Atmospheric Aerosol - American Chemical Society

14

Carbonaceous Urban Aerosol—Primary or Secondary?

LIH-CHING CHU and EDWARD S. MACIAS

Downloaded by STONY BROOK UNIV SUNY on June 4, 2018 | https://pubs.acs.org Publication Date: October 13, 1981 | doi: 10.1021/bk-1981-0167.ch014

Department of Chemistry and CAPITA, Washington University, St. Louis, MO 63130

The relative contribution of primary and secondary carbon to urban aerosol i s discussed i n this paper. Some data from theACHEXstudy i n Los Angeles have been reexamined using new values for the carbon and lead emissions. Data on t o t a l carbon, elemental carbon and lead i n fine p a r t i c l e samples collected i n St. Louis are presented. Lead and elemental carbon have been shown to be useful tracers of primary carbonaceous aerosol. I t i s concluded that secondary carbon i s most l i k e l y to be a significant portion of the urban carbonaceous aerosol i n the summer and i n the middle of the day. Secondary carbon can best be measured with short time resolution sampling (Δt7). T h i s approach a s s u m e s , o f c o u r s e , t h a t l e a d i s p r e s e n t o n l y as a p r i m a r y pollutant. F o r h i g h w a y t r a f f i c composed o f v e h i c l e s and f u e l t y p e s i n t h e same p r o p o r t i o n s as f o r t h e e n t i r e u r b a n a r e a , t h e r a t i o o f a e r o s o l c a r b o n t o l e a d was c a l c u l a t e d t o be a b o u t 4. T h i s i s a b o u t a f a c t o r o f 2 l o w e r t h a n t h a t c a l c u l a t e d f o r 1980 due t o t h e l a r g e number o f a u t o m o b i l e s w h i c h u s e d u n l e a d e d f u e l and t h e l o w e r l e a d c o n t e n t i n l e a d e d f u e l i n 1980 ( 9 ) . F o r t h e o t h e r e x t r e m e where p r i m a r y e m i s s i o n s f r o m a l l s o u r c e s become w e l l mixed, a s i t u a t i o n which i s p o s s i b l e because of the l o n g

Macias and Hopke; Atmospheric Aerosol ACS Symposium Series; American Chemical Society: Washington, DC, 1981.


254

A T M O S P H E R I C AEROSOL

τ

LEAD

CONCENTRATION,

1

μς/m

Figure 1. Plots of aerosol C vs. Pb concentrations for determined in the ACHEX study (\0). The lines indicate for the well-mixed air basin and the highway signature as Dominguez Hills; (Φ) West Covina; (O)

Γ

3

Los Angeles in 1973 as the particulate C/Pb ratio explained in the text; (X) Riverside.



14.

