Compositional Analysis of Size-Segregated Aerosol Samples

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7 Compositional Analysis of Size-Segregated Aerosol Samples

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Thomas A. Cahill and Paul Wakabayashi A i r Quality G r o u p , Crocker Nuclear Laboratory, University of California, Davis, CA 95616-8569

Knowing both the size and composition of fine particles in the air is vital for understanding the sources, transport, transformation, ef­ fects, and sinks of atmospheric aerosols. However, compositional analysis of such size-segregated aerosol samples poses difficulties be­ cause of the small amount of mass available for analysis, the chemical complexity of the particles, and the nonuniform deposits character­ istic of most impactors. Additional problems are posed by the need to measure both a wide range of elements and trace concentrations. Nevertheless, significant progress has been made in the past decade, especially in evaluation of the sources and nature of visibility deg­ radation by fine particles. This chapter is a short summary of the difficulties of obtaining size-specific chemical information and the usefulness of such information once obtained.

THE AMBIENT ATMOSPHERIC AEROSOL

consists of l i q u i d a n d s o l i d particles that can persist for significant periods of t i m e i n air. G e n e r a l l y , most of the mass o f the atmospheric aerosol consists of particles b e t w e e n 0.01 a n d 100 μιη i n d i a m e t e r d i s t r i b u t e d a r o u n d two size modes: a " c o a r s e " o r " m e c h a n ­ i c a l " m o d e c e n t e r e d a r o u n d 10- to 20-μηι p a r t i c l e d i a m e t e r , a n d an " a c c u ­ m u l a t i o n " m o d e c e n t e r e d a r o u n d 0.2- to 0.8-μπι p a r t i c l e d i a m e t e r (1). T h e f o r m e r is p r o d u c e d b y m e c h a n i c a l processes, often natural i n o r i g i n , a n d i n c l u d e s particles s u c h as fine soils, sea spray, p o l l e n , a n d o t h e r materials. S u c h particles are generated easily, b u t t h e y also settle out r a p i d l y because of d e p o s i t i o n velocities of several c e n t i m e t e r s p e r second. T h e a c c u m u l a t i o n m o d e is d o m i n a t e d b y particles generated b y c o m b u s t i o n processes, i n d u s ­ trial processes, a n d secondary particles created b y gases c o n v e r t i n g to p a r 0065-2393/93/0232-0211$06.00/0 © 1993 American Chemical Society

Newman; Measurement Challenges in Atmospheric Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1993.

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MEASUREMENT CHALLENGES IN ATMOSPHERIC CHEMISTRY

t i d e s . T h e a c c u m u l a t i o n - m o d e particles are m o s t l y a n t h r o p o g e n i c . A n ex­ a m p l e of a s i z e - c o m p o s i t i o n profile of a m b i e n t a t m o s p h e r i c aerosols is s h o w n i n F i g u r e 1. F i g u r e 1 shows t h e fraction o f each e l e m e n t a l c o m p o n e n t o f particles that o c c u r r e d i n o n e of eight size ranges f r o m 0.1 to about 15 μηι i n d i a m e t e r . T h e e l e m e n t s i l i c o n , d e r i v e d f r o m the S i O i n soils, is p r e s e n t o n l y i n coarse size ranges. T h u s , it is solely a coarse-mode p a r t i c l e d e r i v e d f r o m m e c h a n i c a l processes. Sulfur, present largely i n t h e f o r m o f a m m o n i u m sulfate, occurs o n l y i n t h e a c c u m u l a t i o n m o d e a r o u n d 0.3 μηι. P o t a s s i u m occurs i n b o t h modes: a coarse m o d e from soil a n d a fine m o d e d e r i v e d f r o m a g r i c u l t u r a l smoke. T h e c h l o r i n e is from sea salt, N a C l , w h i c h is a coarse-mode aerosol that lost its coarsest c o m p o n e n t s d u r i n g transport f r o m oceanic sources about 100 k m u p w i n d o f the site ( D a v i s , California). T h i s figure shows a t y p i c a l b i m o d a l d i s t r i b u t i o n ; such d i s t r i b u t i o n s m a y change. T h u s , t h e challenge for analytical chemists is to generate such data accurately a n d i n e x p e n s i v e l y from c o l l e c t e d aerosols that, as i n t h e instance above, total n o m o r e t h a n a few m i c r o g r a m s for each size range.

