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Cascade Impactors i n the C h e m i c a l a n d Physical

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Characterization o f C o a l - C o m b u s t i o n A e r o s o l Particles John M. Ondov Department of Chemistry, The University of Maryland, College Park, MD 20742 Aerosol particles from combustion of pulverized coal are typically distributed bimodally with respect to size, and contain particles ranging from about 0.01 μm to over 100 μm in diameter. Development of control technology, emissions testing, and prediction of health and environmental effects often require characterization of the size distributions or aerosol particulate mass or the various chemical components. Cascade impaction provides a relatively simple and fundamen tal measurement of the mass-vs-size distribution and can yield size-segregated material in quantities adequate for determining the distributions of chemical, physical, or biologically active constituents. This paper briefly reviews impactor theory, considers the merits and short comings of four cascade impactors, and reviews the principal problems involved in using cascade impactors to measure pro perties of coal combustion aerosols. These problems include errors in measuring narrow distributions, effects of par ticle bounce and reentrainment, diffusive deposition of fine particles, and deposition of condensible/adsorbable gases. Ambiguities in data reduction are also discussed. Cascade impactors have been used extensively to provide sizesegregated particulate samples for characterizing the distributions of mass and chemical constituents in both ambient and source aero sols. In principle, conventional inertial impactors can provide accurate data on particle distributions in the range of 0.2 to 50 ym (1_,^). The lower size limit has been reduced to 0.05 ym with lowpressure impactors (3.»4.,5) id more recently, to 0.026 μπι with a microorifice impactor (6_,7_,8). Large errors in estimates of the distribution parameters can result, however, in cases where the size distribution is narrow (such as that for aerosols modified by highly efficient particulate-control devices), from the effects of particle bounce and reentrainment, and from the deposition of particles and gases from boundary streams onto impaction substrates. These problems are especially important in sampling the bimodal aerosols produced in the combusion of pulverized coal, which contain condensible gases, enormous concentrations of submicrometer particles, and predominantly dry, glassy aluminosilicate spheres that tend to stick poorly to impaction substrates. Because of particle bounce, conaT

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0097-6156/ 86/ 0319-0309$06.00/ 0 © 1986 American Chemical Society

Markuszewski and Blaustein; Fossil Fuels Utilization ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

FOSSIL FUELS UTILIZATION: ENVIRONMENTAL CONCERNS

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t a m i n a t i o n of s u b m i c r o m e t e r - p a r t i c l e f r a c t i o n s w i t h l a r g e r p a r t i c l e s i s an e s p e c i a l l y important problem i n sampling both source and ambient a e r o s o l s . These problems are reviewed below. The

P r i n c i p l e of I n e r t i a l

Impaction

The u n d e r l y i n g p r i n c i p l e of i n e r t i a l i m p a c t i o n i s embodied i n t h e concepts of r e l a x a t i o n time and s t o p p i n g d i s t a n c e . When the v e l o c i t y of a p a r t i c l e - c o n t a i n i n g gas i s changed, f o r example, by a c c e l e r a t i n g the gas through a n o z z l e o r by changing the d i r e c t i o n o f i t s f l o w , the suspended p a r t i c l e , w i t h i t s g r e a t e r i n e r t i a , l a g s behind t h e gas. The time r e q u i r e d f o r e q u i l i b r i u m t o become r e e s t a b l i s h e d i s known as the r e l a x a t i o n t i m e , and the d i s t a n c e the p a r t i c l e t r a v e l s w h i l e e q u i l i b r a t i n g i s known as the s t o p p i n g d i s t a n c e . I n an impac t o r , placement of an o b s t r u c t i o n ( i m p a c t i o n p l a t e ) normal t o t h e d i r e c t i o n of f l o w f o r c e s the gas t o change d i r e c t i o n . A p a r t i c l e i s d e p o s i t e d ( i . e . , c o l l e c t e d ) on the i m p a c t i o n p l a t e i f the s i z e of t h e p l a t e i s l a r g e w i t h r e s p e c t t o the s t o p p i n g d i s t a n c e of the p a r t i c l e . The s t o p p i n g d i s t a n c e (S) o f an a e r o s o l p a r t i c l e i s a f u n c t i o n of the diameter ( D ) , v e l o c i t y ( V ) , and d e n s i t y ( p ) o f the p a r t i c l e , and the v i s c o s i t y of the gas (μ), as shown i n E q u a t i o n 1 ( 9 ) : p

s =

p

P VpCD| 2

p

(

1

)