C H U A N D MACIAS

Carbonaceous

Urban

Aerosol

255

r e t e n t i o n t i m e s i n t h e LA b a s i n , t h e c a l c u l a t e d c a r b o n t o l e a d r a t i o i s a b o u t 7. These two l i m i t s o f p r i m a r y c a r b o n t o l e a d r a t i o s a r e a l s o shown as l i n e s i n F i g u r e 1. A b o u t 40% o f t h e d a t a f a l l b e t w e e n t h e s e two l i n e s and f o r t h e s e d a t a we c o n c l u d e t h a t no o b v i o u s e x c e s s c a r b o n due t o s e c o n d a r y f o r m a t i o n i s p r e s e n t . A b o u t 20% o f t h e d a t a h a v e C/Pb r a t i o s l o w e r t h a n t h e highway s i g n a t u r e . T h i s may be due t o a h i g h e r number o f v e h i c l e s which burn leaded gas, at those sampling times, r e l a t i v e to the b a s i n average (e.g. h i g h e r r a t i o of leaded passenger c a r s to d e i s e l t r u c k s ) . N e a r l y 40% o f t h e p o i n t s h a v e C/Pb r a t i o s g r e a t e r t h a n 7. These d a t a c l e a r l y have e x c e s s c a r b o n r e l a t i v e t o l e a d — a l m o s t c e r t a i n l y due t o s e c o n d a r y c a r b o n f o r m a t i o n . The d a t a w i t h t h e h i g h e s t C/Pb r a t i o s a r e m a i n l y f r o m t h e i n l a n d s i t e s a t R i v e r s i d e and West C o v i n a where p h o t o c h e m i c a l p o l l u t i o n i s t h e most s e v e r e . The e f f e c t o f s e c o n d a r y c a r b o n f o r m a t i o n c a n be s e e n e v e n more c l e a r l y i n F i g u r e 2 where t h e s e d a t a a r e p l o t t e d i n a s l i g h t l y d i f f e r e n t manner. H e r e t h e C/Pb r a t i o i s p l o t t e d a s a f u n c t i o n o f t i m e o f day on t h r e e d i f f e r e n t d a y s . In the e a r l y m o r n i n g t h e C/Pb r a t i o a p p r o a c h e d t h e h i g h w a y v a l u e a t a l l t h r e e s i t e s , i n d i c a t i n g dominant p r i m a r y c a r b o n a e r o s o l a t t h o s e t i m e s . I n t h e m i d d l e o f t h e day a l l s i t e s showed a significant increase i n this ratio. E v e n more t o t h e p o i n t , a t West C o v i n a t h e C/Pb v a l u e s w e r e a b o v e 7 f r o m 1000 u n t i l 1800 and a t R i v e r s i d e h i g h C/Pb v a l u e s w e r e m e a s u r e d f r o m 0800 u n t i l m i d n i g h t . These h i g h C/Pb v a l u e s a r e i n d i c a t i v e o f the presence of s i g n i f i c a n t secondary carbon. I t should also be n o t e d t h a t i f a s i n g l e 24-h s a m p l e had b e e n c o l l e c t e d i n p l a c e of the h i g h e r t i m e - r e s o l v e d samples the e f f e c t of second a r y c a r b o n w o u l d n o t h a v e b e e n d e t e c t e d i n t h e C/Pb r a t i o . Our a n a l y s i s i s i n agreement w i t h t h e e a r l i e r c o n c l u s i o n s f r o m t h e ACHEX d a t a t h a t s e c o n d a r y c a r b o n a e r o s o l i s s i g n i f i c a n t . One m a j o r d i f f e r e n c e i s t h a t o u r new e m i s s i o n i n v e n t o r y a c c o u n t s f o r much more p r i m a r y c a r b o n t h a n was t h o u g h t p r e v i o u s l y , l e a d i n g to the c o n c l u s i o n t h a t d u r i n g the e a r l y morning primary c a r b o n was d o m i n a n t , e v e n d u r i n g v e r y smoggy p e r i o d s . Experimental S t . L o u i s Sample C o l l e c t i o n . Ambient a e r o s o l s were c o l l e c t e d i n S t . L o u i s i n 6-h i n t e r v a l s w i t h a TWOMASS a u t o m a t e d s e q u e n t i a l tape sampler. T h i s sampler f r a c t i o n a t e d the a e r o s o l i n t o two s i z e c l a s s e s , f i n e p a r t i c l e s h a v i n g a e r o d y n a m i c d i a m e t e r s l e s s t h a n 3ym, and c o a r s e p a r t i c l e s w i t h d i a m e t e r s g r e a t e r t h a n 3ym. I t was e q u i p p e d w i t h a b e t a - a t t e n u a t i o n mass m o n i t o r t o d e t e r m i n e f i n e - p a r t i c l e mass ( 1 1 ) . O n l y t h e f i n e p a r t i c l e f i l t e r was examined i n t h i s s t u d y . P a l l f l e x E70 g l a s s - f i b e r f i l t e r tape w i t h a detachable c e l l u l o s e b a c k i n g ( P a l l f l e x I n c . Putnam, CT) was u s e d w i t h t h i s s a m p l e r . An a e r o s o l s a m p l e r o p e r a t i n g f r o m t h e same i n l e t m a n i f o l d as t h e


ATMOSPHERIC


256

AEROSOL

Dominguez Hills IO/IO-II

r —

r

~

F

" S

,

^

I

ι

West Covina

J~T_

8/8-9

1

1

1

1

Riverside

9/5-6

. I Ί— Ο

—ι— 6

•—r-—-——Τ 12

18

TIME (h) Figure 2. Plot of the C/Pb ratio as a function of time for aerosol samples col lected at three sampling sites in Los Angeles in 1973. The data are from the ACHEX study (10).