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£

Drum-Stage Diameter (^m) (1/16)

(1/8)

1/4

1/2

Figure 1. Example of compositionally resolved bimodal and monomodal dis­ tributions of aerosols. The ordinate gives the percent of the species found in the given size fraction of the impactor. The mode near 0.3 μηι is the "accu­ mulation mode", and that above 8 μm is the "coarse mode". The minimum of mass between 1 and 2 μm is typical; the chlorine distribution is anomalous. Chlorine is in fact a coarse-mode marine aerosol that has lost its larger particles during transport from the ocean to Davis, California, a distance of roughly 100 km. (Reproduced with permission from reference 15. Copyright 1988.)

Newman; Measurement Challenges in Atmospheric Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1993.

7.

C A H I L L & WAKABAYASHI

Analysis of Size-Segregated Aerosol Samples

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Compositional Analysis of Atmospheric Aerosols

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Because the a t m o s p h e r i c aerosol consists of a m i x t u r e of gases a n d particles and because the size a n d c o m p o s i t i o n of the particles are u s u a l l y of interest, the particles must b e a n a l y z e d w i t h i n the gases or the particles m u s t b e r e m o v e d f r o m the gases p r i o r to analysis. C o m p o s i t i o n a l analysis of particles w i t h i n a g a s - p a r t i c l e system is h i g h l y desirable a n d p h y s i c a l l y possible, b u t v e r y difficult. T h i s analysis is h i g h l y desirable because it w o u l d p r o v i d e a r e a l - t i m e s i z e - c o m p o s i t i o n analysis to c o m p l e m e n t r e a l - t i m e analyses of meteorology a n d gaseous pollutants as w e l l as r e a l - t i m e analyses of c e r t a i n p r o m p t effects of aerosols, such as v i s ­ i b i l i t y degradation. T h i s m e t h o d is p h y s i c a l l y possible because the e x c i t i n g source (optical, X - r a y , nuclear, etc.) can b e t u n e d to p i c k out particles w i t h o u t exciting the e n o r m o u s l y abundant gases ( N , 0 , A r , C 0 , etc.). L a s e r s c o u l d vaporize particles of a certain size w i t h o u t e x c i t i n g gases, g i v i n g rise to e m i s s i o n spectra. X - r a y s c o u l d excite a l l species b u t filter out soft X - r a y s from gases d o m i n a t e d b y H , C , N , a n d O . H o w e v e r , for a m b i e n t s a m p l i n g , p o t e n t i a l analytical methods are v e r y difficult a n d expensive, a n d t h e y g e n ­ erally lack sensitivity. T h e y are rarely, i f ever, u s e d for a m b i e n t s a m p l i n g today, b u t the n e e d is great a n d the challenges are clear. T h i s , c e r t a i n l y , is one area that needs f u r t h e r w o r k . 2

