18
The the
Effect Owens
of
Owens
Valley
Dry
with
Lake
on
Implications
Air
Quality
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the
in Mono
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Lake Area J. B. BARONE, L. L. ASHBAUGH, B. H. KUSKO, and T. A. CAHILL Air Quality Group, Crocker Nuclear Laboratory and Department of Physics, University of California—Davis, Davis, CA 95616
The suspension of aerosols during dust storms from the Owens Dry Lake i n California has been a subject of great concern to residents i n the Owens Valley. In order to assess the magnitude and extent of aerosols from the lake bed, we measured ambient aerosol concentrations by size, mass, and elemental concentrations at seven sites i n the Owens Valley from February 20 to June 18, 1979. Resuspended samples of material from Owens Lake bed were used to determine the lake bed source signature. The most important effect we measured was a significant increase in sulfate aerosols as far as twenty-five miles downwind of the lake bed. A substantial increase i n gravimetric mass was measured during dust storms for particles i n the 2.5-15µm size range. No s i g n i f i cant increase in mass was measured for particles i n the 0.1-2.5µm size range. Similar measurements on a more limited scale at Mono Lake showed that the sulfur to iron ratio at Mono Lake i s a factor of 6-8 greater than the ratio at Owens Lake. These results are consistent with the known mechanism of sulfate formation (efflorescence) i n alkaline lake beds. The data indicate that Mono Lake has the potential to become a significant source of sulfate aerosols i f the lake bed areas are exposed. The Owens V a l l e y l i e s i n e a s t c e n t r a l C a l i f o r n i a b e t w e e n t h e S i e r r a Nevadas t o t h e w e s t a n d t h e I n y o - W h i t e M o u n t a i n s t o t h e east. The v a l l e y i s 120 m i l e s l o n g , b u t o n l y s e v e n t e e n m i l e s across a t i t s widest point. W h i l e the v a l l e y f l o o r l i e s a t an e l e v a t i o n o f a p p r o x i m a t e l y 4,000 f e e t , t h e m o u n t a i n s o n e i t h e r s i d e t o w e r t o 12-14,000 f e e t . This topography i s important f o r two r e a s o n s . F i r s t , t h e S i e r r a Nevadas c r e a t e a r a i n shadow 0097-6156/81 /0167-0327$05.00/0 © 1981 A m e r i c a n C h e m i c a l Society
Macias and Hopke; Atmospheric Aerosol ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
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AEROSOL
commonly a s s o c i a t e d w i t h t h e d e v e l o p m e n t o f s a l i n e l a k e s ( 1 ) . I n f a c t , s e v e r a l s a l i n e l a k e s and p l a y a s e x i s t i n t h e v i c i n i t y , i n c l u d i n g Owens D r y L a k e , Mono L a k e , S a l i n e V a l l e y , Deep S p r i n g s L a k e , D e a t h V a l l e y , and S e a r l e s L a k e . Second, t h e h i g h mountains on e i t h e r s i d e c o n s t r a i n t h e t r a n s p o r t o f a i r b o r n e c o n t a m i n a n t s w i t h i n t h e v a l l e y , e f f e c t i v e l y t r a p p i n g them u n t i l t h e y e x i t a t the n o r t h o r s o u t h e n d . F u r t h e r m o r e , t h e m o u n t a i n s may s e r v e t o i n c r e a s e w i n d s p e e d s a t t h e v a l l e y s u r f a c e due t o l o c a l c h i n o o k s and v e n t u r i f l o w e f f e c t s b e t w e e n t h e v a l l e y w a l l s ( 2 ) . T h i s e f f e c t c o u l d enhance d e v e l o p m e n t o f d u s t s t o r m s i n t h e v a l l e y ( 3 ) . I n r e c e n t y e a r s , t h e i m p o r t a n c e o f t h e Owens L a k e b e d o n a i r q u a l i t y i n t h e Owens V a l l e y h a s b e e n a s u b j e c t o f much c o n c e r n and d e b a t e . The e x t e n t t o w h i c h t h e l a k e b e d a e r o s o l s a r e t r a n s p o r t e d , as w e l l as whether o r n o t t o x i c s u b s t a n c e s a r e p r e s ent i n t h e l a k e b e d s o i l s , h a s b e e n q u e s t i o n e d b y r e s i d e n t s and r e g u l a t o r y a g e n c i e s . Although