D. H. (1972) Results of the Teesside Investigation. Atomic Energy Research Establishment Report No. AERE-R6983. Ettinger, H. J. and Royer, G. W. (1971). Paper No. 71-81 presented at the 64th Annual Meeting of the Air Pollut. Control Ass. Garland, J. A. and Rae, J. B. (1970) J. Sci. Instrum. 3, 275. Horvath, H. and Noll, K. E. (1969) Atmos. Env. 3, 543. Kretzschmar, J. G. (1975) Atmos. Env. 9, 931. May, K. R. (1967) Airborne Microbes (Edited by P. H. Gregory and J. L. Monteith), p. 60. Cambridge University Press. Middleton, W. E. K. (1958) Vision Through the Atmosphere, p. 19. University of Toronto Press. Olin, J. G., Sere., G. J. and Christenson, D. L. (1971) Am. lnd. Hygiene Ass. J. 32, 209. Sere, G. J. and Tsurubayashi, K. (1975) Am. lnd. Hygiene Ass. 36, 791.
A COMPARISON AEROSOL
OF TECHNIQUES
MASS CONCENTRATION
A. G.
CLARKE,
FOR
AUTOMATIC
MEASUREMENT
M. A. MOGHADASSIand ALAN WILLIAMS
Department of Fuel and Combustion Science, University of Leeds, Leeds, England (Received 21 June 1976; and in revised,]brm 16 August 1976) A b s t r a c t - - M a s s concentrations of ,atmospheric aerosols measured by an integrating nephelometer a n d a piezo electric quartz crystal particle mass monitor have been compared in detail. The results were evaluated relative to a filtration method as standard and the three techniques were found to be equivalent to within + 25/~g/m a over the range 30-200,ug/m 3. The advantages and disadvantages of these instruments for medium term (daily) averages and short term peak concentration measurements are discussed. Differing responses of the methods to small and large particles revealed a sampling error related to wind velocity. A drop in sampled mass concentration by up to a factor of two due to wind speeds up to 9 m/sec was observed.
INTRODUCTION As part of a study of atmospheric aerosols in the Leeds urban area a detailed comparison has been made of three techniques for monitoring aerosol mass concentration. The primary reference measurement was the filtered mass and against this was compared an integrating nephelometer and a particle mass monitor which uses a piezo-electric quartz crystal to weigh collected particles. A unified sampling system was employed so that possible sampling errors could be assessed. The integrating nephelometer is becoming widely used for ambient air monitoring of particulates. This follows extensive development and use in the U.S.A. (Ahlquist and Charlson, 1967; Charlson et al., 1968; Horvath and Noll, 1969) and to a lesser extent in the U.K. (Garland and Rae, 1970, Eggleton and Atkins, 1972). The nephelometer measures the extinction coefficient (b) of atmospheric aerosol due to light scattering. Correlations with mass concentration M have been investigated by a number of workers (Charlson et al., 1968; Ettinger and Royer, 1971; Kretzschmar, 1975) and to a good approximation the relationship b = kM
(1)
is valid. For very clean air it is necessary to allow for the scattering contribution of pure air giving b = A + kM
(2)
With b in units of 10-4m -1 the value of A for pure air is 0.23. This value is based on theoretical calculations for Rayleigh scattering at a wavelength of 500/am (Charlson et al., 1969). Other authors prefer the lower value 0.115 × 10 -4 m -1 which corresponds to a greater visual range in clear air (Davies, 1976bi Middleton, 1958). The difference is negligible for most situations in urban atmospheres since b is dominated by the scattering of light from aerosols. With M in pg/m 3 the range of k values found is quite large. Charlson et al. (1968) in a survey of six different sites obtained values from 0.017 to 0.036 with a mean of 0.026. For low concentrations in a non-urban environment Ettinger and Royer (1971) found k = 0.03 + 0.01 using equation (1). More recently Kretzschmar (1975) fitted his data to equation (2) with A = 0.68 and k = 0.034. At high humidity the relationship breaks down due to the growth of deliquescent aerosol particles by water absorption. The variation of size, mass and extinction coefficient with relative humidity is critically dependent on the chemical composition. 73 A.s. 8/2--A
