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Chapter 28 Aerodynamic Size Associations of Natural Radioactivity with Ambient Aerosols 2

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E. A. Bondietti1,C. Papastefanou, and C. Rangarajan 1Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge,

Downloaded by CORNELL UNIV on May 8, 2017 | http://pubs.acs.org Publication Date: February 5, 1987 | doi: 10.1021/bk-1987-0331.ch028

TN 37831 Nuclear Physics Department, Aristotle University of Thessaloniki,

Thessaloniki 540 06, Greece 3Division of Radiological Protection, Bhabha Atomic Energy Research Centre, Trombay, Bombay 400 085, India The aerodynamic size distributions of Pb-214, Pb-212, Pb-210, Be-7, P-32, S-35 (as SO ), and stable SO were measured using cascade impactors. The activity distribution of Pb-212 and Pb-214 was largely associated with aerosols smaller than 0.52 µm. Based on 46 measurements, the activity median aerodynamic diameter of Pb-212 averaged 0.13 µm (σ = 2.97), while Pb-214 averaged 0.16 µm (σ = 2.86). The larger median size of Pb-214 was attributed to α-recoil depletion of smaller aerosols following decay of aerosol-associated Po-218. Subsequent Pb-214 condensation on all aerosols effectively enriches larger aerosols. Pb-212 does not undergo this recoil-driven redistribution. Low-pressure impactor measurements indicated that the mass median aerodynamic diameter of SO was about three times larger than the activity median diameter of Pb-212, reflecting differences in atmospheric residence times as well as the differences in surface area (Pb-212) and volume (SO ) distributions of the atmospheric aerosol. Cosmogenic radionuclides, especially Be-7, were associated with smaller aerosols than SO regardless of season, while Pb-210 distributions in summer measurements were similar to sulfate but smaller in winter measurements. Even considering recoil following Po-214 a-decay, the average Pb-210-labeled aerosol grows by about a factor of two during its atmospheric lifetime. The presence of 5 to 10% of the Be-7 on aerosols greater than 1µmwas indicative of post-condensation growth, probably either in the upper atmosphere or after mixing into the boundary layer. 2

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The decay of Rn-222 and Rn-220 in the atmosphere produces low vapor pressure progeny that coagulate with other nuclei or condense on existing aerosols. These progeny include 3.0-min (radioactive half-life) Po-218, 26.8-min Pb-214, and 10.6-h Pb-212. A 0097-6156/87/0331-0377$06.00/0 © 1987 American Chemical Society

Hopke; Radon and Its Decay Products ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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l o n g - l i v e d d a u g h t e r i n t h e Rn-222 decay c h a i n , 22-year Pb-210, i s produced about an h o u r a f t e r attachment o f Po-218. Cosmic r a y bombardment o f gases i n t h e atmosphere a l s o p r o d u c e s c o n d e n s a b l e radionuclides. F o r example, N2 and 0 a r e p e r c u r s o r s o f 53.3-d Be-7, and Ar-40 g i v e s r i s e t o 87-d S-35 and 14-d P-32. The atmospheric residence times of these r a d i o n u c l i d e s are c o n t r o l l e d e i t h e r by r a d i o d e c a y o r a e r o s o l r e m o v a l . In a d d i t i o n to these n u c l e a r r e a c t i o n s , myriads of other gas-phase t r a n s f o r m a t i o n s p r o d u c e low-vapor p r e s s u r e s p e c i e s , w i t h t h e o x i d a t i o n o f SO2 and o t h e r reduced s u l f u r s p e c i e s d o m i n a t i n g a e r o s o l f o r m a t i o n and growth. O x i d a t i o n o f SO2 i n t h e gas phase p r o d u c e s H2SO4, a r e a d i l y c o n d e n s a b l e s p e c i e s t h a t e i t h e r combines w i t h o t h e r m o l e c u l e s (new p a r t i c l e f o r m a t i o n ) o r condenses on e x i s t i n g a e r o s o l s . W h i l e c o n d e n s a t i o n a l growth i s p r i n c i p a l l y r e s p o n s i b l e f o r a e r o s o l growth up t o s e v e r a l t e n t h s o f a m i c r o n ( H e r i n g and F r i e d l a n d e r , 1982; McMurray and W i l s o n , 1983), o x i d a t i o n of SO2 a l s o o c c u r s i n t h e a e r o s o l phase t h r o u g h c h e m i c a l r e a c t i o n s . This mode o f SO2 o x i d a t i o n i s b e l i e v e d t o be r e s p o n s i b l e f o r growth o f a e r o s o l s above s e v e r a l t e n t h s o f a m i c r o n ( H e r i n g and F r i e d l a n d e r , 1982; McMurray and W i l s o n , 1983), b e c a u s e growth r a t e s i n c r e a s e w i t h a e r o s o l d i a m e t e r , i n c o n t r a s t t o c o n d e n s a t i o n a l growth, where d i a m e t e r growth d e c r e a s e s as d i a m e t e r i n c r e a s e s ( F r i e d l a n d e r , 1977).