a i r q u a l i t y i n t h e b a s i n i s genera l l y q u i t e good, a t some t i m e s d u r i n g t h e y e a r , p a r t i c u l a t e a e r o sol concentrations are high. I n p a r t i c u l a r , dust storms o c c u r d u r i n g w i n d y p e r i o d s , w h i c h c a u s e p o o r v i s i b i l i t y and may l e a d t o h e a l t h problems ( 2 ) . I n o r d e r t o o b t a i n q u a n t i t a t i v e d a t a on p a r t i c u l a t e a i r q u a l i t y i n t h e Owens V a l l e y , a s t u d y s p o n s o r e d b y t h e C a l i f o r n i a A i r R e s o u r c e s B o a r d was c o n d u c t e d b y t h e A i r Q u a l i t y Group a t UCD. The p r i m a r y o b j e c t i v e was t o d e t e r m i n e t h e i m p a c t o f t h e dry l a k e b e d on t h e a v e r a g e p a r t i c u l a t e c o n c e n t r a t i o n and on t h e dust storm p a r t i c u l a t e c o n c e n t r a t i o n s i n the v a l l e y . In order to a c c o m p l i s h t h i s , i t was n e c e s s a r y t o d e t e r m i n e t h e e l e m e n t a l comp o s i t i o n o f t h e d r y l a k e b e d and t o d e t e r m i n e t h e a v e r a g e w e e k l y and d u s t s t o r m c o n c e n t r a t i o n o f a e r o s o l s . The o b j e c t i v e s w e r e met b y a s t u d y p l a n d e s i g n e d t o a c q u i r e d a t a f o r s e v e n t e e n weeks a t s e v e n s i t e s t h r o u g h o u t t h e v a l l e y . T h i s i n c l u d e d b o t h w e e k l y m o n i t o r i n g and d a i l y i n t e n s i v e s a m p l i n g of a e r o s o l s . The s a m p l e s w e r e w e i g h e d and t h e n a n a l y z e d f o r e l e m e n t a l c o m p o s i t i o n on t h e UC D a v i s c y c l o t r o n . I t s h o u l d be n o t e d t h a t t h e s a m p l e r u s e d i n t h e s t u d y , ( a s t a c k e d f i l t e r u n i t ) , c o l l e c t e d o n l y p a r t i c l e s l e s s t h a n 15um aerodynamic d i a m e t e r , i . e . p a r t i c l e s o f r e s p i r a b l e s i z e . State s t a n d a r d s f o r TSP a r e b a s e d on H i g h Volume s a m p l e r s w h i c h h a v e no inlet cutoff. Hence, p a r t i c l e s a s l a r g e a s 100 m i c r o n s c a n be c a p t u r e d b y t h e s e i n s t r u m e n t s . T h e r e f o r e , measurements made i n t h i s s t u d y may n o t i n d i c a t e w h e t h e r p a r t i c u l a t e s t a n d a r d s h a v e b e e n v i o l a t e d , s i n c e a s i g n i f i c a n t p o r t i o n o f t h e t o t a l suspended p a r t i c u l a t e mass i s n o t m e a s u r e d b y t h e s t a c k e d f i l t e r u n i t (SFU). P a r t i c l e S a m p l i n g and A n a l y s i s The p a r t i c l e s a m p l e r c h o s e n f o r t h i s s t u d y was t h e S t a c k e d F i l t e r U n i t (SFU) d e s c r i b e d b y C a h i l l e t a l ( 4 , 5 ) . P a r t i c l e c o l l e c t i o n i n two s i z e f r a c t i o n s was a c h i e v e d b y p l a c i n g two N u c l e p o r e membrane f i l t e r s i n s e r i e s . The f i r s t f i l t e r , w i t h 8ym
Macias and Hopke; Atmospheric Aerosol ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
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d i a m e t e r p o r e s , has a 50% c o l l e c t i o n e f f i c i e n c y f o r 2.5um d i a m eter p a r t i c l e s . The f i l t e r s w e r e c o a t e d w i t h a t h i n l a y e r o f A p i e z o n t y p e L g r e a s e t o m i n i m i z e p a r t i c l e b o u n c e - o f f and l o s s i n transport. The SFU was f i t t e d w i t h an i n l e t w h i c h e x c l u d e d p a r t i c l e s l a r g e r t h a n 15ym. The s e c o n d f i l t e r was a 0.4um p o r e d i ameter N u c l e p o r e f i l t e r , w h i c h c o l l e c t e d a l l p a r t i c l e s l e s s than 2.5um. A d i a g r a m o f t h e SFU i s shown i n F i g u r e 1, and t h e c o l l e c t i o n e f f i c i e n c y o f e a c h s t a g e i s shown i n F i g u r e 2. Also shown i s t h e b i m o d a l " P a s a d e n a " p a r t i c l e s i z e d i s t r i b u t i o n ( 6 ) , and t h e c o l l e c t i o n e f f i c