Wintertime Carbonaceous Aerosols in Los Angeles - ACS Publications

13 Wintertime Carbonaceous Aerosols in Los Angeles

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 26, 2016 | http://pubs.acs.org Publication Date: October 13, 1981 | doi: 10.1021/bk-1981-0167.ch013

An

Exploration of the Role of Elemental Carbon

M. H. CONKLIN and G. R. CASS California Institute of Technology, Pasadena, CA 91125 L.-C. CHU and E. S. MACIAS Washington University, St. Louis, MO 63130

Aerosol carbon concentrations have been measured under wintertime conditions at two sites in the Los Angeles basin. Samples were analyzed for total carbon by the Gamma Ray Analysis of Light Elements (GRALE) technique and for elemental carbon by reflectance. Total carbon concentrations at downtown Los Angeles averaged 24 µgm and elemental carbon concentrations averaged 9 µgm-3 during the 7 am to 1 pm period. The light absorption e f f i ciency of ambient elemental carbon particles was determined to be 11.9 ± 0.9 m g , and i t was estimated that light absorption by elemental carbon could account for up to 17% of total light extinction in downtown Los Angeles. Baseline conditions for elemental carbon concentrations in the Los Angeles area prior to the introduction of large numbers of diesel passenger cars into the vehicle fleet are established by these measurements. A method for reconstruction of a long time series of past historical a i r monitoring data on elemental carbon concentrations is described. -3

2

-1

Primary carbon p a r t i c l e s c o n s i s t i n g of v o l a t i l e organics plus soot are emitted from most combustion processes. The soot f r a c t i o n of these p o l l u t a n t emissions c o n s i s t s p r i n c i p a l l y of g r a p h i tic or elemental carbon and adsorbed p o l y n u c l e a r aromatic hydrocarbons ( 1_). G r a p h i t i c carbon p a r t i c l e s are thought to be the predominant l i g h t absorbing aerosol species i n the atmosphere ( 2 ) , and have been r e l a t e d to v i s i b i l i t y d e t e r i o r a t i o n i n the Denver region (3.). Carbonaceous aerosols are observed to accumulate i n the r e s p i r a b l e submicron p a r t i c l e s i z e range (4., 5). Soots have been shown to be carcinogenic i n experimental animal studies and hence are of c o n s i d e r a b l e p u b l i c h e a l t h concern (6_) . 0097-6156/81/0167-0235$05.00/0 © 1981 American Chemical Society

In Atmospheric Aerosol; Macias, Edward S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