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Aerodynamic S i z e D i s t r i b u t i o n s o f N a t u r a l l y - R a d i o a c t i v e Aerosols. Measurements o f r a d i o n u c l i d e d i s t r i b u t i o n s u s i n g c a s c a d e i m p a c t o r s i n d i c a t e t h a t Be-7 and Pb-210 a r e a s s o c i a t e d w i t h l a r g e r a e r o s o l s t h a n Pb-212 and Pb-214 (Rôbig e t a l . , 1980; P a p a s t e f a n o u and B o n d i e t t i , 1986). Measurements of Pb-210 a s s o c i a t i o n s o v e r oceans i n d i c a t e d a c t i v i t y median aerodynamic d i a m e t e r s (AMAD) n e a r 0.6 um (Sanak e t a l . , 1981). The i m p a c t o r measurements of Moore e t a l . (1980) on Pb-210, B i - 2 1 0 , and Sr-90 s i z e s i n c o n t i n e n t a l a i r i n d i c a t e d t h a t about 80% o f the a c t i v i t y from a l l t h r e e n u c l i d e s was a s s o c i a t e d w i t h a e r o s o l s below 0.3 μπι. T h a t work a l s o d e t e r m i n e d t h a t t h e mean age o f a e r o s o l Pb-210 was about a week. Knuth e t a l . (1983) compared Pb-210 and s t a b l e Pb s i z e s a t a c o n t i n e n t a l l o c a t i o n and found t h a t 78% o f the Pb-210 found below 1.73 μπι was s m a l l e r t h a n 0.58 um. Young (1974) r e p o r t e d t h a t t h e most o f the Be-7 i n t h e atmosphere was a s s o c i a t e d w i t h submicron a e r o s o l s . T h i s paper summarizes p a r t o f t h e r e s u l t s o f an i n v e s t i g a t i o n d e s i g n e d t o c h a r a c t e r i z e t h e aerodynamic s i z e d i s t r i b u t i o n s o f n a t u r a l r a d i o a c t i v i t y and t o e v a l u a t e t h e r e s u l t s i n t h e c o n t e x t o f s u l f a t e d i s t r i b u t i o n s and r e c e n t advances i n t h e u n d e r s t a n d i n g o f a e r o s o l growth mechanisms. T h i s p a p e r , w h i l e e m p h a s i z i n g our r e s u l t s on Pb-212 and Pb-214, a l s o summarizes our i n i t i a l d a t a f o r longer-lived radionuclides. E x p e r i m e n t a l Methods Measurements on aerodynamic s i z e s o f a t m o s p h e r i c a e r o s o l s and a s s o c i a t e d r a d i o n u c l i d e s were c a r r i e d out w i t h Anderson 2000, I n c . , 1-ACFM Ambient Impactors w i t h o r w i t h o u t t h e Anderson l o w - p r e s s u r e m o d i f i c a t i o n , as w e l l as w i t h S i e r r a model 236 (six-stage) high-volume i m p a c t o r s (HVI). The 1-ACFM d e s i g n o p e r a t e d a t 28