2

2

A l m o s t a l l c h e m i c a l analysis of a t m o s p h e r i c aerosols is based o n r e m o v a l of the particles from gases. T h u s , the p r i m a r y task for aerosol samplers is to separate the aerosol particles f r o m the o v e r w h e l m i n g l y larger mass of the gases i n the atmosphere. T w o procedures are c o m m o n l y u s e d . T h e first, and s i m p l e s t , m e t h o d is to d r a w air t h r o u g h filters, c o l l e c t i n g the particles for future analysis. I n the second, orifices accelerate the g a s - p a r t i c l e stream to h i g h v e l o c i t y a n d t h e n force it to m a k e a sharp b e n d . Particles are r e m o v e d f r o m the air stream a n d i m p a c t e d onto a surface. T h e s e samplers are c a l l e d " i m p a c t o r s " , a n d t h e y are the i n s t r u m e n t s that pose the most difficult c h a l ­ lenges to analytical chemists. V i r t u a l impactors are a subset of these devices that a v o i d surface i m p a c t i o n b y u s i n g filters. Filters. F o r most situations, the most c o m m o n l y u s e d t e c h n i q u e for c o l l e c t i n g aerosols is p u l l i n g air t h r o u g h a filter that collects the particles b u t not the gases. C o m p o s i t i o n a l analyses are t h e n p e r f o r m e d o n the filter. In r e a l i t y , a filter does not collect a l l particles i n the aerosol because the inlet w i l l miss large, w i n d - d r i v e n particles unless great care is t a k e n to achieve isokinetic s a m p l i n g . M o d e r n samplers fix the m a x i m u m size of p a r ­ ticles. T h e o l d e r h i g h - v o l u m e s a m p l e r ( H i - V o l ) has an effective u p p e r cut p o i n t a r o u n d 30 μηι, b u t this value is strongly w i n d - d e p e n d e n t . T h e present standard is for 10-μηι particles ( P M - 1 0 ) , a size that is r e l a t i v e l y constant w i t h different w i n d velocities. F i n e particles (diameters s m a l l e r t h a n 2.5 μηι) are often n e e d e d for h e a l t h or v i s i b i l i t y research, a n d m a n y m e t h o d s exist to p e r f o r m such s a m p l i n g ( P M - 2 . 5 ) . I f the filters are efficient, t h e r e

Newman; Measurement Challenges in Atmospheric Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1993.

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MEASUREMENT C H A L L E N G E S IN ATMOSPHERIC CHEMISTRY

are 0- to 30-μπι, 0- to 10-μιη, or 0- to 2.5-μΐΏ size r e g i m e s , b u t m a n y others have b e e n a c h i e v e d b y devices such as cyclones. W i t h e n o u g h i n t e g r a t e d samples, a f u l l p a r t i c l e size s p e c t r u m can be d e r i v e d f r o m filter data (2). A p r i c e m u s t b e p a i d i n h i g h analytical costs a n d p r o p a g a t i o n o f e r r o r s , b u t a g o o d d e a l of mass can b e c o l l e c t e d , a n d standard filter analyses are possible. If o n l y particles larger t h a n a c e r t a i n size are o f interest, this t e c h n i q u e can b e i n v e r t e d b y u s i n g diffusion to r e m o v e fine particles f r o m t h e a i r s t r e a m a n d l e a v i n g the coarser particles to be c o l l e c t e d o n a filter. A g a i n , t e c h n i q u e s l i k e this, o f w h i c h the diffusion battery is t h e best k n o w n , also y i e l d a standard filter a n d , t h u s , p e r m i t the use of standard analytical m e t h o d s .

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T h e r e are methods that collect particles f r o m b o t h the coarse a n d fine m o d e s s i m u l t a n e o u s l y . A c l e v e r way to achieve c o l l e c t i o n of b o t h m o d e s of particles onto filters is the v i r t u a l i m p a c t o r (VI) (3). T h e g a s - p a r t i c l e m i x t u r e is forced to m a k e a sharp b e n d , a n d particles above 2.5 μηι are e j e c t e d i n t o a s m a l l p o r t i o n (10%) of the gas stream onto a filter, so a l l the coarse particles (typically f r o m the 10 μηι l i m i t set b y the P M - 1 0 i n l e t to 2.5 μιτι) a n d 1 0 % of the fine particles are c o l l e c t e d o n a filter. T h e r e m a i n d e r is t h e n f i l t e r e d ; this p o r t i o n contains no coarse particles b u t 9 0 % o f the fine particles (2.5 to 0 μιη). A f t e r analysis, the fine a d m i x t u r e i n the coarse fraction is r e m o v e d m a t h e m a t i c a l l y . L i m i t a t i o n s i n the process l i m i t its usefulness b e l o w 1 μιη, however. A s e c o n d w a y to achieve c o l l e c t i o n o f coarse- a n d

fine-mode

particles

onto filters is t h r o u g h t a n d e m filtration t h r o u g h the " s t a c k e d filter u n i t " ( S F U ) (4, 5, 6). I n these devices, the c o n v e n i e n t filtration characteristics o f filters