i e n c y o f t h e human u p p e r r e s p i r a t o r y t r a c t ( 7 ) . The SFU t h u s c o l l e c t s p a r t i c l e s i n two s i z e f r a c t i o n s c l o s e l y r e s e m b l i n g t h e f r a c t i o n s d e p o s i t e d i n t h e u p p e r and l o w e r r e s p i r a t o r y t r a c t s , and a l s o s e p a r a t e s t h e c o a r s e , n a t u r a l l y g e n e r a t e d p a r t i c l e s from the f i n e r anthropogenic p a r t i c l e s . The a e r o s o l s a m p l e s c o l l e c t e d by t h e SFU w e r e a n a l y z e d b o t h g r a v i m e t r i c a l l y f o r t o t a l suspended p a r t i c u l a t e mass l e s s t h a n 15um, and by 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 ) f o r e l e m e n t a l content. The f i l t e r s w e r e w e i g h e d b e f o r e and a f t e r s a m p l i n g u s i n g a Cahn 25 e l e c t r o b a l a n c e s e n s i t i v e t o l u g . T y p i c a l p r e c i s i o n o f TSP d e t e r m i n e d by t h i s a n a l y t i c a l method i s f 0 . 5 y g / m f o r samples c o l l e c t e d under c o n d i t i o n s of low a e r o s o l c o n c e n t r a t i o n s (5). A f t e r w e i g h i n g , the f i l t e r s were a n a l y z e d f o r e l e m e n t a l c o n t e n t ( e l e m e n t s h e a v i e r t h a n Na) u s i n g t h e UC D a v i s P I X E s y s tem. T h i s a n a l y s i s t e c h n i q u e i s d e s c r i b e d i n C a h i l l e_t a l ( 8 ) . 3
Study
Design
W e e k l y m o n i t o r i n g o f p a r t i c u l a t e a e r o s o l s b e g a n on F e b r u a r y 20, 1979, and ended on J u n e 18, 1979. A t o t a l o f s e v e n t e e n weeks of m o n i t o r i n g were conducted d u r i n g t h i s p e r i o d . Samplers were r u n f o r s e v e n c o n s e c u t i v e d a y s e a c h week a t a f l o w r a t e o f t e n l i t e r s per minute, except d u r i n g dust storm episodes. During djList s t o r m s , s a m p l e s w e r e c o l l e c t e d d a i l y a t a f l o w r a t e o f t e n l i t e r s per minute. Samples d u r i n g d u s t s t o r m s w e r e c o l l e c t e d on A p r i l 6,7,16,17,23,24, 1979. I n a d d i t i o n , d a t a on w i n d f l o w c o l l e c t e d a t B i s h o p was examined. T h i s s t a t i o n m e a s u r e d w i n d s p e e d and d i r e c t i o n b e t w e e n 7 AM and 7 PM. However, t h e s e d a t a may n o t be r e p r e s e n t a t i v e o f t h e f l o w r e g i m e a t t h e L a k e s i n c e t h e y do n o t i n c l u d e a c o m p l e t e 24-hour r e c o r d and a r e a s i g n i f i c a n t d i s t a n c e f r o m t h e d r y l a k e . The s a m p l i n g s i t e s w e r e c h o s e n i n o r d e r t o i n v e s t i g a t e t h e s p a t i a l d i s t r i b u t i o n of p a r t i c u l a t e p o l l u t a n t s i n the v a l l e y . As an a d d i t i o n a l c o n s i d e r a t i o n , s i t e s w e r e s e l e c t e d t o c o i n c i d e w i t h the major p o p u l a t i o n c e n t e r s i n the v a l l e y i n o r d e r to determine the c o n c e n t r a t i o n of r e s p i r a b l e a e r o s o l s to which v a l l e y r e s i d e n t s a r e e x p o s e d on a d a i l y b a s i s . Seven o f t h e s a m p l i n g s i t e s were i n t h e Owens V a l l e y i t s e l f , and one s i t e was i n t h e Mono L a k e a r e a . S i t e 1 was l o c a t e d n e a r t h e B i s h o p A i r p o r t a t t h e N a t i o n a l Weather S e r v i c e M e t e o r o l o g i c a l s t a t i o n . This s i t e i s a b o u t f i v e m i l e s e a s t o f downtown B i s h o p i n t h e c e n t e r o f t h e
Macias and Hopke; Atmospheric Aerosol ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
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AEROSOL
Airflow' Inlet
stack
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Stack s u p p o r t
Filter
Holder Jaco
holder
support Nut
Parker needle valve for f l o w a d j u s t m e n t Figure 1. Schematic of the stacked filter unit showing the inlet and the 2-stage filter holder
-Support bar of portable stand
To p u m p
OOI D p [μ m )
Figure
2.