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L min"" (1 f t m i n " ) . The s t a g e s had e f f e c t i v e c u t o f f d i a m e t e r s (ECDs) o f 0.4, 0.7, 1.1, 2.1, 3.3, 4.7, 7.0, and 11.0 μπι. The l o w - p r e s s u r e m o d i f i c a t i o n , which a l t e r s t h e i m p a c t o r * s o p e r a t i o n by i n c r e a s i n g t h e r e s o l u t i o n i n t h e s u b m i c r o n region, involves a regulated flow r a t e of 3 L m i n , f i v e l o w - p r e s s u r e (114 mm Hg) s t a g e s f o r the submicron r e g i o n and e i g h t a t m o s p h e r i c p r e s s u r e s t a g e s f o r s e p a r a t i n g a e r o s o l s above 1.4 μπι. The ECDs o f t h e l o w - p r e s s u r e s t a g e s were 0.08, 0.11, 0.23, 0.52, and 0.90 μπι, whereas f o r t h e u p p e r s t a g e s t h e y were 1.4, 2.0, 3.3, 6.6, 10.5, 15.7, 21.7, and 35.0 μπι. The s t a i n l e s s s t e e l p l a t e s s u p p l i e d by t h e m a n u f a c t u r e r were u s e d f o r a e r o s o l collection. E i t h e r p o l y c a r b o n a t e o r g l a s s - f i b e r backup f i l t e r s were used t o c o l l e c t a l l p a r t i c l e s below t h e 0.08-μπι c o l l e c t i o n plate. The S i e r r a HVI had ECDs o f 0.41, 0.73, 1.4, 2.1, 4.2, and 10.2 μπι. No c o l l e c t i o n s u b s t r a t e s were used s i n c e p r e l i m a r y comparisons o f ^Be s i z e s u s i n g two i m p a c t o r s o p e r a t e d i n p a r a l l e l i n d i c a t e d t h a t the u s e o f g l a s s f i b e r s u b s t r a t e s s h i f t s t h e median s i z e o f ^Be from about 0.40 μπι ( b a r e i m p a c t o r s u r f a c e ) t o about 0.6 μπι ( g l a s s f i b e r ) . A l l i m p a c t o r s were o p e r a t e d a t the f l o w r a t e s and p r e s s u r e s s p e c i f i e d i n t h e m a n u f a c t u r e r s o p e r a t i n g manuals. The ECDs used f o r r e p o r t i n g d i s t r i b u t i o n s were t h o s e r e p o r t e d by t h e m a n u f a c t u r e r as we had no method f o r i n d e p e n d e n t l y c a l i b r a t i n g t h e i m p a c t o r s u s i n g a e r o s o l s o f the same geometry and d e n s i t y as t h e ambient a e r o s o l .


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The l e n g t h o f each c o l l e c t i o n p e r i o d v a r i e d from about 3 h f o r Pb-214, t o 30 o r 40 h f o r Pb-212 and 1 t o 14 d f o r Be~7, Pb-210, S-35, and P-32, depending on i m p a c t o r and o b j e c t i v e . The amount o f a i r sampled w i t h t h e L P I was u s u a l l y l e s s t h a n 10 m, w i t h l e s s than 0.1 mg t o t a l So|~ found t o have d e p o s i t e d . The samples were c o l l e c t e d 13 m above t h e ground on t h e r o o f o f t h e E n v i r o n m e n t a l S c i e n c e s D i v i s i o n b u i l d i n g , Oak R i d g e N a t i o n a l Laboratory. The d e p o s i t s on t h e s t a i n l e s s - s t e e l c o l l e c t i o n p l a t e s o f t h e l o w - p r e s s u r e i m p a c t o r ( L P I ) were l e a c h e d w i t h a s o l u t i o n o f 1 M HNO3 and t h e l e a c h a t e r a p i d l y e v a p o r a t e d on 5.08-cm s t a i n l e s s s t e e l p l a t e s (using a hot p l a t e ) . The c o n c e n t r a t i o n s o f Pb-212 and Pb-214 i n t h e impactor p l a t e l e a c h a t e s were measured u s i n g seven ZnS(Ag) a l p h a s c i n t i l l a t i o n c o u n t e r s . The HVI p l a t e s were l e a c h e d w i t h 0.1 M HCI. Be-7 was measured u s i n g i n t r i n s i c germanium c o a x i a l and w e l l d e t e c t o r s . Lead-210 was d e t e r m i n e d 30 days a f t e r c o l l e c t i o n stopped by s e p a r a t i n g and m e a s u r i n g B i - 2 1 0 (Poet e t a l . , 1972). When Pb-210 was measured, t h e u p p e r two HVI s t a g e s were c o a t e d w i t h a t h i n l a y e r o f p e t r o l e u m j e l l y t o m i n i m i z e s o i l p a r t i c l e bounce. S u l f u r - 3 5 was measured by p u r i f y i n g the s u l f a t e from t h e HVI l e a c h a t e s u s i n g an a l u m i n a column ( V e l j k o v i c and M i l e n k o v i c , 1958). A f t e r e l u t i n g t h e s u l f a t e from t h e a l u m i n a column w i t h NH4OH, i t was c o n v e r t e d t o H2SO4 u s i n g a s m a l l Dowex~50(H ) column. The r e s u l t i n g s o l u t i o n o f d i l u t e s u l f u r i c a c i d was c o n c e n t r a t e d and t h e β-activity d e t e r m i n e d u s i n g l i q u i d s c i n t i l l a t i o n techniques. The p u r i t y o f S-35 was checked by f o l l o w i n g t h e decay r a t e o f the i s o l a t e d β-activity. Phosphorus-32 was i s o l a t e d from 1 M HCI by p r e c i p i t a t i o n of z i r c o n i u m p h o s p h a t e ( M u l l i n s and L e d d i c o t t e , 1962). N o n r a d i o a c t i v e s u l f a t e was d e t e r m i n e d by Dionex anion-exchange chromatography methods. 3