74
A.G. CLARKE,M. A. MOGHAt)ASSland ALANWILLIAMS
In the mass monitor, particles are electrostatically precipitated on to a quartz crystal surface. (Olin et al., 1971). The frequency shift of the crystal is linearly dependent upon the deposited mass until the crystal becomes overloaded. This point is dependent upon the nature of the aerosol and the sampling conditions such as relative humidity. For ambient aerosol, overloading has been found in this work to begin in the range 30-50 #g accumulated mass. With a mass concentration of 100#g/m a and a flow rate of one litre per minute the instrument can therefore only give reliable results for 5-8 hr before the crystal needs cleaning. One way of overcoming this problem is to arrange a sequential sampling system so that the particle laden air is only sampled for preset intervals with a given frequency e.g. 1 min every 5 rain, 1 hr every 3 hr etc. For the remaining time clean filtered air is sampled. With this modification the mass monitor moves into the class of instruments valuable for long term monitoring, requiring attention only once a day or less depending on the fraction of total time the ambient air is being sampled. Although there have been a number of nephelometer vs filtered mass and mass minitor vs filtered mass comparisons it was felt that a detailed comparison of all three techniques under the same conditions would be valuable. Mass monitor vs nephelometer comparisons can be made at 5 min intervals or less to establish their equivalence as techniques for monitoring short term fluctuations of mass concentration. Longer period averages and comparison with filtration establishes their absolute accuracy. The results that follow were obtained over a period of 28 days in September/October 1975, sampling for about 8 hr per day. Since instrument comparison was the primary objective no attempt was made in these runs to measure 24 hr averages for the Leeds atmosphere and no discussion of the wider significance of the levels obtained is included. EXPERIMENTAL DETAILS Atmospheric aerosol was sampled from a room on the roof of the Houldsworth School of Applied Science, University of Leeds. The sampling system is shown in Fig. 1. Air was drawn at a flow rate of 400 1/min through a vertical 1.4 m pipe, 50 mm i.d. reaching i m above the store room roof or 4-5 m above the general roof level. The pipe passed through a window to cylindrical chamber (500mm x 150mmdia.) with a conical inlet section from which air streams to the various devices were drawn. The mass monitor inlet, drawing 1 l/min, was designed to give isokinetic sampling. The air then passed via the sequential sampling system to the monitor. (Thermo Systems
'X
monitoIr
\
Mass
]
~ ~ Nephelomet erC~J~ I m01
. I Recorder
Fig. 1. Schematic arrangement of the experimental sampling system. S.S.--sequential sampling; t.h.--thermohygrometer.
Automatic aerosol mass concentrationmeasurement
75
Inc. 3205 A with Model 10320 sequential sampling system). The open faced filter holder was mounted directly in the sampling chamber and employed 7 cm dia. Whatman glass fibre filters, G F C grade, with a flow rate of 2501/min. The nephelometer air stream (1501/min) was drawn through an electrical air heater before entering the instrument. (Meteorology Research Inc., Model 1550). Since the objective was to measure mass concentrations of solid aerosols, but not the visibility, it was necessary to dry out aqueous droplets which occur at high relative humidity. The heater achieved a temperature rise above ambient air of about 20°C. With a residence time of the order of 0.5 sec. before the particles enter the nephelometer this should cause evaporation to dryness of all except very large droplets and those such as H2SO4 which cannot by their nature be dried out. During the sampling period there were no heavy mists or fogs which might contain very large droplets and it is assumed that the nephelometer reading was for dry particles. Provision was made in the sampling chamber for a ThermoHygrometer (Wallac EP-400) and the analogue outputs representing the relative humidity, the mass