18μ where C i s the Cunningham s l i p c o r r e c t i o n f a c t o r , which accounts f o r d i s c o n t i n u i t y i n the t r a n s f e r of momentum t o the p a r t i c l e (as a r e s u l t o f c o l l i s i o n s w i t h gas m o l e c u l e s a t the p a r t i c l e s u r f a c e ) t h a t o c c u r s f o r p a r t i c l e s of s i z e near t h a t of the mean f r e e path of t h e gas m o l e c u l e s (9)· T y p i c a l cascade impactors c o n s i s t of a s e r i e s of n o z z l e p l a t e s , each f o l l o w e d by an i m p a c t i o n p l a t e ; each set of n o z z l e p l a t e p l u s i m p a c t i o n p l a t e i s termed a s t a g e . The s i z i n g c h a r a c t e r i s t i c s o f an i n e r t i a l impactor stage are determined by the e f f i c i e n c y w i t h w h i c h the stage c o l l e c t s p a r t i c l e s of v a r i o u s s i z e s . C o l l e c t i o n e f f i c i e n c y i s a f u n c t i o n of t h r e e d i m e n s i o n l e s s parameters: the i n e r t i a l p a r a meter (Stokes number, S t k ) , the r a t i o of the j e t - t o - p l a t e s p a c i n g t o the j e t w i d t h , and the j e t Reynolds number. The most important of these i s the i n e r t i a l parameter, which i s d e f i n e d by E q u a t i o n 2) as the r a t i o of the s t o p p i n g d i s t a n c e t o some c h a r a c t e r i s t i c dimension of the i m p a c t i o n stage ( 1 0 ) , t y p i c a l l y the r a d i u s of the n o z z l e o r jet (Dj).

s t k

. PpVpCD§/18p

(

2

)

D-j/2 P a r t i c l e c o l l e c t i o n e f f i c i e n c y i s a monotonically increasing function of the i n e r t i a l parameter and has been determined e x p e r i m e n t a l l y (1,11,12). C u s t o m a r i l y , t h i s f u n c t i o n i s r e p r e s e n t e d by a s i n g l e v a l u e , t h a t i s , the v a l u e of the Stokes number ( S t k ) t h a t corresponds to a c o l l e c t i o n e f f i c i e n c y o f 50%. Thus by r e a r r a n g i n g E q u a t i o n 2 ) , the diameter of the p a r t i c l e c o l l e c t e d w i t h 50% e f f i c i e n c y (D50) i s


25.

ONDOV

expressed follows :

Cascade Impactors in the Characterization of Aerosol Particles

311

i n terras of the Stokes number f o r 50% c o l l e c t i o n (Stk5Q) as

(3)

P a r t i c l e s w i t h l a r g e enough s t o p p i n g d i s t a n c e s w i l l be d e p o s i t e d on t h e f i r s t i m p a c t i o n p l a t e ; those w i t h somewhat s m a l l e r s t o p p i n g d i s t a n c e s w i l l be c a r r i e d t o t h e next s t a g e ; and so οη· By i n c r e a s i n g the v e l o c i t y of t h e a e r o s o l i n s u c c e s s i v e s t a g e s , t h e s t o p p i n g d i s t a n c e of the p a r t i c l e i s l i k e w i s e i n c r e a s e d , and p r o g r e s s i v e l y s m a l l e r p a r t i c l e s a r e c o l l e c t e d . The j e t v e l o c i t y can be i n c r e a s e d e i t h e r by r e d u c i n g the number of s i z e of t h e j e t s o r by i n c r e a s i n g t h e o v e r a l l f l o w r a t e of t h e a e r o s o l . Data