Particle
capture
efficiencies of 8.0-^m and OA-^m a face velocity of 5 cm/s.
Nuclepore
filters
for
Also shown is the capture efficiency of the human upper respiratory tract (nasopharynx). Closed symbols are derived from data on capture efficiency using monodispersed aero sols; open symbols are theoretical calculations (see Ref. 4). The lower section of the diagram is a schematic of the "Pasadena" bimodal distribution.
Macias and Hopke; Atmospheric Aerosol ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
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valley. S i t e 2 was l o c a t e d i n B i g P i n e n e a r t h e w e s t e r n edge o f the v a l l e y . The I n d e p e n d e n c e c o u r t h o u s e was t h e l o c a t i o n o f S i t e 3. T h i s s i t e was i n t h e c e n t e r o f I n d e p e n d e n c e n e a r Highway 3 9 5 , S i t e 4 was l o c a t e d a t t h e Lone P i n e H i g h S c h o o l o n t h e s o u t h e a s t s i d e o f town. T h i s s i t e i s a b o u t t e n m i l e s n o r t h o f Owens D r y Lake. The K e e l e r P o s t O f f i c e was c h o s e n f o r S i t e 5. T h i s s i t e was on t h e e a s t e r n edge o f Owens D r y L a k e . S i t e 6 was l o c a t e d a t C a r t a g o on t h e w e s t e r n s i d e o f Owens D r y L a k e . S i t e 7 was l o c a t e d a p p r o x i m a t e l y t w e n t y - f i v e m i l e s s o u t h o f Owens D r y L a k e a t L i t t l e Lake. F i g u r e 3 i s a map o f t h e s t u d y a r e a d e p i c t i n g t h e seven sampling s i t e s i n t h e v a l l e y . S i t e 8 was l o c a t e d a t L e e V i n i n g i n t h e Mono L a k e a r e a on t h e n o r t h w e s t e r n edge o f Mono Lake. A l l e i g h t s a m p l i n g s i t e s were o p e r a t e d by l o c a l r e s i d e n t s . P r e w e i g h e d f i l t e r s w e r e p l a c e d i n f i l t e r h o l d e r s a t U.C. D a v i s and s h i p p e d v i a U.P.S. t o e a c h s i t e . The l o c a l o p e r a t o r w o u l d measure t h e f l o w b e f o r e and a f t e r s a m p l i n g w i t h a s p i r o m e t e r c a l i b r a t e d o r i f i c e m e t e r , and t h e n r e t u r n t h i s i n f o r m a t i o n w i t h t h e exposed f i l t e r s . Upon a r r i v a l a t U.C.D., f i l t e r s w e r e p o s t w e i g h e d and p r e p a r e d f o r x - r a y a n a l y s i s . Results A v e r a g e W e e k l y C o n c e n t r a t i o n o f A e r o s o l s . The w e e k l y m o n i t o r i n g d a t a c o l l e t e d i n t h i s s t u d y p r o v i d e d a n e x c e l l e n t means b y w h i c h t h e p r o f i l e o f p o l l u t a n t s i n t h e v a l l e y c o u l d be d e t e r m i n e d F i g u r e 4 i s a p r o f i l e o f t h e a v e r a g e t o t a l and