concentration and the extinction coefficient were connected to a three pen chart recorder. The design sampling periods for the mass monitor are 0.6 sec., 1 min or 10 min. As well as continuous sampling, the instrument can be set to sample at intervals of 5, 10, 30 or 60 min. The use of other intervals requires an external timing system. To try to obtain short term and long term information on mass concentration variations the system was set to sample one minute in every five. The next option giving a 24 hr capability without overloading the crystal would have been 10 min in each hour. This would have given no information about short term variations. The nephelometer was used with a flashing rate of one per 4 sec and a time constant of 100 sec. The nephelometer output therefore represents a running average for b over a period of about 2 min. Faster flashing rates and shorter time constants are available if required. In practice 5 min averages of the readings were taken for comparison with the mass monitor representative value for that period. These data were used to compute hourly and "daily" (8 hr) averages for statistical analysis and comparison with the daily average filtration results. RESULTS (a) Daily mean mass concentrations The daily (8 hr) mass concentration averages for the mass monitor (Mm.m) are plotted against those obtained by filtration (MF) in Fig. 2. The regression line is: M,,.,, = 13.3 + 0.77 MF,
(3)
with correlation coefficient 0.95 and 95% confidence limits on M,~.m of _+23 #g/m 3. Mm.m = M r at 58 pg/m a. For the nephelometer the daily average extinction coefficients were converted to mass concentrations using the manufacturer's recommended factor: Mn(#g/m 3) = 38b(10 -4 m - 1)
(4)
and the results are plotted against the filtration results in Fig. 3. The regression line is: Mn = 27 + 0.81 MF,
(5)
with correlation coefficient 0.90 and 95% confidence limits on M~ of _+36 #g/m a. At low concentrations the points tend to lie above the M~ = Me line. The main factor appeared to be a sampling error which occurred at high wind speeds and this phenomenon is discussed in detail in section (c) below. If the days on which the daily average wind speed was greater than 6 m.sec -1 are excluded an improved correlation is obtained. The suspect data are those marked by crosses in Fig. 3. The preferred regression line is: M n = 15.4 + 0.85 MF,
(6)
76
A, G. CLARKE, M. A. MOGHADASSI and ALAN WILLIAMS 200
/
I 7 .=
15C
125 =." o
I00
E
75
:~
50
.=
z5
I 25
50
75
i00
Filter,
125
150
175
200
/.¢g.m -3
Fig. 2. Correlation between mass concentrations obtained with the mass monitor M . . filtration Mr. Regression line equation (3).
and
with correlation coefficient 0.92 and 95% confidence limits on M. of + 24 pg/m 3. This equation corresponds to: b = 0.405 + 0.022 M r.
(7)
The pure air extinction coefficient (0.23) contributes over half the intercept. The slope is 157/o less than the average slope of 0.026 found by Charlson et al. (1968) but it is well within their range of values (0.0174).036). For a mass concentration of 100 #g/m 3 the two parameter equation (7) (our results) gives the same b value as the one parameter equation (4) (Charlson's results). The overall mean mass concentrations for the 28 days and the means with the 9 wind affected days excluded are shown in Table 1. The nephelometer response to particles depends on the product Er2n where n is the number of particles per unit volume of radius r and extinction efficiency E. The
5!
200
150 125
; g
10(3
x
75
z
2~
25
50
I
75
Filter,
I
I00
I
125
I
150
I
175
200
/~g.m -3
Fig. 3. Correlation between mass concentrations obtained with the nephelometer M. and filtration M p Data believed to be affected by sampling errors are shown as crosses and were excluded when calculating the regression line equation (6). M , calculated from b using equation (4).
Automatic aerosol mass concentration measurement
77
Table 1. Mean mass concentrations ( p g . m -3) for the total sampling period
Filter M a s s monitor Nephelometer*
All days (28)
Low wind speed days (19)
74 70 86
91 8l 94
* Using M . (~tgm -3) = 0 . 0 2 6 b ( 1 0 - 4 m
1).