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W h i l e n u c l e p o r e p o l y c a r b o n a t e membranes (0.4 um) were p r e f e r r e d as backup f i l t e r s when s u l f a t e was t o be measured, g l a s s - f i b e r f i l t e r s had t o be used i n t h e HVI measurements. Because o f p o t e n t i a l g l a s s f i b e r s u l f a t e a r t i f a c t s , a s e p a r a t e s a m p l i n g f o r t o t a l s u l f a t e was made u s i n g a p o l y c a r b o n a t e f i l t e r .


Results Pb-214 and Pb-212 D i s t r i b u t i o n s . The α-activity r e c o v e r e d from t h e LPI s t a g e s i n i t i a l l y decayed a t a 26.8-min h a l f - l i f e ( i . e . , Pb-214); a f t e r t h r e e h o u r s , t h e r a t e approached t h a t o f 10.6-h Pb-212 ( P a p a s t e f a n o u and B o n d i e t t i , 1986). The measured a l p h a s were a c t u a l l y d e r i v e d from t h e d a u g h t e r s o f t h e s e n u c l i d e s . The c o u n t i n g r a t e d a t a were a n a l y z e d by d i f f e r e n t i a l decay r a t e a n a l y s i s a f t e r background c o r r e c t i o n t o d e t e r m i n e t h e Pb-214 and Pb-212 a c t i v i t y o f t h e samples a t t h e s t o p s a m p l i n g t i m e . The 1 σ c o u n t i n g u n c e r t a i n t i e s were 5% o r l e s s f o r s t a g e s c o r r e s p o n d i n g t o a e r o s o l s below 0.52 um and 3 h) and t h e 15-min minimum d e l a y between t h e time sampling stopped and t h e time c o u n t i n g began. A l p h a s from Pb-210 and o t h e r n u c l i d e s were n e g l i g i b l e as c o n f i r m e d by r e p e a t e d e x a m i n a t i o n o f t h e count r a t e s a f t e r t h e n o r m a l 24-h r e c o r d i n g p e r i o d . Examples o f t h e o b s e r v e d a c t i v i t y s i z e d i s t r i b u t i o n s o f Pb-214 and Pb-212 vs aerodynamic d i a m e t e r (Dp) a r e p r e s e n t e d by t h e f o u r s u b p l o t s i n F i g u r e 1. These examples were s e l e c t e d from 46 measurements made o v e r a ten-month p e r i o d ( P a p a s t e f a n o u and B o n d i e t t i , 1986). About 46% o f t h e measurements showed a r a d i o a c t i v i t y peak i n t h e 0.11- t o 0.23-um r e g i o n ( s u b p l o t a ) , w h i l e 39% showed a peak i n t h e 0.23- t o 0.52 um r e g i o n ( s u b p l o t b). The r e m a i n i n g 15% o f t h e measurements r e s u l t e d i n d i s t r i b u t i o n s s i m i l a r t o t h o s e i n s u b p l o t c (8.7%) o r d ( 6 . 5 % ) , where Pb-214 and Pb-212 a c t i v i t i e s were h i g h e s t i n d i f f e r e n t s i z e ranges i n t h e same spectrum. On t h e average about 76% o f t h e Pb-214 a c t i v i t y and 67% o f t h e Pb-212 a c t i v i t y was found t o be a s s o c i a t e d w i t h a e r o s o l s i n t h e 0.08- t o 1.4-um s i z e range. The a c t i v i t y a s s o c i a t e d w i t h a e r o s o l s s m a l l e r t h a n 0.8-um can a l s o be s u b s t a n t i a l , as i n d i c a t e d i n F i g u r e 1. T a b l e I p r e s e n t s t h e average aerodynamic d i s t r i b u t i o n s o f Pb-212 and Pb-214, as w e l l as t h e f r e q u e n c y w i t h which Pb-214 o r Pb-212 was t h e dominant i s o t o p e i n each s i z e range. The A i t k e n n u c l e i f r a c t i o n (below 0.08 um) c o n t a i n e d a h i g h e r p e r c e n t a g e o f Pb-212 a c t i v i t y compared w i t h Pb-214 i n 69.6% o f t h e measurements. The predominance o f Pb-212 i n t h i s f r a c t i o n i s a l s o i l l u s t r a t e d by t h e d i s t r i b u t i o n s r e p o r t e d i n F i g u r e 1. In the remaining measurements, where Pb-214 was f r a c t i o n a l l y more abundant below 0.08 um, t h e d i s p a r i t y between t h e r e l a t i v e amounts o f each i s o t o p e was n o t n e a r l y as d r a m a t i c . C o n v e r s e l y , F i g u r e 1 and T a b l e I i l l u s t r a t e t h a t Pb-214 i s g e n e r a l l y e n r i c h e d i n t h e a c c u m u l a t i o n mode a e r o s o l , p a r t i c u l a r l y between 0.11 and 0.52 um. where most o f t h e s u r f a c e a r e a and mass o c c u r s .