14.

C H U A N D MACiAS

Carbonaceous

Urban

Aerosol

257

TWOMASS, was u s e d t o c o l l e c t a m b i e n t a e r o s o l on T e f l o n f i l t e r s ( G h i a C o r p . , P l e a s a n t o n , CA). The a e r o s o l i n l e t u s e d was d e s i g n e d so t h a t l a r g e p a r t i c l e s d i d n o t r e a c h t h i s s a m p l e r . Two 6-h s a m p l e s (0900-1500 and 1500-2100) and one 12-h s a m p l e (2100-0900) w e r e c o l l e c t e d e a c h day w i t h t h i s s a m p l e r . A t o t a l o f 154 s a m p l e s w e r e c o l l e c t e d on g l a s s f i b e r f i l t e r s d u r i n g J a n u a r y and F e b r u a r y 1978; J u n e and J u l y , 1979; A u g u s t , 1979; and F e b r u a r y and M a r c h , 1980. D u r i n g the l a s t p e r i o d 24 s a m p l e s were a l s o c o l l e c t e d on t e f l o n f i l t e r s . C h e m i c a l A n a l y s i s . A e r o s o l c a r b o n , s u l f u r and n i t r o g e n c o n c e n t r a t i o n s were m e a s u r e d on t h e g l a s s - f i b e r f i l t e r s w i t h t h e gamma r a y a n a l y s i s o f l i g h t e l e m e n t (GRALE) t e c h n i q u e ( 1 2 ) . Samples were i r r a d i a t e d w i t h 7-MeV p r o t o n s i n t h e W a s h i n g t o n U n i v e r s i t y 135-cm c y c l o t r o n and t h e γ-rays e m i t t e d f o l l o w i n g i n e l a s t i c s c a t t e r i n g were m e a s u r e d i n b e a m . The e l e m e n t a l c a r b o n ( s o o t ) c o n c e n t r a t i o n was m e a s u r e d on t h e g l a s s - f i b e r f i l t e r s by l i g h t r e f l e c t a n c e f r o m t h e f i l t e r ( 1 3 ) . T h i s technique takes advantage of the f a c t t h a t elemental carbon i s the predominant s p e c i e s which i s b l a c k i n the f i n e p a r t i c l e a e r o s o l . T r a c e e l e m e n t s were a n a l y z e d on t e f l o n f i l t e r s by p a r t i c l e induced x - r a y e m i s s i o n (PIXE) a t the U n i v e r s i t y of C a l i f o r n i a Davis (14). Of p a r t i c u l a r i n t e r e s t i n t h i s s t u d y w e r e t h e c o n c e n t r a t i o n s o f l e a d and b r o m i n e . Results F i n e A e r o s o l Mass B a l a n c e . The t e m p o r a l v a r i a t i o n s o f f i n e p a r t i c l e mass, t o t a l c a r b o n , e l e m e n t a l c a r b o n , n i t r o g e n , s u l f u r and b f o r a t y p i c a l summer p e r i o d ( A u g u s t 4-10, 1979) and a t y p i c a l w i n t e r p e r i o d ( M a r c h 1-9, 1980) a r e shown i n F i g u r e s 3 and 4, r e s p e c t i v e l y . The s t r o n g c o r r e l a t i o n b e t w e e n s u l f u r and n i t r o g e n i s s e e n q u i t e s t r i k i n g l y i n t h e s e f i g u r e s and i s shown more q u a n t i t a t i v e l y i n t h e s c a t t e r p l o t ( F i g u r e 5) o f a e r o s o l s u l f u r and n i t r o g e n c o n c e n t r a t i o n s f o r a l l s a m p l e s . These d a t a h a v e a h i g h c o r r e l a t i o n ( r = 0 . 9 1 ) ; t h e l i n e a r l e a s t s q u a r e s f i t t o t h e d a t a y i e l d s a s l o p e o f 1.04±0.04 and i n t e r c e p t o f 0.5±0.2yg m~ . T h i s s l o p e i s c l o s e to the s t o i c h i o m e t r i c r a t i o f o r ammonium s u l f a t e (S/N = 1 . 1 4 ) . T h i s c o r r e l a t i o n and t h e f i n d i n g t h a t a m b i e n t a e r o s o l s u l f u r i n t h e e a s t e r n U.S. i s predominantly i n the form of s u l f a t e (1,15), s t r o n g l y s u g g e s t t h a t a e r o s o l s u l f u r and n i t r o g e n w e r e m a i n l y i n t h e f o r m o f ammonium s u l f a t e d u r i n g t h i s s a m p l i n g p e r i o d . However, i t i s p o s s i b l e t h a t t h e s u l f a t e was u n i n t e n t i o n a l l y n e u t r a l i z e d sometime a f t e r sample c o l l e c t i o n . I t s h o u l d be n o t e d t h a t t h e r e are s e v e r a l samples w i t h s u l f u r c o n c e n t r a t i o n s s i g n i f i c a n t l y greater than n i t r o g e n c o n c e n t r a t i o n s . T h i s may be due t o t h e p r e s e n c e o f an a c i d i c s u l f a t e a e r o s o l o r some o t h e r s u l f u r species at those times (16). A mass b a l a n c e f o r f i n e p a r t i c l e a e r o s o l c a n be c a l c u l a t e d f r o m t h e c a r b o n , s u l f u r , n i t r o g e n , and mass c o n c e n t r a t i o n d a t a s