mass depends on rZn. For a size distribution typical of Los Angeles smog Davies (1975) has shown that the extinction coefficient is largely determined by particles in the range 0.1-0.7/~m with the maximum contribution at 0.25 ttm. The nephelometer does not "see" particles less than 0.1/tm or greater than about 2.5 pm although these contribute significantly to the mass concentration. The good correlation between extinction coefficient and mass concentration therefore demonstrates that the size distributions of aerosols in an urban area do not vary greatly from day to day despite wide variations in concentration. The fairly good agreement between the b vs M equations derived by workers in different locations point to the same conclusion. This is the assumption on which the use of the nephelometer as an instrument for mass concentration measurement is based. One might anticipate a closer agreement between the extinction coefficient and the respirable dust mass concentration. Such a comparison would be possible using the version of the particle mass monitor which employs a cyclone before the crystal collector to remove large particles. (Sem and Tsurubayashi, 1975). (b) D e t a i l e d statistical analysis The statistical significance of any differences between the methods was assessed using t-tests. The usual t-test for the comparison of the means of two sets of data involves the standard deviations of the daily values about the period mean for each method. These standard deviations are largely determined-by day to day variations common to both methods and any systematic instrumental errors tend to be masked. The test was therefore carried out using the "method of paired comparisons" (Davies, 1961) which uses the daily difference between methods, dr -= M~j - Mzj. The expression for t is: (8)
t = dNl/E/sd,
where S2d = Igj(dj - d)2/(N -
1).
(9)
N is the total number of days and the number of degrees of freedom is (N - 1). The hypothesis is tested that d is significantly different from zero. The results are shown in Table 2 both for the 28 days and the 19 days believed to be unaffected by wind. For 95~o confidence that d # 0, t should be greater than 2.05 or less than -2.05. (N = 28). For N = 19 the corresponding figures are +2.09. From Table 2 it can be seen that for the nephelometer and filter methods d is significantly different to zero for the 28 days but not for the 19 days. This confirms the Table 2. t-tests for comparison of methods of measurement of the mass concentration t values (equation 8) N = 28 N = 19 Nephelometer---Filter Mass M o n i t o r - - F i l t e r
2.91 - 1.14
0.73 - 2.66
For 95~o confidence that the differences between methods are insignificant Itl < 2.05 (N = 28) or 2.09 (N = 19).
78
A.G. CLARKE, M. A. MOGHADASSIand ALAN WILLIAMS
qualitative conclusion from Fig. 3 that there is a systematic error present in the 9 excluded results which is believed to be a sampling error. For the mass monitor and filter methods the situation is reversed. For all 28 days the differences are insignificant while for the 19 days they are significant. From Fig. 2 this would appear to be due to the low mass monitor readings relative to the filter at high concentrations. The windy days generally have low concentrations and their exclusion has the effect that the differences at high concentrations become more significant. The mass monitor and filter should be equally affected by any sampling errors at the main air inlet and the results point to a small instrumental error in the mass monitor. (c) Samplin9 errors With a linear velocity of over 3 m. sec-1 in the main sampling tube losses due to diffusion to the walls or sedimentation should be negligible. However the unshielded vertical sampling inlet made possible a reduction in sampling efficiency with increasing horizontal wind speed. The effect on the nephelometer reading would be small since it is mainly determined by the number of 0.1 to 2 particles whose sampling efficiency would be relatively unaffected by wind speed. It is the larger particles which tend to be lost because their higher momentum prevents them following the right-angled bend of the air stream into the sampling inlet. The effect on the collected mass can be severe since a few large particles may contribute significantly to the total. For example Ettinger and Royer (1971) found 30-60~o of the mass lay in particles greater than 5/~m dia. The expected drop in filtered mass Mr relative to the mass M, computed from the nephelometer reading via equation 4 has been observed. Figure 4 shows a plot of the ratio M,/Mv against wind speed. Wind speed data are the means for the period 9 a.m.-6 p.m. at Wilsden Meteorological Station, near Bradford, 20 km west of Leeds. Up to 6 m. sec-1 there is a random scatter of points near the value 1.0. Above this figure there are positive deviations to over 2.0 representing a drop in sampling efficiency on a mass basis to 50~, assuming the b values are unaffected. The mass concentration ratio M(sampled)/M(true) is defined as the aspiration coefficient A. Calculations of A values for vertical sampling tubes in a wind have been made by Davies (1976a) for various particle sizes. Using wind speeds of 10 and 5 m.sec-1, which correspond to wind speed to sampling velocity ratios of 3 and 1.5 respectively, and allowing for the 50 mm inlet dia. the approximate values of the aspiration coefficient shown in Table 3 were calculated by interpolation of Davies' data. For particles 5 #m and below the calculations suggest that the sampling efficiency was greater than 73~o every day and greater than 93~o in all except the 9 wind-affected Wind speed/sampling velocity ratio
I
2
3
2.r. "--+
18 L
~ + / / +
+'-~+ ~ '
+
+
I
+ I.C "
0.f2
i ~.i. l_~I- "I"
++
:
+
-I+
O.E
I
I
J
I
3
,
I
5
Wind speed,
L
I
7
,
I
9
m.s -~
Fig. 4. The effect of wind speed on the mass concentration ratio nephelometer M./filter MF.
