Aerosol Composition in Relation to Air Mass Movements in North

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Aerosol Composition in Relation to Air Mass

Downloaded by SWINBURNE UNIV OF TECHNOLOGY on May 25, 2018 | https://pubs.acs.org Publication Date: October 13, 1981 | doi: 10.1021/bk-1981-0167.ch016

Movements in North China

JOHN W. WINCHESTER, MICHAEL DARZI, and ALISTAIR C. D. LESLIE Department of Oceanography, Florida State University, Tallahassee, FL 32306 WANG MINGXING, REN LIXIN, and LÜ WEIXIU Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, People's Republic of China The elemental composition of aerosol particles has been investigated, as a function of both p a r t i cle size and time, at a nonurban mountain location 110 km NE of the city of Beijing in order to deter mine relationships of composition with large scale air mass movements. The study period, 16-21 March 1980, was selected so as to follow the end (15 March) of much of the residential space heating by coal combustion i n Beijing, thus reducing a i r pollution, and to precede a season of dust storms and the mon soon shift to southerly a i r flow from urbanized eastern China. Earth crustal aerosol and marine aerosol components could be resolved from some pol lution aerosol by use of both meteorological and chemical data. Concentrations of the crustal com ponent, mainly i n a coarse particle mode (Al, S i , K, Ca, T i , Mn, Fe) but with some fine mode admixture (Zn, Μn), varied i n time. The concentration maxima and minima were approximately synchronous with those of the p a r t i a l pressure of water vapor, P , signi fying aerosol concentration changes with a i r masses as they crossed the sampling s i t e . A period of high concentrations of coarse particle Cl corresponded to an incursion of marine a i r . Fine mode S, K, C l , and Pb, also observed, are attributed mainly to a pollu tion component formed from coal combustion and other processes i n the Beijing area. H2O

The B e i j i n g a r e a i n n o r t h C h i n a i s w e l l s i t u a t e d f o r i n v e s t i g a t i n g r e l a t i o n s h i p s between t h e e l e m e n t a l c o m p o s i t i o n o f atmos p h e r i c p a r t i c u l a t e m a t t e r and a i r mass movements. The m u n i c i p a l i t y of B e i j i n g ( B e i j i n g s h i ) has a p o p u l a t i o n o f 7 m i l l i o n , h a l f o f whom l i v e i n t h e c i t y p r o p e r , and p o l l u t i o n s o u r c e s o f t r a c e g a s e s 0097-6156/81 /0167-0287$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.


288

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and a e r o s o l s a r e numerous. D u r i n g w i n t e r c o a l c o m b u s t i o n f o r space h e a t i n g i s a major p o l l u t i o n source type, but t h i s o f f i c i a l l y ends on 15 M a r c h i n B e i j i n g . Some s t e e l and c h e m i c a l i n d u s t r i e s , l o c a t e d s o u t h w e s t and s o u t h e a s t o f t h e c i t y , p r o d u c e a d d i tional a i r pollution. North of the c i t y the p o p u l a t i o n i s mainly engaged i n a g r i c u l t u r e , and n o r t h o f t h e G r e a t W a l l ( e . g . a t B a d a l i n g 50 km n o r t h o f t h e c e n t e r o f B e i j i n g ) t h e p o p u l a t i o n d e n s i t y i s q u i t e l o w o w i n g t o t h e s t e p p e and d e s e r t l a n d s . The o c e a n l i e s some 100 km e a s t o f t h e c i t y , p e r m i t t i n g i n c u r s i o n s o f m a r i n e a i r . D u r i n g t h e c o l d m o n t h s , a i r f l o w i s m a i n l y f r o m t h e n o r t h and w e s t , b r i n g i n g r e l a t i v e l y c l e a n c o n t i n e n t a l a i r f r o m M o n g o l i a and S i b e r i a over the c i t y . D u r i n g t h e warm m o n t h s , a i r f l o w i s m a i n l y f r o m t h e s o u t h and e a s t , b r i n g i n g warm and humid a i r and h e a v y monsoon r a i n f a l l t o t h e c i t y . On b o t h a s e a s o n a l and a s h o r t e r t e r m b a s i s B e i j i n g e x p e r i e n c e s c o n t r a s t i n g w e a t h e r and a i r f l o w c h a r a c t e r i s t i c s , r e f l e c t e d i n c o n t r a s t i n g atmospheric chemical p r o p e r t i e s and a e r o s o l p a r t i c l e c o m p o s i t i o n . The p r e s e n t i n v e s t i g a t i o n was c o n d u c t e d t o document t h e s e i n a p r e l i m i n a r y way i n o r d e r t o l a y a g r o u n d w o r k f o r f u t u r e more d e t a i l e d s t u d i e s o f r e g i o n a l a e r o s o l c h e m i s t r y and l o n g r a n g e a i r p o l l u t i o n t r a n s p o r t . Meteorological Conditions During Aerosol