(Nuclepore) a l l o w a 2.5-μπι cut p o i n t o n the basis o f p o r e size a n d

the face v e l o c i t y of the airstream. S u c h devices are v e r y c o m p a c t a n d i n ­ expensive a n d have b e e n h e a v i l y u s e d i n r e m o t e - a r e a n e t w o r k s (7, 8, 9). A g a i n , h o w e v e r , the l i m i t a t i o n s of the m e t h o d l i m i t the n u m b e r a n d s h a r p ­ ness o f the size cuts so that almost a l l units are o p e r a t e d at 2.5 μιη a n d give coarse a n d fine fractions v e r y m u c h l i k e those of the v i r t u a l i m p a c t o r . E x ­ amples o f the 2.5-μιη cut points o f V I , S F U , a n d impactors are s h o w n in

F i g u r e 2.

However,

cyclones,

virtual impactors,

a n d stacked

filter

units cannot give the sharp, m u l t i p l e cut points of impactors as s h o w n i n F i g u r e 1. N e v e r t h e l e s s , these methods a l l result i n a filter that captures the at­ m o s p h e r i c particles. T h e mass l o a d i n g can b e large, the deposit u n i f o r m , a n d the filter reasonably stable u n d e r transport to a c e n t r a l a n a l y t i c a l l a b ­ oratory. N u m e r o u s papers have treated analysis o f s u c h filters, so this i n ­ f o r m a t i o n is not r e p e a t e d . T h i s chapter focuses o n the p r o b l e m s o f c h e m i c a l analyses o f i m p a c t o r substrates, for w h i c h the p r o b l e m s are m o r e

serious

a n d the solutions e l u s i v e . Impactors.

Impactors w o r k b y f o r c i n g the g a s - p a r t i c l e stream to make

a sharp b e n d . T h i s action causes the larger particles to i m p a c t onto a m e d i u m ,

Newman; Measurement Challenges in Atmospheric Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1993.

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Before Fine Filters

a e r o d y n a m i c diameter (μιη) i

Virtual

Û

Cyclone

ο

Cyclone



Impactor

SFU

Figure 2 . Comparisons of collection efficiencies of various types of aerosol samplers; although all have 5 0 % capture efficiency points of roughly 2 . 5 μιη, the shapes of the collection curves vary. whereas the smaller particles a n d gases c o n t i n u e d o w n s t r e a m . I m p a c t o r s , b y means of t h e i r c o n s t r u c t i o n , can s e q u e n t i a l l y segregate particulate m a t t e r to s m a l l e r a n d s m a l l e r sizes. B y v a r y i n g the orifice size, the n u m b e r o f orifices, the p r e s s u r e , the v e l o c i t y o f the j e t , a n d o t h e r specifications, the d e s i r e d size-selected particles can be c o l l e c t e d . T h e s e i m p a c t o r s have sharper size cutoffs t h a n cyclones, v i r t u a l i m p a c t o r s , or stacked filter units. S o m e examples of a m b i e n t air impactors i n c l u d e the l o w - p r e s s u r e i m ­ pactor ( L P I ) (JO), the B a t t e l l e ( I I , 12), the M u l t i - D a y ( M D ) (13), the D a v i s R o t a t i n g U n i t for M o n i t o r i n g ( D R U M ) (14, 15), the B e r n e r L o w - P r e s s u r e I m p a c t o r ( B L P I ) (16), a n d the M i c r o - O r i f i c e U n i f o r m D e p o s i t I m p a c t o r ( M O U D I ) (17). E a c h has a different w a y o f c o l l e c t i n g p a r t i c u l a t e matter. Because o f the small mass of aerosols, c e r t a i n parameters o f the d e s i g n o f the i m p a c t o r are adjusted so that the aerosol can be a n a l y z e d w i t h a m a x i m u m degree of sensitivity. S o m e of the parameters i n v o l v e the flow rate at w h i c h the i m p a c t o r operates: the m a t e r i a l is c o n c e n t r a t e d to a s m a l l e r area o r the

Newman; Measurement Challenges in Atmospheric Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1993.