Reduction

I n t y p i c a l a e r o s o l s , p a r t i c l e s i z e i s d i s t r i b u t e d l o g n o r m a l l y . The d i s t r i b u t i o n parameters (median and t h e geometric s t a n d a r d d e v i a t i o n ) may be e s t i m a t e d g r a p h i c a l l y from c u m u l a t i v e p l o t s of mass vs p a r t i c l e s i z e on l o g - p r o b a b i l i t y paper, o r m a t h e m a t i c a l l y , by f i t t i n g the d a t a t o a lognorraal d i s t r i b u t i o n f u n c t i o n . By c o n v e n t i o n t h e mass median aerodynamic diameter of an a e r o s o l i s a b b r i v a t e d as MMAD, where as the mass median aerodynamic diameter of a p o p u l a t i o n of p a r t i c l e s t h a t i s a subset of an a e r o s o l i s a b b r e v i a t e d mmad. The l a t t e r c o n v e n t i o n i s used, f o r example, t o i n d i c a t e t h e median p a r t i c l e c o l l e c t e d on a s i n g l e i m p a c t i o n s t a g e . G r a v i m e t r i c o r c h e m i c a l a n a l y s i s of t h e i n d i v i d u a l i m p a c t i o n p l a t e s i s used t o determine t h e a e r o s o l mass o r t h e amount of some c h e m i c a l c o n s t i t u e n t a s s o c i a t e d w i t h the range of p a r t i c l e s i z e s c o l l e c t e d on t h e p l a t e . The s i z e d i s t r i b u t i o n of t h e measured v a r i a b l e can then be deduced by p l o t t i n g t h e d a t a a g a i n s t t h e a p p r o p r i a t e impactor stage parameter, u s u a l l y t h e D50 as determined by E q u a t i o n 3 ) . The D50, however, i s a measure of t h e diameter of a p a r t i c l e c o l l e c t e d w i t h 50% e f f i c i e n c y , and not t h e s i z e of t h e p a r t i c l e c o l l e c t e d on t h e impactor s t a g e . The t r u e d i s t r i b u t i o n of p a r t i c l e s a c t u a l l y c o l l e c t e d on t h e stage i s a f u n c t i o n of the d i s t r i b u t i o n of t h e a e r o s o l sampled and t h e e f f i c i e n c y - v s - s i z e curves f o r t h e i n d i v i d u a l s t a g e s ; o n l y by chance would t h e D50 be e q u a l t o t h e mediam diameter of p a r t i c l e s on the s t a g e . The use of D5QS i n d e t e r m i n i n g a c t u a l d i s t r i b u t i o n s t h e r e f o r e c o n t a i n s an i n t r i n s i c e r r o r . I f e f f i c i e n c y curves a r e a v a i l a b l e , t h i s i n t r i n s i c e r r o r can be reduced by u s i n g d a t a - i n v e r s i o n t e c h n i q u e s (13,14). These t e c h n i q u e s determine t h e d i s t r i b u t i o n t h a t , a f t e r m u l t i p l i c a t i o n by t h e a p p r o p r i a t e s e t s o f c a l i b r a t i o n c u r v e s , best reproduces the amounts of p a r t i c u l a t e mass c o l l e c t e d on each impactor s t a g e . I n g e n e r a l , i f t h e a e r o s o l d i s t r i b u t i o n i s wide enough and t h e mass mediam diameter (MMD) i s w i t h i n t h e range of t h e d e v i c e , D50S can p r o v i d e r e a s o n a b l y a c c u r a t e e s t i m a t e s of the a e r o s o l - d i s t r i b u t i o n parameters. E f f i c i e n c y curves f o r i m p a c t o r s a r e s i g m o i d a l , however, and c o l l e c t i o n e f f i c i e n c y f o r p a r t i c l e s of a l l s i z e s i s non-zero. Thus, when the s i z e d i s t r i b u t i o n i s narrow, as i n a e r o s o l s m o d i f i e d by p a r t i c u l a t e - c o n t r o l d e v i c e s , t h e amount of mass a t t r i b u t e d t o t h e l a r g e r p a r t i c l e s may be i n c o r r e c t . As a r u l e , the s i z e of t h e