Sampling

F o r t h i s i n v e s t i g a t i o n a p r o g r a m o f a e r o s o l s a m p l i n g and me t e o r o l o g i c a l measurements was p l a n n e d a t a n o n u r b a n s i t e 110 km NE o f t h e c e n t e r o f B e i j i n g , commencing on 9 M a r c h and c o n t i n u i n g u n t i l 3 A p r i l 1980. T h i s p e r i o d i n c l u d e d t h e l a s t week o f t h e o f f i c i a l w i n t e r s p a c e h e a t i n g s e a s o n i n B e i j i n g ( t o 15 M a r c h ) and t h e l a s t 3 weeks o f t h e o f f i c i a l h e a t i n g s e a s o n i n r u r a l a r e a s n o r t h o f t h e G r e a t W a l l ( t o 31 M a r c h ) . The p r e s e n t p a p e r i s b a s e d p r i m a r i l y on measurements made 16 t o 21 M a r c h . The s i t e , an a s t r o n o m i c a l o b s e r v i n g s t a t i o n o f t h e C h i n e s e Academy o f S c i e n c e s n e a r X i n g l o n g , i s l o c a t e d i n a m o u n t a i n o u s a g r i c u l t u r a l a r e a o f t h e Yan Shan r a n g e , l a t i t u d e 40°23'Ν and l o n g i t u d e 117°30 E, a t 960 m e t e r s e l e v a t i o n j u s t e a s t o f t h e b o r d e r b e t w e e n B e i j i n g s h i and H e b e i p r o v i n c e and a b o u t 30 km n o r t h and e a s t o f p a r t s o f t h e G r e a t W a l l . W u l i n g Shan, 2100 m e t e r s e l e v a t i o n and a b o u t 25 km n o r t h o f t h e s i t e , was t h e h i g h e s t p e a k i n t h e v i c i n i t y and c o u l d be c l e a r l y s e e n on some d a y s b u t was o b s c u r e d by r e g i o n a l h a z e on o t h e r d a y s d u r i n g t h e measurement p r o g r a m d e s c r i b e d h e r e . Except f o r a small c o a l b u r n i n g h e a t i n g p l a n t and a k i t c h e n n e a r t h e s i t e , l o c a l a i r p o l l u t i o n s o u r c e s w e r e v i r t u a l l y a b s e n t , and t h e a e r o s o l c h a r a c t e r i s t i c s r e p o r t e d h e r e a r e c o n s i d e r e d t o be e s s e n t i a l l y r e g i o n a l with l i t t l e local influence. M e a s u r e m e n t s made d u r i n g t h e s e c o n d week, 16-21 M a r c h 1980, are e s p e c i a l l y r e l e v a n t to the chemical c h a r a c t e r i z a t i o n of a i r m a s s e s . D u r i n g t h i s p e r i o d m e t e o r o l o g i c a l c o n d i t i o n s c a n be sum m a r i z e d as f o l l o w s , b a s e d on w e a t h e r i n f o r m a t i o n f r o m t h e I n s t i t u t e o f A t m o s p h e r i c P h y s i c s and t w i c e d a i l y s u r f a c e and u p p e r l e v e l w e a t h e r maps p u b l i s h e d by t h e J a p a n M e t e o r o l o g i c a l A g e n c y . f



16.