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MEASUREMENT CHALLENGES IN ATMOSPHERIC CHEMISTRY

p e r i o d of s a m p l i n g is e x t e n d e d . B u t i n c r e a s i n g one p a r a m e t e r m a y decrease sensitivity. T w o samplers, the B L P I a n d the M O U D I , r e l y o n an i n c r e a s e d flow rate to collect m o r e m a t e r i a l . T o t a l c o l l e c t e d mass p e r stage is the k e y p a r a m e t e r for these analytical methods, w h i c h r e m o v e the deposit from the c o l l e c t i o n substrate. B o t h of the samplers have m o r e orifices to a l l o w for the increased v o l u m e a n d to achieve the correct s i z i n g of aerosols. A l t h o u g h i n c r e a s i n g the n u m b e r of orifices increases the flow rate, it also increases the spread of aerosols o n a c o l l e c t i o n m e d i a . T h i s s p r e a d i n g has the effect of decreasing the concentration of m a t e r i a l a n d t h e r e b y decreasing the s e n ­ sitivity of c e r t a i n measurements. O n the o t h e r h a n d , the D R U M , the L P I , a n d the B a t t e l l e u t i l i z e a single orifice to concentrate the m a t e r i a l , a n a r ­ rangement that increases sensitivity. H i g h areal d e n s i t y of the deposit, i n grams p e r square c e n t i m e t e r , is a k e y p a r a m e t e r for m e t h o d s that analyze the deposit w i t h o u t r e m o v i n g it from the substrate. B u t p r o b l e m s arise f r o m particle bounce-off a n d f r o m l a y e r i n g of particulate m a t t e r , w h i c h cause p r o b l e m s i n some analysis techniques.

Compositional Analysis of Particulate Samples O n c e the particulate sample has b e e n r e m o v e d f r o m the airstream a n d d e p o s i t e d o n a filter o r an i m p a c t i o n - d i f f u s i o n surface, the analyst c a n e i t h e r r e m o v e the deposit f r o m the surface a n d analyze the r e s u l t i n g gas or l i q u i d or leave the deposit o n the surface a n d analyze the surface a n d deposit together. I n the first m e t h o d , any a n d a l l analytical m e t h o d s are available to the analyst; h o w e v e r , p r o b l e m s arise i n two areas. F i r s t , the analyst m u s t b e sure that the particles are r e m o v e d f r o m the substrate a n d i n c o r p o r a t e d into the aliquot. S e c o n d , the mass of m a t e r i a l is always l i m i t e d , so e x t r e m e analytical sensitivity a n d v e r y p u r e reagents are r e q u i r e d . A n e x a m p l e is the c o l l e c t i o n of fine particles w i t h diameters less t h a n 2.5 μιτι from a 10μ g / m fine aerosol for 4 h at a 20 L / m i n flow rate. T h e total particulate mass c o l l e c t e d is 48 μg. T h i s mass is r e m o v e d from the filter or surface w i t h 0.1 m L o f solvent. T h e total d i s s o l v e d particulate is 480 p p m i n the solvent, a n d this c o n c e n t r a t i o n m u s t b e a n a l y z e d to about 1.1 p p m i n accuracy. T h e analytical m e t h o d needs to b e sensitive at the 0.48-ppb l e v e l , a n d c o n t a m ­ inants i n the solvent m u s t b e h e l d to s u c h levels also. 3

F o r this reason, particulate samplers d e s i g n e d for particulate r e m o v a l have to generate the m a x i m u m possible particulate mass. M o d e r n examples i n c l u d e impactors based o n the h i g h - v o l u m e s a m p l e r ( H i - V o l s ) , the M O U D I (17) of the U n i v e r s i t y of M i n n e s o t a , a n d the B L P I (16). T h e H i - V o l s , i n particular, collect 330 m of a i r i n 4 h , g i v i n g 1100 μ g of deposit for t h r e e size cuts b e l o w a particle d i a m e t e r of 2.5 μηι. T a b l e I shows some k e y parameters for a few w i d e l y u s e d a m b i e n t air impactors for m u l t i p l e size cuts. 3

Newman; Measurement Challenges in Atmospheric Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1993.