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The s h i f t o f Pb-214 t o a s l i g h t l y h i g h e r s i z e d i s t r i b u t i o n compared t o Pb-212 was a l s o found u s i n g 1-ACFM and HVI i m p a c t o r s ( F i g . 2 ) . The h i g h e r f l o w r a t e s o f t h e s e i m p a c t o r s , as w e l l as t h e a b i l i t y t o measure HVI a c t i v i t y by gamma s p e c t r o s c o p y , made u s c o n f i d e n t t h a t t h i s s h i f t was r e a l and n o t a d a t a a n a l y s i s a r t i f a c t . The Pb-214 AMADs, determined w i t h t h e L P I , v a r i e d from 0.10 t o 0.37 urn (mean v a l u e , 0.16 μπι). F o r Pb-212, t h e y v a r i e d from 0.07 t o 0.245 urn (mean v a l u e 0.12 urn) ( P a p a s t e f a n o u and B o n d i e t t i , 1986). These AMAD c a l c u l a t i o n s were made assuming l o g n o r m a l d i s t r i b u t i o n s and u s i n g t h e m a n u f a c t u r e r ' s c u t o f f points. An a b b r e v i a t e d v e r s i o n o f t h e s e r e s u l t s i s p r e s e n t e d i n Table I I . Pb-212 v s SOJ" L P I D i s t r i b u t i o n s . F i g u r e 3 p r e s e n t s a summary o f t h e average Pb-212 AMADs and S0|" MADs (mass median aerodynamic d i a m e t e r s ) determined from a s e r i e s o f L P I measurements made, d u r i n g t h e p e r i o d J a n u a r y t o O c t o b e r , 1985. The Pb-212 d a t a were d e r i v e d from c o l l e c t i o n s made a t t h e same time as S O j " and from measurements made t o compare Pb-212 vs Pb-214. The mean aerodynamic d i a m e t e r o f Pb-212 was about t h r e e t i m e s s m a l l e r t h a n S0^~. Much l e s s s u l f a t e was found i n t h e a e r o s o l f r a c t i o n below 0.08 μπι, compared w i t h Pb-212. W h i l e Pb-212 was l a r g e l y a b s e n t above 0.52 μπι, about 20% o f t h e So|" o c c u r r e d above this size. Be-7, Pb-210, S-35, P-32 and SOJ'. The l o n g e r - l i v e d r a d i o n u c l i d e s were a s s o c i a t e d w i t h l a r g e r a e r o s o l s t h a n Pb-214 o r Pb-212. An example o f t h e s e d i f f e r e n c e s i s p r e s e n t e d b y F i g u r e 4, w h i c h compares t h e d i s t r i b u t i o n o f Pb-212, Pb-210, SOJJ", Be-7, S-35, and P-32 found on t w o - s i x s t a g e HVIs o p e r a t e d c o n t i n u o u s l y f o r one week i n F e b r u a r y , 1985. F i g u r e 4 i l l u s t r a t e s t h a t Pb-210, t h e cosmogenic r a d i o n u c l i d e s , and S0|" were a s s o c i a t e d w i t h l a r g e r a e r o s o l s t h a n Pb-212. I n most o f o u r a n a l y s e s , t h e f r a c t i o n o f Be-7 a s s o c i a t e d w i t h a e r o s o l s above 1.4 μπι was u s u a l l y between 5 and 10%; i n t h i s measurement 4.5% was found i n t h e 1.4 t o 2.1 μπι s i z e range, 1.1% i n t h e 2.1 t o 4.2 μπι s i z e range, and o n l y 0.2% i n s i z e s g r e a t e r t h a n 4.2 μπι. F i g u r e 4 a l s o shows t h a t cosmogenic S-35, measured as soj", does n o t have t h e same a e r o s o l d i s t r i b u t i o n as s t a b l e So|"; t h i s same r e s u l t o c c u r r e d i n two o t h e r measurements. The abundance o f S-35 above 1.4 μπι i s unknown; t h e S-35 and P-32 d i s t r i b u t i o n s p r e s e n t e d i n F i g u r e 4b ( a s w e l l as t h e Pb-210 d i s t r i b u t i o n s i n F i g u r e 4a) r e p r e s e n t o n l y t h e t h r e e c o l l e c t i o n s t a g e s below 1.4 μπι, due t o a c u r r e n t l i m i t a t i o n i n t h e d e t e c t i o n o f low c o n c e n t r a t i o n s o f t h e s e b e t a - e m i t t e r s . T a b l e I I I summarizes t h e median aerodynamic d i a m e t e r s o f Pb-210, Be-7, and S0$~ found i n measurements made t h r o u g h March 1986. Be-7 d i s t r i b u t i o n s a r e s u b s t a n t i a l l y s m a l l e r t h a n S0Jj~, r e g a r d l e s s o f t h e time o f t h e y e a r . The Pb-210 d a t a , w h i l e l i m i t e d , s u g g e s t s t h a t summer s i z e s a r e l a r g e r t h a n w i n t e r sizes. Pb-210 v s Pb-214 D i s t r i b u t i o n s . Pb-210 i s produced from t h e α-decay o f Po-214, t h e e v e n t used t o q u a n t i f y Pb-214 d i s t r i b u t i o n s on t h e L P I i m p a c t o r s . W h i l e t h e r e l a t i o n s h i p between t h e aerodynamic s i z e s o f Po-214 and Pb-210 i s c o m p l i c a t e d b e c a u s e