WINCHESTER E T AL.

Aerosol

Composition

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North

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289

A t m o s p h e r i c c o n d i t i o n s over the s a m p l i n g s i t e were v a r i a b l e , and s e v e r a l d i f f e r e n t a i r masses p a s s e d b y . On 16 M a r c h t h e weath e r map o f t h e n o r t h e r n h e m i s p h e r e shows two h i g h p r e s s u r e s , one o v e r L a k e B a i k a l and t h e o t h e r o v e r S o u t h K o r e a , so t h a t t h e sam p l i n g s i t e was j u s t i n b e t w e e n ; t h e a i r was c a l m , c l o u d y , and c o n d u c i v e f o r p o l l u t a n t s t o b u i l d up. L a t e r the n o r t h h i g h pressure moved s o u t h w a r d s . By m i d d a y o f 17 M a r c h t h e s a m p l i n g s i t e was d o m i n a t e d by h i g h p r e s s u r e . A t t h a t t i m e a i r f l o w o v e r t h e s i t e was m a i n l y f r o m t h e n o r t h w e s t , t h e w i n d s p e e d was 3 m e t e r s / s e c o n d , and t h e w a t e r v a p o r c o n t e n t was v e r y l o w . The s k y was c l e a r , and v e r t i c a l d i l u t i o n was e f f e c t i v e . A f t e r t h e h i g h p r e s s u r e p a s s e d o v e r , a c y c l o n e was f o r m e d t o the n o r t h w e s t o f t h e s a m p l i n g s i t e , w h i c h c a u s e d s o u t h e a s t e r l y w i n d and b r o u g h t warm and m o i s t m a r i n e a i r t o t h e s i t e f r o m l a t e 17 t o 18 M a r c h . L a t e on 18 M a r c h t h e m a r i n e a i r mass moved b a c k w a r d s b e c a u s e o f s t r o n g n o r t h h i g h p r e s s u r e , and a c o l d f r o n t f o r m e d and p a s s e d o v e r t h e s a m p l i n g s i t e s h o r t l y a f t e r i t s f o r m a t i o n e a r l y i n t h e m o r n i n g o f 19 M a r c h . A f t e r the p a s s i n g of the c o l d f r o n t , t h e r e was a 2-day p e r i o d o f c l e a n c o n t i n e n t a l a i r mass moving over the sampling s i t e . On 21 M a r c h a warm a i r mass moved n o r t h w a r d s a g a i n and c a u s e d snow by m i d n i g h t . Experimental