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Analysis of Size-Segregated Aerosol Samples

Table I. Mass-per-Stage Comparison of Several Impactors Volume" (m )

Stages < 2 . 5 μg (nj

Average Mass per Stage (\kg)

1.0

0.24

6

0.40

Flow (LImin)

Sampler Type

3

13

DRUM LPI Battelle MOUDI BLPI

1.0

0.24

7

0.34

1.0 30.0 30.0

0.24 7.20 7.20

6 6 6

0.40 12.00 12.00

MD

30.0

7.20

2

36.00

"Volume per 4-h period. A density of 10 μg/m for particles smaller than 2.5 μπι was assumed. 3

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b

T h e second class of particulate samples is those that are a n a l y z e d w i t h o u t r e m o v a l from the s a m p l i n g substrate. F o r such samples, o n l y l i m i t e d classes of analytical methods can be u s e d , a n d the substrate i t s e l f is c r i t i c a l . A s an e x a m p l e , c o n s i d e r again a 20 L / m i n sample b e i n g c o l l e c t e d from a 10 μg/m fine-particle a m b i e n t aerosol for 4 h . T h e 480 μ g of mass are still c o l l e c t e d , b u t n o w the area of the deposit is critical. I f a 1 2 - c m Teflon filter o f 4 8 0 μ g / c m thickness, such as stretched Teflon [poly(tetrafluoroethylene)], is u s e d , an areal d e n s i t y of 40 μ g / c m of particulate deposit o n a 480 μ g / c m substrate is p r o d u c e d . T h e total particulate filter sample is n o w 520 μ g / c m , a n d a 1 p p m c o m p o s i t i o n a l analysis of the particulate deposit r e ­ quires an analysis sensitivity to 80 p p b . I n other w o r d s , the analytical m e t h o d m u s t b e sensitive to 0.04 μ g / e m . C l e a r l y , the k e y p a r a m e t e r is areal density. I f a filter of 6 c m rather than 12 c m w e r e u s e d , the deposit w o u l d have the same total mass b u t twice the areal density. F o r a g i v e n analytical m e t h o d sensitive to area d e n s i t y , such as X - r a y s , laser absorption, or a b e t a gauge, h a l v i n g the area gives r o u g h l y a factor of 2 gain i n sensitivity. T a b l e I I shows a c o m p a r i s o n of analytical sensitivity for a few w i d e l y u s e d a m b i e n t air impactors for m u l t i p l e size cuts. 3

2

2

2

2

2

2

2

2

T h e extreme examples of such samplers are the single-jet impactors s u c h as the B a t t e l l e of F l o r i d a State U n i v e r s i t y , the L P I , o r the D R U M , a l l of Table II. Analytical Sensitivity Comparisons for a 2-ng/cm Detectable L i m i t 2

Sampler Type DRUM LPI Battelle MOUDI BLPI MD

Stages < 2 . 5 μg M

Analysis Area (cm )

6 7 6 6 6 2

0.084 0.071 0.071 9.620 13.040 18.600

2

E

Volume (m )

Collection Sensitivity (m lcm )

Minimum Detectable Limit (nglm )

0.24 0.24 0.24 7.20 7.20 7.20

2.90 3.40 3.40 0.75 0.55 0.39

0.7 0.6 0.6 2.7 3.6 5.1

3

3

2

N O T E : All parameters are identical to those in Table I. "Area per stage. The number of orifices varies from 25 to 232 below 2.5 μπι.

Newman; Measurement Challenges in Atmospheric Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1993.

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w h i c h operate at o n l y 1.1 L / m i n . I n 4 h i n a 10 μ g / m aerosol, t h e c o l l e c t e d mass for fine particles is o n l y 2.4 μg, generally spread out o v e r five separate stages. Yet, because the orifices are t i n y , the deposit falls almost e n t i r e l y w i t h i n a 1 . 1 - m m - d i a m e t e r c i r c l e , so an areal d e n s i t y of 320 μ g / c m is p r o ­ d u c e d . G o o d sensitivities despite the small mass can b e o b t a i n e d w i t h a m e t h o d , s u c h as p a r t i c l e - i n d u c e d X - r a y e m i s s i o n ( P I X E ) (18), that uses a focused i o n b e a m that o n l y irradiates the deposit area p l u s 1.1 m m i n a l l directions. 3