312

largest p a r t i c l e collected on the f i r s t stage of the impactor should be determined by microscopy or by inference from other measurements. Additional s i z i n g errors, resulting from nonideal behavior, are discussed below. Nonideal Behavior The p r i n c i p a l problems i n determining size d i s t r i b u t i o n parameters with cascade impactors are wall losses, i n e f f i c i e n t c o l l e c t i o n due to p a r t i c l e bounce, deposition of gas-phase species on impaction substrates, and deposition of fine p a r t i c l e s from boundary layers. As discussed above, an i n t r i n s i c error can result from using D5QS to i n f e r size distributions from the impactor data. Depending on the sampling conditions and on the chemical and physical properties of the aerosol sampled, each of the other errors can also be s i g n i f i cant, and generally must be considered i n sampling coal combustion aerosols. Wall losses. Ideally, a l l of the aerosol sampled i n an impactor should be deposited on the c o l l e c t i o n plates or be captured by the a f t e r - f i l t e r . In practice, some of the p a r t i c l e s c o l l e c t on other i n t e r i o r surfaces and are t y p i c a l l y excluded from the analysis. Wall losses result from d i f f u s i o n of p a r t i c l e s i n turbulent eddies, from sedimentation of large p a r t i c l e s (especially i n large-particle stages with large internal volume and corresponding low gas v e l o c i t i e s ) , from e l e c t r o s t a t i c e f f e c t s , and from p a r t i c l e bounce. Wall losses appear to depend on size and hence on stage. Therefore, i f they are s i g n i f i c a n t , both the t o t a l mass concentration of the aerosol and i t s inferred size d i s t r i b u t i o n w i l l contain an error. Experience has shown that wall losses are greater for large, hard, dry p a r t i c l e s , t y p i c a l l y composed of aluminosilicate materials, than for small (submicrometer) carbonaceous or sulfate p a r t i c l e s . P a r t i c l e bounce. When p a r t i c l e s bounce off the c o l l e c t i o n surface, they may be carried to subsequent stages, where they may stick or again bounce o f f . The result i s that subsequent stages c o l l e c t more mass than i s appropriate, and the inferred p a r t i c l e - s i z e d i s t r i b u t i o n i s biased towards the smaller p a r t i c l e s . Apparently, because of increasing v e l o c i t y , p a r t i c l e s that bounce off one stage continue to bounce off the subsequent stages and are f i n a l l y collected on the a f t e r f i l t e r . As discussed below, such c o l l e c t i o n can severely l i m i t the u t i l i t y of a f t e r f i l t e r data. T y p i c a l l y , sticky substances are applied to impaction surfaces to reduce p a r t i c l e bounce. Compounds that can be "wicked" by the collected p a r t i c l e s tend to be the most effective. The significance of these and other real-world d i f f i c u l t i e s are discussed below for sampling coal combustion aerosols. Problems in Measuring Coal Combustion Aerosols In an e a r l i e r study (15) we addressed some of the problems i n obtaining accurate concentration-vs-particle-size d i s t r i b u t i o n s for elements i n stack aerosols collected downstream of an e l e c t r o s t a t i c p r e c i p i t a t o r and a Venturi wet scrubber at a c o a l - f i r e d power plant. The problems investigated were error associated with the use of the