Automatic aerosol mass concentration measurement
79
Table 3. Theoretical aspiration coefficients(sampling efficiencies)for the experimental sampling system Aerodynamic particle diameter (#m)
Wind speed 10 m.sec -1 5 m.sec -1
1
5 10 50
1
1
0.73 0:20 0.05
0.93 0.68 0.05
days. The extinction coefficient can therefore be considered to be unaffected by sampling errors to within the accuracy of the instrument. At high wind speeds over half the 10 p m and virtually all the 50 p m particles are lost and Fig. 4 indicates the importance of these particles to the filtered mass. This type of error could probably have been reduced by shielding the inlet so that the air is sampled from a nearly stagnant zone. (May, 1967, Davies, 1976a). (d) Short period results The data for several days were analysed to compare the performance of the nephelometer and mass monitor as instruments for peak concentration measurements. In our experiment five minute values were taken but it should be noted that while these refer to 5 min average values for the nephelometer they refer to one minute's sampling out of five for the mass monitor due to the sequential sampling system. F o r constant concentration conditions the standard deviation of 5 rain values, $5, about the daily mean would show r a n d o m fluctuations due to the instrument. These would be indistinguishable from short term r a n d o m fluctuations in concentration if only one instrument was considered. Over a period of an hour these r a n d o m fluctuations should be averaged out and the standard deviation of hourly means about the daily mean $6o shows the extent of n o n - r a n d o m variations in concentration. This variation also increases S 5. It is interesting to plot $5 against $6o as shown in Fig. 5 using 9 days results. For the nephelometer the points lie on a straight line of unit slope passing nearly through the origin i.e. S 5 g $6o. This indicates that the 5 rain values show little fluctuation about the hourly means. For the mass monitor there is considerably more short term fluctuation giving $5 > $6o Even when $6o tends to zero corresponding to essentially constant mass concentrati6ns during the day there is a standard deviation of 20-30 #g m - 3.
6C
5c ,_~ 4C
•
,
~4 .~ 3c 2(?
y, I
I0
I
20
60min
I
30
S D.,
I
40
I
50
I
60
~g m - 3
Fig. 5. Plot of the standard deviations of 5 min values against the standard deviations of the hourly mean values for both mass monitor (full line) and nephelometer (dashed line) for 9 days. Both standard deviations relative to the daily mean.