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H o w e v e r , analytical m e t h o d s that analyze t h e deposit a n d substrate together, s u c h as P I X E a n d X - r a y fluorescence ( X R F ) , have a serious p r o b l e m p o s e d b y a n o n u n i f o r m deposit. F o r an i o n b e a m , s u c h as protons, i n c i d e n t u p o n an aerosol sample p l a c e d 45° to the b e a m , the ions pass t h r o u g h the sample a n d are c o l l e c t e d i n a F a r a d a y c u p to p r o v i d e absolute concentrations. I f t h e e x i t i n g radiation, w h e t h e r it b e ions, X - r a y s , electrons, o r l i g h t , is u n i f o r m across the deposit or i f the deposit itself is u n i f o r m , t h e n t h e result is accurate. H o w e v e r , i f b o t h the b e a m a n d sample are n o n u n i f o r m i n e i t h e r p l a n e , a c o n v o l u t i o n integral is r e q u i r e d to o b t a i n the c o n c e n t r a t i o n o n the substrate. I n practice this integral is n e v e r d o n e , so analytical accuracy is critically d e p e n d e n t o n b e a m a n d sample u n i f o r m i t y , b o t h o f w h i c h are usually suspect. T h e l i m i t s to the areal density of deposit for filters are set b y c l o g g i n g of the filter that sets i n at t y p i c a l l y 100 μ g / e m . T h e l i m i t o f areal d e n s i t y for impactors is set b y the p r o b l e m of p a r t i c l e b o u n c e . T h i s is a serious p r o b l e m for coarse, d r y aerosols b u t less so for fine, w e t , secondary aerosols. N e v e r t h e l e s s , sticky substrates are u n i v e r s a l l y u s e d (19), a n d deposits are generally l i m i t e d to a few monolayers of particles for a 2.5-μπι p a r t i c l e . T h i s l i m i t amounts to no m o r e than 7 μπι of deposit, or, for 1 . 5 ^ g / m aerosols (per stage), about 1000 μ g / c m of deposit. 2

3

2

I n s u m m a r y , c h e m i c a l analysis of the c o l l e c t e d particulate m a t t e r o n a m u l t i p l e - s t a g e d i m p a c t o r poses serious difficulties: 1. T h e r e is o n l y a severely l i m i t e d a m o u n t of mass available for analysis, a n d efforts to increase size i n f o r m a t i o n t h r o u g h m o r e stages s i m p l y makes the available mass e v e n less. A t t e m p t s to collect m o r e mass b y l o n g e r runs are l i m i t e d b y p a r t i c l e b o u n c e effects. 2. A t t e m p t s to p r o v i d e accurate particulate size i n f o r m a t i o n u s u ­ ally r e q u i r e adhesives on the c o l l e c t i o n surfaces. B u t these adhesives i n t u r n m a k e r e m o v a l o f particles for analysis dif­ ficult, contaminate the deposit, a n d p r o v i d e b a c k g r o u n d a n d contaminant p r o b l e m s for m e t h o d s that analyze the substrates a n d deposit together. T h e p r o b l e m s of p a r t i c l e b o u n c e are, o f course, greatly r e d u c e d for s u b m i c r o m e t e r hygroscopic or o r ­ ganic aerosols, w h i c h usually i n c l u d e the i m p o r t a n t a n t h r o ­ p o g e n i c sulfates, nitrates, a n d organic c o m p o u n d s .

Newman; Measurement Challenges in Atmospheric Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1993.

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Analysis of Size-Segregated Aerosol Samples

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3. I m p a c t o r deposits are t y p i c a l l y h i g h l y n o n u n i f o r m ; this n o n u n i f o r m i t y reduces accuracy a n d p r e c i s i o n for t e c h n i q u e s that analyze the substrate a n d deposit together.