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313

s

^50 > w a l l l o s s e s , and c o n t a m i n a t i o n of a f t e r f l i t e r s by p a r t i c l e bounce. I n a l a t e r s t u d y , we observed t h e a r t i f a c t u a l d e p o s i t i o n o f gas-phase components d u r i n g sampling ( 1 6 ) . I n both s t u d i e s we used t h e U n i v e r s i t y of Washington Mark I I I and Mark V Source Test Cascade Impactors ( 1 7 ) . Our s p e c i f i c o b j e c t i v e s were t o v e r i f y the s i z e s o f p a r t i c l e s c o l l e c t e d on i m p a c t i o n s t a g e s and a f t e r f i l t e r s and t o e s t i mate impactor e f f i c i e n c y r e l a t i v e t o c o l l e c t i o n on f i l t e r s . The u l t i m a t e g o a l was t o determine e l e m e n t a l e m i s s i o n r a t e s as a f u n c t i o n of p a r t i c l e s i z e . The Mark I I I and Mark V i m p a c t o r s used i n these s t u d i e s a r e , r e s p e c t i v e l y , 7- and 11-stage m u l t i c i r c u l a r j e t u n i t s w i t h i n t e g r a l backup f i l t e r s . These were operated i s o k i n e t i c a l l y , i n - s t a c k , w i t h Nuclepore p o l y c a r b o n a t e i m p a c t i o n s u b s t r a t e s coated w i t h vacuum g r e a s e . E l e m e n t a l c o n s t i t u e n t s of t h e p a r t i c l e s c o l l e c t e d were determined by n e u t r o n a c t i v a t i o n a n a l y s i s , and number-size d i s t r i b u t i o n s ( f o r each s t a g e ) were determined by c o u n t i n g p a r t i c l e s i n d i s c r e t e s i z e ranges a f t e r s o n i c d i s p e r s i o n i n hexane. P a r t i c l e s i z e s were determined from s c a n n i n g e l e c t r o n microscope (SEM) pho t o g r a p h s o r by use of a Quantimet image a n a l y z e r w i t h an i n t e r f a c e t o the SEM. Impactor stage d a t a . I n F i g u r e 1 a r e p l o t t e d both t h e c u m u l a t i v e mass d i s t r i b u t i o n s of A l i n f l y - a s h p a r t i c l e s as determined from s t a g e D5QS and the d i s t r i b u t i o n parameters f o r each stage as d e t e r mined from SEM a n a l y s e s . The SEM n u m b e r - d i s t r i b u t i o n (nmd) parame t e r s f o r each stage were t r a n s f o r m e d t o the c o r r e s p o n d i n g volume parameters and a d j u s t e d f o r t h e measured p a r t i c l e d e n s i t y (2.44 g/cm^) and s l i p c o r r e c t i o n f a c t o r t o o b t a i n e s t i m a t e s of t h e mass mediam aerodynamic diameters (mmad) f o r t h e s t a g e . N e i t h e r t h e nmds nor the mmads determined from t h e microscopy a n a l y s i s a r e d i r e c t l y comparable t o the D50 stage parameters. Values corresponding t o the D5QS can, however, be i n t e r p o l a t e d from t h e c u m u l a t i v e curves ( F i g u r e I ) and a r e l i s t e d i n Table I . Comparison of these d a t a shows t h a t use of t h e D5QS s u p p l i e d w i t h the i m p a c t o r ( t h e s e a r e c a l c u l a t e d v a l u e s , and g e n e r a l l y agree w e l l w i t h c a l i b r a t i o n s ) r e s u l t s i n s e r i o u s o v e r e s t i m a t i o n of the amount of mass a s s o c i a t e d w i t h l a r g e p a r t i c l e s . As i n d i c a t e d i n F i g u r e 1, a e r o s o l MMAD was o v e r e s t i m a t e d by a f a c t o r of two when the D5QS were used, and t h e e s t i m a t e d g e o m e t r i c s t a n d a r d d e v i a t i o n was 40% t o o l a r g e . I n t h i s c a s e , t h e a e r o s o l c o n t a i n e d r e l a t i v e l y few p a r t i c l e s w i t h d i a m e t e r s comparable to the D5QS of the f i r s t impactor s t a g e s . The presence of p a r t i c l e s w i t h diameters much s m a l l e r than the D50S i s a t t r i b u t e d t o t h e i r l o w , but s i g n i f i c a n t , c o l l e c t i o n e f f i c i e n c y . W a l l l o s s e s were e s t i m a t e d by comparing t h e mass c o n c e n t r a t i o n s of v a r i o u s e l e m e n t a l c o n s t i t u e n t s of a e r o s o l s c o l l e c t e d w i t h a s e r i e s of a l t e r n a t e l y c o l l e c t e d f i l t e r and i m p a c t o r samplers. F o r samples c o l l e c t e d downstream of an e l e c t r o s t a t i c p r e c i p i t a t o r (ESP), where t h e a e r o s o l MMAD was 11.5 ym, the average amount of mass c o l l e c t e d i n t h e i m p a c t o r s was about 60% of t h a t c o l l e c t e d by the f i l t e r s ( T a b l e I I ) . When f i n e (MMAD 1.0