ATMOSPHERIC


236

AEROSOL

T h e r e a r e p r e s e n t l y no l a r g e h i s t o r i c a l a i r q u a l i t y monitoring data bases f o r elemental carbon. The d e v e l o p m e n t o f an h i s t o r i c a l data base i s n e c e s s a r y f o r the d e s i g n of e m i s s i o n control s t r a t e g i e s b e c a u s e i t i s i m p o r t a n t t o u n d e r s t a n d t h e n a t u r e o f an e x i s t i n g a i r q u a l i t y problem b e f o r e the e f f e c t of altered emissions c a n be e v a l u a t e d . Numerous s h o r t - t e r m s t u d i e s c o m p l e t e d i n S t . L o u i s (7_) , Los A n g e l e s (8.,9.), B e r k e l e y (10.), D e n v e r ( 3 ) , and Austria (11) show t h a t e l e m e n t a l c a r b o n a e r o s o l s a r e p r e s e n t i n s i g n i f i c a n t q u a n t i t i e s , but d a t a s u f f i c i e n t to d e f i n e long-term averages and t o e v a l u a t e s p a t i a l and t e m p o r a l t r e n d s o v e r p e r i o d s of y e a r s a r e a b s e n t . The l a c k o f a c o m p l e t e d a t a b a s e on soot concentrations i n Los Angeles i s of particular importance. As n o t e d by P i e r s o n (12)« Los A n g e l e s a i r q u a l i t y i s l i k e l y t o be h e a v i l y a f f e c t e d by i n c r e a s e d s o o t e m i s s i o n s i f l a r g e numbers o f d i e s e l p a s s e n g e r c a r s are introduced i n t o the v e h i c l e f l e e t . I t i s important to charact e r i z e existing a i r quality before this occurs. The p u r p o s e o f t h i s p a p e r i s t o r e p o r t p r o g r e s s t o d a t e on the r e c o n s t r u c t i o n of a long-term h i s t o r i c a l d a t a base f o r element a l c a r b o n c o n c e n t r a t i o n s i n t h e Los Angeles basin. Since the m i d - 1 9 5 0 ' s , t h e Los A n g e l e s A i r P o l l u t i o n C o n t r o l D i s t r i c t and i t s s u c c e s s o r a g e n c i e s have c o l l e c t e d c o n s e c u t i v e h o u r l y average data using instruments which measure p a r t i c u l a t e m a t t e r i n Km u n i t s . The Km u n i t i s a r e f l e c t a n c e measurement o f the "blackness" of p a r t i c u l a t e m a t t e r d e p o s i t e d on a f i l t e r . The d a r k e n e d a p p e a r a n c e of a m b i e n t p a r t i c u l a t e samples has b e e n a t t r i b u t e d t o t h e element a l c a r b o n c o n t e n t o f t h e sample ( 2 ) . A w o r k i n g h y p o t h e s i s a t t h e o u t s e t o f t h i s p r o j e c t was t h a t t h e Km s a m p l e r d a t a can be r e l a t e d to e l e m e n t a l c a r b o n c o n c e n t r a t i o n s . To v e r i f y t h i s h y p o t h e s i s , low volume s a m p l e r s have b e e n operated i n p a r a l l e l w i t h t h e a i r p o l l u t i o n c o n t r o l d i s t r i c t ' s Km samplers. The samples c o l l e c t e d were a n a l y z e d o p t i c a l l y f o r elemental carbon and by t h e Gamma Ray A n a l y s i s o f L i g h t E l e m e n t s (GRALE) t e c h n i q u e f o r t o t a l carbon. These d a t a w e r e u s e d to assess the c o n c e n t r a t i o n o f e l e m e n t a l and t o t a l c a r b o n a e r o s o l s p r e s e n t d u r i n g t h e w i n t e r months i n Los A n g e l e s . I t was established t h a t t h e Km s a m p l e r s c a n be c a l i b r a t e d t o r e a d e l e m e n t a l c a r b o n c o n c e n t r a t i o n s . T h i s c a l i b r a t i o n can be used to reconstruct historical elemental c a r b o n l e v e l s a t s e v e n s i t e s i n Los Angeles.

Experimental

Design

A s a m p l i n g s c h e d u l e was d e s i g n e d w h i c h w o u l d encounter peak levels of ambient e l e m e n t a l c a r b o n . Peak l e v e l s were o f i n t e r e s t f o r two r e a s o n s : t o i n s u r e t h a t f i l t e r l o a d i n g s w o u l d be within the a p p r o p r i a t e r a n g e f o r maximum a n a l y t i c a l s e n s i t i v i t y and t o expand t h e r a n g e o f v a l u e s o v e r w h i c h t h e Km samplers could be calibrated. To determine when peak e l e m e n t a l c a r b o n c o n c e n t r a t i o n s w o u l d be l i k e l y t o o c c u r , h i s t o r i c a l d a t a on c a r b o n m o n o x i d e



13.

CONKLIN E T AL.