f i n e g r a v i m e t r i c mass measured a t e a c h s i t e i n t h e b a s i n . The e r r o r b a r s r e p r e s e n t t h e s t a n d a r d e r r o r o f t h e mean. A map o f t h e s t u d y a r e a i s i n c l u d e d a t the bottom o f the f i g u r e t o p r o v i d e a b e t t e r unders t a n d i n g o f t h e r e l a t i o n s h i p b e t w e e n t o p o g r a p h y and t h e measured p o l l u t a n t c o n c e n t r a t i o n s . The d o m i n a n t c h a r a c t e r i s t i c o f t h i s p r o f i l e i s t h e marked peak i n t o t a l mass a t t h e K e e l e r s a m p l i n g site. T h i s p e a k , c o u p l e d w i t h t h e d e c r e a s i n g t o t a l mass v a l u e s measured f a r t h e r n o r t h f r o m t h e d r y l a k e b e d , s u g g e s t s t h a t t h e d r y l a k e bed i s a s i g n i f i c a n t s o u r c e o f c o a r s e a e r o s o l s . The f i n e a e r o s o l mass d o e s n o t f o l l o w t h e same p a t t e r n as t h e t o t a l mass, s u g g e s t i n g t h a t t h e l a k e b e d may n o t be a s i g n i f i c a n t source of f i n e a e r o s o l s . A s i m i l a r p r o f i l e d e p i c t i n g s e l e c t e d t r a c e e l e m e n t s i s shown i n F i g u r e 5. The e r r o r b a r s h e r e a r e a g a i n t h e s t a n d a r d e r r o r o f t h e mean. S i l i c o n and i r o n a r e u s u a l l y s o i l - d e r i v e d a e r o s o l s , w h i l e s u l f u r i s g e n e r a l l y produced by gas t o p a r t i c l e c o n v e r s i o n o f s u l f u r d i o x i d e t o s u l f a t e a e r o s o l s or by d i r e c t p r o d u c t i o n o f s u l f a t e s through the burning o f f o s s i l fuels. The s o u r c e o f i r o n and s i l i c o n i n t h e Owens V a l l e y a p p e a r s t o be l o c a l s o i l m a t e r i a l w i t h o n l y s l i g h t enhancement due t o t h e d r y l a k e b e d . T h e r e i s no a n t h r o p o g e n i c s o u r c e o f c o a r s e s u l f u r a e r o s o l s near the d r y l a k e . Thus, t h e s e a e r o s o l s a r e bei n g suspended from t h e d r y l a k e bed.
Macias and Hopke; Atmospheric Aerosol ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
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332
Figure
3.
Map of the Owens Valley
with the aerosol sampling (O)
TOTAL MASS
1
"
F
I
r
ι
I
-
^
sites indicated by
/
^
1
i
i
I
I
1 KEELER
Τ V
I
^
LONE / ·' OWENS PINE/> LAKE \\
TINEMAHA
.^-^j^--^ BISHOP
AEROSOL
LITTLE INDEPENDENCE
CARTAGO
Figure 4. Average for all weeks of aerosol mass less than 15.0 μπι (total mass) and mass less than 2.5 \*m (fine mass) at each sampling site. A map of the sampling sites is included at the bottom of the figure. Error bars represent standard error of the mean.