Ga, T i , Eu, A l

>1.4, >1.7, >2.0, >2.1

Dy, Th, L a , Na

>2.2, >2.6, >2.7, >3.3

Adapted

from r e f e r e n c e 15

F i g u r e 3. P a r t i c l e s c o l l e c t e d on an impactor s t a g e w i t h a D^Q of 6.4 ym; s c a n n i n g e l e c t r o n m i c r o s c o p y , 2,400X m a g n i f i c a t i o n .


25. ONDOV


319

v a l u e of the s l i p c o r r e c t i o n f a c t o r (C) o r by r e d u c i n g the d i a m e t e r of the j e t s . The v a l u e of C i s i n c r e a s e d by r e d u c i n g t h e gas p r e s s u r e , and such an i n c r e a s e i s t h e b a s i s f o r l o w - p r e s s u r e and h i g h p r e s s u r e - d r o p i m p a c t o r s such as those designed by M c F a r l a n d e t a l . 03), H e r i n g e t a l . (4_), and Pilât e t a l . (5_). C h a r a c t e r i s t i c s o f t h e f i n a l stage of each of these i m p a c t o r s a r e l i s t e d i n Table I V . The M c F a r l a n d and H e r i n g i m p a c t o r s use c o m m e r c i a l l y a v a i l a b l e preimpact o r s f o r supermicrometer s i z i n g and have t h r e e and f o u r a d d i t i o n a l s t a g e s , r e s p e c t i v e l y , f o r s i z i n g submicrometer p a r t i c l e s . The submicrometer s i z i n g stages o p e r a t e a t a f i n a l stage p r e s s u r e of 24.3 mm Hg (McFarland i m p a c t o r ) o r 8 mm Hg ( H e r i n g i m p a c t o r ) . The lower l i m i t diameter f o r both i m p a c t o r s i s 0.05 ym. The McFarland i m p a c t o r samples a t a r a t e of 28 £pm and, by u s i n g m u l t i p l e j e t s (600 o r 1,762), a c h i e v e s j e t v e l o c i t i e s w i t h i n t h e range r e q u i r e d f o r accept a b l e p a r t i c l e bounce. U n f o r t u n a t e l y , however, the u n i t r e q u i r e s a vaccum pump w e i g h i n g between 400 and 500 l b t o support the 28 £pm s a m p l i n g r a t e a t t h e low f i n a l - s t a g e p r e s s u r e . Because of i t s l a r g e s i z e and power r e q u i r e m e n t s , the u n i t has not been adopted f o r r o u t i n e f i e l d use and has never been f u l l y e v a l u a t e d . The H e r i n g impactor i s more p o r t a b l e and has been used t o study the d i s t r i b u t i o n of submicrometer s u l f a t e a e r o s o l s ( 2 2 ) . I n t h i s i m p a c t o r a c r i t i c a l o r i f i c e s e p a r a t e s the atmospheric and l o w p r e s s u r e stages and determines the 1-ilpm sampling r a t e . The l o w p r e s s u r e stages each use a s i n g l e j e t , which even a t t h e r e l a t i v e l y low sampling r a t e produces j e t v e l o c i t i e s r a n g i n g from 9,300 t o 30,000 cm/s. These j e t v e l o c i t i e s a r e so h i g h t h a t p a r t i c l e bounce i s severe f o r the submicrometer p a r t i c l e s ( 2 3 ) . The s i n g l e - j e t d e s i g n i s not conducive t o a n a l y s i s by x - r a y f l u o r e s e n c e , and t h e low f l o w - r a t e hampers the c o l l e c t i o n of s u f f i c i e n t m a t e r i a l f o r a n a l y s e s of t r a c e c o n s t i t u e n t s i n ambient a i r . I n another study ( 2 4 ) , we used a U n i v e r s i t y of Washington Mark V i m p a c t o r t o determine t h e d i s t r i b u t i o n s of t r a c e elements i n submicrometer combustion a e r o s o l s from a 430-MW(e) c o a l - f i r e d power p l a n t . The Mark V i s s i m i l a r i n d e s i g n t o the Mark I I I , but i t has 11 impactor stages and may be operated as a h i g h p r e s s u r e - d r o p impact o r . The o r i f i c e p l a t e s of the l a s t f o u r stages of t h e Mark V a r e q u i t e s i m i l a r t o those of the U n i v e r s i t y of Washington Mark I V , which was designed s p e c i f i c a l l y f o r l o w - p r e s s u r e o p e r a t i o n . The u n i t can p r o v i d e up t o s i x submicrometer p a r t i c l e - s i z e f r a c t i o n s . The Mark V was operated a t f l o w r a t e s of about 7 £pm a t f i n a l - s t a g e p r e s s u r e s no l o w e r than 345 mm Hg. T h e o r e t i c a l v a l u e s of stage c u t o f f diameters can be c a l c u l a t e d f o r the impactor i f the gas p r e s s u r e s can be a c c u r a t e l y e s t i m a t e d o r measured. Pilât e t a l . (25) have e x t e n s i v e l y measured t h e p r e s s u r e s on each stage of the Mark IV and have prepared t h e o r e t i c a l curves g i v i n g stage c u t o f f diameters as a f u n c t i o n of gas temperature and s a m p l i n g r a t e f o r a s p e c i f i c f i n a l - s t a g e p r e s s u r e . Because of t h e c o n s t r a i n t s of i s o k i n e t i c sampling and t h e somewhat h i g h p r e s s u r e drop f i l t e r r e q u i r e d f o r o u r c h e m i c a l a n a l y s e s , and p o s s i b l y because the amount of gas leakage around the j e t stages i n our study may have d i f f e r e d from t h a t i n the study of Pilât e t a l . ( 2 5 ) , we were unable t o o b t a i n the f i n a l - s t a g e p r e s s u r e s and sample f l o w r a t e s c a l l e d f o r by the t h e o r e t i c a l c u r v e s . Because t h e degree of leakage between s t a g e s was unknown, and because the e f f e c t s of p a r t i c l e bounce a r e not accounted f o r by t h e o r y , we r e l i e d on SEM t e c h n i q u e s t o determine