c

a

t

3


ATMOSPHERIC


258

Figure 3. Temporal variations of fine particle mass, total C, elemental C, N, S, and bsrat in St. Louis for summer 1979

AEROSOL

START TIME

Ε Ο

·ΝΗ4 ·'··:: .·;.·ν·'ΝΗΐ:··::>'.'

< 20

S0| 26±ll%

Figure 6. Mass balance of St. Louis fine aerosol broken down into summer and winter sampling periods

MASS-*(/jg/m3)

\I2±6%°: S0| 33 ±15%

Summer

Winter

40±I9

38 ±19



C H U A N D MACIAS

Carbonaceous

Urban

Aerosol

261

Figure 7. (a) Frequency distribution of St. Louis fine aerosol mass concentration; (b) mass balance of St. Louis fine aerosol divided into high- and low-sulfate con centration regimes for summer and winter. The median sulfate concentration of 8.4 μgm was used as the separation point for high- and low-sulfate regimes. 3


262

ATMOSPHERIC

AEROSOL

E l e m e n t a l - T o t a l Carbon C o r r e l a t i o n s The r e l a t i o n s h i p b e t w e e n e l e m e n t a l c a r b o n and t o t a l c a r b o n i n a l l s a m p l e s i s shown i n F i g u r e 8 a . The d a t a a r e s c a t t e r e d and n o t w e l l c o r r e l a t e d ( r = 0.63). T h i s i s d i f f e r e n t from the r e s u l t s r e p o r t e d by R o s e n e t a l . ( 5 ) , h o w e v e r , o u r d a t a f o r 6-h s a m p l e s w h i l e R o s e n examined 24-h s a m p l e s . I n o r d e r t o more a c c u r a t e l y compare t h e two d a t a s e t s we h a v e a v e r a g e d t h e 6-h s a m p l e s t o d e t e r m i n e t h e 24-h a v e r a g e c o n c e n t r a t i o n s o f e l e m e n t a l and t o t a l c a r b o n f o r t h e summer and w i n t e r , r e s p e c t i v e l y ( F i g u r e 8b and c ) . The w i n t e r d a t a show a s t r o n g c o r r e l a t i o n ( r = 0.86) i n agreement w i t h t h e p r e v i o u s work ( 5 ) ; t h i s c o r r e l a t i o n i s n o t s e e n i n t h e summer d a t a ( r = 0 . 1 4 ) . We conclude from t h i s t h a t i n the w i n t e r sampling p e r i o d i n S t . Louis there i s v e r y l i t t l e secondary o r g a n i c a e r o s o l present. What i s t h e r e i s c o v a r y i n g w i t h t h e p r i m a r y c a r b o n a e r o s o l . On e i t h e r t i m e s c a l e i n t h e summer t h e r e a p p e a r s t o be s i g n i f i c a n t amounts o f s e c o n d a r y c a r b o n w h i c h v a r y i n d e p e n d e n t l y o f t h e p r i mary c a r b o n a e r o s o l i n S t . L o u i s . This d i f f e r e n c e i n the c o r r e l a t i o n s on d i f f e r e n t t i m e s c a l e s may a l s o i m p l y t h a t d i u r n a l p a t t e r n s o f p r i m a r y and s e c o n d a r y c a r b o n a e r o s o l a r e d i f f e r e n t .