Procedures

The methods e m p l o y e d i n t h i s i n v e s t i g a t i o n w e r e p r i n c i p a l l y b a s e d on a e r o s o l s a m p l i n g and e l e m e n t a l a n a l y s i s by p a r t i c l e i n duced X - r a y e m i s s i o n , ΡΙΧΕ, as r e v i e w e d by J o h a n s s o n and J o h a n s s o n (1). T h e s e methods a r e d i s c u s s e d w i t h s p e c i f i c r e f e r e n c e t o t h e p r e s e n t s t u d y b y Wang e t a l . ( 2 ) . B e c a u s e o f t h e i n h e r e n t s e n s i t i v i t y o f t h e P I X E method and i t s s u i t a b i l i t y f o r a n a l y s i s o f sam p l e s h a v i n g a few mm2 a r e a , P I X E - c o m p a t i b l e a e r o s o l s a m p l e r s w e r e e m p l o y e d . T h e s e w e r e : (1) t i m e s e q u e n c e f i l t e r s a m p l e r s , " s t r e a k e r s " , w h i c h c o l l e c t an a e r o s o l d e p o s i t a s a s t r e a k a l o n g t h e s u r f a c e o f a 0.4 ym p o r e s i z e N u c l e p o r e f i l t e r s t r i p by means o f a c o n t i n u o u s l y m o v i n g s u c k i n g o r i f i c e d r a w i n g a i r a t 0.7 l i t e r / m i n u t e t h r o u g h t h e f i l t e r w h i l e s l i d i n g a l o n g i t s smooth b a c k s i d e ( 3 ) ; and (2) s i n g l e o r i f i c e c a s c a d e i m p a c t o r s w i t h 8 s t a g e s and p a r t i c l e s i z e c u t s ( f o r 1 l i t e r / m i n u t e f l o w r a t e ) a t 0.25, 0.5, 1, 2, 4, 8, and 16 ym a e r o d y n a m i c d i a m e t e r (ymad). (Actual s i z e cuts a r e s l i g h t l y g r e a t e r a t t h e 0.8 l i t e r / m i n u t e f l o w r a t e u s e d h e r e . ) The s t r e a k e r s p r o v i d e up t o 14 days o f s a m p l e s t r e a k (168 mm l o n g ) on a s i n g l e f i l t e r w h i c h c o u l d be a n a l y z e d i n 4-hour t i m e s t e p s u s i n g a 2-mm c o l l i m a t e d p r o t o n beam. The i m p a c t o r s w e r e f a b r i c a t e d f r o m p l a s t i c ( P I X E I n t e r n a t i o n a l , I n c . , T a l l a h a s s e e , F L , U.S.A.) and o p e r a t e d f o r 10 o r 12 h o u r t i m e p e r i o d s t o c o l l e c t 8 p a r t i c l e s i z e f r a c t i o n s on V a s e l i n e o r p a r a f f i n c o a t e d M y l a r f i l m o r , f o r e x h i b i t e d v a r i a b i l i t y w i t h t i m e a t t h e X i n g l o n g s i t e w h i c h c o r r e s p o n d e d t o t h e s u c c e s s i o n o f a i r masses w h i c h passed over the s i t e . F i g u r e 1 shows t h i s v a r i a b i l i t y t o g e t h e r w i t h t h e p a t t e r n s o f c o n c e n t r a t i o n v a r i a b i l i t y o f F e , S, a n d C I c o l l e c t e d by t h e N u c l e p o r e f i l t e r o f t h e s t r e a k e r and d e t e r m i n e d by P I X E . A l s o i n d i c a t e d a r e a p p r o x i m a t e t i m e s o f o c c u r r e n c e o f the d i f f e r e n t a i r masses, i n f e r r e d from b o t h m e t e o r o l o g i c a l con d i t i o n s and c h e m i c a l c h a r a c t e r i s t i c s : a p e r i o d o f r a t h e r p o l l u t e d a i r ( P ) o n 16-17 M a r c h , one w i t h m a r i n e a i r m i x e d w i t h c o n t i n e n t a l (M+C) o n 17-18 M a r c h , t h e n c o n t i n e n t a l a i r w i t h some p o l l u t a n t s (C+P) o n 18-19 M a r c h , f o l l o w e d b y r a t h e r u n p o l l u t e d c o n t i n e n t a l a i r (C) o n 20 M a r c h , and f i n a l l y c o n t i n e n t a l a i r w i t h r i s i n g p o l l u t i o n l e v e l s (C+P) o n 21 M a r c h . A l o n g t h e t o p o f F i g u r e 1 a r e g i v e n t h e m i d p o i n t s o f a l t e r n a t i n g 10 a n d 12 h o u r p e r i o d s o f s a m p l e s c o l l e c t e d by cascade impactors, d i s c u s s e d f u r t h e r below. Fe i s r e p r e s e n t a t i v e o f a g r o u p o f e l e m e n t s i n p a r t i c u l a t e matter d e r i v e d l a r g e l y from e a r t h c r u s t m i n e r a l s . I t s trace i n F i g u r e 1 shows t i m e s o f maximum and minimum c o n c e n t r a t i o n s w h i c h u s u a l l y o c c u r w i t h i n a few hours o f a s i m i l a r p a t t e r n i n Ρ ^ Ο · * f we i n t e r p r e t t h e m a j o r v a r i a t i o n s o f P ^ O r e s u l t of a i r mass c h a n g e s , s u c h a s b y f r o n t a l p a s s a g e s , t h e n l a r g e c h a n g e s i n t

o

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WINCHESTER

ET AL.