Carbonaceous

Aerosols

in

Los

Angeles

237

and total o x i d e s o f n i t r o g e n c o n c e n t r a t i o n s i n Los A n g e l e s were examined as t r a c e r s f o r t h e p r e s e n c e o f p r i m a r y c o m b u s t i o n pollutants. I t was f o u n d t h a t c o n c e n t r a t i o n maxima f o r t h e s e p o l l u t a n t s o c c u r d u r i n g t h e m o r n i n g peak t r a f f i c h o u r s throughout the w i n t e r m o n t h s . U s i n g t h i s i n f o r m a t i o n , t h e s a m p l i n g s c h e d u l e was s e t f o r 10 weekday m o r n i n g s d u r i n g t h e p e r i o d J a n u a r y 15 t o F e b r u a r y 26, 1980. Low-volume s a m p l e r s were o p e r a t e d in parallel with South Coast A i r Q u a l i t y Management D i s t r i c t a i r m o n i t o r i n g equipment a t t h e D i s t r i c t ' s P a s a d e n a and downtown Los A n g e l e s stations. As shown i n F i g u r e 1, t h e downtown Los A n g e l e s sampling site p r e s e n t l y i s l o c a t e d w i t h i n a r i n g of freeway i n t e r c h a n g e s . I t i s in an industrial area to the n o r t h e a s t of the c e n t r a l business district. R a i l r o a d s w i t c h i n g yards are east of that s i t e . The Pasadena monitoring station is situated in a mixed r e s i d e n t i a l / c o m m e r c i a l d i s t r i c t two b l o c k s s o u t h o f t h e I n t e r s t a t e 210 F r e e w a y . T h r e e pump and f i l t e r a s s e m b l i e s were o p e r a t e d s i m u l t a n e o u s l y at each location. Sample d u r a t i o n o f 51.5 m i n u t e s e a c h h o u r and sample i n l e t l i n e c o n f i g u r a t i o n r e p l i c a t e d t h e Km s a m p l e r s . Samp l i n g b e g a n a t 700 h o u r s P a c i f i c S t a n d a r d Time (PST) and c o n t i n u e d o v e r c o n s e c u t i v e h o u r s u n t i l 1300 PST. F i l t e r s u b s t r a t e s were c h o s e n t h a t w e r e s u i t e d f o r s u b s e q u e n t determinations o f a e r o s o l 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 , t r a c e m e t a l s and a e r o s o l l i g h t a b s o r p t i o n c o e f f i c i e n t ( ° ^ ) * The three filter assemblies c o n t a i n e d , a 47 mm P a l l f l e x T i s s u q u a r t z f i l t e r (2500 QAO), a 47 mm N u c l e p o r e p o l y c a r b o n a t e f i l t e r (0.40 ym p o r e s i z e ) and a 13 mm P a l l f l e x T i s s u q u a r t z f i l t e r (2500 QAO). The 47 mm P a l l f l e x T i s s u q u a r t z f i l t e r s were c h o s e n b e c a u s e o f their low carbon content ( 1 3 ) . F i l t e r p r e p a r a t i o n c o n s i s t e d of p r e f i r i n g t h e s e f i l t e r s t o 900 C i n a i r f o r 1-1/2 h o u r s t o f u r t h e r r e d u c e ^the carbon levels. These f i l t e r s were masked l e a v i n g a 0.32 cm a r e a exposed t o t h e a i r f l o w . A m b i e n t a i r was drawn through e a c h f i l t e r a t a r a t e o f a p p r o x i m a t e l y 12 1/min. F o l l o w i n g sample c o l l e c t i o n e a c h o f t h e f i l t e r s was p l a c e d i n a plastic p e t r i d i s h , s e a l e d and c h i l l e d . F i l t e r d e p o s i t s were s u b s e q u e n t l y a n a l y z e d f o r t o t a l c a r b o n by t h e Gamma Ray A n a l y s i s o f L i g h t Elements (GRALE) t e c h n i q u e (13) . E l e m e n t a l c a r b o n d e t e r m i n a t i o n on t h e same f i l t e r s was o b t a i n e d by reflectance calibrated against the c a r b o n c o n t e n t o f b u t a n e s o o t s t a n d a r d s w h i c h had b e e n h e a t e d g r a d u a l l y i n a i r t o 300° C and h e l d at t h a t temperature for g r e a t e r t h a n 10 m i n i n o r d e r t o remove o r g a n i c s p r e s e n t . The e l e mental carbon c o n c e n t r a t i o n of ambient samples a l s o can be o b t a i n e d by t h e GRALE t e c h n i q u e a f t e r s i m i l a r h e a t t r e a t m e n t t o e l i m i n a t e v o l a t i l e o r g a n i c s . The e q u i v a l e n c e o f the reflectance and GRALE t e c h n i q u e s f o r measurement o f a m b i e n t e l e m e n t a l c a r b o n c o n c e n t r a t i o n s i s d e m o n s t r a t e d by D e l u m y e a , Chu and M a c i a s (7_). Their experiments i n S t . L o u i s show t h a t t h e r e f l e c t a n c e method c a l i b r a t e d a g a i n s t heated butane soot w i l l reproduce the elemental carbon c o n c e n t r a t i o n (EC) o f a m b i e n t samples as m e a s u r e d by GRALE w i t h a near u n i t s l o p e : a