80
A.G. CLARKE,M. A. MOGHADASSIand ALAN WILLIAMS
This short term fluctuation is partly due to instrument limitations and partly to variations in the sampled mass. The mass monitor sensitivity is specified in terms of the frequency shift of the crystals and is quoted as 180 Hz//ag with a measurement error of 1 Hz corresponding to 0.006/ag. At a mass concentration of 50/ag/m 3 the total sampled mass in one minute is 0.05 #g so that the instrumental error is of the order of 11~o. Since the observed standard deviation on the hourly mean can be about 30/ag/m 3 (60~o) a substantial part of this variation must be due to the actual sampled mass, unless the instrument is performing below specification. A variation in mass concentration could be due to changing numbers of particles over the whole size spectrum or to fluctuations mainly in the number of large particles. Since the nephelometer output is relatively constant the latter explanation is most likely. One 40#m dia. spherical particle or eight 20 #m particles of density 2 have a mass of 0.06/ag--more than the figure for the total sampled mass in l min quoted above. So there is a fundamental limitation to this type of measurement. The aerosol mass concentration is not well defined in a volume of 1 litre, however well mixed, because of the random fluctuations in the very small number of giant particles. Reliable measurements with the mass monitor in ambient air do require at least a 10min sampling period i.e. a 101 sampled volume. For shorter sampling periods to give accurate results would require the development of an instrument with a higher flow rate. CONCLUSIONS
1. Comparison of the mass concentrations obtained by the particle mass monitor, nephelometer and filtration indicate that they can give the same results for daily means to within +25 #g/m 3 with 95~o confidence in the range 30-200/ag/m 3. The differences found were not entirely random for the mass monitor which gave slightly lower results than filtration at high concentrations. 2. For filtration and the mass monitor particular care is required in the design of the sampling system inlet if accurate results are to be obtained. High wind speeds ( > 6 m . s e c -1) are believed to have led to the loss of 50~ of the mass through the failure of large particles to enter the sampling tube. For the nephelometer the sampling system requirements are less stringent since the instrument responds largely to the sub. 2/am particle size range which are efficiently sampled for most combinations of wind speed, sampling tube diameter and flow velocity likely to be employed. 3. The mass monitor can be used for 24 hr continuous sampling if fitted with a sequential sampling system giving for example a 1 min in 5 or 10min per hour capability. This arrangement is necessary to avoid overloading the crystal. Because of the considerable random fluctuations in the one minute values the extension of the overall sampling period is made at the cost of a loss in accuracy for the monitoring of peak concentrations of less than one hour's duration. 4. The correlation between the light extinction coefficient and the mass concentration is good, a correlation coefficient of 0.92 being found in this work. However the range of the slopes of the regression lines between b and the filtered mass found by different workers is wide (___50~o). These differences may be partly due to experimental factors and partly to variations in size distribution and chemical composition of the measured aerosol. Consequently the nephelometer cannot be regarded as a primary instrument for the measurement of total mass concentration. Its advantages are that it can combine short term measurements of peak concentrations with long term measurements in one instrument requiring minimal maintenance and infrequent calibration. Acknowledyement This research was supported by a grant from the Science Research Council whose assistance is gratefully acknowledged.
REFERENCES Ahlquist, N. C. and Charlson, R. J. (1967) J. Air Pollut. Control Ass. 17, 467. Charlson, R. J., Ahlquist, N. C. and Horvath, H. 0968) Atmos. Env. 2, 455.
Automatic aerosol mass concentration measurement
81
Charlson, R. J., Ahlquist, N. C., Selvidge, H. and MacCready, P. B. (1969) J. Air Pollut. Control Ass. 19, 937. Davies, C. N. (1976a) Physics of Sampling Particles in Flowing Gases, p. 624. Chemistry and Industry. Davies, C. N. (1975) J. Aerosol Sci. 6, 335. Davies, C. N. (1976b) Private communication. Davies, O. L. (Editor) (1961) Statistical Methods in Research and Production, p. 56. Oliver and Boyd, London. Eggleton, A. E. J. and Atkins, D. H. (1972) Results of the Teesside Investigation. Atomic Energy Research Establishment Report No. AERE-R6983. Ettinger, H. J. and Royer, G. W. (1971). Paper No. 71-81 presented at the 64th Annual Meeting of the Air Pollut. Control Ass. Garland, J. A. and Rae, J. B. (1970) J. Sci. Instrum. 3, 275. Horvath, H. and Noll, K. E. (1969) Atmos. Env. 3, 543. Kretzschmar, J. G. (1975) Atmos. Env. 9, 931. May, K. R. (1967) Airborne Microbes (Edited by P. H. Gregory and J. L. Monteith), p. 60. Cambridge University Press. Middleton, W. E. K. (1958) Vision Through the Atmosphere, p. 19. University of Toronto Press. Olin, J. G., Sere., G. J. and Christenson, D. L. (1971) Am. lnd. Hygiene Ass. J. 32, 209. Sere, G. J. and Tsurubayashi, K. (1975) Am. lnd. Hygiene Ass. 36, 791.