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4. M a n y techniques used for c o m p o s i t i o n a l analysis of filters w i l l not w o r k for most impactors (e.g., g r a v i m e t r i c mass). T h e s e factors c o m b i n e to make impactors less precise a n d accurate t h a n filters. V e r y few comparisons have b e e n m a d e b e t w e e n s i z i n g impactors a n d those that have p r o v i d e d m i x e d results. T h e 1977 E n v i r o n m e n t a l P r o t e c t i o n A g e n c y - D e p a r t m e n t of E n e r g y S a m p l e r I n t e r c o m p a r i s o n i n c l u d e d the M u l t i - D a y S a m p l e r , w h i c h p e r f o r m e d w e l l ( ± 1 5 % ) for fine aerosols s u c h as sulfur, l e a d , a n d z i n c (15). T h e 1986 C a r b o n a c e o u s Species tests at G l e n d o r a , C a l i f o r n i a , i n c l u d e d the D R U M sampler. It p e r f o r m e d w e l l for sulfur ( ± 1 8 % ) , as c o m p a r e d to the fine filter s a m p l e r ( P M - 2 . 5 ) , b u t no o t h e r s i z i n g i m p a c t o r was available for c o m p a r i s o n a n d no e l e m e n t other t h a n sulfur was r e p o r t e d . D R U M versus filter comparisons w e r e r e p o r t e d as part of the S o u t h e r n C a l i f o r n i a A i r Q u a l i t y S t u d y of 1987 (2). A g a i n , no other i m p a c t o r was available for c o m p a r i s o n , a n d the comparisons w i t h filters w e r e o n l y fair ( r « 0.7; r , l i n e a r c o r r e l a t i o n coefficient). 2

P r o b a b l y the first side-by-side c o m p a r i s o n of multiple-stage impactors o c c u r r e d as part of the Salt R i v e r Project's G r a n d C a n y o n S t u d y of 1 9 8 9 1990; the D R U M , L P I , a n d M O U D I w e r e a l l used. T h i s study h a d o p e r ­ ational p r o b l e m s . P o s s i b l y because some o f the samplers w e r e r e n t e d out to the study a n d operated b y t h i r d parties, a great d e a l of data w e r e lost. N e v e r t h e l e s s , the o v e r a l l b e h a v i o r of the major aerosol species was u s u a l l y r e p r o d u c e d i n size a n d concentration. A l t h o u g h the D R U M was r e p o r t e d to have b e e n less precise a n d accurate t h a n the M O U D I or L P I , a l l cor­ relations w e r e far worse t h a n similar correlations for filter samples, a n d slopes versus filters w e r e w e l l b e l o w u n i t y (20). M o r e o v e r , a study was p e r f o r m e d at S h e n a n d o a h N a t i o n a l Park i n 1991, d u r i n g w h i c h filters a n d the M O U D I w e r e operated versus three co-located D R U M samplers. F i g u r e 3 shows the results o v e r a 3-week p e r i o d . A g a i n , fair agreement is e v i d e n t , b u t the data w i t h r = 0.78 m u s t be c o m p a r e d to side-by-side filter samples at the same site, w h i c h a c h i e v e d r = 0.96. I n an attempt to i m p r o v e this situation, w e d e v e l o p e d a f a m i l y of samplers w i t h the rotating d r u m a n d slit configuration of the M u l t i - D a y I m p a c t o r , w h i c h i t s e l f was a modification of the L u n d g r e n Impactor. F l o w was raised to 12 L / m i n , a n d a n e w analysis system was b u i l t that was d e d i c a t e d to such difficult samples. F i g u r e 4 shows the result of a side-by-side c o m p a r i s o n of two I M P R O V E d D R U M Samplers for particles w i t h diameters b e t w e e n 2.5 a n d 0.34 μπι. A l t h o u g h this was o n l y a p r e c i s i o n test, the results are better t h a n other side-by-side tests of a m b i e n t s i z i n g impactors; values of r are as h i g h as 0.96, a n d slopes are w i t h i n 5 % of u n i t y . 3

3

2

T h e a r g u m e n t can be made that some lack of p r e c i s i o n a n d accuracy is o n l y to be e x p e c t e d , g i v e n the f o r m i d a b l e difficulties i n accurate c o l l e c t i o n and c o m p o s i t i o n a l analysis of aerosols b y size. T h u s , w h i l e efforts are b e i n g

Newman; Measurement Challenges in Atmospheric Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1993.

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M E A S U R E M E N T C H A L L E N G E S IN A T M O S P H E R I C

CHEMISTRY

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