Measurement Challenges in Atmospheric Chemistry - American

7 Compositional Analysis of Size-Segregated Aerosol Samples

Downloaded by RUTGERS UNIV on December 28, 2017 | http://pubs.acs.org Publication Date: June 1, 1993 | doi: 10.1021/ba-1993-0232.ch007

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.


£

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.)


7.

C A H I L L & WAKABAYASHI

Analysis of Size-Segregated Aerosol Samples

213

Compositional Analysis of Atmospheric Aerosols


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


214

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 .


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 ,


7.



215


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



216


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


7.


217


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


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.


3

218


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


2

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 .


7.



219

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.


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


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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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1 -§ Downloaded by RUTGERS UNIV on December 28, 2017 | http://pubs.acs.org Publication Date: June 1, 1993 | doi: 10.1021/ba-1993-0232.ch007

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Measurement Challenges in Atmospheric Chemistry - American

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