Macias and Hopke; Atmospheric Aerosol ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
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E f f e c t o f D u s t S t o r m E p i s o d e s on t h e A v e r a g e W e e k l y A e r o s o l Concentrations. The t o t a l and f i n e g r a v i m e t r i c mass a v e r a g e d o v e r a l l s i t e s f o r e a c h week, i s d e p i c t e d i n F i g u r e 6. The e r r o r b a r s f o r t h e Owens V a l l e y c u r v e s r e p r e s e n t t h e s t a n d a r d d e v i a t i o n o f t h e mean. The e r r o r s on t h e Mono L a k e c u r v e r e p r e s e n t t h e s a m p l i n g s y s t e m e r r o r o f ±15%. The mean w e e k l y v a l u e s do n o t i n c l u d e t h e t h r e e d u s t s t o r m e p i s o d e s sampled s e p a r a t e l y , b u t do i n c l u d e s e v e r a l a d d i t i o n a l dust storms. Table I l i s t s a l l the d u s t s t o r m s r e p o r t e d by t h e s a m p l e r o p e r a t o r s . The F i n e G r a v i m e t r i c Mass i s v i r t u a l l y unchanged o v e r t h e monitoring period. The T o t a l G r a v i m e t r i c Mass (TGM) measurements, however, i l l u s t r a t e s e v e r a l i m p o r t a n t p o i n t s . F i r s t , a general upward t r e n d f r o m w i n t e r t o summer i s o b s e r v e d . T h i s t r e n d has b e e n o b s e r v e d i n d a t a f r o m o t h e r s i t e s i n U t a h and c a n be a t t r i b u t e d t o h i g h e r l e v e l s o f s o i l a e r o s o l s suspended i n t h e atmos p h e r e as t h e s o i l s u r f a c e d r i e s o u t . S u p e r i m p o s e d on t h e g e n e r a l t r e n d , h o w e v e r , a r e p e a k s o f TGM w h i c h a r e a f a c t o r o f 1.3 t o 2.3 t i m e s t h e non-peak l e v e l s . Each o f the peaks corresponds to a d u s t s t o r m o r h i g h w i n d s p e r i o d as shown i n T a b l e I . F u r t h e r more, t h e d u s t s t o r m e p i s o d e s w h i c h w e r e sampled s e p a r a t e l y i n t h e f i r s t , t h i r d , and f o u r t h weeks o f A p r i l , w o u l d r a i s e t h e TGM i n t h o s e p e r i o d s by a f a c t o r o f 1.7. I t s h o u l d be n o t e d t h a t t h e t h i r d week o f A p r i l a l r e a d y i n c l u d e s one d u s t s t o r m p e r i o d w h i c h was n o t sampled s e p a r a t e l y . F i n a l l y , although i t i s d i f f i c u l t to draw i n f e r e n c e s f r o m t h e s e d a t a a b o u t t h e p e r s i s t e n c e o f s u s p e n d ed d u s t a f t e r t h e s t o r m , t h e f r e q u e n c y o f o c c u r r e n c e o f e l e v a t e d TGM l e v e l s i s w e l l documented. D u r i n g t h e s e v e n t e e n week m o n i t o r i n g p e r i o d , n i n e weeks e x h i b i t e l e v a t e d TGM l e v e l s i f t h e s e p a r a t e dust storm samples a r e i n c l u d e d i n the weekly average v a l ues. Hence, t h e s e d a t a i n d i c a t e t h a t d u s t s t o r m e p i s o d e s h a v e a s i g n i f i c a n t i m p a c t on t h e a v e r a g e a e r o s o l c o n c e n t r a t i o n i n t h e Owens V a l l e y . M a g n i t u d e and S p a t i a l E x t e n t o f L a k e Bed A e r o s o l s . The magn i t u d e and s p a t i a l e x t e n t o f t h e c o n t r i b u t i o n o f l a k e bed a e r o s o l s t o the measured a t m o s p h e r i c a e r o s o l c o n c e n t r a t i o n i n t h e v a l l e y c a n be i d e n t i f i e d u s i n g d a t a f r o m t h e i n t e n s i v e d u s t s t o r m s a m p l i n g p r o g r a m , t h e w e e k l y m o n i t o r i n g s t u d y , and t h e l a k e bed s o i l s a m p l e a n a l y s i s . G r a v i m e t r i c mass and f o u r e l e m e n t s , s u l f u r , c h l o r i n e , s i l i c o n , and i r o n , w e r e s e l e c t e d f o r a n a l y s i s . Based upon d a t a o b t a i n e d by r e s u s p e n d i n g l a k e bed and s a m p l i n g s i t e s o i l s a m p l e s , two e l e m e n t s ( s u l f u r and c h l o r i n e ) w e r e i d e n t i f i e d as b e i n g p r i m a r i l y g e n e r a t e d i n t h e v a l l e y f r o m r e s u s p e n d ed l a k e bed m a t e r i a l s . The c o a r s e a e r o s o l f r a c t i o n (15um t o 2.5um) o f t h e s e e l e m e n t s i s a good t r a c e r o f l a k e bed m a t e r i a l s f o r t h r e e r e a s o n s . F i r s t , s i n c e these elements were o n l y found i n l a k e b e d s o i l s a m p l e s , no o t h e r " n a t u r a l " s o u r c e o f t h e s e a e r osols i s l i k e l y to e x i s t i n the v a l l e y . Second, s i n c e the o n l y anthropogenic source of s u l f u r a e r o s o l s i n the v a l l e y (automotive e x h a u s t ) w o u l d p r o d u c e f i n e p a r t i c l e s u l f u r , (