t

Automotive Tracers. The w o r k o f C a s s , Boone and M a c i a s (9) i n d i c a t e s t h a t i n L o s A n g e l e s 90% o f t h e e l e m e n t a l c a r b o n a e r o s o l and 7 1 % o f t h e p r i m a r y t o t a l c a r b o n a e r o s o l i s due t o mobile s o u r c e s . Although a complete carbonaceous a e r o s o l e m i s s i o n i n v e n t o r y has n o t b e e n c a r r i e d o u t f o r S t . L o u i s , i t i s r e a s o n a b l e t o assume t h a t m o b i l e s o u r c e s o f c a r b o n a c e o u s a e r o s o l a r e q u i t e i m p o r t a n t i n t h a t a i r s h e d as w e l l . A u t o m o t i v e e x h a u s t i s by f a r t h e m a j o r s o u r c e o f p a r t i c u l a t e l e a d i n u r b a n a t m o s p h e r e s ( 1 8 ) . I n t h i s s t u d y we f o u n d a s t r o n g c o r r e l a t i o n b e t w e e n l e a d and b r o m i n e ( r = 0.91) d u r i n g t h e w i n t e r o f 1980, a s shown i n F i g u r e 9 a . T h i s i s as e x p e c t e d f o r a u t o m o b i l e e m i s s i o n s b e c a u s e t h e p r i n c i p a l l e a d component i n the e x h a u s t i s P b B r C l . The B r / P b r a t i o o f 0.16 i s l o w e r t h a n the s t o i c h i o m e t r i c v a l u e o f 0.39 due t o t h e l o s s o f B r f r o m t h e a e r o s o l w i t h t i m e . T h u s , l e a d a p p e a r s t o be a good t r a c e r f o r auto emissions i n the St. L o u i s a e r o s o l . I n t e r e s t i n g l y , the e l e m e n t a l c a r b o n and l e a d a r e w e l l c o r r e l a t e d ( r = 0 . 8 4 ) , as shown i n F i g u r e 9b, and y e t l e a d comes m a i n l y f r o m t h e c o m b u s t i o n of l e a d e d g a s o l i n e w h i l e t h e b u l k o f t h e e l e m e n t a l c a r b o n comes from unleaded d e i s e l f u e l . The h i g h c o r r e l a t i o n i s most l i k e l y a r e s u l t of the f a i r l y constant mixture of leaded, unleaded, and d e i s e l v e h i c l e s i n t h e a u t o m o t i v e t r a f f i c . T o t a l carbon and l e a d a r e a l s o w e l l c o r r e l a t e d ( r = 0.82) as shown i n F i g u r e 9c d u r i n g t h i s w i n t e r p e r i o d w h i c h a g r e e s w i t h t h e h i g h c o r r e l a t i o n b e t w e e n e l e m e n t a l c a r b o n and t o t a l c a r b o n ( F i g u r e 9d). Thus d u r i n g t h e w i n t e r p e r i o d p r i m a r y e m i s s i o n s f r o m m o b i l e s o u r c e s seem t o d o m i n a t e t h e c a r b o n a c e o u s a e r o s o l . The l e a s t s q u a r e s f i t s t o t h e d a t a o f t o t a l c a r b o n and e l e m e n t a l



C H U A N D MACIAS

Carbonaceous

TOTAL CARBON (jug/rrr

Urban

Aerosol

263

Figure 8. Plot of elemental C concentration vs. total C concentration for particulate samples collected in St. Louis: (a) all samples (6 h); (b) 24-h averages for summer samples; (c) 24-h averages for winter samples.


ATMOSPHERIC


264

AEROSOL

Winter 1980, St. Louis

10 0

Ε

16

TC = 0.6 + 2.2Pb r = 082

/ A

/

c

ro

d

0 32 TC r = 0 93

EC =

Ε

\

3

σ»8|

Ζ '21 Ο m

Atmospheric Aerosol - American Chemical Society

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