s

ATMOSPHERIC

238


EC

(

9

GRALE » ° · ± ° ·

2 )

EC

REFLECT

+

(

3

i

4

)

AEROSOL

( 1 )

and c o r r e l a t i o n c o e f f i c i e n t of 0.96. They found that the butane soot standards have r e f l e c t a n c e p r o p e r t i e s r e p r e s e n t a t i v e of ambient elemental carbon but that a small amount of non-elemental carbon may remain i n samples and standards a f t e r heat treatment i n a i r at 300° C. The Nuclepore f i l t e r s used are t r a n s l u c e n t , l i g h t i n weight and non-hygroscopic, which makes them d e s i r a b l e f o r mass measurements and l i g h t absorption measurements. F i l t e r p r e p a r a t i o n cons i s t e d of removing any s t a t i c charge and weighing each f i l t e r . Following sample c o l l e c t i o n at a flowrate of approximately 20 1/min, the f i l t e r s were sealed and c h i l l e d . A f t e r reweighing to determine the c o l l e c t e d a e r o s o l mass, the Nuclepore polycarbonate filters were d i v i d e d i n t o p i e shaped wedges f o r determination of Dabs and t r a c e metal concentrations. Measurement of b was performed using the opal g l a s s i n t e g r a t i n g p l a t e technique (14) as modified by Ouimette (15). In t h i s procedure an exposed f i l t e r i s i l l u m i n a t e d by coherent light at a wavelength of 0.6328 ym transmitted through a 0.25 mm diameter o p t i c a l fiber. Light transmitted through the sample i s detected by a photo t r a n s i s t o r . L i g h t absorption by the a e r o s o l deposit i s c a l c u l a t e d from the ratio of the i n t e n s i t y of l i g h t transmitted through the a e r o s o l deposit to that transmitted through a blank edge of the same filter: a

b , = abs

— V

In

—

D

g

(2)

I

where Dabs

i s the a e r o s o l l i g h t absorption c o e f f i c i e n t (m * ) ;

A

i s the f i l l e r surface area covered by the aerosol deposit (m );

V

i s the volume of a i r drawn through the f i l t e r

I

i s the i n t e n s i t y of l i g h t transmitted through an unexposed p o r t i o n of the f i l t e r ; and

I

i s the i n t e n s i t y of l i g h t transmitted through the a e r o s o l deposit on the exposed area of the f i l t e r .

3 (m );

At l e a s t f i v e r e p l i c a t e measurements of transmission were made on different p o r t i o n s of the f i l t e r sample to account f o r any v a r i a tions i n the f i l t e r loading. The standard d e v i a t i o n of the



13.

CONKLIN E T AL.