320


the s i z e d i s t r i b u t i o n of p a r t i c l e s c o l l e c t e d on i n d i v i d u a l s t a g e s . The mrads f o r i n d i v i d u a l s t a g e s t h a t c o l l e c t e d submicrometer p a r t i c l e s ranged from 0.77 t o 0.11 ym. I n t h i s study we used T e f l o n - f i b e r a f t e r f l i t e r s t o a c h i e v e the d e s i r e d f l o w r a t e , but these c o u l d not be a n a l y z e d by SEM. The s i z e d i s t r i b u t i o n s observed f o r the l a s t f o u r s t a g e s ( F i g u r e 4) were g e n e r a l l y " w e l l behaved" — i . e . , they f i t w e l l the lognormal d i s t r i b u t i o n f u n c t i o n , and tended t o be f r e e of the l a r g e p a r t i c l e s t h a t s h o u l d have been c o l l e c t e d on p r e v i o u s s t a ges. We b e l i e v e t h a t the use of coated s u b s t r a t e s and the numerous i m p a c t o r stages helped t o m i n i m i z e the e f f e c t s of p a r t i c l e bounce. More r e c e n t l y , Kuhlmey, e t a l . (6) developed a u n i f o r m - d e p o s i t m i c r o - o r i f i c e impactor i n which n o z z l e s w i t h diameters of 100 ym were made i n 50-ym-thick p l a t e s by a p h o t o c h e m i c a l e t c h i n g p r o c e s s . The o r i g i n a l impactor used 200 j e t s per stage and c o u l d c o l l e c t 0.1 ym p a r t i c l e s a t a f l o w r a t e of 10 £,pm w i t h an o v e r a l l p r e s s u r e drop of o n l y a few i n c h e s of Hg. Because of the l a r g e number of n o z z l e s , t h e r e l a t i v e l y h i g h f l o w r a t e i s a c h i e v e d a t somewhat lower j e t v e l o c i t i e s than a c h i e v e d i n low p r e s s u r e i m p a c t o r s . The n o z z l e s were p l a c e d i n a s p i r a l p a t t e r n and the c o l l e c t i o n p l a t e was r o t a t e d t o a c h i e v e u n i f o r m d e p o s i t i o n . The advantages of u n i f o r m d e p o s i t i o n are t h a t p a r t i c l e s do not p i l e up under the n o z z l e s and a l t e r the j e t - t o p l a t e s p a c i n g , thus changing the c u t o f f diameter; and t h a t x - r a y f l u o r e s c e n c e can be used f o r a n a l y s i s . A l a t e r , h o r i z o n t a l l y c o n f i g u r e d f i v e - s t a g e d e s i g n w i t h up t o 2,000 j e t s p e r stage p e r m i t t e d sampling a t 30 i,pm w i t h a mimimum c u t o f f s i z e of 0.06 ym. M a r p l e (8) has r e c e n t l y b u i l t two v e r t i c a l l y c o n f i g u r e d m i c r o o r i f i c e i m p a c t o r s h a v i n g a f i n a l c u t o f f s i z e of 0.026 ym (see T a b l e V ) . These v e r t i c a l l y c o n f i g u r e d u n i t s use f o u r - s t a g e c o n v e n t i o n a l a t m o s p h e r i c - p r e s s u r e p r e i m p a c t o r s t o remove supermicrometer p a r t i c l e s , and they a l s o operate a t 30 Jlpm. A l l of these m i c r o o r i f i c e i m p a c t o r s can be used w i t h s t a n d a r d low-volume r o t a r y carbon-vane vacuum pumps. We have used both the h o r i z o n t a l l y and v e r t i c a l l y c o n f i g u r e d i m p a c t o r s t o sample c o a l - c o m b u s t i o n a e r o s o l s i n e l e v a t e d plumes and i n plumes a t ground l e v e l near Deep Creek Lake i n western M a r y l a n d . Few problems were encountered i n t h i s study however, p a r t i c l e bounce c o u l d not be e v a l u a t e d f o r ambient samples because the i n d i v i d u a l submicrometer p a r t i c l e s c o u l d not be d i s c e r n e d . M a r p l e and Rubow ( 8 ) have performed e x t e n s i v e c a l i b r a t i o n s w i t h monodisperse a e r o s o l s ; these show t h a t i m p a c t i o n p l a t e s covered w i t h aluminium f o i l have s l i g h t l y l a r g e r D5QS than p l a t e s covered w i t h T e f l o n - f i b e r f i l t e r s . P a r t i c l e bounce problems i n the m i c r o o r i f i c e impactor have n o t been a d e q u a t e l y e v a l u a t e d w i t h r e a l - w o r l d a e r o s o l s . As j e t v e l o c i t i e s i n the l a t e r few stages are g r e a t e r than the 3000 t o 4000 cm/sec d e s i g n c r i t e r i a quoted by M c F a r l a n d , e t a l . ( 3 ) , p a r t i c l e bounce problems may be expected f o r stages E, F, G. Note t h a t even a t lower j e t v e l o c i t i e s , some amount of p a r t i c l e bounce i s a n t i c i p a t e d , and i n g e n e r a l , i t i s e x t r e m e l y d i f f i c u l t t o o b t a i n submicrometer p a r t i c l e f r a c t i o n s c o m p l e t e l y f r e e of l a r g e r p a r t i c l e s . P a r t i c l e bounce can l e a d t o v e r y l a r g e e r r o r s i n the t o t a l a e r o s o l mass and the conc e n t r a t i o n of m a t r i x - t y p e elements ( e . g . , A l , S i , r a r e e a r t h s , e t c . ) w h i c h are t y p i c a l l y a s s o c i a t e d w i t h supermicrometer p a r t i c l e s . This phenomenon i s f r e q u e n t l y o v e r l o o k e d by r e s e a r c h e r s i n v e s t i g a t i n g submicrometer mass and c o m p o s i t i o n . C o n s e q u e n t l y , much o f the e x i s t i n g i n f o r m a t i o n d e r i v e d from i m p a c t o r s on e l e m e n t a l c o m p o s i t i o n o f f i n e


25. ONDOV

T a b l e IV·


C h a r a c t e r i s t i c s of

Jet Diameter cm

Impactor

M c F a r l a n d , et a l . Hering, et a l . Pilât, e t a l .

T a b l e V.

0.0208 0.140 0.0198

321

Three Reduced P r e s s u r e Impactors

No. of Jets

Jet Velocity cm/sec

D50 ym

Final Stage Pressure Torr

Flow Rate 1pm

1762

4158

0.05

2.43

28

1

30,000

0.05

8

1.0

60

13,900

20.1

345

7

The New M i c r o - O r i f i c e Impactor P e r m i t s Submicrometer S i z i n g A t H i g h Flow R a t e s , Low P r e s s u r e Drops, and Lower J e t Velocities. T o t a l Flow Rate 30 1pm

Stage

Jet Width, cm

No. Jets

Jet Velocity cm/sec

D50 ym

inches

ΔΡ mercury

C

0.0123

2000

2,180

1.6

1.0

D

0.0100

2000

3,330

0.55

1.3

Ε

0.00694

2000

7,200

0.20

2.4

F

0.00551

2000

12,700

0.092

7

G

0.00371

2000

23,100

0.024

14

Adapted from Marple and Rubow ( 8 ) .