Aerosol

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IMPACTOR

SAMPLE

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China

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NUMBER


ττπχττττ 3132

3334

35" 36

3738

DATE, Figure

1.

Water

vapor

centrations

pressure

(Ph o 2

(ng/m ) measured 3

39" 40

MARCH in mbars)

4h 42

43" 44

45" 46

47" 48

1980 and aerosol

using streaker sampler at

Fe, S, and CI

con

Xinglong.

Approximate times of polluted (P), continental (C), and marine (M) air masses are indi cated based on synoptic weather maps and consistent with aerosol composition measure ments. Times of impactor samples, taken in duplicate concurrently with the streaker, are indicated at the top as midpoints of alternating 10- and 12-h sampling periods.


ATMOSPHERIC

292

t h e c r u s t a l a e r o s o l c o n c e n t r a t i o n s may a l s o r e p r e s e n t c h a n g e s i n r e l a t i v e l y l a r g e s c a l e a i r mass c o m p o s i t i o n c h a r a c t e r i s t i c s . We r e g a r d t h e c a l i b r a t i o n o f s t r e a k e r t i m i n g t o b e a c c u r a t e t o 0.5-1 t i m e s t e p (2-4 h o u r s ) . S i n c e some o f t h e t i m e d i f f e r e n c e s b e t w e e n c o r r e s p o n d i n g f e a t u r e s o f t h e F e and P ^ O c e s a r e greater than t h i s u n c e r t a i n t y , t h e r e may be s m a l l b u t r e a l d i f f e r e n c e s i n a r r i v a l t i m e s o f t h e s e c o n s t i t u e n t s o f s u c c e s s i v e a i r masses a t t h e sampling s i t e . S o c c u r s g e n e r a l l y a t a r o u n d 2000 ng/m^ ( h i g h e r o n 16 M a r c h ) and w i t h a p a t t e r n o f v a r i a b i l i t y u n l i k e t h a t o f F e . We b e l i e v e t h a t a e r o s o l S h e r e i s m a i n l y a n a i r p o l l u t a n t and t h a t i t s c o n c e n t r a t i o n v a r i a t i o n s a r e governed by s e v e r a l f a c t o r s . These i n c l u d e g a s e o u s SO2 c o n c e n t r a t i o n s , t h e r a t e o f g a s - t o - p a r t i c l e c o n v e r s i o n d u r i n g t r a n s p o r t , and a t m o s p h e r i c m i x i n g p r o c e s s e s . The h i g h e r i n i t i a l c o n c e n t r a t i o n s may be a t t r i b u t e d i n p a r t t o h i g h e r SO2 p o l l u t i o n s o u r c e s t r e n g t h s on 15 M a r c h , t h e l a s t day p e r m i t t e d f o r much o f t h e r e s i d e n t i a l s p a c e h e a t i n g b y c o a l c o m b u s t i o n i n B e i j i n g , b u t s u b s e q u e n t a i r p o l l u t i o n b y SO2 f r o m o t h e r u t i l i z a t i o n of coal i n t h i s part of north China s t i l l maintains aerosol S l e v e l s f a r above n a t u r a l c o n t i n e n t a l l e v e l s . Thus, a e r o s o l S cann o t be c o n s i d e r e d a s c h a r a c t e r i s t i c o f a i r mass c o m p o s i t i o n i n t h e same s e n s e a s c a n a e r o s o l F e . C I was g e n e r a l l y f o u n d o n t h e N u c l e p o r e f i l t e r o f t h e s t r e a k e r s a m p l e r a t l e v e l s v e r y much l o w e r t h a n i n c a s c a d e i m p a c t o r s operated c o n c u r r e n t l y . This r e s u l t , discussed f u r t h e r below, i s a t t r i b u t e d t o v o l a t i l i t y o f C I on t h e f i l t e r , s u c h a s by c h e m i c a