Carbonaceous

Aerosols

in Los

239

Angeles

transmission measurements made on a s i n g l e sample ranged between 1% and 4% of the mean value f o r that sample. The t h i r d sample taken each hour was c o l l e c t e d on a 13 mm P a l l f l e x T i s s u q u a r t z (2500 QAO) f i l t e r at a f l o w r a t e of 20 1/min (nominal). Those f i l t e r s also were p r e f i r e d to 900°C f o r 1-1/2 hours p r i o r to use to lower t h e i r carbon content. The 13 mm quartz f i l t e r s were archived f o r f u t u r e use. Hourly measurements of other monitored pollutants and m e t e o r o l o g i c a l c o n d i t i o n s were obtained from instruments located at each sampling s t a t i o n : Ν 0 , NO, N0 χ

2

(Thermoelectron Corp Model 14

CO (Mine Safety Appliances Co LIRA Model 8501-SCA (Los Angeles))

B/E)

(Pasadena)

and

P a r t i c u l a t e matter i n Km u n i t s (A.R. Chaney Chemical t o r i e s , Inc., Autosampler)

Bendix

Labora

Wind Speed Wind D i r e c t i o n Wintertime Carbonaceous

A e r o s o l s i n Los Angeles

The average t o t a l carbon and elemental carbon concentrations observed at downtown Los Angeles and Pasadena are i l l u s t r a t e d i n F i g u r e 2. Values shown are averages of one-hour samples taken during the period 7 am to 1 pm. Both elemental and t o t a l carbon concentrations are higher i n downtown Los Angeles. This was expected as elemental carbon a e r o s o l i s a primary p o l l u t a n t emit ted from motor v e h i c l e s and stationary combustion sources. Figure 3 shows the d i u r n a l trend i n elemental carbon concentra tions measured at Pasadena and Los Angeles. Elemental carbon concentrations d e c l i n e from e a r l y morning u n t i l mid-day i n a manner s i m i l a r to hourly t r a f f i c d e n s i t y (Figure 4 ) . Wintertime samples at Pasadena and downtown Los Angeles from the present study show elemental carbon to average about 31% and 37% of t o t a l carbon, r e s p e c t i v e l y . Considerable s c a t t e r i n the r a t i o of elemental to t o t a l carbon was observed between i n d i v i d u a l short-term samples. These data can be compared to winter studies i n other c i t i e s . Samples taken i n Denver during November 1973 showed t o t a l carbon concentrations averaging 15.7 ygm , of which about 68% could not be removed by solvent e x t r a c t i o n . The i n s o l u b l e carbon present was considered as an upper l i m i t on elemental carbon (3.). Gundel (10) reported i n s o l u b l e carbon concentrations averaging 48% of t o t a l carbon present i n Berkeley, C a l i f o r n i a dur ing the winter of 1978. Each of these studies would confirm that elemental carbon forms a s u b s t a n t i a l f r a c t i o n of the carbon con t a i n i n g p a r t i c u l a t e matter observed i n c i t i e s during the winter months.


ATMOSPHERIC


240

AEROSOL

PACIFIC OCEAN

Figure 1.

The metropolitan Los Angeles area showing sampling sites in relation the local freeway network (( © ) air monitoring station)

to

PASADENA

Figure 2. centrations

Average total ((O) by grale) and elemental ((%) by reflectance) C conobserved at downtown Los Angeles and Pasadena during the hours 7 am to 1 pm, January 15-February 26, 1980 (units in pg/m ) 3


CONKLIN E T A L .

Carbonaceous

Aerosols

in Los

Angeles


1/15/80 20 10

-I

0 1/22/80

1/24/80

20 -

20

ίο H

10

0 2/5/80

1/31/80

20 -

20 -

1 0 -

10 -

0 2/19/80

2/14/80

20 -

20

10 -

10 0 2/26/80

20 -

20

1 0-

ioH

0

ο 8

9

10

II

12

13

TIME

LOS Figure 3.

10

M

12

13

TIME

ANGELES

Temporal

9

trends in elemental

PASADENA C concentrations

(pg/m*)


ATMOSPHERIC

242

ce

AEROSOL

lOf

Ο

ce

LU CL


< CC

Wintertime Carbonaceous Aerosols in Los Angeles - ACS Publications

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