25.

ONDOV


323

p a r t i c l e s s h o u l d be viewed w i t h s k e p t i c i s m u n l e s s the impact of p a r t i c l e bounce has been r i g o r o u s l y i n v e s t i g a t e d * I n g e n e r a l , sub micrometer p a r t i c l e s i z i n g and p a r t i c l e bounce o f f s h o u l d be checked by m i c r o s c o p y t e c h n i q u e s . F i n e p a r t i c l e d i s t r i b u t i o n parameters s h o u l d a l s o be o b t a i n e d by a u x i l i a r y methods, f o r example, e l e c t r i c mobility analysis.

Literature Cited 1. Ranz, W.E., and Wong, J.B., Indust. Eng. Chem. 44, 1371-1381 (1952). 2. Mercer, T.T., Amer. Industr. Hyg. Assoc. J . 26, 236-241 (1965). 3. McFarland, A.R., Nye, H.S., Erikson, C.H., Development of a Low Pressure Impactor. U.S. Environmental Protection Agency, Report No. EPA-650/2-74-014 (1973). 4. Hering, S.V., Flagan, R.C., Friedlander, S.K., Environ. Sci. Technol. 667-673 (1978). 5. Pilat, M.J., Powell, E.B., Carr, R.C., Ρroc. 70th Annual Meeting, Air Pollut. Control Assoc. 4, 35.2 (1977). 6. Kuhlmey, G.A., Liu, B.Y.H., Marple, V.A., Amer. Indust. Hyg. Assoc. J . 42, 790-795 (1981). 7. Marple, V.A., Liu, B.Y.H., Kuhlmey, G.A., J . Aerosol Sci. 12, 333-337 (1981). 8. Marple, V.A., Rubow, K.L., Development of a Microorifice Uniform Deposit Cascade Impactor. University of Minnesota, Particle Technology Laboratory Publ. No. 532 (1984). 9. Fuchs, N.A., The Mechanics of Aerosols. MacMillan Co., New York, (1964). p. 71. 10. Hidy, G.M., Brock, J.R., The Dynamics of Aerocolloidal Systems. Pergamon Press, Oxford (1970). Vol. 1, p. 72. 11. May, K.J., J . Sci. Instrument. 22, 187-195 (1945). 12. McFarland, A.R., Zeller, H.W., Study of a Large Volume Impactor for High Altitude Aerosol Collection. Report of the Division of Technical Information Extension of the U.S. Atomic Energy Commission, TID-18624 (1963). 13. Cooper, D.W., Spielman, L.A., Atmos. Environ. 10, 723-729 (1976). 14. Dzubay, T.G., Hasan, Η., Size Distributions of Species in Fine Particles in Denver Using a Micro-Orifice Impactor. In prepara tion. 15. Ondov, J.M., Ragaini, R.C., Biermann, A.H., Atmos. Environ. 12, 1175-1185 (1978). 16. Ondov, J.M., Biermann, A.H., Ralston, H.R., Composition and Distribution Characteristics of Aerosols Emitted from a Coal-Utility Boiler Equipped with a Hot-Side Electrostatic Precipitator. Presented at the Annual ACS Meeting, Miami Beach, FL, September 10-15 (1979). 17. Allegrini, I., DeSantis, F . , DiPalo, V., Liberti, Α., J . Aerosol. Sci 15, 465-471 (1984). 18. Pilat, M.J., Ensor, D.S., Bosch, J.C., Atmos. Environ. 4, 671-679 (1970). 19. Andersen, A.A., Amer. Indust. Hyg. Assoc. J . 27, 160-165 (1966). 20. Marple, V.A., Amer. Ind. Hyg. Conf. Abs. N.P. p. 24 (1977). 21. Dahneke, B., J . Colloid Inter. Sci. 45, 584-590 (1973). 22. Hering, S.V., Friedlander, S.K., Atmos. Environ. 16, 2647-2656 (1982). 23. Hering, S.V., Personal communication (1981). Markuszewski and Blaustein; Fossil Fuels Utilization ACS Symposium Series; American Chemical Society: Washington, DC, 1986.


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24. Ondov, J.M., Biermann, A.H., Heft, R.E., Koszykowski, R.F., Elemental Composition of Atmospheric Fine Particles Emitted from Coal Burned in a Modern Electric Power Plant Equipped with a Flue-Gas Desulfurization System. American Chemical Society Symposium Series no. 167, 173-186 (1981). 25. Pilat, M.J., University of Washington, Personal communication (1979). RECEIVED May 29, 1986


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