l r e a c t i o n w i t h a c i d i c o r o t h e r r e a c t i v e a g e n t s drawn t h r o u g h t h e filter. The c a s c a d e i m p a c t o r s s h o u l d h a v e much l e s s t e n d e n c y f o r such v o l a t i l i z a t i o n , owing t o t h e a i r p a s s i n g b y , r a t h e r than t h r o u g h , t h e i m p a c t i o n s u r f a c e s w h i c h c o l l e c t p a r t i c l e s down t o a b o u t 0.25 ymad. T h e r e f o r e , t h e p a t t e r n o f C I v a r i a b i l i t y i s u s u a l l y n o t t o be i n t e r p r e t e d as r e p r e s e n t i n g o n l y changes i n aeros o l C I c o n c e n t r a t i o n i n t h e atmosphere b u t i n s t e a d as t h e r e s u l t a n t o f b o t h t h e c o n c e n t r a t i o n i n a i r and c h e m i c a l r e a c t i v i t y on t h e filter. T h i s may b e e s p e c i a l l y t r u e when s a m p l i n g p o l l u t e d a t m o s pheres w h i c h c o n t a i n s u l f u r i c a c i d and o t h e r r e a c t i v e s u b s t a n c e s . However, i n one p a r t o f t h e r e c o r d o f F i g u r e 1, 17-18 M a r c h , C I c o n c e n t r a t i o n s on t h e s t r e a k e r f i l t e r w e r e u n u s u a l l y h i g h a n d n o t l o w e r t h a n C I c o n c e n t r a t i o n s made b y s i m u l t a n e o u s i m p a c t o r measurements. T h i s p e r i o d c o r r e s p o n d s t o an i n c u r s i o n o f marine a i r from t h e e a s t w h i c h was a l s o i n f e r r e d f r o m m e t e o r o l o g i c a l d a t a . C o n c e n t r a t i o n s o f t h e e l e m e n t s S i , A l , K, C a , and T i v a r i e d i n time w i t h p a t t e r n s n e a r l y i d e n t i c a l t o that of Fe. Figure 2 shows t h a t t h e i r r a t i o s t o Fe a r e q u i t e i n v a r i a n t a n d c l o s e t o t h e a v e r a g e v a l u e s f o r e a r t h c r u s t m a t e r i a l s g i v e n by Mason ( 6 ) . The r a t i o Ca/Fe f l u c t u a t e s s l i g h t l y , and T i / F e a v e r a g e s somewhat h i g h e r on 20-21 M a r c h t h a n p r e v i o u s l y , b u t p r i n c i p a l l y a t e r r e s t r i a l dust source f o r a l l s i x elements i s considered l i k e l y i n t h e s e s a m p l e s . F o r Mn a n d Z n , h o w e v e r , s t r a i g h t f o r w a r d d i s p e r s i o n of t e r r e s t r i a l dust i s not the only s i g n i f i c a n t source process. t r a


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WINCHESTER

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Ti/Fe

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0.02 0.1 0.05

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AEROSOL

Zn/Fe

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Figure

2.

Weight

ratios of elements

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20

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21

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Average ratios for Si, Al, K, Ca, and Ti to Fe approximate earth crust ratios, indicated by arrows at left. These elements occur mainly in coarse particles (Figures 3 and 4). The Mn/Fe ratio is somewhat greater and Zn/Fe is 20 times greater than earth crust ratios. These two elements have appreciable fine-mode concentrations (Figure 4). All 8 elements may occur characteristically in continental air masses.