384

80 |1-ACFM[


212

Pb (53 mBq

"?b_

Pb (42 mBq

3

m" ) 3

P b (lj.5 Bq m" )

60

40

20

0.1

10

10

0.1

Figure 2. Aerodynamic size (D ) distributions of Pb-212 and Pb-214 activity (R) found with 1-ACFM and high-volume (HVI) impactors illustrating the particle shift of Pb-214. Lower D limits are arbitrary. p

p


28.

Table I I .

385


BONDIETTI ET AL.

Summary o f Mean M o n t h l y A c t i v i t y Median Aerodynamic D i a m e t e r s (AMAD) and G e o m e t r i c S t a n d a r d D e v i a t i o n s ( σ ) o f Radon and T h o r o n Daughter S i z e D i s t r i b u t i o n s i n Ambient A e r o s o l s &


Pb-214 Period

Number o f samples

(month)

Pb-212

AMAD (um)

AMAD (μια)

December 1984

2

0.,16

2.,35

0. 15

2,.80

J a n u a r y 1985

2

0.,17

2.,46

0. 09

2. 96

March

5

0, 17

2. 33

0. 12

2.,46

April

5

0..18

2. 08

0. 17

2.,25

May

3

0,,20

2,.28

0. 20

2.,35

June 7

4

0..21

2,.98

0. 17

2.,38

July

6

0,.16

3..27

0. 23

3.,52

August

6

0 .12

32 .51

0 .10

3 .11

10

0 .13

3 .68

0 .11

3 .44

October

2

0 .09

3 .13

0 .09

2 .85

November

1

0 .07

3 .20

0 .12

6 .70

December

2

NR*

NR*

0 .13

4 .35

September

*NR

= not recorded.



386


AMAD = 0.13 μπ\ MMAD = 0.38 jum

212

P b (N=32) 2

3

SO ^

Aerodynamic Size Associations of Natural ... - ACS Publications

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