294

ATMOSPHERIC

AEROSOL

F i g u r e 2 i n d i c a t e s Mn/Fe t o be somewhat above t h e c r u s t a l r a t i o t h r o u g h 19 M a r c h , and t h e r e a f t e r a marked i n c r e a s e i s s e e n . The a e r o s o l r a t i o Zn/Fe a v e r a g e s a b o u t 20 t i m e s g r e a t e r t h a n i n t h e e a r t h c r u s t (somewhat g r e a t e r on 20-21 M a r c h ) , s h o w i n g " a n o m a l o u s a t m o s p h e r i c e n r i c h m e n t o f Zn f i r s t r e c o g n i z e d by Rahn ( 7 ) . Since p a r t i c l e s i z e d i s t r i b u t i o n m e a s u r e m e n t s , d i s c u s s e d b e l o w , show s u b s t a n t i a l f i n e p a r t i c l e c o n c e n t r a t i o n s o f b o t h Zn and Mn, t h e p r o c e s s e s f o r t h e i r t r a n s f e r t o t h e a t m o s p h e r e must be d i f f e r e n t f r o m t h o s e f o r t h e o t h e r s i x e l e m e n t s o f F i g u r e 2. However, t h e i r c o n c e n t r a t i o n v a r i a t i o n s i n t i m e s t i l l r e s e m b l e t h o s e o f Fe shown i n F i g u r e 1 and t h e r e f o r e t h e s e e l e m e n t s may a l s o be r e l a t i v e l y l a r g e s c a l e c h a r a c t e r i s t i c s of a i r masses, i n c o n t r a s t to S where r e g i o n a l p o l l u t i o n s o u r c e s and a e r o s o l f o r m a t i o n p r o c e s s e s must be important. The a v e r a g e p a r t i c l e s i z e d i s t r i b u t i o n s f o r f o u r p r e d o m i n a n t l y c r u s t a l e l e m e n t s , A l , S i , Ca, and T i , a r e shown i n F i g u r e 3. They a r e e s s e n t i a l l y i d e n t i c a l . I t s h o u l d be p o i n t e d o u t t h a t t h e down t u r n o f t h e r e l a t i v e c o n c e n t r a t i o n s above 8 ymad ( i m p a c t o r s t a g e 6) i s the combined r e s u l t of the a c t u a l d i s t r i b u t i o n of p a r t i c l e s i z e s i n t h e a t m o s p h e r e and t h e e f f i c i e n c y w i t h w h i c h t h e s e v e r y c o a r s e p a r t i c l e s c a n e n t e r (upward) i n t o t h e c a s c a d e i m p a c t o r . This ef f i c i e n c y must d e c r e a s e w i t h i n c r e a s i n g p a r t i c l e s i z e and g e n e r a l l y depend on i n l e t d e s i g n and w i n d s p e e d . N e v e r t h e l e s s , i t i s i m p o r t a n t to note here t h a t the p a t t e r n s of the f o u r elements a r e s i m i l a r , i m p l y i n g a common a e r o s o l s o u r c e . F i g u r e 4 p r e s e n t s p a r t i c l e s i z e d i s t r i b u t i o n s f o r s i x elements w h i c h d i f f e r among t h e m s e l v e s and a l s o f r o m t h o s e i n F i g u r e 3. Somewhat s u b j e c t i v e l y , we may i d e n t i f y t h r e e p a t t e r n s i n t h e s e d i s t r i b u t i o n s : (a) A c o a r s e mode, t y p i f i e d by Ca and t h e o t h e r e l e ments o f F i g u r e 3, w h i c h may r e p r e s e n t a t e r r e s t r i a l d u s t o r i g i n . T h i s mode c a n a c c o u n t f o r c o a r s e p a r t i c l e c o n c e n t r a t i o n s o b s e r v e d f o r F e , K, Mn, and S. (b) A f i n e mode w i t h somewhat g r e a t e r c o n c e n t r a t i o n s i n t h e 0.5-1 ymad f r a c t i o n t h a n i n 1-2 ymad p a r t i c l e s . The amounts i n t h i s

Aerosol Composition in Relation to Air Mass Movements in North

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